NCS000335_DOD SJAFB 2015 CWPP Post Construction Program_20151011

Comprehensive Watershed Protection Plan
Seymour Johnson AFB, NC

May 2015

Installation Management Flight,
4TH CES/CEI
Seymour Johnson
AFB, NC 27531

TABLE OF CONTENTS
1.0 INTRODUCTION ............................................................................................... 4
1.1 BACKGROUND ............................................................................................................ 4
1.1.1 Energy Independence and Security Act (EISA) ................................................... 6
1.1.2 North Carolina Municipal Separate Storm Sewer System (MS4) Permit .............. 8
2.0 TECHNICAL APPROACH .............................................................................. 12
2.1 BMP SIZING ............................................................................................................... 12
2.2 LID TOOLBOX OVERVIEW ...................................................................................... 13
2.3 PLAN AREA ............................................................................................................... 15
3.0 DATA REVIEW AND PREPARATION .......................................................... 17
3.1 SUMMARY OF DATA ................................................................................................ 17
3.2 LAND USE .................................................................................................................. 17
3.2.1 Current Land Use ............................................................................................. 17
3.2.2 Demolition Credit ............................................................................................ 17
3.2.3 Future Land Use ............................................................................................... 18
3.3 IMPERVIOUSNESS .................................................................................................... 18
3.4 TOPOGRAPHY ........................................................................................................... 18
3.5 STORMWATER DRAINAGE SYSTEM ..................................................................... 18
3.6 AERIAL IMAGERY .................................................................................................... 18
3.7 SOILS .......................................................................................................................... 20
3.8 GROUNDWATER ....................................................................................................... 20
3.9 CLIMATE DATA ........................................................................................................ 20
4.0 METHODOLOGY ............................................................................................ 24
4.1 SUBWATERSHED DELINEATION............................................................................ 24
4.2 HYDROLOGIC RESPONSE UNITS AT SEYMOUR JOHNSON AFB ........................ 27
4.2.1 The HRU Concept ............................................................................................ 27
4.2.2 Creation of HRUs at Seymour Johnson AFB ..................................................... 27
4.2.3 Generation of HRU Runoff Time Series ........................................................... 31
4.3 IMPERVIOUS AREA ASSESSMENT ......................................................................... 32
4.3.1 Current Imperviousness .................................................................................... 32
4.3.2 Future Imperviousness ..................................................................................... 32
4.3.3 Summary of Existing and Future Imperviousness .............................................. 33
4.4 EXISTING AND FUTURE RUNOFF CONDITIONS .................................................. 34
4.5 ANALYSIS FOR OPTION 1 ........................................................................................ 35
4.6 ANALYSIS FOR OPTION 2 ........................................................................................ 35
4.6.1 BMPs for SUSTAIN Analysis .......................................................................... 36
4.6.2 Drainage Network ............................................................................................ 37
4.6.3 Option 2 SUSTAIN Analysis ............................................................................ 38
5.0 COMPARISON OF BMP SIZING APPROACHES ......................................... 39
5.1 OPTION 1 RESULTS................................................................................................... 39
5.2 OPTION 2A: TREATING NEW IMPERVIOUSNESS ONLY ...................................... 39
5.3 OPTION 2B: TAPPING INTO (RETROFITTING) EXISTING
IMPERVIOUSNESS .................................................................................................... 41
5.4 OPTION 2C: PHASED IMPLEMENTATION OF TAPPING INTO
(RETROFITTING) EXISTING IMPERVIOUSNESS ................................................... 47

