NCS000331_10_Appendix G (Fort Bragg SCM Inspection & Maintenance Manual)_20210517

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Table of Contents
ACRONYMS
GLOSSARY
CHAPTER 1 INTRODUCTION....................................................................................................................... 1-1
1.1 Background...........................................................................................................1-1
1.2 Purpose.................................................................................................................1-2
1.3 How to Use this Manual.........................................................................................1-2
1.4 Disclaimer..............................................................................................................1-3
1.5 Contact Information...............................................................................................1-3
CHAPTER 2 INSPECTION............................................................................................................................ 2-1
2.1 Inspections............................................................................................................2-1
2.2 Frequency of Inspection.........................................................................................2-2
2.3 Site Visit Preparation..............................................................................................2-2
2.4 Safety Considerations.............................................................................................2-3
2.5 Reporting and Record Keeping.................................................................................2-3
CHAPTER 3 MAINTENANCE....................................................................................................................... 3-1
3.1 Maintenance of Stormwater Controls......................................................................3-1
3.2 Routine Maintenance.............................................................................................3-2
3.3 Emergency Maintenance........................................................................................3-9
CHAPTER 4 REPORTING AND RECORD KEEPING................................................................................4-1
4.1 Reporting Overview...............................................................................................4-1
4.2 Inspection Documentation.....................................................................................4-1
4.3 FOS Ratings............................................................................................................4-2
4.4 Reporting Special Circumstances..............................................................................4-3
4.5 Stormwater Management Database (SMD)....................................................................4-3
CHAPTER 5 BIORETENTION BASIN.......................................................................................................... 5-1
5.1 Stormwater Control Overview................................................................................5-2
5.2 Bioretention Components......................................................................................5-2
5.3 Inspection and Maintenance..................................................................................5-3
5.4 Inspection and Maintenance Summary.................................................................. 5-13
CHAPTER 6 INFILTRATION BASIN............................................................................................................6-1

6.1 Stormwater Control Overview................................................................................6-2
6.2 Infiltration Basin Components.................................................................................6-3
6.3 Inspection and Maintenance..................................................................................6-4
6.4 Inspection and Maintenance Summary.................................................................. 6-11

Table of Contents (continued)
CHAPTER 7 DRY DETENTION BASIN...................................................................................................... 7-1
7.1 Stormwater Control Overview.................................................................................7-2
7.2 Dry Detention Basin Components............................................................................7-3
7.3Inspection and Maintenance...................................................................................7-4
7.4Inspection and Maintenance Summary..................................................................7-14
CHAPTER 8 WET DETENTION BASIN..................................................................................................... 8-1
8.1 Stormwater Control Overview................................................................................8-2
8.2 Wet Detention Pond Components...........................................................................8-3
8.3 Inspection and Maintenance..................................................................................8-4
8.4Inspection and Maintenance Summary..................................................................8-13
CHAPTER 9 STORMWATER WETLAND................................................................................................................ 9-1
9.1 Stormwater Control Overview.........................................................................................9-2
9.2 Stormwater Control Components.....................................................................................9-3
9.3 Inspection and Maintenance...........................................................................................9-4
9.4Inspection and Maintenance Summary.........................................................................9-14
CHAPTER10 SWALE...............................................................................................................................................10-1
10.1 Stormwater Control Overview.....................................................................................10-2
10.2 Swale Components......................................................................................................10-3
10.3 Inspection and Maintenance.......................................................................................10-4
10.4 Inspection and Maintenance Summary.......................................................................10-8
CHAPTER 11 LEVEL SPREADER.........................................................................................................................11-1
11.1 Stormwater Control Overview.....................................................................................11-2
11.2 Level Spreader Components.........................................................................................11-3
11.3 Inspection and Maintenance.......................................................................................11-4
11.4 Inspection and Maintenance Summary.......................................................................11-9
REFERENCES
APPENDIX A INSPECTION CHECKLISTS

Acronyms
BMP
Best Management Practice
EPA
Environmental Protection Agency
FEMA
Federal Emergency Management Agency
NCDEMLR
North Carolina Division of Energy, Mineral and Land Resources
NCDENR
North Carolina Department of Environment and Natural Resources
NCDOT
North Carolina Department of Transportation
NCDWQ
North Carolina Division of Water Quality (NCDENR)
NCHRP
National Cooperative Highway Research Program
NPDES
National Pollutant Discharge Elimination System
NRC
National Response Center
PSH
Preformed Scour Hole
PSRM
Permanent Soil Reinforcement Matting
PVC
Polyvinyl Chloride
SC
Stormwater Control
SCM
Stormwater Control Measure
SMD
Stormwater Management Database
WMS
Water Management Section
NCDOT HSP 2010-01

Glossary
Glossary
Attenuate The reduction of flow in volume and/or force.
Best Management A general term most often used to describe methods that are the most
Practice (BMP) effective and practical means of preventing or minimizing pollution. The
term BMP is often used to describe the structures that are built to reduce
stormwater pollution.
Channelization The process by which concentrated flow erodes a channel through areas in
or around a stormwater control that can lead to compromised
performance of the control or even failure.
Concentrated Flow Water that is flowing in a channel or pipe, or that has otherwise collected
and is flowing in a manner not consistent with sheet flow.
Confined Space A space that (1) is large enough and so configured that an employee can
bodily enter and perform assigned work; (2) has limited or restricted means
for entry or exit (for example, tanks, vessels, silos, storage bins, hoppers,
vaults, and pits); and (3) is not designed for continuous employee
occupancy (29 CFR 1910.146).
Diffuse Flow Another term used to describe sheet flow.
Drawdown The lowering of a water level in a controlled manner.
Dredging Most often done with a machine (commonly on a boat) used to scoop up
sediment, gravel, or obstructions from submerged areas (in this case,
stormwater controls), so as to deepen them or restore them to their original
volume.
Embankment An earthen berm, constructed from fill material, used to store runoff in
basins.
Erosion The process by which a surface is worn away. In the context of this
manual, erosion refers to the process by which runoff washes away soil.
Function of Service (FOS) A rating system used by Ft Bragg that is assigned to each SCM
based on its condition after inspecting. Ratings are FF, PF, NF, and
Mon. See Chapter 4 for a description of each rating.
Impervious Surface A land cover through which water cannot infiltrate; examples include
concrete, asphalt, rooftops.

Infiltration The act of water soaking through the surface of the soil.
Intermittent Stream An intermittent stream has flowing water during certain times of the year,
when groundwater provides water for stream flow. During dry periods,
intermittent streams may not have flowing water. Runoff from rainfall is a
supplemental source of water for stream flow.
Percolation The act of water traveling downward through the soil.
Perennial Stream A perennial stream has flowing water year-round during a typical year.
The water table is located above the stream bed for most of the year.
Groundwater is the primary source of water for stream flow. Runoff from
rainfall is a supplemental source of water for stream flow.

Glossary
Permanent Soil A material used to hold soil in place so that vegetation can take root and
Reinforcement Matting prevent erosion; commonly used in areas with steep slopes or other erosive
(PSRM) conditions.
Riparian Buffer The term riparian refers to an area adjacent to a body of water. Riparian
buffers are naturally vegetated (undisturbed) areas through which
stormwater runoff passes as sheet flow so that infiltration of runoff and
filtration of pollutants occur before stormwater reaches the body of
water.
Runoff Precipitation on land that eventually reaches streams and rivers.
SA Waters Tidal salt waters that are used for commercial shellfishing or
marketing purposes and are also protected for all Class SC and Class SB
uses. All SA waters are also High Quality Waters by
supplemental classification.
Sediment Soil that has been eroded and deposited elsewhere; typically contains
pollutants and has a negative impact on aquatic environments.
Seepage The slow discharge or escape of water.
Sheet Flow A shallow layer of water flowing over land at a low velocity, enabling
pollutants to settle out.
Stormwater A term used to describe water that originates from precipitation; often used
interchangeably with the term runoff.
Stormwater Control A structure designed to manage stormwater and/or treat stormwater
Measure (SCM) pollutants in an effort to reduce surface water pollution.
Suspended Solids Small particles carried in stormwater as a result of erosion; regarded as an
indicator of water quality.
Toe (of embankment) The bottom of the embankment where it meets the natural ground.
Vegetated Shelf Also known as aquatic benches or shelves, vegetated shelves are those
shallow areas around the edge of stormwater controls with permanent
pools that support aquatic vegetation, both submerged and
emergent.

CHAPTER 1 Introduction
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1.1 Background
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Introduction
When it rains or snow melts, part of the water soaks into the ground, part of it evaporates, and part of
it flows over land. The part that flows over land is called stormwater runoff. In natural settings, water
from storm events is slowed by vegetation and other obstacles as it flows over land. As a result, most
of the stormwater infiltrates into the soil. This is beneficial to the environment because it helps to
maintain a relatively constant source of groundwater to streams, rivers, and other bodies of water.
Unfortunately, this is not the case in developed areas. In developed areas, much of the land is covered
by hard surfaces such as buildings and pavement. These hard surfaces, due to their impervious nature,
allow little to no infiltration and stormwater in essence becomes a transportation system for
pollutants. As stormwater flows over hard surfaces it picks up oil, antifreeze, salt, cigarette butts, paper
and other pollutants. It also picks up soil and organic material as it flows from hard surfaces and erodes
less stable surfaces. Pollutants transported by stormwater are deposited into streams, rivers and other
bodies of water where they destroy habitat and have a detrimental effect on aquatic insects, fish,
birds, mammals, and our drink water supply.

Fort Bragg has installed a number of stormwater controls measures (SCMs) across the installation to
reduce the amount of pollutants found in stormwater. These are engineered structures or devices that
are generally designed to slow down or hold the water for a short period of time and remove
pollutants before it is released to receiving waters. Stormwater controls are widely referred to as best
management practices or SCMs. They are also called "post -construction" stormwater controls because
they are designed to stay in place and treat runoff after an impervious surface is built, as opposed to
temporary erosion control practices used during a construction project (such as silt fences and
sediment basins).
1.2 Purpose
SCMs must be routinely inspected and maintained to ensure they continually function as designed. If
proper maintenance is not provided, adverse environmental impacts such as the discharge of
pollutants into ground and surface waters may occur.
In 2011, Fort Bragg was issued a National Pollutant Discharge Elimination System (NPDES) Phase II permit
by the North Carolina Department of Environment and Natural Resources (NCDENR) Division of Water
Quality (DWQ). The NPDES Phase II permit authorizes Fort Bragg & Camp Mackall to discharge
stormwater runoff and continue operation of oil water separators not associated with wastewater into
North Carolina's waterways, following Part II section F Post- Construction site runoff controls. Fort Bragg
NPDES Phase II Permit Part II.F.1.b&c requires development and implementation of strategies which
include a combination of structural and /or non-structural best management practices (BMPs) and ensure
long-term operation and maintenance of SCMs. This document will outline the regular inspection
frequency, an inspection checklist, 'how-to' instructions for regular maintenance, evaluation and
reporting procedures for routine, non -routine maintenance, and an inspection and maintenance tracking
mechanism."
This manual, entitled Fort Bragg Storm Water Control Measures Inspection and Maintenance Manual
fulfills this requirement of the NPDES Phase II permit and its intended use is to serve as a source of
guidance to those individuals responsible for the inspection and maintenance of SCMs on Fort Bragg
NC.
1.3 How to Use this Manual
This manual is intended to be a practical tool to aid in the inspection and maintenance of SCMs. The
first several chapters include a general discussion of inspection requirements, maintenance
considerations, and reporting requirements for SCMs. The remaining chapters address individual SCMs
types.
CHAPTER 2 — INSPECTION
Chapter 2 lists some of the common inspection procedures required for most SCMs; however, it is
critical that one refer to the individual chapters for specific inspection guidance. On occasion, one may
encounter SCM types with unique features that are not covered in this manual.

Introduction
CHAPTER 3 — MAINTENANCE
Chapter 3 outlines routine and emergency maintenance procedures that may be needed when
maintaining SCMs.
CHAPTER 4 — REPORTING AND RECORD KEEPING
Chapter 4 discusses the procedure for reporting and documenting inspection and maintenance activities
in the Fort Bragg Stormwater Management Data Base.
CHAPTERS 5 THROUGH 13 — STORMWATER CONTROL MEASURES SCMs
Chapters 5 through 13 provide an overview of different types of SCMs. Included is a general description
of the SCM and its components, guidance on inspection and maintenance, photos, and detailed diagrams
of the SCM.
APPENDIX A — INSPECTION CHECKLISTS
Appendix A provides an inspection checklist for SCMs. This inspection form, which covers structural
components and areas of maintenance, is to be filled out at the time of the inspection.
1.4 Disclaimer
This manual provides guidance for inspection and maintenance procedures for typical SCMs owned
by Fort Bragg. These guidelines are not intended to be a comprehensive reference on SCM inspection and
maintenance. Additional references should be consulted as needed to maintain safe and functional
structural controls. Further, it would not be possible to properly address every configuration or issue
that might arise. Unique circumstances may require deviation from this guidance. Sound
professional judgment, resourcefulness, and ingenuity are expected and encouraged.
1.5 Contact Information
The information provided in this manual is intended to serve as a guideline to those individuals responsible
for maintaining SCMs. If a situation should require deviation from the procedures, and/or criteria
presented in this manual, contact the Water Management Section at [phone] or [phone]
for additional guidance. Further information is available at WMS website,
http://pao.bragg.army.mil/directorates/dpw/WMB
1-3

Inspection
CHAPTER 2 Inspection
2.1 Inspections
Due to the substantial number of SCM's scattered across the installation, a goal of this program is to best
manage time and resources by conducting routine maintenance in conjunction with scheduled inspections.
SCMs must be routinely inspected and have the necessary maintenance performed on them to be certain
they continually function as designed. Specific inspection details for each SCM are presented in Chapters 5-
13 of this manual. Refer to the appropriate chapter when completing the inspection checklist. The
following problems that may be encountered during an inspection.
General Site Conditions
• Trash and debris
• Animal burrows
• Algae, stagnation, and odors
• Vandalism
Structural/Mechanical
• Obstructions of the inlet or outlet devices by trash, debris, and vegetative growth
• Cracks and deterioration of inlets, outlets, pipes, and catch basins

Inspection
• Malfunctioning valves, sluice gates, locks, and access hatches
• Slow -draining infiltration devices
• Inadequate outlet protection
• Water seepage or ponding
Vegetation
• Distressed aquatic shoreline vegetation
• Poor and distressed stands of grass
• Unwanted weeds, grasses, and woody plants
• Bare ground
Earthworks
• Excessive erosion/sedimentation, particularly in emergency spillways, filter strips, orforebays
• Cracks or settling in the embankment or berms
• Deterioration of downstream channels
Spills/Releases
• Hazardous spill
• Illicit discharge illegal dumping
2.2 Frequency of Inspection
Individual State permitted SCM's shall be inspected semi-annually. All non -permitted SCM's shall be
inspected annually. The schedule is automatically generated from WMS Stormwater Management
Database (SMD).
2.3 Site Visit Preparation
Be prepared before going into the field to conduct inspections. The followinglist
summarizes items that may be needed.
• A copy of this manual
• Copies of appropriate inspection checklists
• GPS unit and local maps (if available)
• Trash bags
• A digital camera (if available)

• A shovel, bush axe, and/or rake
• Hand pruners and/or loppers and Weed eater/Brush saw
• Water Management Section Educational Brochures
• Flashlight

Inspection
2.4 Safety Considerations
Safety considerations must be a constant focus during SCM inspections. Prior to conducting the inspection,
anticipate any potential hazards based on recent or current conditions (i.e., flooding after a heavy rain).
Always avoid hazardous conditions and document them on the inspection checklist.
The following list highlights common safety concerns when performing SCM inspections:
• Never conduct inspections of confined spaces alone, without proper training in accordance with
confined space regulations.
• Take precautions around standing water; depths are likely unknown and the ground may be
unstable.
• Be alert and take proper precautions when entering and exiting road ways.
• Park in areas that provide safe entrance and exit of work area. Do not create potential conflicts with
other vehicles or equipment operating in the work area; and provide maximum protection for
workers entering and exiting the vehicle, especially along open road ways.
• Always wear appropriate protective clothing, i.e. boots, gloves, and safety vests.
• Be aware of large vertical drops and avoid standing on retaining walls or other structures that
present a fall hazard. Make a note of hazard areas on the inspection form.
• If toxic, hazardous or unknown substances are discovered in the area, leave the vicinity and
report the findings by contacting 911 and your supervisor.
• Be aware of loose material (such as riprap), excavation drop-off, tripping hazards, uneven
ground, and other obstructions.
• Be aware of poisonous plants, insects, and snakes.
2.5 Reporting and Record Keeping
Observations made at the time of inspection pertaining to the status of the SCM shall be documented
according to the reporting procedures presented in Chapter 4 of this manual. All inspection findings and
maintenance activities should be noted on the inspection form (see Appendix A), and then entered into
SM D. Refer to Chapter 4 for further guidance on documentation and entering information into SM D.
2-3

Maintenance
CHAPTER 3 Maintenance
3.1 Maintenance of SCMs
SCMs require two basic types of maintenance: (1) routine maintenance and (2) emergency
maintenance. All routine maintenance and/or emergency repair needs found at the time of inspection
should be identified and reported. Visual observations, contacts made, maintenance performed, and any
maintenance recommended at the time of inspection must be documented using the reporting
procedures presented in Chapter 4 of this manual.
If emergency maintenance needs are found, the inspector should either take immediate action to correct
the problem(s) or alert the responsible parties of maintenance and/or repair needs. A follow-up
inspection should be made to ensure that corrective actions have been satisfactorily completed and
normal operation has been restored. All correspondence and corrective actions shall be documented.
This chapter describes the maintenance activities most commonly required when maintaining SCMs.
Actual maintenance needs may vary, depending on specific site conditions.
3-1

Maintenance
3.2 Routine Maintenance
Routine maintenance is any procedure performed on a regular basis to maintain the proper working order
of a SCM. Tasks associated with routine maintenance include, but are not limited to, the following:
• Periodic maintenance of grasses, trees, shrubs, and other desirable plant species
• Removal of undesirable plant species
• Removal of trash and debris
• Upkeep of mechanical/structural components
Care should be taken to avoid using equipment that can cause soil compaction in or around SCMs. Heavy
equipment with narrow tracks or narrow tires, rubber tires with large lugs, or high-pressure tires can cause
excessive compaction resulting in reduced infiltration and damage to underdrain systems. When mowing in
or around a SCMs, use riding mowers or tractor mowers with turf -type tires, push mowers and/or weed
eaters.
Safety considerations must be a constant focus when conducting routine maintenance. Refer to section 2.4
for a list of common safety concerns.
3.2.1 DESIRABLE VEGETATION MAINTENANCE
Desirable vegetation is an important component of many SCMs. Turf -type grasses, native/ornamental grasses,
trees, shrubs, and herbaceous plants help control erosion, provide structural stability, and remove pollutants
from stormwater runoff. Desirable vegetation can also enhance the aesthetic appeal of SCMs and enable
them to blend into the landscape.
Periodic maintenance of desirable vegetation is required to ensure that it remains healthy and established.
Climatic conditions, lack of proper maintenance, storm events, vehicular/equipment traffic, and vandalism
can have a detrimental effect on plant material. Provisions may be needed to supplement or replant some
vegetated areas due to plant loss. Supplemental and replacement planting should occur during the
appropriate planting season for the particular plant species being utilized. These new plantings will require
additional care until they are established.
The following are general guidelines for the maintenance of turf -type grasses, native/ornamental grasses,
trees, shrubs, and herbaceous plants used in association with stormwater controls. Additional information and
guidance can be found in NCDENR BMP Manual.
3-2