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5.5 SUMMARY ................................................................................................................. 50
6.0 IMPLEMENTATION STRATEGY .................................................................. 51
6.1 OVERVIEW OF TOOLBOX CAPABILITIES ............................................................. 51
6.2 LID SIZING TOOLBOX DEVELOPMENT ................................................................. 51
6.2.1 Main Interface .................................................................................................. 52
6.2.2 Subwatershed Characteristics Interface ............................................................. 53
6.2.3 Interface for BMP Implementation ................................................................... 53
6.2.4 Interface for BMP Design Specification ............................................................ 55
6.2.5 Interface for BMP Tracking Log ....................................................................... 56
6.2.6 Interface for Certificate of Applicability............................................................ 57
7.0 REFERENCES .................................................................................................. 60
APPENDIX A: EXAMPLE APPLICATION OF THE LID TOOLBOX AT A BASE-
WIDE SCALE ................................................................................................... 62
APPENDIX B: EXAMPLE LID TOOLBOX SITE-SCALE PROJECT ..................... 85
APPENDIX C: IMPERVIOUS SURFACE ACRE CALCULATIONS ....................... 92
APPENDIX D: TETRA TECH TOOLBOX PROJECT TRACKING SOFTWARE ... 94
LIST OF FIGURES

Figure 1-1. Timeline of LID Regulatory Policies ...................................................................................... 5
Figure 2-1. Conceptual LID Toolbox Schematic ..................................................................................... 14
Figure 2-2. Location of Seymour Johnson Air Force Base ....................................................................... 16
Figure 3-1. Current land use in the Seymour Johnson AFB ..................................................................... 19
Figure 3-2. Soils in the Seymour Johnson AFB ...................................................................................... 22
Figure 3-3. Monitored groundwater levels in the Seymour Johnson AFB ................................................ 23
Figure 4-1. Schematic for analyzing two options to meet EISA requirements .......................................... 24
Figure 4-2. Subbasin delineation for the Seymour Johnson AFB ............................................................. 26
Figure 4-3. The Seymour Johnson AFB HRUs for the year of 2007 ........................................................ 29
Figure 4-4. The Seymour Johnson AFB HRUs for the year of 2011 ........................................................ 30
Figure 4-5. Sectional view of a typical bioretention unit.......................................................................... 36
Figure 4-6. The SUSTAIN model routing network for Seymour Johnson AFB ........................................ 37
Figure 5-1. The tradeoff relationship and the near-optimal solution identified for treating
new imperviousness at the Seymour Johnson AFB ..................................................................... 40
Figure 5-2. Existing imperviousness that can be tapped into and nearby available
pervious spaces for potential BMP implementation .................................................................... 44
Figure 5-3. Tradeoff between total BMP costs and the annual average runoff volume for
tapping into existing imperviousness in Seymour Johnson AFB.................................................. 45
Figure 5-4. Accumulative and step increase in percentages of runoff volume being
retained as the BMPs are implemented in a 10 percent size step ................................................. 48
Figure 5-5. Comparison of allowable new imperviousness between Option 2b (project-
by-project) and Option 2c (phased) as in a three-step implementation scheme. ........................... 49
Figure 6-1. Main interface of the LID Sizing Toolbox for Seymour Johnson AFB................................... 52
Figure 6-2. Interface for the presentation of subwatershed characteristics. ............................................... 53

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Figure 6-3. Interface for performing the BMP implementation analysis ................................................... 54
Figure 6-4. Interface for user-specified BMP design inputs. .................................................................... 55
Figure 6-5. Example bioswale design cut sheet ....................................................................................... 56
Figure 6-6. Example Certificate of Compliance page 1. .......................................................................... 58
Figure 6-7. Example Certificate of Compliance page 2. .......................................................................... 59
LIST OF TABLES