Maintenance
Turf -Type Grasses
Turf -type grasses are common to most SCMs. They can be the major component in a SCM makeup or
simply used to stabilize the areas surrounding a SCM.
Turf -type grasses provide soil stability, reduce water flow velocities, and help maintain the structural
integrity of SCMs. They serve as pretreatment by slowing the overland runoff and filtering out sediment
and pollutants. If maintained properly, turf -type grasses can also improve access to SCMs, making
inspection and maintenance easier. The following is a list of the turf -type grasses most commonly used in
association with SCMs.
Turf -Type Grasses
Eremochloa ophiuroides
Centipedegrass
Cynodon dactylon
Bermudagrass
Zoysia japonica
Zoysia
Festuca arundinacea
Tall Fescue
lr �9� .JA_1a1 c-L1W1 au
CENTIPEDEGRASS, SOURCE: VIRGINIA TECH WEED ID GUIDE
Provisions should be made to reestablish a uniform cover of turf -type grass on those areas damaged
by sediment accumulation, stormwater flow, and/or vehicular/equipment traffic. Failure to maintain
a uniform turf -grass cover could result in structural failure and sediment loss. Any turf found not in a
healthy growing state should be evaluated to determine the maintenance needs (i.e., fertilization, repair
seeding, sodding).
Mowing should be tailored to the specific site conditions, turf -grass type, and seasonal variations.
Ideally, turf -type grasses should be mowed at a frequency to maintain a maximum height of 4 inches for
warm -season grasses and 6 inches for cool -season grasses. Mowing activities should include trimming
grass around fences and structures. All clippings discharged from mowers or string trimmers should be
directed away from the stormwater device so they do not add nutrients to the water.
Herbaceous Plants and Native/Ornamental Grasses
Some SCMs have as a part of their makeup a carefully designed mixture of herbaceous plants and
native/ornamental grasses. This plant material is essential to the operation and function of the SCM.
Planted within certain SCMs, this plant material not only stabilizes the soil and prevents erosion, but it
also removes nutrients, metals, and other pollutants from stormwater runoff. The following is a list of
the native/ornamental grass and herbaceous plant species most commonly used in association with
SCMs.
3-3

Maintenance
Herbaceous Plants
Aster laevis
Echinacea purpurea
Eupatorium fistulosum
Helianthus angustifolius
Heliopsis helianthoides
Hibiscus moscheutos
Iris virginica
Lobelia cardinalis
Monarda fistulosa
Rudbeckia subtomentosa
Solidago speciosa
Tradescantia virginiana
Verbena noveboracensis
Native/Ornamental Grasses
Andropogon gerardii
Andropogon glomeratus
Chasmanthium latifolium
Juncus effusus
Muhlenbergia capillaris
Panicum virgatum
Scirpus cypennus
Sorghastrum nutans
Spartina olternifolia
Spartina bakeri
Smooth Aster
Purple Cone Flower
Joe Pye Weed
Swamp Sunflower
Ox Eye Sunflower
Marsh Mallow
Blue Flag Iris
Cardinal Flower
Wild Bergamot
Sweet Coneflower
Showy Goldenrod
Virginia Spiderwort
New York Ironweed
Big Bluestem
Bushy Bluestem
River Oats
Soft Rush
Pink Muhly Grass
Switch Grass
Wool Grass
Indian Grass
Smooth Cord Grass
Cord Grass
JOE PYE WEED, SOURCE: NCDOT
WOOL GRASS, SOURCE: NCDOT

Herbaceous plant material should be allowed to die -back at season's end. Stalks and other dead plant
material may be cut as needed to maintain the SCMs aesthetic appeal. All cut material should be
removed from the site and disposed of properly.
Native and ornamental grasses, with the exception of evergreen species, should be cut back every two to
three years based on the plant's growth during that period. Use a chain saw, hedge trimmer, or any
other cutting device that will not pull the crown from the ground. Top growth should be cut to a height
of 4-12 inches, and all cut material should be removed from the site and disposed of properly.
Cut back of native and ornamental grasses should be delayed until the late winter months (mid- February
to mid -March) to take advantage of their ornamental quality. While in their dormant state, most native
and ornamental grasses have an ornamental quality that is desirable until new growth begins in the
spring. If these grasses are cut back too early, this added benefit will be lost.
3-4
NCDOT HSP 2010- 01

Maintenance
Note: Evergreen native grass species such as Juncus
should not be cut back. Evergreen species have
ornamental qualities year round and provide an added
bonus of pollution uptake while other native grasses
are in their dormant state. Cutting back can be
detrimental to evergreen grass species. Established
plant material is very slow to recover when
compromised, and the end result is often death of
the plant.
JUNCUS, SOURCE: NCDOT
Trees and Shrubs
Trees and shrubs are essential to the operation and function of certain SCMs. Often used on the floor
of wetlands and wet detention basins, trees and shrubs provide valuable shade which helps regulate
water temperatures. High water temperatures can be harmful to aquatic animals and direct sunlight
upon the water's surface can cause an increase in algal blooms. Sometimes trees and shrubs are used
on the side -slopes and outside edges of those stormwater controls (i.e., bioretention basin) that contain
underdrain systems. Trees and shrubs used in this capacity can increase safety by discouraging people
from entering the stormwater control. They can also enhance the aesthetic appeal of stormwater
controls enabling them to blend into the landscape. The following is a list of the tree and shrub species
commonly used in association with some SCMs.
Shrubs
Aronia arbutifolia
Callicarpa americans
Cephalanthus occidentalis
Cie thra aInifolia
Corpus sericea
Ilex glabra
Ilex vomitoria
Red Chokeberry
Beautyberry
Buttonbush
Summersweet
Redosier Dogwood
I nkberry
Yaunon Holly
BEAUTYBERRY, SOURCE: NCDOT
3-5

Maintenance
Trees
Amelanchier arborea
Corpus florida
Ilex opaca
Juniperus virginiana
Magnolia virginiana
Nyssa sylvatica
Downy Serviceberry
Flowering Dogwood
American Holly
Eastern Red Cedar
Sweetbay Magnolia
Black Gum
. _ I.
YAUPON MOLLY, SOURCE. U.S. FISH & WILDLIFE
Trees and shrubs should be maintained in a healthy condition. Check routinely for any disease or
insect infestation problems and treat accordingly. Prune dead, broken, and damaged branches and stems
as needed. Remove pruning debris from grounds and dispose of properly.
Turf grasses, native/ornamental grasses, trees, shrubs, and herbaceous plants should be
maintained in a healthy growing state and mulch plays an important role in the performance of most
plantings. Mulch also helps to maintain soil moisture, promotes infiltration, prevents soil erosion, and
provides a habitat for microorganisms in a SCM. A mulch layer of aged, double -shredded hardwood
bark should cover the entire surface of the planted area and should be maintained at a maximum depth
of four inches
3.2.2 FERTILIZATION
Fertilizer should only be used when necessary and not as a routine seasonal practice. Spread fertilizer
uniformly over the targeted area; during application, use extreme care to prevent the fertilizer from
contaminating the stormwater control or adjacent streams, ponds, lakes, or other bodies of water.
Immediately incorporate fertilizers into the soil when seeding. Do not apply topdressing fertilizers to
grass swales, filter strips, or buffer areas that drain to nutrient sensitive water bodies unless allowed by
rule or approved by the appropriate resource agency.
Standards [NCAC 2007]) provide the following guidance for riparian buffer areas:
No fertilizer shall be used other than a one-time application to reestablish vegetation. Ongoing
fertilizer application is prohibited.
3-6

Maintenance
For one-time applications of fertilizer for reestablishment of vegetation in riparian buffer areas that
allow such application, place slow -release fertilizer into the hole dug for the plant rather than broadcasting
fertilizer after planting.
3.2.3 UNDESIRABLE VEGETATION REMOVAL
If proper maintenance is not provided, undesirable vegetation such as weeds, grasses, and woody plants
(trees and shrubs) will invade the vegetated areas of stormwater controls. In time, this vegetation can
inhibit the ability of a SCM to store, treat, and/or convey water.
Desirable plant material can even become undesirable if it becomes established in areas where it is not
wanted. For example, turf -type grasses become undesirable vegetation when they invade
planted/mulched areas. Trees and shrubs become undesirable when they invade and establish
themselves on the floors of SCMs that have under -drain piping.
The following is a list of the most common undesirable plant species found during SCM inspection and
maintenance activities. Although some of these plants are native species to North Carolina, due to their
aggressive nature and/or seeding habits they are not wanted where species diversity is desired.
Broadleaf Weeds
Ambrosia artemisiifolia
Cardamine hirsute
Cerastium vulgatum
Chamaesyce maculate
Chenopodium album
Conyza canadensis
Diodia virginiana
Geranium carolinianum
Glechoma hederacea
1pomoea sp.
Lactuca serriola
Common Ragweed
Hairy Bittercress
Chickweed
Spotted Spurge
Lambsquarter
Horseweed
Virginia Buttonweed
Carolina Geranium
Ground Ivy
Morning Glory
Prickly Lettuce
COMMON RAGWEED, SOURCE: USDA
3-7

Maintenance
Broadleaf Weeds(continued)
Rumex acetosella Red Sorrel
Rumexcrispus Curly Dock
Stachys floridana Florida Betony
Trifolium spp Hop Clover
Vcia sp. Vetch
Grasses, Sedges and Grass -like Plants
Cyperusesculentus Yellow Nutsedge
Cyperusrotundus Purple Nutsedge
Microstegium vimineum Japanese Stiltgrass
Typha sp. Cattail
Trees
Acerrubrum Red Maple
Ailanthus altissima Tree of Heaven
Liquidambarstyraciflua Sweetgum
Paulownia tomentosa Princess Tree
Salix nigra Black Willow
YELLOW NUTSEDGE, SOURCE: NCCES
Remove undesirable vegetation before it becomes established. Once established, this plant material
can have an adverse effect on the survivability of desirable plants and the aesthetic appeal of SCMs.
The best time to do this is during routine mowing or plant maintenance activities. Undesirable plants
can be removed by physical, mechanical, and/or herbicidal practices. Dispose of the trimmed plant
material properly; do not discard into waterways because the material could clog the waterways and
add nutrients to the water.
3.2.4 TRASH AND DEBRIS REMOVAL
Trash and other debris can pollute surface waters and damage stormwater control devices. The removal of
floating trash and other debris will not only improve water quality, it will reduce the potential for outlet
clogging during storm events and improve the overall aesthetic appeal of a SCM. Trash should be
removed on a routine basis as part of the maintenance activities. Remove trash and debris from outlet
orifices, trash racks, basin and swale floors and side slopes, and other components, as well as from the
area surrounding the SCM.
3-8

Maintenance Maintenance
3.2.5 MECHANICAUSTRUCTURAL COMPONENT MAINTENANCE
Mechanical/structural components need to be maintained regularly in accordance with the manufacturer's
recommendations to ensure that they remain functional at all times. All mechanical components, including
valves, sluice gates, pumps, fences, gates, trash racks, and access hatches, should be operated during
each inspection to ensure that they function properly.
Check security components such as fences, gates, and locks for soundness. Repair any fence, gate, or
lock found damaged in a timely manner in order to restore site security and safety.
3.3 Emergency Maintenance
Emergency maintenance is a nonroutine repair performed to correct a problem and restore a SCM to its
proper working order. Tasks associated with emergency maintenance include, but are not limited to:
' Sediment removal
' Structural repair
■
Erosion repair/bank stabilization
Care should be taken to avoid using equipment that can cause soil compaction in or around stormwater
controls. Heavy equipment with narrow tracks or narrow tires, rubber tires with large lugs, or high-
pressure tires can cause excessive compaction resulting in reduced infiltration and damage to underdrain
systems. When mowing in or around a SCM, use riding mowers or tractor mowers with turf -type tires,
push mowers and/or weed eaters.
Safety considerations must be a constant focus when conducting routine maintenance. Refer to section 2.4
for a list of common safety concerns.
3.3.1 SEDIMENT REMOVAL
Sediment will eventually accumulate in every type of stormwater SCM. The degree to which it accumulates
will depend on the upstream sediment source, rainfall intensity, and the amount of runoff that a SCM
receives.
Any sediment found blocking the inlet or outlet of a SCM should be removed. If sediment buildups are
allowed to block inlets or outlets, stormwater may be diverted to areas of the SCM not designed for
concentrated water flow and cause these areas to erode.
Sediment that has accumulated and is inhibiting the function of a SCM must be removed. In general,
sediment should be removed when it exceeds 50% of the forebay's storage capacity or the original
design sediment storage depth (See As -built drawings). The dredged or removed sediment must be
transferred to a waste pile or area that is protected from stormwater run-on. Make sure the
removed sediment is not left in the vicinity of the SCM where stormwater could come into contact with
it and transport it back to the SCM or nearby receiving waters. If there is evidence of pollution (a
sheen on the sediment or odor), contact DPW Environmental Compliance Branch Hazardous Waste
Section for assistance with disposal (396-2141).
3-9

3.3.2 MECHANICAL/STRUCTURAL REPAIR
Mechanical/structural repairs should be made promptly by qualified personnel. Equipment, materials,
and personnel should be readily available to perform repairs on short notice. The following conditions
could lead to structure failure and may necessitate an emergency repair: a broken sluice gate; cracks in
concrete outlet structures; settling, scouring, cracking, sloughing, or furrowing on embankments; or seepage
around an outflow pipe.
3.3.3 EROSION REPAIR/SOIL STABILIZATION
It is necessary that a uniform vegetative cover be maintained to prevent soil loss, to maintain structural
integrity, and to enhance the pollutant removal benefits of a SCM. Failure to maintain a uniform
vegetative cover could result in structural failure and sediment loss.
Take corrective actions when erosion is found. Repair activities should be tailored to the specific site
conditions, grass type, and seasonal variations. Repair may include the use of one or a combination of the
following measures: erosion control blankets, riprap, matting, sodding, and/or seeding and mulching.
3.3.4 UNDESIRABLE WOODY VEGETATION REMOVAL
Although plant roots are important for soil stabilization, they can present problems if volunteer trees
and shrubs are allowed to mature in locations they are not wanted. Mature trees and shrubs can impede
and redirect water flow in inlet and outlet ditches. As trees and shrubs mature, their root systems can
penetrate deep into the basin and clog the underdrain system. Decaying plant roots can create voids in
the dams and embankments when mature trees die or are cut. This can result in structural failure if the
situation is not addressed. Woody vegetation not removed can also impede access to SCM control
structures.
Remove undesirable woody vegetation when found and dispose of it at the Ft Bragg land fill. Any void
created by removal activities should be completely filled in and properly compacted using an appropriate
earthen material. Reestablish desirable vegetation to stabilize the area and prevent erosion.
3.3.5 ANIMAL BURROW REPAIR
Voids created by animal burrows can weaken dams and embankments, and result in structure failure.
Animal burrows found in the dams, berms, and embankments of SCMs should be filled in as soon as
possible. Burrows should be completely filled in and properly compacted using an appropriate earthen
material. Reestablish desirable vegetation to stabilize the area and prevent erosion. If burrowing
problems persist, DPW wildlife officials should be consulted for information regarding preventive
tactics or animal removal. Contact DPW, Wildlife Branch at [phone].
'�-10

Reporting and Record Keeping
CHAPTER 4 Reporting and Record Keeping
4.1 Reporting Overview
Section F (1) a-c of Fort Bragg's NPDES Phase II Permit requires Ft Bragg to develop and
implement strategies which include a combination of structural and/or non-structural best
management practices (BMPs) appropriate for the base; and ensure adequate long-term operation
and maintenance of SCMs. This manual will outline the regular inspection frequency, an
inspection checklist, `how-to' instructions for regular maintenance, evaluation and reporting
procedures for non -routine maintenance, and an inspection and maintenance tracking
mechanism." This manual and the Database described in the following paragraph were developed
to comply with this requirement.
4.2 Inspection Documentation
Each inspection must be documented using the corresponding inspection report form presented in
Appendix A of this manual. Proper documentation ensures that Ft Bragg is performing its inspection and
maintenance responsibilities as required by the stormwater NPDES Phase II Permit.
4-1

Reporting and Record Keeping
Each completed report should contain the following: Date of
■ inspection
■
Type of inspection (routine or follow-up)
. If a Level of Function (FOS) of a "PF" or "NF" was issued and maintenance was necessary, a
follow-up inspection must be performed after the maintenance has been completed.
■ Name of the inspector
■
Type of SCM
■
Current condition
■
Description of necessary maintenance (if applicable)
■
■
Description of any corrective actions taken at the time of inspection (if applicable) Names of
contacts made (if applicable)
A copy of each completed inspection report is maintained at a central location in the WMS for a period of
three to five years and made available to the Director of the Division of Water Quality, or an authorized
representative, immediately upon request.
4.3 FOS Ratings
After each inspection, the SCM must be given a Function of Service rating. The following table describes the
ratings; however, it is important to note that ratings are subjective, and the overall functionality of the SCM
must always be considered. For example, debris preventing the sluice gate of a Wet Detention Pond from
closing may initially appear as a minor impediment; but, until that debris is removed, the FOS should be
rated as an "NF" because the debris is inhibiting the function of the basin.
FOS Category
FOS Description
FF
Fully Functional (No Repairs needed) = Pass for Inspection
PF
Partially Functional (Repairs Needed) = Pass the Inspection. Maintenance needs were found,
but function of the device has not been significantly affected.
NF
Not Functional =Fail Inspection. Device is no longer functional due to the general or
complete failure of a major structural component and/or the lack of adequate
MON
Monitor to determine root cause of problem
4-2

Reporting and Record Keeping
4.5 Stormwater Management Database (SMD)
The Stormwater Management Database or SMD is available on the WMS drive ( Refer to appendix A -
Fort Bragg Stormwater Management Database User's Manual). This is a database management tool
used to track stormwater control inspection and maintenance activities. SMD is to be used in
conjunction with the hard copy inspection checklists provided in Appendix A of this manual. The general
process for documenting inspection and maintenance of stormwater SCM's is as follows:
1. Perform the inspection and complete the hard copy of the appropriate inspection checklist.
Note any necessary maintenance on the checklist, as well as any notable findings or
observations.
2. Upon returning to the office, log in to SMD and transfer the inspection information to the
database inspection form for each specific stormwater control inspected. SCMs can be
searched by its unique identification code (select the desired control from the automated
dropdown list).
3. Enter any relevant or notable information into the description text box below the
inspection questions. Assign the FOS in the space below the inspection checklist.
4. If maintenance is necessary, enter the maintenance needed or maintenance that was done in
the appropriate space and select "Save."
a. If maintenance is performed NOT in association with an inspection, this information
can later be entered and the FOS can be changed IF the respective maintenance
improved the FOS of the control.
A follow-up inspection must be conducted and entered into SMD after all significant maintenance is
performed on a control with an existing FOS of a PF or NF to verify that each need has been properly
addressed and to ensure that the FOS is properly updated.
4-3

Bioretention Basin
CHAPTER 5 Bioretention Basin
Overview
A BIORETENTION BASIN is a type of filtration basin with landscaped shrubs and other various plants, filter
media, and a mulch cover to enhance pollutant removal.
PURPOSE AND DESCRIPTION
Bioretention basins are structural SCM's designed to temporarily capture stormwater runoff,
filter and retain pollutants, and reduce peak flows.
Inflow to the bioretention basin is filtered through engineered media or amended soil. The filtered
water in some SCMs exits through an underdrain system at the bottom of the filter media.
INSPECTION
Ponded water should infiltrate into the filter media within 48-72 hours of a storm event;
prolonged ponding indicates that the filter media or underdrain system requires maintenance.
The mulch layer and vegetation should be carefully inspected. Use landscaping plans as an aid
during the inspection.
Inspections should occur semi-annually, unless more frequent inspections are deemed necessary.
TYPICAL MAINTENANCE
Trash and debris should be removed from inlet grates, forebays, the basin, and trash racks.
Note: This is a specialized device and if major maintenance is needed, such as the flushing of the
underdrain system, please contact WMS for guidance.
5-1

Bioretention Basin
5.1 Stormwater Control Measure Overview
A Bioretention Basin (BB) is a soil and plant -based filtering system designed to remove pollutants
from stormwater runoff. It works by temporarily retaining stormwater runoff and allowing it to slowly
infiltrate into the soil over a period of 48 hours. As stormwater slowly passes through the soil, it comes
in contact with soil particles and plant roots where pollutants are absorbed and removed. In some
installed systems the filtered water then enters the basin's underdrain system where it exits the
bioretention basin and flows to the nearest stream, river, or lake.
Figure 5-1 illustrates the flow path and treatment processes in a typical bioretention basin.
Figure 5-1. Cutaway of a bioretention basin layout and treatment processes
5.2 Bioretention Components
Figure 5-2 illustrates the various components of a typical bioretention basin. Note that layouts vary.
Some bioretention basins are designed to include a flow bypass structure. For these systems, runoff
enters the bioretention basin via a flow bypass structure, which is designed to divert a set amount of
runoff to the bioretention basin. Excess flow bypasses the bioretention basin and is discharged
downstream via a filter strip and/or swale. This configuration is typical of infiltration basins. See Chapter
10 for inspection and maintenance information and figures for flow bypass systems. The typical
bioretention basin configuration includes the following components:
5-2

4 Met Drainage10
� �
System Er�er
Cleanout 6pilLVa
Underdrain
Forebay------------
Transition �' �, Trash
Berm a
„�. f Rack
Cutlet Control
Structure
Landscaping';
Filter Media
Embankment ^
outlet
Protection
ou let
pipe
�.-
lote:. Mosfstdrrriwa `frols are not located adjacent to a water body„ the
pove diagram may d t, but rely on water conveyances such as ditches, swales, -
buffers to transport treated runoff to the nearest river, lake, or stream.