Table 3-1. Summary of data ................................................................................................................... 17
Table 4-1. Summary of HRUs in Seymour Johnson AFB ........................................................................ 28
Table 4-2. Loading rates for nutrients (adapted from Hunt and Lucas 2003) ............................................ 31
Table 4-3. Loading rates for TSS (adapted from Simmons 1993 and Tetra Tech 2012) ............................ 31
Table 4-4. Summary of existing and future imperviousness in Seymour Johnson AFB
subwatersheds ........................................................................................................................... 34
Table 4-5. Summary of runoff conditions for different land use scenarios in the Seymour
Johnson AFB ............................................................................................................................ 35
Table 4-6. Design parameters for the bioretention BMP .......................................................................... 36
Table 5-1. BMP size needed for EISA Option 1 at Seymour Johnson AFB .............................................. 39
Table 5-2. Comparison of annual average runoff volume and water quality performances
among different land use scenarios in Option 2a for complying with EISA
regulations at Seymour Johnson AFB ........................................................................................ 40
Table 5-3. Comparison of peak flow statistics between existing, future, and future with
optimal BMP land use conditions following Option 2a at Seymour Johnson AFB ....................... 41
Table 5-4. Comparison of total BMP costs between Option 1 and Option 2a of complying
with the EISA regulations at Seymour Johnson AFB .................................................................. 41
Table 5-5. Summary of identified impervious surfaces for tapping and corresponding
pervious spaces in subwatersheds with high infiltration rates in Seymour Johnson
AFB .......................................................................................................................................... 42
Table 5-6. Comparison of annual average runoff volume and water quality performances
among different land use scenarios in Option 2b for complying with EISA
regulations at Seymour Johnson AFB ........................................................................................ 46
Table 5-7. Comparison of peak flow statistics between existing, future, and future with
optimal BMP land use conditions following Option 2b at Seymour Johnson AFB ....................... 46
Table 5-8. Comparison of total BMP costs between Option 1, Option 2a, and Option 2b
of complying with the EISA regulations at Seymour Johnson AFB ............................................ 47
Table 5-9. Comparison of the BMP cost and treatment benefits between Option 2b and
the three phase implementation scheme for Option 2c ................................................................ 49

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1.0 INTRODUCTION
In response to growing concerns about adverse environmental impacts resulting from urban stormwater
runoff, regulatory agencies continue to adopt stringent requirements to limit or mitigate downstream
impacts. Two such requirements such as section 438 of the Energy Independence and Security Act (EISA) and
the National Pollutant Discharge Elimination System (NPDES) Permit, require tight controls on runoff volume,
rate, pollutant loading, and temperature and can therefore be challenging to implement. Developing a strategy
to meet those challenges required Seymour Johnson Air Force base (SAFB) to face the key question of how to
best achieve compliance by implementing low-impact development (LID) techniques given the distribution of
land use, impervious cover, and soil type on the base. Answering that question requires analysis of land
characteristics, runoff generating surfaces, site constraints, LID effectiveness, and LID costs, the
combinations of which would be difficult to numerate. The ultimate goal of this project is to reduce
stormwater pollution impacts to the watershed in which SJAFB resides (Middle Neuse Watershed –Cataloging
Unit: 03020202).

This updated plan, first developed by Tetra Tech October 2013, undertakes the task for Seymour Johnson AFB,
North Carolina to protect the Middle Neuse Watershed. The objectives for this project are twofold:

1. Optimization: Utilize a stormwater model of the base to identify the optimal BMP configuration,
sizes, and locations to achieve regulatory compliance.

2. BMP Management Tool: Depending on the optimization results, use a BMP sizing approach for siting
and sizing BMPs based on an installation-wide, Watershed Management strategy that achieves the best
numerical objectives.

Coincident with the introduction of EISA 438 regulations in December 2007, which require BMP
implementation for significant increases in impervious area at federal facilities, Seymour Johnson AFB
began an impervious surface demolition program that has since reduced the base’s impervious
footprint by over 69 acres, or 8 percent of the 2007 impervious area baseline. Tables showing
reduction in impervious surfaces for the years 2007, 2011, and 2014 can be found at Appendix C.
Impervious surface data was retrieved from the SJAFB GIS System. As a result, it is anticipated that the
total annual runoff volume and peak flow rate have been reduced and that much of the future
development on base could be constructed (without any LID or structural BMPs) without raising these
values above the 2007 conditions. The analysis presented in this plan determines the hydrologic impacts
of the impervious area demolition and development; and, establishes 2007 as the baseline year for
regulatory compliance (year EISA was enacted). Further, optimization techniques are used to
determine the most cost effective approach for designing BMPs for all future development that result
in an increase in flows or volumes beyond the 2007 baseline. EISA 438 gives you two options for
demonstrating post-construction compliance:

 Option 1 is to retain or store the 95th percentile rainfall event on-site,
 Option 2 is to determine predevelopment hydrology on the basis of site-specific conditions
through the use of continuous simulation modeling techniques, published data, studies, or
other established tools to determine the volume of water to be managed on-site.

SJAFB has decided to utilize the benefits of continuous modeling techniques of Option 2. This implementation
strategy will tap into (retrofitting) existing imperviousness at high infiltration subwatersheds (mission
dependent) and implement effective BMPs in three major phases. This option has demonstrated more
substantial cost savings than Option 1. This strategy does involve a lot of coordination in site planning, tracking,
design, construction, and maintenance to restore, to the maximum extent technically feasible, the
predevelopment hydrology of the property. Option 2 also includes pre-processed hydrologic calculations made

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at the installation scale, rather than at the individual project scale.

Although EISA compliance is an important driver for this stormwater protection plan, the implementation of
BMPs on base will also assist Seymour Johnson AFB in complying with requirements of its Municipal
Separate Storm Sewer System (MS4) permit – NCS000335. This plan allows SJAFB to meet Section F(2b),
Post-Construction Site Runoff Control requirements, in providing a strategy for implement a Comprehensive
Watershed Protection Plan (CWPP). The CWPP objective is to develop, implement, and enforce a program
to address storm water runoff from new development and redevelopment projects, including public
transportation maintained by SJAFB, that disturb greater than or equal to one acre, including projects less than
one acre that are part of a larger common plan of development or sale, that discharge into the base MS4.

Consequently, to effectively manage the Water Protection Program, any development and redevelopment project
that has a land disturbance > 5,000 square feet will be tracked and then reviewed to determine if EISA 438 and/or
MS4 requirements apply. Eligible projects will be coded in the Air Force “ACES” program. The current, tracked,
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project, listing can be found in Appendix D - “Toolbox Project Tracking”. The 4 Civil Engineering Squadron
design engineers will need to look at impervious areas to be tapped and ensure tapped areas can be drained to a
BMP for treatment. It will be beneficial to simultaneously identify impervious areas to be tapped and the
associated space for BMPs to be implemented. Also, note that “Phased” BMP implementations and tapped areas
selections will begin after all “credits” retrieved from demolitions determined by the “Toolbox” have been
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exhausted. Lastly, the 4 CE Engineering “Base Comprehensive Plan” will be used to effectively restrict growth
away from identified sensitive areas to areas that can support the base without compromising water quality and
DoD mission requirements.

1.1 BACKGROUND

The National Research Council’s 2008 report on urban stormwater determined that current control efforts are
not fully adequate to protect the nation’s water resources (National Research Council 2008). Those
conventional approaches to stormwater control focus on state or local flood control programs and water
quality objectives that are directed at municipal and industrial discharges. Conventional approaches to
stormwater management (e.g., extended detention approaches) mitigate the impacts of peak runoff rates,
which are primarily due to an increase in developed land and impervious surfaces. Many of those
approaches do not protect downstream hydrology thereby resulting in degraded aquatic ecosystems.
Studies have determined that detention ponds and similar strategies do not adequately address management
of smaller and more frequent storms have a tendency to increase the duration of peak flows, and have minimal
impact on volume reduction. Conventional design strategies have their limitations and can lead to increased
stream erosion and stream instability (USEPA 2009a).