Figure 5-2. Bioretention basin layout and components
All filtration basins, including bioretention basins, could have an underdrain system. If an underdrain
system is present, cleanouts will be found within the basin.
5.3 Inspection and Maintenance
Bioretention basins should be inspected periodically to determine whether they are functioning as
intended. If any part of the bioretention basin is not functioning properly, determine the cause and
restore to working order as soon as possible. Figures 5-3 and 5-4 illustrate a plan and profile view of the
areas that should be inspected and maintained in a typical bioretention basin configuration.
5-3
NCDOT- HSP- 2010- 01

Bioretention Basin
TRANSITION BERM
FOREBAY
1 N FLOW
VARIOUS PLANT
EMERGENCY SPILLWAY
SPECIES
TOP OF
EMBANKMENT
UTFALL
-
RECEIVING
CHANNEL
OUTLET CONTROL
STRUCTURE
FILTER MEDIA
Aun1112•3=uai
Figure 5-3. Plan view of a bioretention basin and its components
TRASH RACK
VARIOUS PLANT
_ SPECIES
I INFLOW �•.
FOREBAY�'
r
TRANSITION CLEANOUT
BERM UN❑ERDRAIN
FILTER MEDIA -
EMERGENCY
SPILLWAY
EMBANKMENT
OUTLET
PROTECTION
z �
OUTLET CONTROL
STRUCTURE
Figure 5-4. Profile view of a bioretention basin and its components
5-4

Bioretention Basin
All inspection findings and maintenance activities should be noted on the inspection form (see Appendix
A) and then entered into the Stormwater Control Inspection Management System Database. Photos
should be taken to track the status of the SCM and document the maintenance activities conducted.
These, too, can be uploaded to SMD. Refer to Chapter 4 for further guidance.
The following sections describe the function of a bioretention basin and provide guidance on the inspection
and maintenance of its major components.
5.3.1 INLET DRAINAGE SYSTEM
Function
The inlet drainage system collects and conveys water to the bioretention basin. Inlet drainage systems
can consist of open channels, pipes, curb and gutter, and catch basins. Some bioretention basin
designs use flow bypass structures to divert a predetermined amount of runoff to the SCM while
bypassing excess flow so that the basin, outlet control structure, and embankment do notfail. If this is
the case, refer to Sections 10.3.2 through 10.3.4 in Chapter 10, Infiltration Basin, for inspection and
maintenance guidance on flow bypass systems.
Inspection
Stormwater runoff should be allowed to flow freely into the basin. Inspect all ditches, pipes, and/or
catch basins draining to the bioretention basin for trash, sediment, and debris. Inspect the ground
surface above buried pipes and structures for depressions and other signs of pipe breakage or
separation. Inspect ditches for signs of erosion and undesirable vegetation. Bioretention basins are
particularly susceptible to sedimentation. The area draining to the basin should be carefully inspected for
bare soil (NCDWQ, 2007). Desirable vegetation includes grass cover in an open ditch to prevent soil
erosion. Undesirable vegetation includes any woody plants or invasive species that may impede the inlet
drainage system.
Maintenance
Remove and properly dispose of debris, undesirable vegetation, and major sediment accumulations.
Repair all eroded areas and damaged pipes. Refer to Chapter 3 for additional guidance on maintenance
techniques.
5.3.2 FOREBAY
Function
A forebay is a basin located at the inlet of a SCM that pre -treats stormwater (see Figure 5-5). A forebay
reduces the runoff velocity of stormwater, which in turn allows sediment in the stormwater to settle
before the stormwater enters the bioretention basin. A transition berm is the component located at the
downstream end of the forebay that acts as a weir and releases runoff to the SCM. The presence of a
forebay reduces the risk of SCM failure.
5-5

Bioretention Basin
Some forebays within bioretention basins have been designed with special transition weirs that function
much like a level spreader lip. A hard, flat material is used to convert concentrated water flow into
sheet water flow. This process prevents mulch from being washed out of the area immediately
downstream of the forebay, which would lead to erosion within the basin. Figure 5-6 illustrates two
examples of transition weirs.
Inspection
Inspect forebays for trash, debris, and undesirable vegetation. Inspect to determine whether forebays
are structurally sound and that they do not contain excessive amounts of accumulated sediment. Inspect
the embankment and transition berm for structural integrity and signs of erosion. Inspect the forebay
for undesirable vegetation. Forebays may have grass -covered embankments and berms, but they are
not designed to support vegetation. Minimal plant growth is tolerable, but if it becomes excessive, the
storage volume for sediment is reduced and dredging becomes more difficult. If any invasive species are
present, it is generally preferable to remove them before they become well -established.
Maintenance
Remove trash, debris, and undesirable vegetation and properly dispose of it off -site. Remove sediment
and dispose of it off -site if it appears to occupy more than 50% of the forebay's storage capacity. Replace
erosion protection materials (i.e., riprap) as needed. If the surrounding soil is disturbed during cleanout of
the forebay, or at any other time, reseed any areas of bare soil with grass to prevent erosion. Repair the
transition berm as necessary, taking care to maintain the original elevation of the berm. Refer to Chapter
3 for additional guidance on sediment disposal.
Figure 5-5. Typical forebay (foreground)
5-6

� 5IF P ,
Figure 5-6. Two examples of transition weirs
5.3.3 BASIN AND SURROUNDING AREA
Function
Bioretention Basin
The basin is the component of the bioretention basin that reduces the rate of runoff and filters out
contaminants (see Figure 5-7). The basin bottom has special filter media and an underdrain system.
See Section 5.3.5 for further information regarding underdrain systems. Filter media in bioretention basins
is typically well -mixed engineered media consisting of washed sand, fine clay and silt, and organics.
The media is covered with a layer of mulch and planted with landscape vegetation. See Section 5.3.4
for further information regarding the mulch layer and vegetation.
Ponded water should take 48 hours to infiltrate into the filter media. This is known as the drawdown
period. Some basins are designed with a shorter drawdown period. If available, consult design
drawings to determine whether this is the case for a given bioretention basin.
Inspection
Inspect the basin for structural integrity. Note any signs of erosion (see Figure 5-8) or burrowing animals.
Inspect the embankment or berms for settling, scouring, cracking, sloughing, and furrowing, and for
invasive shrubs and trees. Also, inspect the downstream toe of the embankment for seepage
(NCDWQ 2007). Note trash, debris, and/or sediment found in the basin or surrounding areas.
Sediment should be removed if it is clogging the filter media or if it has reached a depth of 3 inches
(NCDWQ 2007). Refer to Chapter 3 for guidance on sediment removal.
5-7

Bioretention Basin
-11 N't 'A
Figure 5-7. Typical basin area
ACV
Figure 5-8. Examples of erosion within a bioretention basin
l ligi 1%% c ly
S101,11 n% Off •r
Inspect the basin for evidence that water remains in the basin longer than the design drawdown period.
Cattails and other wetland vegetation are good indicators that water is remaining in the basin longer
than intended. If this is the case, try to determine the cause of the standing water. Likely causes include
a clogged underdrain system or clogged filter media.
Maintenance
The permeability of the filter media must be maintained orthe bioretention basin will not function
properly. Vehicles and construction equipment should not be driven on the basin bottom (NCDWQ,
2007). Use best professional judgment when selecting landscaping equipment for use in the basin
and minimize foot traffic on the basin bottom.
5-8

Bioretention Basin
Flush the underdrain system if the basin is not draining properly (see Section 5.3.5). Consider flushing
the underdrain system annually as a preventative maintenance measure if the system appears to have
the tendency to plug. If the problem persists, remove and replace the first few inches of the filter
media. Finally, as a last resort, excavate the underdrain system and repair or replace it if necessary, and
replace the original filter media with new media (NCDWQ 2007). These components should be
repaired or replaced to meet the original design specifications unless they are deemed insufficient.
For additional guidance, consult the North Carolina Division of Water Quality Stormwater Best
Management Practices Manual.
Repair areas with erosion, channelization, or animal burrows by regrading and reestablishing the proper
cover (see Section 5.3.4). Take care to preserve the filter media's grade and mixture. Consult the
design drawings or landscaping plan if necessary. If soil testing indicates a pH problem or toxic
conditions, the addition of lime or replacement of the filter media may be warranted. If the filter media
is to be replaced, consult the original design specification to ensure that the correct mixture and testing
procedures are followed.
5.3.4 LANDSCAPING
Function
The mulch layer and vegetation in the basin are important in the removal of stormwater pollutants.
Mulch regulates soil conditions, prevents weed establishment, and protects plants. Mulch also plays an
important role in removing heavy metals from runoff. Other stormwater pollutants are filtered out as
runoff soaks through the filter media.
Contact with vegetation allows for biological treatment of metals and nutrients through primarily microbial
processes (NCHRP, 2006). Plant roots improve soil structure and enhance the soil's ability to filter out
stormwater pollutants (NCDWQ 2007). Photos of some plants appropriate for use in bioretention
basins are presented in Figure 5-9.
Inspection
Note any undesirable vegetation. Consult the design drawings or landscaping plan to determine whether
the appropriate vegetation appears to be present. Bioretention basins may have several species of plants
and identifying them all may not be practical. If this is the case, verify that there is a variety of healthy
species, making sure that one species is not out -competing the others. Inspect the basin to verify that
desirable vegetation is healthy. Note any areas in and around the basin that have exposed soil. All areas
should be covered with mulch, landscaped vegetation, grass, or erosion protection materials (i.e.,
riprap).
The basin floor should be covered with a layer of mulch 3 to 4 inches thick. Note areas that require
additional mulch. The surrounding area should be stabilized with at least a cover of grass. Inspect the
grass to ensure that it is being mowed at a frequency to maintain a height of 6 to 15 inches.
5-9

Bioretention Basin
Figure 5-9. Examples of plant species used in North Carolina bioretention basins
Maintenance
The permeability of the filter media must be maintained or the bioretention basin may fail. Vehicles and
construction equipment should not be driven on the basin bottom (NCDWO, 2007). Use best professional
judgment when selecting landscaping equipment for use in the basin and minimize foot traffic on the
basin bottom.
Mow grass to the ideal height. Turf grass is typically mowed at a frequency to maintain a maximum
height of 4 inches for warm -season grasses and 6 inches for cool -season grasses.
Add mulch and reestablish grass or herbaceous groundcover where cover is insufficient. Use double -
shredded hardwood bark where mulch is needed. Remove undesirable vegetation, taking care not to
damage desirable vegetation. Replace dead or unhealthy plants using the original design drawings or
landscaping plan if necessary. Fertilize turf grass and/or plants upon reestablishment if necessary. The
use of fertilizer may be restricted at some locations. Refer to Chapter 3 when considering the use of
fertilizer and for additional information on caring for vegetation.
5-10

5.3.5 UNDERDRAIN SYSTEM
Function
Bioretention Basin
Bioretention basins May be equipped with underdrain systems to drain runoff that has passed through
the filter media. Positioned beneath the filter media, the underdrain system is usually constructed
of perforated PVC pipe. The perforated pipe is wrapped in filter fabric to prevent clogging and installed
in a shallow layer (typically less than 12 inches) of No. 57 stone. Cleanouts provide access to the
underdrain system for inspection and maintenance activities.
Inspection
Inspect the cleanouts to ensure that their caps are in place and undamaged. Damaged or missing caps
will allow stormwater to exit the basin untreated. Cleanouts can also be used to drain the basin if the
filter media has become clogged. To inspect, remove the caps from the cleanouts and observe the
inside for sediment or standing water using a flashlight. If water is available, pour water in the
cleanouts using a bucket or hose and observe the flow in the outlet control structure or outlet pipe. If
the water does not exit freely, the underdrain system may be clogged. If sediment is flushed from the
underdrain system, the filter fabric may have been damaged and will need to be excavated and
repaired. Also, the basin should be inspected for depressions, which is another indication that the filter
fabric is damaged and soil is entering the underdrain system.
Maintenance
A high-pressure hose can be used to flush out underdrain system by spraying water into cleanouts.
Replace all cleanout caps that are missing, cracked, or otherwise damaged. Consider flushing underdrain
systems annually if they appear to have a tendency to plug. If roots or soil are present in the
underdrain system, it is likely that the filter fabric protecting the perforated PVC pipe is damaged. If
this is the case, the underdrain system should be excavated and repaired or replaced. All repairs
should be made in accordance with the original design specifications. To avoid crushing the
underdrain system, heavy equipment should not be driven in the basin. Refer to Chapter 3 for
additional information.
5.3.6 OUTLET CONTROL STRUCTURE (Box)
Function
Typically, the underdrain system connects to an outlet control structure, which is designed to release
excess stormwater during large storm events. Outlet control structures for bioretention basins generally
resemble catch basins that are elevated, typically 12 inches or less (NCDWQ 2007), above the basin
bottom. Figure 5-10 shows two examples of outlet control structures.
5-11

Bioretention Basin
Figure 5-10. Bioretention basin outlet control structures
Inspection
Inspect the outlet control structure thoroughly for any sign of damage such as cracks, holes, or leaks.
Confirm that the outlet box remains covered with a trash rack or grate and that the trash rack or grate
is structurally sound. Note the amount of trash and debris buildup on the trash rack.
Maintenance
Repair any damaged areas of the outlet control structure, and remove sediment and debris. Replace
metal components if necessary.
5.3.7 OUTLET DRAINAGE SYSTEM
Function
The outlet drainage system conveys water from the SCM to the downstream drainage system or receiving
water. Components of this system can include channels, pipes, catch basins, manholes, culverts, and other
structures.
Inspection
Inspect pipes and drainage structures for cracks or leaks. Inspect the ground surface above buried
pipes and structures for depressions or other signs that might indicate pipe breakage or separation.
Inspect ditches for signs of erosion and undesirable vegetation. If outlet protection materials (i.e., riprap)
are present, verify that these materials are adequate to protect against erosion.
Maintenance
Remove and properly dispose of debris, undesirable vegetation, and major sediment accumulations.
Repair all eroded areas and damaged pipes. Replace outlet protection materials as necessary. Refer to
Chapter 3 for additional guidance on maintenance techniques.
5-12

Bioretention Basin
5.3.8 EMERGENCY SPILLWAY (IF PRESENT
Function
The emergency spillway serves as an overflow structure that is typically constructed as a channel in
natural ground.
Inspection
The emergency spillway should remain free of trash and debris at all times. Emergency spillways
are typically covered with grass, but concrete and riprap are also used. For grass channels, verify that
grass is maintained between 6 and 15 inches. Inspect concrete to verify that it is in good condition, or
verify that adequate riprap is present. Note all erosion and undesirable vegetation.
Maintenance
Emergency spillways must be free of trash, debris, and undesirable vegetation or they may become
blocked and will not function properly. Remove these if present. Grass should be carefully maintained
at a height of 6 to 15 inches. If applicable, repair concrete or replace riprap as necessary. Repair areas of
erosion.
5.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection report form and entered into SMD. Observations recorded would include a general
description of the SCM and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
inspection must be documented on the appropriate SCM inspection report form and entered into
SMD.
5-13

Infiltration Basin
CHAPTER 6 Infiltration Basin
Overview
An INFILTRATION BASIN is a stormwater control that uses the natural filtering ability of the soil to
remove pollutants from stormwater runoff.
PURPOSE AND DESCRIPTION
Infiltration basins are structural stormwater controls measures (SCMs) designed to temporarily
capture stormwater runoff, allowing it to soak into the soil.
Infiltration helps to reduce peak flows and recharge groundwater.
As runoff percolates through the soil, fine sediment and associated pollutants are absorbed and
INSPECTION
Infiltration basins should drain completely during dry periods; standing water in the basin may
indicate the need for maintenance.
Pretreatment SCs, such as forebays, are of particular importance to the function of the infiltration
basin. If pretreatment SCs are neglected, excessive sedimentation in the basin can occur, which
may require intensive maintenance to correct.
Inspections should occur semi-annually, unless more frequent inspections are deemed necessary.
6-1

TYPICAL MAINTENANCE
A four -inch layer of clean sand or a dense cover of turf grass must be maintained at all times.
Any structural deficiencies should be corrected.
Undesirable vegetation, especially woody vegetation in the embankment, should be removed.

Infiltration Basin
6.1 Stormwater Control Measure Overview
An Infiltration Basin is a shallow impoundment constructed over a permeable soil that uses the natural
filtering ability of the soil to remove pollutants found in stormwater runoff. During storm events, runoff
is directed into the basin where it is temporarily detained and allowed to infiltrate slowly into the soil
and eventually into the water table. As runoff slowly passes through the soil, fine sediment and
associated pollutants are absorbed and removed from the runoff as it makes contact with soil particles.
This SCM has high pollutant removal efficiency and can also help recharge the groundwater.
On Fort Bragg, infiltration basins are installed in natural deposits where sandy soils promote infiltration.
All of the stormwater in the basin should infiltrate into the soil within five days after a storm.

Figure 6-1 illustrates flow entering the basin, being detained, and infiltrating into the soil. The figure also
Figure 6-1. Cutaway of an infiltration basin flow diagram and treatment

shows the treatment processes that occur in a typical dry detention basin.