Site development and land cover changes result in increased imperviousness, soil compaction, loss of
vegetation, and loss of natural drainage patterns. Those changes lead to alterations of the natural hydrology
and disrupt the water balance of a site. Ultimately, the modifications increase the volume and peak flow of
runoff, the duration of discharge, pollutant loadings, and the temperature of runoff (USEPA 2009a).

In recent years an approach called LID has evolved that promotes the use of systems and practices that
mimic the natural hydrology of a site to (1) infiltrate and recharge, (2) evapotranspire, and (3) harvest and use
precipitation near to where it falls on the site (USEPA 2009a).

LID strategies can be incorporated into most sites where vegetation or infiltration techniques can be
integrated into the landscape. LID BMP strategies include rain gardens, vegetated swales, pocket wetlands,
infiltration planters, porous and permeable pavements, vegetated median strips, green roofs, and reforestation
and revegetation of riparian buffers and floodplains. LID also includes stormwater harvesting approaches
such as rain barrels and cisterns that can be used to capture and reuse rainfall for watering plants or other
non-potable uses.

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Many documents have come into play that interpret or define regulations concerning stormwater runoff and
LID over the last couple decades. Figure 1-1 shows a timeline of the major events and guidance.

• LID originates in Prince George's County, Maryland
1990

• Office of the Federation Environmental Executive created

1993

• Presidential Memo on Federal Landscaping

1994

• EO 13148 Greening the Government Through Leadership in Environmental

Management
2000

• DoD LID Manual & Army Strategy for the Environment

2004

• EO 13423 Strengthening Federal Environmental, Energy, & Transportation

Management and EISA
2007

• EO 13514 Leadership in Environmental, Energy & Economic Performance

• EPA Guidance for Implementation of EISA Section 438
2009

Figure 1-1. Timeline of LID Regulatory Policies

Since 2004, LID strategies for stormwater control have been considered for many projects as part of the original
UFC Low Impact Development Manual (UFC 2010). The application of LID to new or existing infrastructure is
practical and achievable for most sites but requires a change of thinking on the part of the site designer, engineer,
and maintenance personnel.

LID differs from conventional stormwater management principles in that it does not store and release stormwater
in the traditional fashion. Because of that, the approach for planning, incorporating, and maintaining LID BMPs
on sites relies on a water balance more typical of natural systems in which water is captured and lost through
infiltration, evapotranspiration, or beneficial use.

1.1.1 Energy Independence and Security Act (EISA)

The regulatory drivers for this plan are EISA Section 438 and the SJAFB NPDES Permit NCS000335.
Congress enacted EISA in December 2007. Section 438 of EISA legislation establishes strict stormwater
runoff requirements for federal development and redevelopment projects. The legislation reads as follows:

“Storm water runoff requirements for federal development projects. The sponsor of any
development or redevelopment project involving a Federal facility with a footprint that
exceeds 5,000 square feet shall use site planning, design, construction, and maintenance
strategies for the property to maintain or restore, to the maximum extent technically feasible, the
predevelopment hydrology of the property with regard to the temperature, rate, volume, and
duration of flow.”

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Section 438 is intended to address the inadequacies of current approaches to managing stormwater and
promote practices that maintain or restore predevelopment site hydrology. Although Congress did not
prescribe specific strategies to comply with Section 438, it can be inferred that one of the goals of the act is to
promote the use of sustainable stormwater management approaches, designs, and practices that better protect
receiving water quality and better address volume control (USEPA 2009a). LID is the preferred approach
that can be used to meet the criteria of Section 438.

To assist federal agencies, the U.S. Environmental Protection Agency (US EPA) developed a Technical
Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of
the Energy Independence and Security Act. The document is intended solely as technical guidance for federal
facilities. It is not a regulation and does not impose any legally binding requirements. It is important to
note that, for Department of Defense (DoD) facilities, the DoD policy memorandum, summarized below, takes
precedent over EPA’s technical guidance document. EPA’s technical guidance document describes two options
for demonstrating compliance with EISA Section 438 requirements, each of which is intended to achieve the
outcome of maintaining or restoring predevelopment hydrology. Option 1 is to retain the 95th percentile
rainfall event on-site, and Option 2 is to determine predevelopment hydrology on the basis of site-specific
conditions through the use of continuous simulation modeling techniques, published data, studies, or other
established tools to determine the volume of water to be managed on-site.