Infiltration Basin
6.2 Infiltration Basin Components
Figure 6-2 illustrates the various components of a typical infiltration basin. Infiltration basins are
typically designed to include a flow bypass system. Runoff enters the infiltration basin via a flow bypass
structure, which is designed to divert a set amount of runoff to the basin. Excess flow bypasses the
infiltration basin and is conveyed by ditch or pipe directly downstream.
In contrast, infiltration basins without a flow bypass system will receive all runoff regardless of the flow
rate. Both configurations may or may not have an emergency outlet control structure.
Infiltration basins may include the following components:
■ Inlet Drainage System
■ Flow Bypass System
• Forebay
■ Basin
■
Emergency Outlet Control Structure
■ Outlet Drainage System

Figure 6-2. Components of a typical infiltration basin
6-3
NCDOT- HSP- 2010- 01

Infiltration Basin
6.3 Inspection and Maintenance
Infiltration basins should be inspected to determine whether they are functioning as intended. If an
infiltration basin is found to not be functioning properly, determine the cause and restore to
working order as soon as possible. Figures 7-3 and 7-4 illustrate a plan and profile view of the areas that
should be inspected and maintained in a typical infiltration basin configuration.
All inspection findings and maintenance activities should be noted on the inspection form (see Appendix
A) and then entered into the SMD. Photos should be taken to track the status of the stormwater control
and document the maintenance activities conducted. These, too, can be uploaded to SMD. Refer to
Chapter 4 for additional guidance.
Note that the ability of the soil in the infiltration basin to percolate water is critical to the SCM's function.
To minimize compaction, heavy machinery should not be used in the basin.
The following sections describe the function of an infiltration basin, and provide inspection and
maintenance guidance for its major components.
SWALE PRETREATMENT DEVICE
TO
TO SWA (SHOWN HERE AS A FOREBAY)
FILTER STRIP TOP OF
I EMBANKMENT
BASIN BOTTOM LINED — —
INFLOW'' WITH 4" MIN. OF CLEAN — 8
SAND OR DENSE
° VEGETATION OUTLET
°off° °
FLOW 0 PROTECTION
BYPASS
STRUCTURE EMERGENCY OUTLET
CONTROL STRUCTURE
(OPTIONAL)
TRANSITION BERM
INFLOW
NOTE: FLOW BYPASS CONFIGURATION
VARIES.
Figure 6-3. Plan view of an infiltration basin and its components
6-4

Infiltration Basin
FOREBAY TRASH RACK EMBANKMENT
OUTLET
INFLOW ' ' PROTECTION
4" MIN. CLEAN SAND
FLOW OR DENSE VEGETATION
BYPASS TRANSITION BERM EMERGENCY OUTLET CONTROL
STRUCTURE STRUCTURE (OPTIONAL)
Figure 6-4. Profile view of an infiltration basin and its components
6.3.1 INLET DRAINAGE SYSTEM
Function
The inlet drainage system collects and conveys water to the SCM. Inlet drainage systems can consist
of open channels, pipes, curb and gutter, and catch basins.
Inspection
Stormwater runoff should be allowed to flow freely into the flow bypass structure, pretreatment device,
and infiltration basin. Inspect all ditches, pipes, and/or catch basins draining to the infiltration basin for
trash, sediment and debris. Inspect the ground surface above buried pipes and structures for
depressions and other signs of pipe breakage or separation. If the inlet drainage system consists of
an earthen ditch, inspect the ditch for signs of erosion and undesirable vegetation. Infiltration basins
are particularly susceptible to sedimentation. The area draining to the basin should be carefully inspected
for bare soil.
Maintenance
Remove and properly dispose of debris, undesirable vegetation, and major sediment accumulations.
Repair all eroded areas and damaged pipes. Refer to Chapter 3 for additional guidance on maintenance
techniques.
6.3.2 FLOW BYPASS SYSTEM
Because the primary outlet is infiltration through the soil, infiltration basins are typically installed
offline. This means that a flow bypass system diverts a predetermined amount of runoff to the SCM while
bypassing excess flow so that stormwater does not overtop the banks of the basin during heavy rainfall.
The bypass system generally consists of a flow bypass structure in conjunction with a swale and/or
filter strip. The flow bypass structure directs the flow, and the swale and/or filter strip provides
treatment of stormwater that bypasses the infiltration basin. Refer to Chapter 10 for guidance on
swales. Refer to the subsequent sections within this chapter for guidance on filter strips and flow bypass
structures.
6-5

Infiltration Basin
6.3.3 FLOW BYPASS STRUCTURE
Function
The flow bypass structure diverts runoff to the infiltration basin under usual circumstances, but will
allow high flow to bypass the basin via a swale and/or filter strip during large storm events.
Inspection
The flow bypass structure uses a weir or other configuration, and must be free of any sediment, trash,
and debris to function properly. Inspect the flow bypass structure for holes and cracks, and for any
erosion that would allow runoff to flow around the structure. If applicable, inspect metal components for
excessive corrosion. Note any undesirable vegetation that might prevent the flow bypass structure
from functioning properly.
Check the basin and swale/filter strip for visual signs that they are receiving flow and have not been
overwhelmed. If the basin does not appear to be receiving flow or if there is evidence of excessive flow,
the flow bypass structure is not functioning properly. Erosion protection materials may have been
installed at the bypass structures' outlet to the swale or filter strip. If this is the case, inspect to ensure
that these materials remain in place and are properly installed.
Maintenance
Remove all undesirable vegetation, sediment, trash, and debris. If the flow bypass structure is clogged,
remove sediment and properly dispose of it off -site. Repair any cracks and holes, and eroded areas
associated with the flow bypass structure. Replace components as necessary. Replace or repair any
materials (e.g., riprap, permanent soil reinforcement matting [PSRM]) used to protect the outlet of the
bypass structure to the filter strip or swale.
6.3.4 FILTER STRIP
Function
Some flow bypass systems use filter strips to treat runoff that has bypassed the infiltration basin. Filter
strips promote infiltration and biological uptake of pollutants, and filter out sediment.
Inspection
The filter strip should be vegetated with a uniform, dense cover of desirable vegetation. Inspect for
signs of erosion and channelization. Also, verify that the vegetation is being mowed at the proper
frequency. Ideally, turf grasses should be mowed at a frequency to maintain a maximum height of 4
inches for warm -season grasses and 6 inches for cool -season grasses.
Note any undesirable vegetation growing in the filter strip.
M.

Infiltration Basin
Maintenance
Remove trash and undesirable vegetation. Remove debris that could cause channelization. Repair
areas affected by erosion or channelization. Mow grass and repair or replace PSRM as necessary. Ensure
that grass remains dense for optimum removal of pollutants. If excessive sediment has accumulated,
remove the sediment and regrade the filter strip. Reestablish vegetation where necessary. (NCDWQ,
2007).
6.3.5 PRETREATMENT CONTROL
Function
Infiltration basins are prone to failure when heavy sediment loads are allowed to enter the basin.
Stormwater that has not been pretreated to remove large solids will quickly clog the first few inches of
soil, reducing the infiltration rate of the basin. Once the basin is clogged, it will begin to retain water,
potentially causing a mosquito hazard and erosion of the emergency spillway from overuse.
Swales or forebays are often used to pretreat runoff upstream of the infiltration basin. Refer to Chapter
10 for in-depth guidance on swales. Guidance on forebays follows.
A forebay reduces the runoff velocity of stormwater, which in turn allows solids suspended in the
stormwater to settle before the stormwater enters the infiltration basin. A transition berm is the
component located at the downstream end of the forebay that acts as a weir and releases runoff to
the stormwater control. The presence of a forebay reduces the risk of SCM failure.
Inspection (Forebay)
Inspect to ensure that all forebays remain free of trash, debris, and undesirable vegetation. Inspect
to determine whether forebays are structurally sound and that they do not contain excessive
amounts of accumulated sediment. Inspect the transition berm to determine whether it is structurally
sound and shows signs of erosion.
Maintenance (Forebay)
Remove trash, debris, and undesirable vegetation and properly dispose of it off -site. Remove sediment
and properly dispose of it off -site if it appears to occupy more than 50% of the forebay's storage
capacity. Replace erosion protection materials (i.e., riprap) as needed. If necessary, reestablish
vegetation for earthen forebays. Repair the transition berm as necessary, taking care to maintain the
original elevation of the weir. Refer to Chapter 3 for further guidance on sediment disposal.
W

Infiltration Basin
6.3.6 BASIN AND SURROUNDING AREA
Function
The main pollutant removal processes occur in the basin. Runoff is detained and allowed to infiltrate
into the soil and eventually into the water table. Infiltration of stormwater into the basin soil is
the primary outlet for this SC. The permeability of the soil beneath the basin must be
maintained or the infiltration basin will fail. Vehicles and construction equipment should not be
operated within the basin (NCDWQ, 2007). Use best professional judgment when selecting mowing and
other landscaping equipment for use in the basin. Avoid the use of riding lawnmowers and minimize
foot traffic on the basin bottom. Some flow bypass structure configurations require the addition of
an emergency outlet in the form of an emergency outlet control structure or an emergency spillway.
Emergency outlet systems are also installed if failure of the bypass structure is a concern.
Inspection
The bottom of the basin may be vegetated with grass or covered with a layer of sand. If sand was
used, it should form a uniform cover of at least 4 inches. The basin's interior and exterior side slopes,
and all areas surrounding the basin should be stabilized with a uniform cover of turf grass. Inspect grass
to ensure that it is being mowed at a frequency to maintain a desired height of 6 to 15 inches.
Inspect the basin for structural integrity. Note any signs of erosion, burrowing animals, or undesirable
vegetation. Inspect the embankment for settling, scouring, cracking, sloughing, and furrowing, and for
shrubs or trees. Also, inspect the downstream toe of the embankment for seepage (NCDWQ, 2007).
Note remove all trash and debris found in the basin and surrounding areas.
The basin bottom should be relativelyflat; standing water can accumulate in localized low areas. Inspect
for evidence that excessive ponding is occurring. Water should drain from the basin within five days.
Cattails and other wetland vegetation are indicators that water is remaining in the basin longer than
intended. If this is the case, try to determine the cause of the standing water. For example, the soil
may no longer be capable of percolating stormwater because of sedimentation or compaction. A high
groundwater table or shallow bedrock can also inhibit infiltration.
If the basin appears to be retaining water longer than 5 days, it may need to be pumped out to correct
the problem. If this is the case, visually inspect for obvious signs of pollutants. Check for an oily sheen or
excessive suspended solids. If either of these is present, do not drain the basin. Call the WMB for
guidance.
Maintenance

Remove all undesirable vegetation (using mechanical or herbicidal treatment) and mow grass to the
ideal height. Reestablish turf grass where erosion has occurred (seed or sod). Refer to Chapter 3
when considering the use of fertilizer, which may be restricted at some locations. Replace sand where
needed and repair all animal burrows. If there is channelization, reestablish the proper grade of the
basin bottom by removing sediment and filling in, then reestablishing vegetation if applicable. Sediment
should be removed as necessary to prevent clogging. Soil clogged with sediment should be removed
and the basin bottom tilled. To minimize compaction, heavy equipment should not be used in the basin.
Refer to Chapter 3 for additional information.
6.3.7 EMERGENCY OUTLET CONTROL STRUCTURE (IF PRESEN
Function
An emergency outlet control structure may have been installed to release excess stormwater during
large storm events. The emergency outlet control structure is composed of a riser and a discharge pipe.
See Section 6.3.9 for information regarding the discharge pipe. The riser is typically made of concrete
for durability. Many risers have an open top with a trash rack over the opening. Some outlet structures
may have sluice gates that enable the basin to be drained for maintenance purposes. If this is the case and
standing water is present, drain the basin and return the sluice gate to the closed position. Perform
inspection and maintenance tasks for components that were previously inaccessible.
Inspection
Inspect the emergency outlet control structure thoroughly for any sign of damage such as cracking,
holes, or leakage. Inspect to ensure that the outlet box remains covered with a trash rack and that
the trash rack is not excessively corroded. Note the amount of trash and debris buildup on the trash
rack. If a sluice gate is present, open and close it to ensure that it can be operated through its entire
range of motion. I nspect the sluice gate for excessive corrosion. Verify that there is no sediment or
debris below the sluice gate that could prevent it from being fully closed.
Maintenance
Repair any damaged areas of the emergency outlet control structure, and remove sediment and debris if
it is accessible. Replace metal components if necessary. Lubricate sluice gates with a marine -type
grease. It is recommended that a cover be fabricated to protect the sluice gate's worm gear from
corrosion and increase the visibility of the device. The cover can be made by placing a cap on one end of a
section of PVC pipe (see Figure 6-5). Perform additional maintenance and repairs as described in the
manufacturer's instructions. If the outlet is clogged or blocked and standing water prevents access to
the structure, hip waders or a small boat may be needed to make the necessary repairs. Ensure that the
proper safety procedures are followed when working in or around standing water.

Infiltration Basin
Figure 6-5. Two large sluice gates with PVC covers
6.3.8 EMERGENCY SPILLWAY (IF PRESENT)
Function
An emergency spillway may be present as an alternative to the emergency outlet control structure.
An emergency spillway is an overflow channel, typically constructed in natural ground.
Inspection
The emergency spillway should remain free of trash and debris at all times. Emergency spillways are
typically covered with grass, but concrete and riprap are also used. For grass channels, verify that
grass is maintained between 6 and 15 inches. Inspect concrete to verify that it is in good condition, or
verify that adequate riprap is present. Note all erosion and undesirable vegetation.
Maintenance
Emergency spillways must be free of trash, debris, and undesirable vegetation at all times to function
properly. Remove these if present. Grass should be carefully maintained at a height of 6 to 15 inches.
If applicable, repair concrete or replace riprap as necessary. Repair areas of erosion.
6.3.9 OUTLET DRAINAGE SYSTEM
Function
The outlet drainage system conveys water from the SCM to the downstream drainage system or receiving
water. The outlet drainage system includes conveyance facilities downstream of the emergency outlet
control structure, the emergency spillway, and/or the flow bypass system.
Inspection
Inspect the inside of pipes (where feasible) to ensure that they are free of sediment and debris. Inspect
the ground surface above buried pipes and structures for depressions and other signs of
0-10

Infiltration Basin
pipe breakage or separation. If the outlet drainage system consists of an earthen ditch, inspect for
signs of erosion and undesirable vegetation. If outlet protection materials are present, verify that these
materials are adequate.
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment accumulations.
Repair all eroded areas and damaged pipes. Replace outlet protection materials (i.e., riprap) as
necessary. Refer to Chapter 3 for additional guidance on maintenance techniques.
6.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection checklist form and entered into SMD. Observations recorded would include a general
description of the SCM and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
inspection must be documented on the appropriate SCM inspection checklist form and entered into
SMD.
6-11

Dry Detention Basin
CHAPTER 7 Dry Detention Basin
Overview
PURPOSE AND DESCRIPTION
Dry detention basins are structural SCMs designed to temporarily capture stormwater runoff and
reduce flow velocity.
Inflow to the SCM is detained and released slowly from a primary outlet control structure over a
period of 48 — 72 hours.
Dry detention basins are designed with a drawdown component that keeps the basin dry
INSPECTION
Dry detention basins should be dry between storm events; standing water in the basin may
indicate that maintenance is needed.
The outlet control structure is critical to the proper function of the dry detention basin and should
be inspected carefully.
Inspections should occur semi-annually, unless more frequent inspections are deemed necessary.
TYPICAL MAINTENANCE
Trash and debris should be removed from inlet grates, trash racks, orifices, and forebays.
7-1

Any structural deficiencies should be repaired or the components replaced.

Dry Detention Basin
7.1 Stormwater Control Measure Overview
A Dry Detention Basin is a stormwater device whose outlet has been designed to temporarily detain
contaminated runoff and release it slowly over a period of 48-72 hours. It is during this time that runoff
velocities are lowered and physical processes work to remove pollutants. As stormwater is detained
and runoff velocities are lowered, suspended solids and other associated pollutants settle out and are
removed from the runoff before it exits the basin and enters the nearest stream, river, or lake.
The primary pollutant removal process is sedimentation. Vegetation in the dry detention basin can
also filter out and take up pollutants. Additionally, some water may infiltrate through the bottom of the
basin, which recharges aquifers.
The basin's main outlet is small and is located near the bottom of the basin. This outlet is referred
to as the drawdown orifice, and it allows the basin to temporarily hold water during storm events and
drain completely afterward. One or more additional outlets allow water to overflow during large
storms.
Figure 7-1 illustrates flow entering the basin, being detained, and exiting the basin. The figure also
shows the treatment processes that occur in a typical dry detention basin.
Figure 7-1. Cutaway of a dry detention basin flow diagram and treatment processes
7-2

Dry Detention Basin
7.2 Dry Detention Basin Components
Figure 7-2 illustrates the various components of a typical dry detention basin. Note that layouts vary.
Some systems will have additional components, whereas others may lack certain components. Dry
detention basins may include the following:
■
■
Inlet Drainage System
Forebay
■
Basin
■
■ Outlet Control Structure
Drawdown Orifice
■
7-3

Inlet Drainage
System
Farebay
Drawdown
_ = Orifice
Trash
Transition Basin - Rack
- Berm �
Embankment
Outlet Control
Structure
Emergency
Spillway
Outlet
Protecti,
Outlet
pipe
Ca Most storm' Wgter controls1lre not }oca `ce 't to a Water-ttody, as the
boVi diagram may depict, but rely on water conveyances such as ditches, swales
nd buffers to transport treated runoff to the nearest river, lake, or stream.

Spillway

Dry Detention Basin
7.3 Inspection and Maintenance
Dry detention basins should be inspected to determine whether they are functioning as intended. If a
dry detention basin is found to not be functioning properly, determine the cause and restore to
working order as soon as possible. Figures 7-3 and 7-4 illustrate a plan and profile view of the areas
that should be inspected and maintained in a typical dry detention basin configuration.
All inspection findings and maintenance activities should be noted on the inspection form (see Appendix
A) and then entered into the Stormwater Management Database. Photos should be taken to track the
status of the SCM and document the maintenance activities conducted. These, too, can be uploaded to
SMD. Refer to Chapter 4 for further guidance.
7-4

The following sections describe the function of a dry detention basin, and provide inspection and
TRANSITION BERM
FOREBAY
I NFLOW
EMERGENCY SPILLWAY
TOP OFF
EMBANKMENT
UTFALL
'RECEIVING
CHANNEL
OUTLET CONTROL
STRUCTURE
Figure 7-3. Plan view of a dry detention basin and its conapm%M&
_ TRASH RACK SPILLWAY
EMBANKMENT
7�
F .ti-4. Profile view of a dry detenti �n basin and its compormtST
.._ r PROTECTION
FOREBAY
CLEANOUT ' �1
e.
TRANSITION J UNDERDRAIN
BERM OUTLET CONTROL
' STRUCTURE
DRAWDOWN —'
ORIFICE

maintenance guidance for its major components.

7.3.1 INLET DRAINAGE SYSTEM
Function
Dry Detention Basin
The inlet drainage system collects and conveys water to the dry detention basin. Inlet drainage systems
can consist of open channels, pipes, curb and gutter, and catch basins.
Inspection
Stormwater runoff should be allowed to flow freely into the basin. Inspect ditches, pipes, and/or catch
basins draining to the dry detention basin for trash, sediment, and debris. Inspect the ground surface
above buried pipes and structures for depressions and other signs of pipe breakage or separation.
Inspect ditches for signs of erosion and undesirable vegetation. Desirable vegetation includes grass
cover in an open ditch to prevent soil erosion. Undesirable vegetation includes any woody plants or
invasive species that may impede the inlet drainage system.
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment
accumulations. Repair eroded areas and damaged pipes. Refer to Chapter 3 for additional guidance
on maintenance techniques.
7.3.2 FOREBAY
Function
A forebay is a basin located at the inlet of a SCM that pretreats stormwater. Forebays reduce the runoff
velocity of stormwater, which in turn allows sediment in the runoff to settle before the stormwater
enters the dry detention basin. A transition berm is the component located at the downstream end of
the forebay that acts as a weir and releases runoff to the stormwater control. The presence of a forebay
reduces the risk of SCM failure.
Inspection
Inspect forebays for trash, debris, and undesirable vegetation. Inspect to determine whether forebays
are structurally sound and that they do not contain excessive amounts of accumulated sediment. Inspect
the embankment and transition berm to determine whether it is structurally sound and shows signs of
erosion. Forebays may have grass -covered embankments and berms, but they are not designed to
support vegetation in the basin. Minimal plant growth is tolerable, but if it becomes excessive, the
storage volume for sediment is reduced and dredging becomes more difficult. If any invasive species
are present, it is generally preferable to remove them before they become well -established.
Maintenance
Remove trash, debris, and undesirable vegetation and properly dispose of it off -site. Remove sediment
and dispose of it off -site if it appears to occupy more than 50% of the forebay's storage capacity. Replace
erosion protection materials (i.e., riprap) as needed. If the surrounding soil is disturbed during cleanout of
the forebay, or at any other time, reseed any areas of bare soil with
7-5

Dry Detention Basin
grass to prevent erosion. Repair the transition berm as necessary, taking care to maintain the original
elevation of the berm. Refer to Chapter 3 for additional guidance on sediment disposal.
7.3.3 BASIN AND SURROUNDING AREA
Function
The basin is the component of the dry detention basin that reduces the rate of runoff and allows suspended
solids to settle.
Inspection
The floor of the basin, its interior and exterior side slopes, and the area surrounding the basin should be
stabilized with at least a cover of turf -type grass. Verify that grass is being mowed at a frequency to maintain
a height of 6 to 15 inches. Inspect the basin for structural integrity. Note any signs of erosion, burrowing
animals, or undesirable vegetation. Inspect the embankment for settling, scouring, cracking, sloughing,
and furrowing, and for invading shrubs and trees. Also, inspect the downstream toe of the embankment
for seepage (NCDWO, 2007). Note all trash and debris found in the basin or in surrounding areas. Also,
inspect the basin for evidence that water remains in the basin longer than 5 days. Cattails and other
wetland vegetation are good indicators that water is remaining in the basin longer than intended. If
this is the case, try to determine the cause of the standing water. Possible causes include a high
groundwater table, clogged drawdown orifice(s), and localized low areas.
Some dry detention basins have been planted with certain grasses, trees, shrubs, and herbaceous plant
material to enhance their aesthetic appeal and increase their pollutant removal ability (See Figure 7-5). If
applicable, verify that this plant material remains healthy and uniformly established. A layer of mulch
should be present to help maintain soil moisture, protect against surface sealing, prevent soil erosion,
and provide a suitable microclimate for microorganisms. The mulch layer should be approximately 3-4
inches thick and relatively level, with no exaggerated channels cut through as a consequence of
concentrated water flow.
It is important to note that dry detention basins with special plant material are not as common as
grassed dry detention basins and can be easily confused with bioretention basins. However, dry
detention basins do not typically have underdrain systems and cleanouts will not be present. All
dry detention basins have a distinct outlet control structure with a restrictive outlet (drawdown
orifice or slightly opened sluice gate) near the bottom of the basin.
Baffles may have been installed to increase the effective flow length in the basin. If this is the case,
verify that the baffles are undamaged.
7-6

Dry Detention Basin
Figure 7-5. Dry detention basin with planted landscaped vegetation
Maintenance
Remove undesirable vegetation (using a mechanical or herbicidal treatment) and mow grass to the
ideal height. Turf grass is typically mowed at a frequency to maintain a maximum height of 4 inches for
warm -season grasses and 6 inches for cool -season grasses.
If erosion has occurred, reestablish turf grass (seed or sod) or replace mulch with double- shredded
hardwood bark. Replace dead or unhealthy plant material, and fertilize turf grass and/or plant material
upon reestablishment. The use of fertilizer may be restricted at some locations. Refer to Chapter 3
when considering the use of fertilizer. Contact WMS if guidance is needed in maintaining landscaped dry
detention basins.
Repair all animal burrows. If there is channelization, reestablish the proper grade of the basin bottom
by removing sediment and filling in, then reestablishing vegetation. Repair damaged baffles if
applicable. Sediment should be removed if it occupies more than 25% of the basin's original storage
capacity.
7.3.4 UNDERDRAIN SYSTEM (IF PRESENT)
Function
Some large dry detention basins are equipped with underdrain systems to ensure that the basin is properly
drained between storm events. In these systems, the drawdown orifice is still the primary outlet while
the underdrain system is used to prevent small pools from holding water for periods longer tha n intended.
Underdrain systems are usually constructed using perforated PVC pipe. The perforated pipe is wrapped
in filter fabric to prevent clogging and installed in a
7-7

Dry Detention Basin
shallow layer (typically 12 inches) of No. 57 stone. To avoid crushing the underdrain system, heavy
equipment should not be driven in the basin. Refer to Chapter 3 for additional information.
Cleanouts are typically visible from the surface if an underdrain system is present.
Inspection
Verify that cleanout caps are in place and undamaged. Damaged or missing caps will allow stormwater
to exit the basin untreated. Remove the caps from the cleanouts and inspect the inside for sediment
or standing water using a flashlight. If water is available, pour water in the cleanouts using a bucket or
hose and observe the outlet control structure or outlet pipe for flow. If the water does not exit freely,
this indicates the underdrain system is clogged. If sediment is flushed from the underdrain system, it
may be an indication that the filter fabric has been damaged and will need to be excavated and
repaired. The basin should be inspected for depressions that are another indication that soil is
entering the underdrain system.
If the basin is holding water longer than 5 days, (NCDWQ 2007) the under drain system (if present) can
be used to drain the basin via its cleanouts so that any plant or soil material which may be preventing
infiltration can be remediated. Before doing so, the water must first be visually inspected for
obvious signs of pollutants. Check for an oily sheen or excessive suspended solids. If either of these is
present, do not drain the basin. Call the WMS for guidance.
Maintenance
A high-pressure hose can be used to flush out underdrain systems by spraying water into cleanouts.
Replace cleanout caps that are missing, cracked, or otherwise damaged. Consider flushing underdrain
systems annually if it appears that they have a tendency to plug. Repair or replace underdrain systems in
accordance with the original design specifications. If additional guidance is needed, consult the North
Carolina Division of Water Quality Stormwater Best Management Practices Manual (NCDWQ 2007).
7.3.5 OUTLET CONTROL STRUCTURE
Function
The outlet control structure is used to drain the dry detention basin. Outlet control structures can have
several components, including a trash rack, a trash screen, one or more drawdown orifices, a sluice
gate, and an outlet pipe. Inspection and maintenance guidance for these specific components is
provided in subsequent sections of this chapter. Figure 7-6 illustrates the configuration of a typical
outlet control structure. The drawdown orifice(s) is located at the bottom of the outlet control
structure and slowly drains the basin, reducing peak flow and allowing solids to settle. Typically, the
outlet control structure has a second, larger opening that allows flow from large rainfall events to escape
without overtopping the basin. Usually, this is in the form of an open -top outlet structure with a trash
rack. Figure 7-7 offers a photograph of a typical outlet control structure.
Standing water in the basin may conceal parts of the outlet structure. If access to submerged
components is needed, the basin can be drained or pumped out; however, the water must first be
7-8

Dry Detention Basin
visually inspected for obvious signs of pollutants. Check for an oily sheen or excessive suspended
solids. If either of these is present, do not drain the basin. Call the WMS for guidance.
Inspection
Treated water should be allowed to flow freely upon entering the outlet control structure. Inspect the
outlet control structure thoroughly for any signs of damage such as cracking, holes, or leakage.
Verify that the outlet box remains covered with a trash rack. See subsequent sections for guidance on
specific components.
Maintenance
Repair any damaged areas of the outlet control structure, and remove sediment and debris if it is accessible.
If the outlet is clogged or blocked and standing water prevents access to the structure, hip waders
or a small boat may be needed to make the necessary repairs. Ensure that the proper safety
procedures are followed when working in or around standing water.
PVC WORM WORM GEAR
GEAR COVER
SLUICE
GATE
TRASH RACK
DRAWDOWN
ORIFICE TRASH
SCREEN
BOTTOM
STEEL PLATE WITH
DRAWDOWN
J ORIFICES
Figure 7-6. Components of an outlet control structure
7-9

Dry Detention Basin
Figure 7-7. An outlet control structure
7.3.6 TRASH RACK
Function
Positioned atop the outlet control structure (box), the trash rack protects the overflow of the outlet
structure from becoming clogged with debris.
Inspection
Inspect the trash rack for debris and excessive corrosion.
Maintenance
Remove trash and debris. Replace the trash rack if it is corroded or otherwise damaged. The
replacement trash rack should be consistent with the design specifications for the SCM.
7.3.7 TRASH SCREEN
Function
Positioned near the base of the outlet control structure, the trash screen helps keep the orifice from
becoming clogged.
7-10

Dry Detention Basin
Inspection
If it is accessible, inspect the trash screen for accumulated sediment and debris. If the basin is holding
water above the level of the orifice and water is not flowing into the outlet control structure, this is a
good indication that the trash screen may be clogged.
Maintenance
If there is evidence that the trash screen is clogged, open the sluice gate to drain the basin or pump it
out after properly inspecting the water for pollution. Once the basin is drained, remove any sediment,
trash, or debris from the trash screen. Remember to return the sluice gate to its original position.
7.3.8 DRAWDOWN ORIFICE
Function
Positioned near the base of the outlet control structure, the drawdown orifice is a restricting device
that allows stormwater to slowly enter the outlet box. In most cases, the drawdown orifice consists
of either one or more 2- to 3-inch holes drilled directly into the side of the box or a predrilled steel plate
attached to the side of the outlet control structure covering a manufactured outlet opening.
Inspection
If the orifice is accessible, verify that stormwater flows freely through it into the outlet structure. If a
steel plate was used to restrict flow, check the plate for excessive corrosion. Standing water above the
level of the orifice is an indication that the orifice may be blocked.
Maintenance
Remove sediment and debris blocking the flow into the orifice. If the water level in the basin is above the
orifice, follow proper precautions before opening the sluice gate or pumping out the basin. After the
basin has been drained, lift the trash screen to gain access to the orifice. Remember to return the
sluice gate to its original position. Replace the steel orifice plate if necessary.
7.3.9 SLUICE GATE (IF PRESENT)
Function
Sluice gates are typically installed for emergency maintenance needs or to prevent spills from entering
water bodies. In these cases, the sluice gate covers an additional opening that is larger than the
drawdown orifice. The sluice gate can be opened to rapidly drain the basin. In some instances, sluice
gates are used in place of drawdown orifices. If a basin's outlet control structure is equipped with both
an orifice and a sluice gate, the sluice gate should remain closed at all times. If a sluice gate is used to
restrict water flow, it should be left open to provide a 1-inch opening.
7-11

Dry Detention Basin
Inspection
Two types of sluice gates are currently in use: (1) a screw -type gate and (2) a lift -type gate. Figures 7-8
and 7-9 show a screw -type sluice gate and a lift -type sluice gate, respectively. Open and close the sluice
gate to verify that it is operable through its entire range of motion. Inspect the sluice gate for
excessive corrosion. Verify that there is no sediment or debris below the sluice gate that could
prevent it from being fully closed.
Maintenance
7-12

If lubrication is necessary, lubricate with a marine -type grease. It is recommended that a cover be

fabricated to protect the sluice gate's worm gear from corrosion and increase the visibility of the
device. The cover can be made by placing a cap on one end of a section of PVC pipe (see Figure 7-10).
Remove any sediment and debris near the sluice gate.

Figure 7-9. A lift -type sluice gate
Figure 7-10. Two large sluice gates with PVC covers
7.3.10 OUTLET DRAINAGE SYSTEM
Function
Dry Detention Basin
7-13

The outlet drainage system conveys water from the SCM to the downstream drainage system.
Typically, a pipe conveys water from the outlet control structure through the embankment to a swale or
other open channel.
NCDOT- HSP- 2010- 01

Dry Detention Basin
Inspection
Inspect the inside of the outlet box and pipe(s) for sediment and debris. Inspect the ground surface
above buried pipes and structures for depressions or other signs that might indicate pipe breakage or
separation. Inspect ditches for signs of erosion and undesirable vegetation.
Maintenance
Remove any sediment or debris that is accessible. Repair eroded areas and damaged pipes. Replace
outlet protection materials (i.e., riprap) as necessary. Refer to Chapter 3 for additional guidance on
maintenance techniques.
7.3.11 EMERGENCY SPILLWAY (IF PRESENT
Function
The emergency spillway serves as an overflow structure that is typically constructed as a channel in
natural ground. The emergency spillway is necessary to minimize the potential for overtopping the
basin, which can damage the embankment and lead to SCM failure and downstream flooding.
Inspection
The emergency spillway should remain free of trash and debris. Emergency spillways are typically
covered with grass, but concrete and riprap are also used. For grass channels, verify that grass is
maintained between 6 and 15 inches. Inspect concrete to verify that it is in good condition, or verify that
adequate riprap is present. Note all erosion and undesirable vegetation.
Maintenance
Emergency spillways must be free of trash, debris, and undesirable vegetation to maintain their
functionality. Remove these if present. Grass should be carefully maintained at a height of 6 to 15
inches. If applicable, repair concrete or replace riprap as necessary. Repair areas of erosion.
7.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection report form and entered into SMD. Observations recorded would include a general
description of the SCM and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
7-14

inspection must be documented on the appropriate SCM inspection report form and entered into
SMD.

Wet Detention Basin
CHAPTER 8 Wet Detention Basin
OVERVIEW
A WET DETENTION BASIN/POND is a stormwater control that maintains a permanent pool of water,
reduces peak stormwater flows, promotes the settling of suspended solids and biological uptake of
pollutants, and reduces erosive velocities downstream of the outlet control structure.
PURPOSE AND DESCRIPTION
Wet detention basins/ponds improve water quality by allowing sediment to settle and
promoting the growth of wetland plants.
The basin has additional capacity to detain and slowly release stormwater from a primary outlet
INSPECTION
The water level should remain near the drawdown device or orifice; low water level or frequent
overflowing indicates that maintenance is necessary.
The outlet control structure is critical to the proper function of the wet detention basin and
should be inspected carefully.
Inspections should occur semi-annually, unless more frequent inspections are deemed
TYPICAL MAINTENANCE
Trash and debris should be removed from inlet grates, forebays, orifices, and trash racks.
Any structural deficiencies should be repaired or the components replaced.
Undesirable vegetation, especially woody vegetation on the embankment, should be removed.
8-1

Wet Detention Basin
8.1 Stormwater Control Overview
A Wet Detention Basin is a constructed basin with a permanent pool of water that is used to detain and
treat contaminated runoff. During storm events, runoff is directed into the pond where it is detained
until it is displaced by runoff from the next storm. It is during this detention period that suspended solids
and other associated pollutants found in stormwater runoff settle- out.

A-Permane
f Emergency
Spillway;!
Forebay captures Wilt
rativn +
sediment °ours Stormwater is detained and
slowly released through the
drawdewn orifice(s)
T) The permanent pool and resulting wetland 2) Overflow exits the basin via the
communities enhance the pollutant removal emergency spillway to prevent 5C failu
capabilities of the wet detention basin and downstream flooding

shows the treatment processes that occur in a typical wet detention basin.

Wet Detention Basin
8.2 Wet Detention Pond Components
Figure 8-2 illustrates the various components of a typical wet detention basin. Note that layouts vary. Some
systems will have additional components while others may lack certain components. Wet detention basins
may include the following:
Inlet Drainage System
Forebay
' Basin
' Outlet Control Structure
' Outlet Drainage System
Emergency Spillway
Figure 8-2. Components of a typical wet detention basin
8-3

Wet Detention Basin
8.3 Inspection and Maintenance
Wet detention basins should be inspected to determine whether they are functioning as intended. If a
wet detention basin is found not to be functioning properly, determine the cause and restore to
working order as soon as possible. Figures 8-3 and 8-4 illustrate a plan and profile view of the areas
that should be inspected and maintained in a typical wet detention basin configuration.
All inspection findings and maintenance activities should be noted on the inspection form (see Appendix
A), and then entered into the Stormwater Management Database (SMD). Photos should be taken to track
the status of the SCM and document the maintenance activities conducted. These, too, can be uploaded
to SMD. Refer to Chapter 4 for further guidance.
The following sections describe the function of a wet detention basin, and provide inspection and
maintenance guidance for its major components.
TRANSITION BERM
FORE13AY
INFLOW
EMERGENCY SPILLWAY
TOP OF -\
EMBANKMENT
RECEIVING
CHANNEL
OUTLET CONTROL
STRUCTURE
Figure 8-3. Plan view of a wet detention basin and its components
PERMANENT TRASH RACK
POOL ELEV.
1
,1
INFLOW _
FOREDAY J�
TRANSITION ❑RAWDOWN
BERM ❑EV ICEIORI F ICE
EMERGENCY
SPILLWAY
EMBANKMENT
OUTLET
PROTECTION
OUTLET CONTROL
STRUCTURE
Figure 8-4. Profile view of a wet detention basin and its components
8-4

8.3.1 INLET DRAINAGE SYSTEM
Function
Wet Detention Basin
The inlet drainage system collects and conveys water to the wet detention basin. Inlet drainage systems
can consist of open channels, pipes, curb and gutter, and catch basins.
Inspection
Stormwater runoff should be allowed to flow freely into the basin. Inspect ditches, pipes, and/or catch
basins draining to the wet detention basin for trash, sediment, and debris. Inspect the ground
surface above buried pipes and structures for depressions and other signs of pipe breakage or
separation. Inspect ditches for signs of erosion and undesirable vegetation.
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment
accumulations. Repair eroded areas and damaged pipes. Refer to Chapter 3 for additional guidance
on maintenance techniques.
8.3.2 FOREBAY
Function
A forebay is a basin located at the inlet of a SCM that pretreats stormwater. A forebay reduces the runoff
velocity of stormwater, which in turn allows sediment in the stormwater to settle before the
stormwater enters the wet detention basin. A transition berm is the component located at the downstream
end of the forebay that acts as a weir and releases runoff to the SCM. The presence of a forebay
reduces the risk of SCM failure.
Inspection
Inspect forebays for trash, debris, and undesirable vegetation. Inspect to determine whether forebays
are structurally sound and that they do not contain excessive amounts of accumulated sediment.
Sediment accumulation should be less than 50% of the forebay's storage capacity. Inspect the
embankment and transition berm for structural integrity and signs of erosion. Inspect the forebay for
undesirable vegetation. Forebays may have grass -covered embankments and berms, but they are not
designed to support vegetation in the basin. Minimal plant growth is tolerable, but if it becomes
excessive, the storage volume for sediment is reduced and dredging becomes more difficult. If any
invasive species are present, it is generally preferable to remove them before they become well
established.
Maintenance
Remove trash, debris, and undesirable vegetation and properly dispose of it off -site. Remove sediment
and dispose of it off -site if it appears to occupy more than 50% of the forebay's storage capacity. Replace
erosion protection materials (i.e., riprap) as needed. If the surrounding soil is disturbed during cleanout of
8-5

the forebay, or at any other time, reseed any areas of bare soil with grass to prevent erosion. Repair the
transition berm as necessary, taking care to maintain the original elevation of the berm. Refer to
Chapter 3 for additional guidance on sediment disposal.