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For Option 1, EPA’s 95percentile methodology is used to determine the design storm. That method
allows for a practical and reasonable approach to determine LID volumes. The design storm event is based
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on the regional 95percentile, 24-hour rainfall depth over a minimum of 10 years. The design storm is
used to calculate post-development runoff volumes to size LID BMPs to retain on site runoff from all
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rainfall events less than or equal to the 95percentile rainfall event. LID BMPs are encouraged throughout the
site design to ensure control and water quality objectives are met (USEPA 2009a).

Option 2 can be used to determine predevelopment hydrology through a site-specific performance design
objective. The methods for calculating, modeling, and sizing stormwater runoff are based on continuous
simulation analyses. Recognized modeling software and consistent hydrological assessment tools are to be
used and appropriately documented.

EPA’s technical guidance document describes how site designers can comply with Section 438 for both
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Option 1 (retaining the 95percentile rainfall event) and Option 2 (site-specific hydrologic analysis). The
document provides detailed methods and data needed to estimate the predevelopment hydrology and
provides guidance on documenting Section 438 compliance. To comply with Section 438, EPA also
highlights technologies that federal agencies would typically use to mimic the natural hydrologic cycle
processes of infiltration, evapotranspiration, and surface/subsurface flow. The LID strategies suggested for
federal agencies include the planning techniques such as construction footprint reduction (e.g., building up
instead of out) to reduce their stormwater impact. Specific post construction measures identified in
EPA’s guidance document include biological systems and engineered systems (USEPA 2009a) and include
the following:

 Rain gardens, bioretention, and infiltration planters
 Porous pavements
 Vegetated swales, bioswales, Green roofs
 Trees and tree boxes
 Pocket wetlands
 Reforestation/revegetation using native plants
 Protection and enhancement of riparian buffers and floodplains
 Rainwater harvesting for use (e.g., irrigation, HVAC make-up, non-potable indoor uses)

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In January 2010, the DoD released a memorandum that directs DoD facilities to implement EISA Section
438 using LID techniques in accordance with the methodology illustrated in Figure 1 of that document
(DoD 2010). The following paragraphs provide an overview of DoD’s interpretation of EPA’s technical
guidance document, including clarifying how predevelopment hydrology and maximum extent feasible are
defined according to the DoD. That policy memo highlights the use of EPA’s technical guidance
document, summarized in Section 1.1.2 (USEPA 2009a).

EISA Section 438 requirements are applicable to all DoD construction projects that have a footprint
greater than 5,000 gross square feet, or expand the footprint of existing facilities by more than 5,000 gross
square feet. According to the DoD, the project footprint is defined as all horizontal hard surfaces and
disturbed areas associated with the project development, including both building area and pavements (such
as roads, parking, and sidewalks). Those requirements do not apply to internal renovations,
maintenance, or resurfacing of existing pavements (DoD 2010). EISA also does not apply if there is no
building involved in the development or redevelopment project. Any construction of the permanent retention or
detention ponds is strongly discouraged. If retention/detention option is selected, written documentation for
options considered and justification for the choice should be included in the design analysis. (See DUSD (I&E)
19 January 2010 policy memorandum, subject: DoD Implementation of Storm Water Requirements under
Section 438 of the Energy Independence and Security Act). See also UFC 3-210-10, Low Impact Development,
15 November 2010 for DoD implementing guidance. For EISA 438 applicable projects (MILCON, O&M,
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NAF, etc.) 4 CES/CEN updates ACES-PM MAJCOM unique by selecting "EISA 438" and input in the value
field 'Yes' or 'No' to indicate whether EISA 438 has been addressed for the project. Estimated design and
construction costs for implementing EISA Section 438 shall be documented in the project cost estimate as a
separate line item.

The overall stormwater management objective for each project is to maintain predevelopment hydrology and
prevent any net increase in stormwater runoff. The DoD defines predevelopment hydrology as the pre-project
hydrologic conditions of temperature, rate, volume, and duration of stormwater flow from the project site. The
analysis of the predevelopment hydrology must include site-specific factors (such as soil type, ground cover, and
ground slope) and use modeling or other recognized tools to establish the design objective for the water
volume to be managed from the project site (DoD 2010). The term “predevelopment hydrology” is not
specifically defined in EPA’s technical guidance document; however, it is worth noting that predevelopment
hydrology can often be defined as the hydrological conditions of a site before any land-disturbing activities
occur. The DoD definition takes precedent over any other definitions of predevelopment hydrology. The
net result is that EISA Section 438 requirements are triggered only by new development greater than
existing impervious area footprints and require management of only the additional impervious area.

Section 438 also requires project site design to achieve the design objective to the maximum extent
technically feasible. According to the DoD, the maximum extent technically feasible criterion requires full
employment of accepted and reasonable stormwater retention and reuse technologies (e.g., bioretention
areas, permeable pavements, cisterns/recycling, and green roofs), subject to site and applicable regulatory
constraints (e.g., site size, soil types, vegetation, demand for recycled water, existing structural limitations,
state or local prohibitions on water collection). Before finalizing the design for a redevelopment project,
DoD components must also consider whether natural hydrological conditions of the property can be
restored, to the extent practical (DoD 2010). Some streams have been severely impacted by urbanization as a
result of changes in their hydrologic and sediment regimes, loss of stream bank vegetation, and channel
alterations (e.g., piping, straightening, channelization, and/or revetment with concrete or gabions). All site-
specific technical constraints that limit the full attainment of the design objective must be documented. If the
design objective cannot be met within the project footprint, LID measures may be applied at nearby locations
on DoD property (e.g., downstream from the project) within available resources. Such an interpretation of
EPA’s document allows DoD engineers to evaluate compliance with EISA at the DoD facility property
boundary, rather than at the project site boundary. That interpretation provides flexibility to manage
stormwater more cost-effectively within the facility rather than on site that can provide significant cost

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 8

savings. The BMP implementation strategy to take advantage of this is introduced in this plan.

1.1.2 North Carolina Municipal Separate Storm Sewer System (MS4) Permit

Seymour Johnson AFB received its National Pollutant Discharge Elimination System (NPDES) Phase I,
MS4 Permit (NCS000335) from the State of North Carolina in March of 2011, effective April 1, 2011.
The permit contains requirements rooted in the federal Clean Water Act and Phase II stormwater regulations,
state statutes, and state regulations adopted by the North Carolina Environmental Management Commission.
The SJAFB NPDES Permit NCS000335 does not have any requirements in it to meet EISA 438. However, the
MS4 Permit requires SJAFB to enforce a program to address stormwater runoff from new development and
redevelopment projects, including public transportation (roads and bridges) maintained by SJAFB. Discharge
of stormwater is authorized under the permit but is subject to a number of limitations and monitoring and
reporting requirements. Continued operation of oil water separators not associated with wastewater discharges
is also authorized. The permit covers current and future activities (post-construction requirements for
development and redevelopment projects > 1 acre), the management of which must be implemented to the
maximum extent practicable.

1.1.2.1 Stormwater Plan

The Stormwater Plan must be developed detailing the base’s stormwater management program over the five-
year term of the permit. The Stormwater Plan describes each of the measures required by the permit
including the six minimum measures required by the federal Phase II stormwater rules. These program areas
are:

a. Public education and outreach,
b. Public involvement and participation,
c. Illicit discharge detection and elimination,
d. Construction site runoff controls,
e. Post-construction site runoff controls, and
f. Pollution prevention and good housekeeping.