8.3.3 BASIN AND SURROUNDING AREA
Function
Wet Detention Basin
The basin is the component of the wet detention basin in which sedimentation and biological uptake of
pollutants occurs. A permanent pool of water is maintained in the basin. The basin is designed with
additional capacity, above the permanent pool, for detaining and slowly releasing runoff from storm
events. The sides of the basin may be covered with grass, riprap, and/or wetland vegetation.
If sediment removal or access to submerged components is necessary, the basin can be drained or pumped
out; however, the water must first be visually inspected for obvious signs of pollutants. Check for an oily
sheen, any unusual odors, or excessive suspended solids. If any of these are present, do not drain the
basin. Call the WMS for guidance.
Inspection
Inspect the sides of the basin and the surrounding area for structural integrity, noting any signs of burrowing
animals, erosion, inadequate vegetative cover, or undesirable vegetation. Verify that grass is being
mowed at a frequency to maintain a height of 6 to 15 inches. Inspect the embankment for settling,
scouring, cracking, sloughing, and furrowing, and for invading shrubs and trees. Also, inspect the
downstream toe of the embankment for seepage (NCDWQ, 2007). Remove any trash and debris found
in the basin or in surrounding areas.
The water level should be at or near the invert of the drawdown device/orifice except after storm events
and during prolonged dry periods. If the water level appears to be too high given recent weather
conditions, one or more components are likely clogged. Follow the inspection procedures outlined
previously before draining or pumping out the basin, and then follow the maintenance guidance given in
subsequent sections of this chapter to correct the problem. Low water levels when there is adequate
rainfall indicates that the infiltration rate in the basin is higher than originally anticipated. If this is an
aesthetic problem or causing adverse effects on the vegetation, a liner can be installed (NCDWQ, 2007).
Contact the WMS if there are chronic problems with low water levels.
Try to determine the depth of sediment that has accumulated in the basin. If levels are
approaching the drawdown orifice, sediment needs to be removed before it impedes the
functionality of the basin.
Algae are going to occur in wet detention basins as a result of: 1) nutrient enriched water,
2) shallow water, and 3) intense sun light. When the algal population is controlled, algae provide nutrient
removal. Generally, if the algae mats are not so thick as to provide mosquito refuge, they should not require
removal. However, unhealthy algal growth can occur when poor vegetative density increases the water
temperature of the basin or from excessive nutrient inputs.

If algal growth covers more than 50% of the basin, develop a management plan to remove and prevent
reoccurrence of such growth (NCDWQ, 2007). Physical removal is one option. Available chemical control
options are discussed in the N.C. Agricultural Chemicals Manual (NCSU, 2015). Applicators require a
commercial pesticide applicators license with an aquatic endorsement.
Some wet detention basins have a vegetated shelf at the perimeter that is planted with wetland
vegetation to increase pollutant removal. Consult the design drawings or landscaping plan to determine
whether the appropriate vegetation is present. Note any undesirable vegetation and verify that plants
remain healthy and uniformly established. Cattails or other invasive plants should be removed if they
cover more than 50% of the basin area (NCDWQ, 2007).
Baffles may have been installed to increase the effective flow length in the basin. If this is the case,
verify that the baffles are undamaged.
Maintenance
Remove undesirable vegetation by hand if possible or by wiping them with pesticide. Use aquatic
herbicides and other appropriate products per label instructions in and around the basin. Mow grass to
the ideal height. Turf grass is typically mowed at a frequency to maintain a maximum height of 4
inches for warm -season grasses and 6 inches for cool -season grasses. However, on highway right-of-
way facilities, it is more practical to maintain a grass height between 6 and 15 inches.
If erosion has occurred, reestablish grass, wetland vegetation, or riprap as appropriate. Replace dead or
unhealthy plant material. Fertilize upon reestablishment only if needed. The use of fertilizer may be
restricted at some locations. Refer to Chapter 3 when considering the use of fertilizer.
Repair all animal burrows. Repair damaged baffles if applicable. Remove sediment if it is suspected to
have exceeded the original design sediment storage depth or if it is impeding the function of the basin.
8.3.4 OUTLET CONTROL STRUCTURE (BOX)
Function
The outlet control structure regulates the water level and slowly releases stormwater. Outlet control
structures can have several components, including a trash rack, a drawdown device or orifice, a sluice
gate, and an outlet pipe. Inspection and maintenance guidance is provided in subsequent sections of this
chapter. Figure 8-5 illustrates the configuration of a typical outlet control structure. Drawdown
devices or orifices are located a few inches to a few feet below the top of the outlet control structure.
The outlet control structure has a second larger opening at the top allowing flow from large rainfall events
to escape without overtopping the basin. Usually, the outlet structure has an open -top with some form
of a trash rack.
:.

8-7

Wet Detention Basin
Inspection
Treated water should be allowed to flow freely upon entering the outlet control structure. Inspect the
outlet control structure thoroughly for any signs of damage such as cracks, holes, or leaks. Verify that
the outlet box remains covered with a trash rack. See subsequent sections for guidance on specific
components.
Maintenance

Repair any damaged areas of the outlet control structure, and remove sediment and debris if it is accessible.
PVC WORM ' 4NORM GEAR
GEAR COVER M
TRASH RACK
SLUICE
GATE
Figure 8-5. Compor,'ents of an outlet control structure
DRAWDOWN
DEVICE
BASIN
BOTTOM

If the outlet is clogged or blocked, hip waders or a small boat may be needed to make the necessary
repairs. Ensure that the proper safety procedures are followed when working in or around standing
water.

8.3.5 TRASH RACK
Wet Detention Basin
Figure 8-6. Two views of an outlet control structure
Function
Positioned atop the outlet control structure (box), the trash rack protects the overflow of the outlet
structure from becoming clogged with debris (see Figures 8-5 and 8-6).
Inspection
Inspect the trash rack for debris and excessive corrosion.
Maintenance
Remove trash and debris. Replace the trash rack if it is corroded or otherwise damaged. The
replacement trash rack should be consistent with the design specifications for the SCM.
8.3.6 DRAWDOWN DEVICE OR ORIFICE
Function
Positioned a few inches below the top of the outlet control structure, the drawdown device or orifice is
a restricting device that allows stormwater to slowly enter the outlet box. Several configurations exist.
The simplest type is one or more orifices in the shape of slots or holes directly in the side of the outlet
control structure.

Wet Detention Basin
Figure 8-7. Outlet control structures showing two types of drawdown devices,
slotted (left) and PVC elbow (right).
Drawdown devices have been installed on some outlet control structures to protect the orifice from
becoming clogged with floating debris. For example, a small baffle, a wall constructed around the
portion of the riser with the orifice(s), can protect the orifice(s) from floating debris while allowing runoff
to flow under the baffle and out through the orifices. Another type of drawdown device has a PVC
elbow or tee that maintains the entrance to the orifice below the water level so it does not become
clogged with floating debris. Figure 8-7 illustrates this configuration.
Inspection
If it is accessible, inspect the drawdown device to ensure that stormwater can freely enter the outlet
control structure. If metal components were used to restrict flow or guard against floating debris, check
them for excessive corrosion. If the drawdown device is not visible and is suspected to be submerged
after the 2 — 5 day drawdown period (check design specifications), it is likely clogged.
Maintenance
Remove sediment and debris blocking the flow into the orifice. If the orifice is submerged, follow
guidance given in Section 8.3.4 before pumping out or draining the basin. Replace damaged or
corroded components.
8.3.7 SLUICE GATE (IF PRESENT)
Function
Sluice gates are installed for emergency maintenance needs. The sluice gate covers an additional opening at
the invert of the basin that is larger than the drawdown orifice. The sluice gate can be opened to rapidly
drain the basin. Figure 8-8 is a photograph of a screw -type sluice gate.
Figure 8-9 shows a lift -type sluice gate.
&10

Figure 8-8. An open screw -type sluice gate
Figure 8-9. A Lift -type sluice gate
Wet Detention Basin
8-11

Wet Detention Basin
Inspection
Two types of sluice gates are currently in use: (1) a screw -type gate and (2) a lift -type gate. Make sure
the sluice gate has not frozen shut by slightly testing the functionality, taking care not to open it, which
could allow sediment or debris to escape or get lodged in the outlet. Inspect the sluice gate for excessive
corrosion. Verify that there is no sediment or debris below the sluice gate that could prevent it from
being fully closed.
Maintenance
If lubrication is necessary, lubricate with marine -type grease. It is recommended that a cover be
fabricated to protect the sluice gate's worm gear on a screw -type gate from corrosion and to increase the
visibility of the device. The cover can be made by placing a cap on one end of a section of PVC pipe
(see Figure 8-10). If accessible, remove any sediment and debris near the sluice gate.
Figure 8-10. Two large sluice gates with PVC covers
8.3.8 OUTLET DRAINAGE SYSTEM
Function
The outlet drainage system conveys water from the SCM to the downstream drainage system or receiving
water. Typically, a pipe conveys water from the outlet control structure through the embankment to a
swale or other open channel.
Inspection
Inspect the inside of structures and pipe(s) to verify that they are free of sediment and debris. Inspect
the ground surface above buried pipes and structures for depressions or other signs that might indicate
pipe breakage or separation. Inspect ditches for signs of erosion and undesirable vegetation.
&12

Wet Detention Basin
Maintenance
Remove any sediment or debris that is accessible. Repair eroded areas and damaged pipes. Replace
outlet protection materials (i.e., riprap) as necessary. Refer to Chapter 3 for additional guidance on
maintenance techniques.
9.3.9 EMERGENCY SPILLWAY (IF PRESENT)
Function
The emergency spillway serves as an overflow structure that is typically constructed as a channel in
natural ground. The emergency spillway is necessary to minimize the potential for overtopping the
basin, which can damage the embankment and lead to SCM failure and downstream flooding.
Inspection
The emergency spillway should remain free of trash and debris. Emergency spillways are typically
covered with grass, but concrete and riprap are also used. For grass channels, verify that grass is
maintained between 6 and 15 inches. Inspect concrete channels to verify that they are in good
condition, or verify that adequate riprap is present. Note all erosion and undesirable vegetation. Observe
the point of discharge to evaluate if erosion protection is adequate.
Maintenance
Emergency spillways must be free of trash, debris, sediment and undesirable vegetation to maintain
proper function. Remove these if present. Grass should be carefully maintained at a height of 6 to 15
inches. Repair concrete or replace riprap as necessary. Repair areas of erosion.
8.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection report form and entered into SMD. Observations recorded would include a general
description of the SCM and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
inspection must be documented on the appropriate SCM inspection report form and entered into
SMD.
8-13

Stormwater Wetland
CHAPTER 9 Stormwater Wetland
h
i S
i•
OVERVIEW
A STORMWATER WETLAND is An engineered marsh or swamp with dense wetland vegetation
designed to remove stormwater pollutants primarily through biological processes.
PURPOSE AND DESCRIPTION
Stormwater wetlands, as opposed to naturally occurring wetlands, have distinct inlet and outlet
structures.
Vegetation grows throughout the wetland.
Pools of standing water are usually present, although some wetlands are designed to treat runoff
below around -
INSPECTION
The density and diversity of vegetation is critical to the success of the stormwater wetland.
During inspection, vegetation should be compared to the landscaping plan.
The water level should remain near the drawdown device, except under drought conditions.
Inspections should occur semi-annually, unless more frequent inspections are deemed
necessary.
743

Stormwater Wetland
TYPICAL MAINTENANCE
Trash and debris should be removed from inlet grates, forebays, orifices, and trash racks.
Undesirable vegetation should be removed, and diseased or dying vegetation should be
replanted.
If an algae bloom is present, an action plan should be developed and followed.
9.1 Stormwater Control Measure Overview
Stormwater Wetlands are constructed stormwater controls that mimic natural wetlands, and are
designed with multiple treatment zones of different water depths. Each treatment zone supports a
diverse aquatic ecosystem. The primary treatment zones are deep pools, shallow water, and shallow
land. Deep pools (18-36 inches deep) dissipate flow energy, trap sediment, remove nitrogen, and
promote infiltration. Deep pools are also important for mosquito control by providing habitat for fish.
The shallow water zone (2-4 inches) provides habitat for diverse wetland plants that provide additional
nitrogen and phosphorus removal. The shallow land zone (also referred to as the temporary inundation
zone) is submerged only for a short period of time as a result of storm events. Otherwise this area is
dry and should be 0-12 inches above the normal pool elevation. This area supports a wide variety of
vegetation that grows in wet or dry conditions and increases the diversity of the wetland.
A constant supply of water, generally provided by groundwater in addition to stormwater, is needed to
maintain a shallow water level. Therefore, most stormwater wetlands are located in low-lying, typically
wet areas. Figure 9-1 illustrates the treatment processes that occur in a typical stormwater wetland.
.tiTJ.

Figure 9-1. Cutaway of a stormwater wetland flow diagram and treatment processes
9.2 Stormwater Wetland Components
Stormwater wetlands include, but are not limited to, the following components:
• Inlet Drainage System
• Forebay(s)
• Basin
• Outlet Control Structure
• Outlet Drainage System
• Emergency Spillway
Atypical stormwater wetland layout and its components are shown in Figure 9-2.

ft
jk
Figure 9-2. Components of a typical stormwater wetland
r
As runoff enters the stormwater wetland forebay, the runoff velocity is quickly reduced allowing large
trash, debris, and solids to be removed. After runoff leaves the forebay, runoff moves slowly through
the wetland vegetation in the shallow water zone. Contact with vegetation allows for biological
treatment of metals and nutrients primarily through microbial processes (NCHRP, 2006). As runoff
continues through the wetland, it enters the deep pools. Because the deep pools tend to be stagnant,
they provide oxygen -free environments for additional nutrient removal by anaerobic microbes (Hunt et
al., 2007).
As opposed to natural systems, stormwater wetlands have distinct inlet and outlet control systems
to manage the shallow water elevation. The outlet control structure is typically a riser structure
composed of low -flow and high -flow options. A low -flow orifice maintains the shallow water level in
the wetland. High -flow options (e.g., an open -throat riser, broad -crested weir) detain and slowly release
stormwater from large storm events over a period of 2-5 days. This reduces the runoff velocity of the
discharge, and allows additional physical and chemical processes to remove pollutants while the water is
detained in the stormwater wetland.
9.3 Inspection and Maintenance

Stormwater wetlands should be inspected to determine whether they are functioning as intended.
Most important, it should be verified that the outlet control structure is functioning properly and that the
shallow water level is being maintained. If a stormwater wetland is found not to be functioning
properly, determine the cause and restore to working order as soon as possible. Figures 9-3 and 9-4
illustrate a plan and profile view of the areas that should be inspected and maintained in a typical
stormwater wetland configuration.
All inspection findings and maintenance activities must be noted on the inspection form (see Appendix
A) and then entered into the SMD. Photos should be taken to track the status of the SCM and
document the maintenance activities conducted. These, too, should be uploaded to SMD. Refer to
Chapter 4 for further guidance.
The following sections describe the function of a stormwater wetland, and provide inspection and
maintenance guidance for its major components.
C
Figure 9-3. Plan view of a stormwater wetland and its components
FOREGAY
SHALLOW WATER
INFLOY LEVEL
ENERGY RISSIPATU
DEEP
9*r= ■1 "
TRASH RACK
SHALLOW EMBANKMENT
LfaNO .
EMERGENCY
SPILLWAY
ENERGY
�•,' �OISSIPATOR
[XiTLET CONTROL
POOL STRUCTURE
ANTIFLOTATION MATERIN-S

Stormwater Wetland
Figure 9-4. Profile view of a stormwater wetland and its components
9.3.1 INLET DRAINAGE SYSTEM
Function
The inlet drainage system collects and conveys water to the stormwater wetland. Inlet drainage systems
can consist of open channels, pipes, curb and gutter, and catch basins.
Inspection
Stormwater runoff should be allowed to flow freely into the basin. Inspect ditches, pipes, and/or catch
basins draining to the stormwater wetland for trash, sediment, and debris. Inspect the ground
surface above buried pipes and structures for depressions and other signs of pipe breakage or
separation. Inspect ditches for signs of erosion and undesirable vegetation. Desirable vegetation
includes grass cover in an open ditch to prevent soil erosion. Undesirable vegetation includes any woody
plants or invasive species that may impede the inlet drainage system.
Maintenance
Remove and properly dispose of debris, undesirable vegetation, and major sediment
accumulations. Repair eroded areas and damaged pipes. Refer to Chapter 3 for additional guidance
on maintenance techniques.
9.3.2 FOREBAY
Function
A forebay is a basin located at the inlet of a SCM that pretreats stormwater. A forebay reduces the velocity
of stormwater, which in turn allows sediment in the runoff to settle before the stormwater enters
the stormwater wetland. A transition berm is the component located at the downstream end of the
forebay that acts as a weir and releases runoff to the SCM. The presence of a forebay reduces the risk of
SCM failure.
Inspection
Inspect forebays for trash and debris. Inspect to determine whether forebays are structurally sound
and that they do not contain excessive amounts of accumulated sediment. Inspect the embankment and
transition berm for structural integrity and signs of erosion. Inspect the forebay for undesirable
vegetation. Forebays may have grass -covered embankments and berms, but they are not designed to
support vegetation in the basin. Minimal plant growth is tolerable, but if it becomes excessive, the
storage volume for sediment is reduced and dredging becomes more difficult. If any invasive species
are present, it is generally preferable to remove them before they become well established.

Stormwater Wetland
Maintenance
Remove trash, debris, and undesirable vegetation and properly dispose of it off -site. Remove sediment
and dispose of it off -site if it appears to occupy more than 50% of the forebay's storage capacity. Replace
erosion protection materials (i.e., riprap) as needed. If the surrounding soil is disturbed during cleanout of
the forebay, or at any other time, reseed any areas of bare soil with a turf -type grass to prevent erosion.
Repair the transition berm as necessary, taking care to maintain the original elevation of the berm.
Refer to Chapter 3 for additional guidance on sediment disposal.
9.3.3 BASIN AND SURROUNDING AREA
Function
The basin is the component of the stormwater wetland in which wetland plants and aquatic organisms
remove nutrients and other pollutants. Generally, the wetland supports a shallow water elevation
interspersed with deep pools. The wetland is also designed with additional capacity, above the
shallow water elevation, for detaining and slowly releasing runoff from storm events. The interior
and side slopes of the stormwater wetland are covered with very specific wetland and non -wetland
vegetation, depending on the water level in that area of the SCM.
If access to submerged components is necessary, the stormwater wetland can be drained or pumped
out; however, the water must first be visually inspected for obvious signs of pollutants. Check for an oily
sheen, any unusual odors, or an excessive amount of suspended solids. If any of these are present, do
not drain the basin. Call WMS for guidance.
Inspection
Inspection of the stormwater wetland basin should focus particularly on the health, density, and diversity
of wetland vegetation. The design planting plan should be obtained and referenced during the
inspection. The plants in the planting plan are chosen to provide one or more specific functions such as
pollutant uptake, shade, or mosquito control. If over 30% of the wetland vegetation appears to be
dead or dying, determine the source of the problem (i.e., inadequate water supply, disease) and
record it on the inspection form (Appendix A). Ideal vegetation density should be greater than 90% in
planted areas (NCDWQ, 2007).
If the vegetation density is clearly reduced from the intended design, provide a note on the inspection
form, including the general location within the stormwater wetland.
Aquatic weeds and other invasive species can be detrimental to a stormwater wetland because they
will very quickly take over the desirable wetland plants. Once established, it can be impossible to
remove the invasive species without also harming the native plants. Therefore, early intervention is
important. Invasive or aggressive species should be removed as soon as they are noticed, because by
the following year they will be much more difficult to kill. Examples of exotic invasive species that
can be problems for stormwater wetlands include hydria, phragmites, and purple loosestrife. Cattails

Stormwater Wetland
can also be a problem species. Although they are native and provide some pollutant removal, cattails
multiply quickly and can reduce the device's storage capacity, crowd out other desirable species, and
create a breeding habitat for mosquitoes (Hunt et al., 2007).
Many stormwater wetlands contain some algae. Algae are going to occur in stormwater wetlands as a
result of: 1) nutrient enriched water, 2) shallow water, and 3) intense sun light. When the algal
population is controlled, algae provide nutrient removal. Generally, if the algae mats are not so thick
as to provide mosquito refuge, they should not require removal. However, unhealthy algal growth can
occur when poor vegetative density increases the temperature of the water in the wetland or when the
wetland receives excessive nutrient inputs. If algal growth covers more than 50% of the stormwater
wetland, develop a management plan to remove and prevent reoccurrence of such growth (NCDWQ
2007). Physical removal is one option. Approved chemical control options are discussed in the N.C.
Agricultural Chemicals Manual (NCSU, 2015). Applicators are required to have a commercial pesticide
applicators license with an aquatic endorsement.
Inspect the sides of the stormwater wetland and the surrounding area for structural integrity, noting
any signs of burrowing animals, erosion, or inadequate cover. If areas around the stormwater wetland
are grassed, verify that grass is being mowed at a frequency to maintain a height of 6 to 15 inches.
Inspect the embankment for settling, scouring, cracking, sloughing, and furrowing, and for invasive shrubs
and trees. Also, inspect the downstream toe of the embankment for seepage (NCDWQ, 2007). Note
trash and debris found in the stormwater wetland or in surrounding areas.
The water level should be at or near the invert of the drawdown device/orifice except after storm events
and during prolonged dry periods. Generally, the deep pools of the wetland should not dry out during
prolonged dry periods, except for significant droughts. If the deep pools are dry and the region is not
undergoing an extreme drought, the stormwater wetland may be experiencing excessive infiltration.
Make a note of the condition on the inspection form. The underlying soils may need to be tamped down
or a clay soil supplement added to the wetland (Hunt et al., 2007).
Maintenance
Avoid spraying nonaquatic chemicals in and around the basin. Remove undesirable vegetation by
hand if possible or by wiping plants with pesticide. It is important to dispose of vegetative cuttings
off -site so they do not contribute additional nutrients to the stormwater wetland.
Mow grass to the ideal height around the wetland and on the embankment. Turf grass is typically mowed at
a frequency to maintain a maximum height of 4 inches for warm -season grasses and
6 inches for cool -season grasses.
If inadequate plant density is noted in the inspection, replace vegetation according to the planting plan.
Fertilize upon establishment, following best practices to avoid excessive input of nutrients into the
wetland. The use of fertilizer may be restricted at some locations. Refer to Chapter 3 when considering

the use of fertilizer. If inadequate wetland vegetation is a widespread problem throughout the SCM or if the
wetland is dry during non -drought periods it may be necessary to perform soil amendments or to re -
grade the basin.
If excessive algae are noted during the inspection, refer to the information under "Inspection" of this
section and/or follow any appropriate actions in the algae management plan for the SCM.
Repair animal burrows.
Unlike most other stormwater controls, solids that have settled in the stormwater wetland cannot be
removed by dredging. Solids should be removed from the stormwater wetland only if it is deemed critical to
the functioning of the stormwater wetland. The top layer of dredged material should be replaced over
the wetland to reestablish vegetation (NCDWQ, 2007). Proper maintenance of the forebay should prevent
more costly dredging of the stormwater wetland basin.
9.3.4 OUTLET CONTROL STRUCTURE (BOX
Function
The outlet control structure maintains the shallow water level in the wetland and regulates the slow
release of stormwater. Outlet control structures can have several components, including a trash rack, a
drawdown device or orifice, a sluice gate, and an outlet pipe. Inspection and maintenance guidance
for these specific components is provided in subsequent sections of this chapter. Figure 9-5 illustrates
the configuration of a typical outlet control structure.
Figure 9-6 illustrates the configuration of a flashboard riser, sometimes used on stormwater wetlands to
provide more flexibility with water levels. Generally, a drawdown device, such as an orifice installed
directly into the riser or an inverted pipe, controls the elevation of the shallow pool. The outlet control
structure often has a second, larger opening at the top that allows flow from large rainfall events to
escape without overtopping the basin. Usually, this is in the form of an open -top outlet structure with a
trash rack. Flashboard risers consist of a riser with one wall or edge composed of movable boards to
create adjustable water levels. With this type of riser, the shallow water level and large rainfall
elevations may be adjusted in a variety of ways to handle maintenance needs. Figure 9-7 shows
photos of the two outlet control structures.

Stormwater Wetiand
Inspection
Treated water should be allowed to flow freely upon entering the outlet control structure. Inspect the
outlet control structure thoroughly for any signs of damage such as cracks, holes, or leaks. Verify that
the outlet box remains covered with a trash rack. See subsequent sections for guidance on specific
components. For flashboard riser structures, check the connection between boards for any signs of leaks
orseepage.
Maintenance
Repair any damaged areas of the outlet control structure, and remove sediment and debris if it is accessible.
If the outlet is clogged or blocked, hip waders or a small boat may be needed to make the necessary
repairs. Ensure that the proper safety procedures are followed when working in or around standing
water.
PVC WORM WORM GEAR
GEAR COVER yr
TRASH RACK
i
i
i
i
SLUICE
GATE
i
i
i
i
i
i
i
i
OUTLET
PIPE + --
DRAWDOWN
DEVICE
BASIN
BOTTOM
Figure 9-5. Components of a typical outlet control structure for a stormwater wetland

Stormwater Wetiand
TRASH RACK
FLASHBOAREI
INSERT
OUTLET PIPE
SHALLOW WATER
LEVEL ORIFICE
CROSS BRACE
FLASHBOARD INSERTS WITH
ORIFICES FOR ADJUSTABLE
WATER LEVEL
SHALLOW WATER
LEVEL
Figure 9-6. Components of a flashboard riser outlet control structure
for a stormwater wetland
Figure 9-7. Two examples of outlet control structures: a standard riser (left)
and a flashboard riser (right)
9.3.5 TRASH RACK
Function
Positioned atop the outlet control structure, the trash rack protects the overflow of the outlet control
structure from becoming clogged with debris (see Figures 9-5 and 9-6).
9-10

Stormwater Wetland
Inspection
Inspect the trash rack for debris and excessive corrosion.
Maintenance
Remove trash and debris. Replace the trash rack if it is corroded or otherwise damaged. The
replacement trash rack should be consistent with the design specifications for the SCM.
9.3.6 DRAWDOWN DEVICE OR ORIFICE
Function
The drawdown device or orifice is a restricting device that allows stormwater to slowly enter the outlet
box. Several configurations exist. The simplest type is one or more orifices in the shape of slots or
holes directly in the side of the outlet control structure. Another type of drawdown device has a PVC
elbow or tee that maintains the invert to the outlet control structure below the water level so it does not
become clogged with floating debris.
Inspection
If it is accessible, inspect the drawdown device to ensure that stormwater can freely enter the outlet
control structure. If metal components were used to restrict flow or guard against floating debris, check
them for excessive corrosion. If the drawdown device is not visible, it is likely submerged due to being
clogged.
Maintenance
Remove sediment and debris blocking the flow into the orifice. If the water level in the basin is above the
orifice, open the sluice gate and drain the basin until the drawdown device/orifice is accessible, then clean
it out. Replace damaged or corroded components.
9.3.7 SLUICE GATE (IF PRESENT)
Function
Sluice gates are installed for emergency maintenance needs. The sluice gate covers an additional opening at
the invert of the basin that is larger than the drawdown orifice. The sluice gate can be opened to rapidly
drain the basin. Figure 9-8 is a photograph of a screw -type sluice gate.
Figure 9-9 shows a lift -type sluice gate.
Inspection
Two types of sluice gates are currently in use: (1) a screw -type gate and (2) a lift -type gate. Make
sure the valve has not frozen shut by slightly testing the functionality, taking care not to open it, which
could allow sediment or debris to escape or get lodged in the outlet. Inspect the sluice gate for
excessive corrosion. Verify that there is no sediment or debris below the sluice gate that could prevent
it from being fully closed.
9-11

Stormwater Wetland
c* r
"s
ti
it
1.
Figure 9-8. An open screw -type sluice gate
Figure 9-9. A lift -type sluice gate
9-12

Stormwater Wetland
Maintenance
If lubrication is necessary, lubricate with marine -type grease. It is recommended that a cover be
fabricated to protect the sluice gate's worm gear from corrosion and to increase the visibility of the
device. The cover can be made by placing a cap on one end of a section of PVC pipe (see Figure 9-10).
Remove any sediment and debris near the sluice gate.
Figure 9-10. Two large sluice gates with PVC cover
9.3.8 OUTLET DRAINAGE SYSTEM
Function
The outlet drainage system conveys water from the SCM to the downstream drainage system or receiving
water. Typically, a pipe conveys water from the outlet control structure through the embankment to a
swale or other open channel.
Inspection
Inspect the inside of structures and pipe(s) to verify that they are free of sediment and debris. Inspect
the ground surface above buried pipes and structures for depressions or other signs that might indicate
pipe breakage or separation. Inspect ditches for signs of erosion and undesirable vegetation.
Maintenance
Remove any sediment or debris that is accessible. Repair eroded areas and damaged pipes. Replace
outlet protection materials (i.e., riprap) as necessary. Refer to Chapter 3 for additional guidance on
maintenance techniques.
9-13

Stormwater Wetland
9.3.9 EMERGENCY SPILLWAY (IF PRESEN
Function
The emergency spillway serves as an overflow structure that is typically constructed as a channel in
natural ground. The emergency spillway is necessary to minimize the potential for overtopping the
basin, which can damage the embankment and lead to SCM failure and downstream flooding.
Inspection
The emergency spillway should remain free of trash and debris. Emergency spillways are typically
covered with grass, but concrete and riprap are also used. For grass channels, verify that grass is
maintained between 6 and 15 inches. Inspect concrete channels to verify that they are in good
condition, or verify that adequate riprap is present. Note all erosion and undesirable vegetation.
Maintenance
Emergency spillways must be free of trash, debris, and undesirable vegetation to maintain their
functionality. Remove these if present. Grass should be carefully maintained at a height of 6 to 15
inches. Repair concrete or replace riprap as necessary. Repair areas of erosion.
9.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection report form and entered into SMD. Observations recorded would include a general
description of the SCM and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
inspection must be documented on the appropriate SCM inspection report form and entered into
SMD.
9-14

CHAPTER 10 Swale
OVERVIEW
Swale
A SWALE is a ve etated channel that treats and conveys runoff from small draina e areas.
PURPOSE AND DESCRIPTION
Swales are broad and shallow so that water will flow slowly.
Swales remove suspended solids, metals, and nutrients through sedimentation, vegetated
INSPECTION
Swales should be carefully monitored for erosion so they do not contribute sediment to receiving
waters.
Swales should be inspected periodically to verify that the ideal vegetation type is maintained at
the proper density and height.
Inspections should occur semi-annually, unless more frequent inspections are deemed necessary.
TYPICAL MAINTENANCE
Sediment and debris should be removed before it negatively impacts vegetation growth or inhibits
infiltration.
Mowing should be done at a frequency that maintains aesthetics and prevents the growth of
undesirable vegetation.
Eroded areas in and around the swale should be repaired as needed.
10-1

Swale
10.1 Stormwater Control Measure Overview
Swales convey and treat peak runoff from small drainage areas. Swales decrease runoff velocity to
promote infiltration and physical filtration. Swales also increase contact time between runoff and
vegetation to promote biological uptake of pollutants.
Swales are similar to roadside ditches in appearance and function. During storm events, swales collect
water from roadways and transport it downstream. The main difference between a swale and a roadside
ditch is that swales are especially broad and shallow. The broad and shallow design and the vegetation
in the swale help to slow the speed of the runoff.
The reduction in speed enhances both the pollutant filtering and runoff infiltration processes Suspended
solids and other associated pollutants can be filtered and settle -out, contact with vegetation allows for
biological treatment of metals and nutrients through primarily microbial processes, and the potential for
downgrade erosion is greatly reduced. Figure 10-1 illustrates the treatment processes and flow paths in
two different swale configurations.
1Q2

qL
Area w�l.ere-fittra
- __.¢trd7nfiltration occur
_ I
.Velocily and shallow
e low for treatment
Area where fillration'
and Infiltration occur
Note: Generally, swales are configured so that
stormwater exits the swale and flows overland until
it infitrates or reaches a receiving water body
sabovej, but sometimes stormwater is collected and
conveyed from the swale via an outlet drainage
system (below).

10.3 Swale Components
Figure 10-2 illustrates the various components of a typical swale. Note that layouts vary. Some
systems will have additional components while others may lack certain components. Swale
systems may include the following:
Inlet Drainage System
Forebay, Preformed Scour Hole, or Splash Pad
Swale
Check Dams (not illustrated in Figure 10-2)
Outlet Drainage System
Swale
10-3

Forebay, Preformed Scour Hole,
or Splash Pad
Inlet Drainage System
Outlet Drainage System

Swale
10.3 Inspection and Maintenance
Swales and the drainage structures associated with them should be inspected to determine whether they
are functioning as intended. If the system is found to not be functioning properly, determine the cause and
restore to working order as soon as possible.
All inspection findings and maintenance activities should be noted on the inspection form (see Appendix A)
and then entered into SMD. Photos should be taken to track the status of the SCM and document the
maintenance activities conducted. These, too, can be uploaded to SMD. Refer to Chapter 4 for
further guidance.
The following sections describe the function of a swale, and provide inspection and maintenance guidance
for its major components.
10.3.1 INLET DRAINAGE SYSTEM (IF PRESENT)
Function
The inlet drainage system collects and conveys water to the SCM. Inlet drainage systems can consist
of open channels, pipes, curb and gutter, and catch basins. An inlet drainage system may not be present
if the swale receives runoff directly from the source via overland flow.
While swales sometimes include a forebay at the inlet, riprap and filter fabric are often used for erosion
protection.
Inspection
Stormwater runoff should be allowed to flow freely into the swale. Inspect ditches, pipes, and/or catch
basins draining to the swale for trash, sediment, and debris. Inspect the ground surface above buried
pipes and structures for depressions and other signs of pipe breakage or separation. Inspect ditches for
signs of erosion and undesirable vegetation.
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment accumulations.
Repair eroded areas and damaged pipes. If riprap has been placed at the inlet, inspect for undercutting
or erosion. Refer to Chapter 3 for additional guidance on maintenance techniques.
10.3.2 FOREBAY. PREFORMED SCOUR HOLE. OR SPLASH PAD (IF PRESENT
Function
Forebays, splash pads, and/or preformed scour holes (PSHs) are energy dissipaters that may all be
used at the inlet(s) of a swale to pretreat stormwater and protect against erosion. A forebay reduces the
flow velocity, which in turn allows suspended solids in the stormwater to settle before the
stormwater enters the swale. Forebays facilitate maintenance by collecting sediment in one area
where it can be removed easily, increasing the life of the swale. Splash pads simply protect against
erosion where runoff exits a pipe, in this case, at the inlet to the swale. Preformed
10-4

Swale
scour holes resemble forebays, but are designed to dissipate flow as it exits the PSH, similar to a level
spreader. PSI -Is are square or round in shape, and use riprap and permanent soil reinforcement
matting to protect against erosion. Figure 10-3 gives a side -by -side comparison of a forebay and a PSH.
Guidance for forebays and splash pads follows.
Inspection
Inspect forebays and splash pads for trash, debris, and undesirable vegetation. Inspect to determine
whether these facilities are structurally sound and that they do not contain excessive amounts of
accumulated sediment. Splash pads are not typically designed to store sediment; however, any
sediment accumulations that have the potential to wash downstream or cause problems in the swale
should be removed.
Maintenance
Remove sediment, trash, debris, and undesirable vegetation and properly dispose of it off -site. Sediment
should be removed from a forebay when it exceeds 50% of a forebay's storage capacity. Correct any
structural deficiencies and replace erosion protection materials (i.e., riprap) as needed. If necessary,
reestablish vegetation for earthen forebays. Refer to Chapter 3 for further guidance on sediment
disposal.
10.3.3 SWALE
Figure 10-3. A well maintained forebay (left) and
a newly constructed preformed scour hole (right)
Function
A swale decreases runoff velocity to promote infiltration and filtration of stormwater runoff. By slowing
runoff, suspended solids and associated pollutants settle out of stormwater before being conveyed
downstream. Typically, swales work best on relatively flat grades. However, check dams (see section
10.3.4) may be installed in areas with steep slopes to help slow the flow. Swales often drain directly
into receiving waters and other stormwater controls. For this reason, it is important that swales are
10-5

Swale
carefully inspected and maintained to prevent excessive pollutants from being conveyed downstream.
Figure 10-4 shows two photos of typical swales in a highway setting.
Inspection
The base of the swale, its interior and exterior side slopes, and areas surrounding the swale should be
stabilized with a dense cover of turf -type grass. Inspect the swale and surrounding areas for erosion.
Inspect for undercutting of swale side slopes, which can cause the slopes to become steeper than 3:1,
the maximum allowed. Steep side slopes can cause excessive channelization and an increase in flow
velocity, which decreases the effectiveness of the swale.
Figure 10-4. Linear highway applications of swales
Verify that grass is being mowed at a frequency to maintain the desired height. Ideally, turf grasses
should be mowed at a frequency to maintain a maximum height of 4 inches for warm- season grasses
and 6 inches for cool -season grasses.
Inspect the swale and surrounding areas for bare soil, sediment deposition, trash, debris, and
undesirable vegetation.
Maintenance
Trash, debris, and undesirable vegetation should be removed from the swale. Excessive accumulations
of sediment should be removed when typical flow is altered or grass can no longer be seen. Stabilize
any eroding surfaces in or around the swale, and repair undercut slopes by restoring the proper grade
and reseeding. Refer to Chapter 3 for further guidance.
10.3.4 CHECK DAMS (IF PRESENT
Function
Checks dams are small riprap dams constructed to enhance the water quality benefits of swales. Checks
dams, are installed in swales to create storage volume or to slow flow in areas with steep slopes. Figure 10-
5 shows an example of check dams implemented in a swale. Check dams are permanent riprap dams
constructed to enhance water quality and should not be confused with a sediment and erosion control
temporary device used during construction.

Inspection
Inspect check dams for trash, debris, undesirable vegetation, excessive sediment, and erosion around
the sides of the rock check. Verify that check dams retain the proper dimensions. Check dams should be
approximately one foot high along the base and sides of the swale and approximately 4.5 feet in width
(in the direction of flow). The main structure of the check dam should be composed of Class B riprap. A 12-
Figure 10-6. Check dam

inch (approximate) layer of No. 57 stone should be located on the upstream side of the check dam to
capture sediment. Figure 10-6 shows the basic configuration of a check dam.

Swale
Maintenance
If possible, maintenance on the permanent check dam should be performed when the swale is dry so as
not to allow accumulated sediment to wash downstream. Remove and properly dispose of trash, debris,
undesirable vegetation, and sediment. Replace riprap and No. 57 stone, repair erosion, and rebuild or
reshape check dams as necessary. When mowing swales, be sure not to damage the check dam's
structure. Handheld equipment should be used when mowing around check dams.
10.3.5 OUTLET DRAINAGE SYSTEM (IF PRESENT)
Function
The outlet drainage system conveys water from the storm water SCM to the downstream drainage
system or receiving water. Components of this system can include channels, pipes, catch basins, manholes,
culverts, and other structures. Swales often end at a grassy, open area or forested buffer. An outlet
drainage system will not be present in these situations.
Inspection
Inspect pipes and drainage structures for cracks or leaks. Inspect the ground surface above buried
pipes and structures for depressions or other signs of pipe breakage or separation. Inspect ditches for signs
of erosion and undesirable vegetation. If outlet protection materials are present, verify that these
materials are adequate.
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment accumulations.
Repair eroded areas and damaged pipes. Replace outlet protection materials (i.e., riprap) as
necessary. Refer to Chapter 3 for additional guidance on maintenance techniques.
10.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection report form and entered into SMD. Observations recorded would include a general
description of the SCM and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
inspection must be documented on the appropriate SCM inspection report form and entered into
SMD.
1'

Level Spreader
CHAPTER 11 Level Spreader
S:• �'
PIZ
OVERVIEW
A LEVEL SPREADER is a structural stormwater control that redistributes concentrated stormwater flow
into sheet flow.
PURPOSE AND DESCRIPTION
A level spreader provides a non -erosive outlet for concentrated runoff by diffusing the water
uniformly across a stable slope.
INSPECTION
Level spreaders are designed to convert concentrated flow to sheet flow before it enters a
buffer or filter strip; erosion within the buffer or filter strip is an indication that the level spreader is
not functioning properly.
The level spreader lip must be level to promote uniform, diffuse flow along its entire length.
Inspections should occur semi-annually, unless more frequent inspections are deemed necessary.

TYPICAL MAINTENANCE
Sediment, trash, and debris should be removed from the level spreader trough and other
components as applicable.
Any structural deficiencies, especially pertaining to the level spreader lip, should be repaired or
the components replaced.
Repair eroded areas within the buffer or filter strip after determining the root cause of the erosion.

Level Spreader
11.1 Stormwater Control Measure Overview
A Level Spreader is a stormwater device used to slow runoff velocities, trap sediment and promote
infiltration. It works by dispersing concentrated runoff uniformly over the ground's surface. During
storm events concentrated runoff from parking lots, roadways, or ditches is directed into the level
spreader where it is spread out and released as sheet flow over a vegetated area (filter strip). The
resultant sheet flow helps reduce the erosion potential and enhances the pollutant filtering and runoff
infiltration processes. Suspended solids and other associated pollutants are filtered and settle -out,
while dissolved metals and nutrients are taken -up by the soil and plant roots. Figure 11-1 illustrates
concentrated water flow entering a level spreader and sheet water flow exiting a level spreader.
Figure 11-1. Water flow through a level spreader
Figure 11-2 illustrates the treatment processes that occur in a typical level spreader system.
112

Level Spreader
Figure 11-2. Level spreader treatment processes
11.2 Level Spreader Components
Figure 11-3 illustrates the various components of a typical level spreader system. Currently, level
spreaders are designed to include a flow bypass structure. For these systems, runoff enters the level
spreader via a flow bypass structure, which is designed to divert a set amount of runoff to the trough of
the level spreader. Excess flow bypasses the level spreader and is conveyed by ditch or pipe directly
downstream. Level spreaders constructed before the flow bypass system requirement was in place may
not include a flow bypass system. For these level spreaders, all runoff will enter the level spreader
regardless of the flow rate.
Level spreader systems may include, but are not limited to, the following components: Inlet
■ Drainage System
■
■ Flow Bypass Structure/System
■ Level Spreader Trough and Lip
■ Filter Strip and/or Buffer
Bypass Swale/Pipe or Outlet Ditch
11-3

Level Spreader
50' BUFFER ZONE
FROM INLET BUFFER BUFFER
DRAINAGE ZONE ZONE
SYSTEM FLOW
— BYPASS
STRUCTURE
LIP
VEGETATED
OR CONCRETE BYPASS
TROUGH SWALE OR
PIPE
44' L
fi. c f •HL 'i' 1'.�. L�. - _ ..
as r•
• .: .r ii i it - {�...y
31
X.
OPTIONAL PERMANENT SOIL
DRAWDOWN REINFORCEMENT MATTING `\
DEVICE l�
RECEIVING WATER
BODY
Figure 11-3. Components of a typical level spreader system
(not all components may be present)
11.3 Inspection and Maintenance
Level spreaders should be inspected to determine whether they are functioning as intended. Most
importantly, it should be verified that the level spreader is distributing flow evenly. If a level spreader
is found not to be functioning properly, determine the cause and restore to working order as soon as
possible.
All inspection findings and maintenance activities should be noted on the inspection form (see Appendix
A) and then entered into the Stormwater Management Database. Photos should be taken to track the
status of the stormwater control (SC) and document the maintenance activities conducted. These, too,
can be uploaded to SMD. Refer to Chapter 4 for further guidance.
The following sections describe the function of a level spreader, and provide inspection and
maintenance guidance for its major components.
114

Level Spreader
11.3.1 INLET DRAINAGE SYSTEM
Function
The inlet drainage system collects and conveys water to the SCM. Inlet drainage systems can consist
of open channels, pipes, curb and gutter, and catch basins.
Inspection
Stormwater runoff should be allowed to flow freely into the level spreader or flow bypass structure.
Inspect ditches, pipes, and/or catch basins draining to the level spreader for trash, sediment, and
debris. Inspect the ground surface above buried pipes and structures for depressions and other signs
of pipe breakage or separation. Inspect ditches for signs of erosion and undesirable vegetation.
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment
accumulations. Repair eroded areas and damaged pipes. Refer to Chapter 3 for additional guidance
on maintenance techniques.
11.3.2 FOREBAY (IF PRESENT)
Function
A forebay is a basin located at the inlet of a SCM that pretreats stormwater. A forebay reduces the runoff
velocity of stormwater, which in turn allows suspended particles in the stormwater to settle before
the stormwater enters the level spreader. The presence of a forebay reduces the risk of SCM failure due
to clogging of the filter strip and sediment accumulation in the trough.
Inspection
Inspect the forebay for trash, debris, and undesirable vegetation. Inspect to determine whether forebays
are structurally sound and that they do not contain excessive amounts of accumulated sediment.
Maintenance
Remove trash, debris, and undesirable vegetation and properly dispose of it off -site. Remove sediment
and dispose of it off -site if it appears to occupy more than 50% of the forebay's storage capacity.
Reestablish vegetation in earthen forebays. Refer to Chapter 3 for further guidance.
11.3.3 FLOW BYPASS STRUCTURE (IF PRESENT)
Function
Level spreaders can easily become overwhelmed during large rainfall events. If flow in excess of the
design amount is allowed to pass through the level spreader, it will likely re -concentrate and erode
11-5

areas of the filter strip or buffer. Flow bypass structures allow excess flows from large storm events to
bypass the level spreader via a bypass swale or pipe and discharge directly to the receiving stream.
Level Spreader
Inspection
The flow bypass structure should be free of any sediment, trash, and debris. Inspect the flow bypass
structure for holes and cracks, and for any erosion that would allow runoff to flow around the structure.
Note any undesirable vegetation that might prevent the flow bypass structure from performing its function.
If applicable, inspect metal components for excessive corrosion. Check the level spreader and
buffer/filter strip for visual signs that it is receiving flow and has not been overwhelmed. If the level spreader
does not appear to be receiving flow or if there is evidence of excessive flow, the flow bypass structure is
not functioning properly.
Maintenance
Remove undesirable vegetation, sediment, trash, and debris. If the flow bypass structure becomes
clogged, remove sediment and dispose of it off -site. Repair any cracks and holes, and eroded areas
associated with the flow bypass structure. Replace components as necessary.
11.3.4 LEVEL SPREADER TROUGH AND LIP
Function
The level spreader trough collects stormwater runoff, and the non -erosive lip evenly distributes the
runoff to downgrade areas as diffuse flow. The trough may be vegetated or concrete. The lip may be
constructed of various materials. The level spreader may also be equipped with a drawdown system
(e.g., weep holes leading to a drainage system) if prolonged periods of standing water are not desired.
Inspection
The level spreader trough and lip should be level and remain relatively free of sediment and debris or
the level spreader will quickly become overwhelmed and lose its effectiveness. Inspect the lip for erosion,
undesirable vegetation, and other impairments that could render it uneven or otherwise ineffective. Verify
that the area immediately downstream of the level spreader lip is free of woody vegetation, which can
cause re -concentration of flows and reduce flow across the filter strip. If a drawdown system is present,
check to make sure it is not clogged with sediment or debris. Pervious bags filled with stone may have
been placed over the drawdown system's inlets to keep sediment from entering. If sand bags are
present, inspect them for damage.
Maintenance
Repair any areas of the level spreader trough that are damaged or eroded. If there is erosion around
the ends of the level spreader, regrade the soil, build a berm that is higher than the lip, and stabilize
the area with vegetation (NCDWQ, 2007). Sediment should be removed before it interferes with the
level spreader's ability to distribute flow evenly. Repair the lip if it has eroded or has become cracked
or otherwise damaged. Remove any vegetation growing over the lip or any woody vegetation that
11-6

could channelize flow. If there is a drawdown system and it has become clogged, remove any
sediment and debris. Replace damaged pervious rock bags. See Figure 11-4 for examples of level
spreaders with typical maintenance needs.
Figure 11-4. Typical maintenance concerns: erosion at the level spreader lip and
channelization in the filter strip (left), and woody debris in need of removal (right)
All
..�.. �.. .. -�.•� -. '• ir.. � � .r�� yam, .l•?i5 ' . ^ 44
�^ x ��t ��,L��'`n'�'�'!��. ��' sir• �. . .r;' �
'1� • .1,�"' �:titl�,}p� �1 y1/�`i7 � �.y. , 1. 1' . �jr�'+.
, dense cover of desirable vegetation. Inspect for
signs of erosion and channelization. Also, verify that the vegetation is being mowed at the proper
frequency. Ideally, turf grasses should be mowed at a frequency to maintain a maximum height of 4
inches for warm -season grasses and 6 inches for cool -season grasses.
Buffers may be used instead of a grassed filter strip. Buffer areas, which are typically forested, are
particularly susceptible to channelized flow and erosion. Carefully inspect the buffer for evidence of
these processes, which indicates that the level spreader is not functioning properly. Also, verify that
sediment has not accumulated in the filter strip or buffer to the degree that vegetation is dying
(NCDWQ 2007). If PSRM has been installed, inspect it for damage and verify that it is properly toed in
and anchored.
Maintenance
Remove trash and undesirable vegetation. Remove debris that could cause channelization. Repair
areas affected by erosion or channelization. Mow grass to the appropriate height. Repair or replace
PSRM as necessary. Note that PSRM may not be visible and should not be disturbed if vegetation is well
established and adequate to protect against erosion. Ensure that grass remains dense and thick for
optimum removal of pollutants. If excessive sediment has accumulated, remove the sediment,
reestablish vegetation, and regrade if necessary (NCDWQ 2007).

Level Spreader
Figure 11-5. Example of a level spreader and filter strip with
a drawdown device consisting of weep holes with rock bags
11.3.6 BYPASS SWALE /PIPE OR OUTLET DITCH
Function
A level spreader equipped with a flow bypass system has a swale or pipe that conveys runoff from the
flow bypass structure to the receiving stream during large rainfall events. Conversely, a level spreader
with a filter strip, but no bypass system, will discharge flow into an outlet ditch. If the ditch or pipe
empties directly into a stream, it should do so in a manner that does not degrade the stream channel or
banks. Typically, this requires the use of riprap or another type of protection/energy dissipation.
Inspection
Stormwater should be allowed to flow freely from the flow bypass structure or filter strip. All
conveyances should be inspected for trash, sediment, and debris. For vegetated, open channels, inspect
the area for erosion and undesirable vegetation. Also, note the mowing frequency, as mentioned
previously. For other open channels, verify that the channel is properly protected from erosion. If a
pipe is used, inspect the surface above the buried pipe for signs of pipe breakage or separation.
When applicable, inspect the area where the bypass swale or pipe discharges to the stream.

Level Spreader
Maintenance
Remove and properly dispose of debris, unwanted vegetation, and major sediment
accumulations. Repair eroded areas and damaged pipes. Replace or repair any material
(i.e., riprap) or structure used to prevent the discharge to the stream from causing erosion. Refer to
Chapter 3 for additional guidance on maintenance techniques.
11.4 Inspection and Maintenance Summary
Observations made while inspecting stormwater controls must be documented on the appropriate SCM
inspection report form and entered into SMD. Observations recorded would include a general
description of the SC and those maintenance needs identified such as the removal of accessible trash,
sediment, and unwanted vegetation.
If during an inspection emergency maintenance needs are identified (i.e., a sluice gate frozen in the
open position), the inspector should either correct the problem at that time or contact the party(s)
responsible for emergency maintenance repairs. If emergency maintenance needs are not corrected
at the time of the inspection, a follow-up inspection should be conducted to verify that the responsible
party(s) has taken action and all needs have been addressed. Observations made during the follow-up
inspection must be documented on the appropriate SCM inspection report form and entered into
SMD.
11-9

References
References
Code of Federal Regulations, Title 29, Labor, Part 1910, Occupational Safety and Health Standards,
SubpartJ, General Environmental Controls, 146, Permit -Required Confined Spaces.
FEMA, 2005. Dam Owner's Guide to Plant Impact on Earthen Dams. FEMA L-263. Sept. 2005.
<http://www.damsafety.org/media/Documents/PDF/fema 1263 .pdf> (accessed 19 January
2009).
Hunt, W.F., Burchell, M.R., Wright, J.D., and Bass, K.L.. 2007. Stormwater Wetland Design Update: Zones,
Vegetation, Soil and Outlet Guidance. NC Cooperative Extension, Raleigh, NC.
NCAC. 2007. Title 15A, Environment and Natural Resources, Chapter 02, Environmental Management.
NCDWQ "Redbook. "Surface Waters and Wetlands Standards. North Carolina Administrative
Code, effective May 1, 2007.
NCDOT. 1998. Vegetation Management Manual. North Carolina Department of
Transportation, Roadside Environmental Unit, Raleigh, NC.
NCDOT. 2004. Workplace Safety Manual. By Karen H. Goodall. North Carolina Department of
Transportation, Transportation Mobility and Safety Division, Raleigh, NC.
NCDOT. 2006. Roadway Standard Drawings. North Carolina Department of Transportation, Raleigh, NC.
NCDOT. 2008a. Invasive Exotic Plants of North Carolina. By Cherri Smith. North Carolina Department of
Transportation, Raleigh, NC.
NCDOT. 2008b. Stormwater Best Management Practices Toolbox. North Carolina Department of
Transportation, Hydraulics Unit, Raleigh, NC.
NCDWQ. 2007. Stormwater Best Management Practices Manual. North Carolina Department of
Environment and Natural Resources. Division of Water Quality. 27 Sept. 2007.
<http://h2o.enr.state.nc.us/su/SCM forms.htm> (accessed 6 Aug. 2008).
NCHRP. 2006. Evaluation of Best Management Practices and Low Impact Development for Highway
Runoff Control. User's Guide for SCM/LIDSelection. National Cooperative Highway Research
Program, Transportation Research Board, Washington, DC.
NCSU. 2015. 2015 North Carolina Agricultural Chemicals Manual. North Carolina State University.
College of Agriculture and Life Sciences. November, 2015. <http://ipm.ncsu.edu/agchem/8-
toc.pdf> (accessed 6 Apr. 2015).
USDA. 2009. USDA-NRCS Plants Database Image Gallery.
<http://Plants.usda.gov/gallery.html> (accessed 20June 2009).

References
NCCES. 1998. Horticulture Information Leaflet 647 (Yellow Nutsedge photograph). By Joseph C. Neal.
North Carolina State University, North Carolina Cooperative Extension Service. December, 1998.
Virginia Cooperative Extension. 2009 (accessed). Virginia Tech Weed Identification Guide.
(Centipedegrass photograph) <http://www.ppws.vt.edu/scott/weed id/ereop.htm> (accessed 30
June 2009).
U.S. Fish and Wildlife Service. 2007. (Yaupon Holly photograph).
<http://en.wikipedia.org/wiki/File:llex vomitoria fws.lpg> (accessed 30June 2009).

APPENDIX A

FT BRAGG Stormwater Control Measure Inspection Report
PERMIT # SCM# Date:
INSPECTOR NAME:
Frequency: 0 Other 0 Semiannual OQ Annually
Check appropriate box
Bioretention Basin 0
Dry Pond 0
Wet Pond *
Infiltration trench 0
Underground Detention0
Wetland 0
Sand Filters 0
Vegetated Swale 0
Bioswale
Filter Strip/Level
Spreader
Proprietary Infiltration
Gallery
Infiltration Basin 0
Structural Integrity
(Ref: BMP Inspection & Maintenance Manual)
Yes
No
N/A
MON
Does the facility show any signs of settling, cracking, bulging, misalignment, or any
other structural deterioration?
0
❑
Does embankment, emergency spillways, side slope, orinlet/outletstructure show
signs ofexcessive erosion orslumping?
0
❑
Is the inletand outletpipe damaged orotherwise notfunctioning properly?
0
0
❑
Does impoundment area show erosion?
0
0
❑
Are trees, shrubs, or saplings present on the embankment?
0
0
❑
Are animal burrows present?
0
0
❑
Do grass areas require mowing and /or are clippings building up?
0
0
❑
Is vegetation healthy
0
0
❑
Working Conditions(Ref: BMP Inspection & Maintenance Manual)
Are sand filters clogged?
0
❑
Is there standing water present after allowable designed drainage rate?
a
❑
Is water quality orifice, weir, grate, trash rack completely clear of debris and completely
open?
0
0
❑
Does the depth of sediment or other factors suggesta loss ofstorage volume?
0
0
❑
Is there an accumulation offloating debris and/ortrash?
0
d
0
❑
Other Inspection Items(Ref: BMP Inspection & Maintenance Manual)
Is there evidence of encroachments or improper use of impoundment areas?
0
0
❑
Does the fence gate or other safety device need repair?
0
0
❑
Is there excessive algae growth or has one type of vegetation taken overthe facility?
0
0
❑
Is there evidence of oil, grease, or other automotive fluids entering and clogging the
Hazwaste @396-2141)
0
0
0
El
In Biorententions does mulch coverthe entire area, is specified numberand types of
still in place?
0
0
0
El
Maintenance performed /Monitor description
Check appropriate function of service (FOS) d FF 0 PF 0 NF 0 MON
F F Fully Functional (No Repairs needed), P F Partially Functional (Repairs Needed) Maintenance needs were found, but function of the device has not been
significantly affected. N F Not Functional (Device is no longer functional due to the general or complete of a major structural component),
MON (Monitor to determine root cause of problem/failure)