Beyond the six minimum measures listed above, there are several additional sections of the permit.
Section H describes requirements for managing industrial areas under either a Stormwater Plan (SWP) for
each facility with an industrial activity or a plan for the base as a whole. Section J, K, and L describe
monitoring requirements, special requirements if impaired waters are present and special requirements related
to any existing Total Maximum Daily Loads (TMDLs) that the base may be subject to. The remaining parts of
the permit speak to aspects of program assessment, reporting and record keeping, compliance and liability,
operation and maintenance, monitoring, and other miscellaneous items.
For each program area identified in the permit, the Stormwater Plan must identify the structural and
nonstructural best management practice (BMP) used, the frequency of management, measurable goals and a
performance assessment, an implementation schedule, and funding. This project develops a BMP
implementation strategy for the base, which constitutes compliance with post-construction runoff control
requirements of the permit (as opposed to meeting specific in-stream compliance standards).

1.1.2.2 Post-construction Site Runoff Controls

The post-construction site runoff controls are most applicable to the BMP sizing tool presented in this plan.
The objective of post-construction site runoff controls laid out in the permit is to implement and maintain
structural and/or nonstructural BMPs for new development and redevelopment projects (including roads and
bridges) that disturb greater than or equal to one acre. This includes projects less than one acre that are part
of a common plan of development. Those projects must be included as a part of the base comprehensive
watershed planning process. SJAFB has decided that the entirety of base development should be
considered part of a common plan of development and BMPs planned accordingly.

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To meet the post-construction objective of NPDES Permit NCS000335, Section F2b (the primary reason for this
plan), SJAFB will:

 Develop and implement a Comprehensive Watershed Protection Plan (CWPP), approved by DWQ, to
meet part or all of the post-construction program requirements. Requirements and status of the CWPP
will be reported to DWQ in the Storm Water Annual Report.

The technical work presented in this project will provide the foundation for implementing a comprehensive
watershed protection program that will meet all the post-construction program requirements of the base
MS4 Permit.

1.1.2.3 Impaired Waters

Section K of the permit discusses requirements when impaired waters (303d list) are present. The 2012
Middle Neuse Watershed shows no impairments for the receiving streams in areas around SJAFB.
The permit identifies a number of steps (timeline identified in permit) Seymour Johnson AFB must
complete if a stream is on the state’s 303(d) list in the future. These include identifying the impaired water and
outfalls draining to it, describing the likely causes of impairment, and describing existing programs and
strategies (including controls) implemented by the base to address the impairment.

1.1.2.4 TMDLs

Section L discusses TMDL related requirements, many of which mirror the impaired waters section.
There are no local TMDLs for SJAFB designated at the present. There is a long established nutrient
TMDL for the Neuse River Estuary. The state has developed regulations to address this TMDL. However; the
base is not specifically named in the urban stormwater management section of the regulations (15A NCAC
02B .0235). Though the base is subject to the buffer requirements that generally require protection of existing
riparian buffers (15A NCAC 2B .0233).

1.1.2.5 Plan Implications

Overall, the MS4 stormwater discharge permit appears to provide some flexibility to the base for how it
meets the state requirements and manages its stormwater. For example, there are a number of options as
presented above for complying with the post-construction site runoff controls section. The option that takes
advantage (exclusively or in part) of the plan being developed to meet EISA seems logical. Option #2 appears
to be the best option.

This plan assesses the extent to which the reduction in impervious area affects stormwater runoff quantity
and quality discharged from the base with the intention that this reduction of impervious area since 2007 will
also satisfy retrofit requirement.
The goal of the state regulations is to protect receiving waters by allowing comprehensive watershed
management planning, monitoring requirements, and management techniques on the discharge of
stormwater from Seymour Johnson AFB. The MS4 permit for Seymour Johnson AFB does not explicitly
provide numerical stormwater quality or quantity discharge requirements. Thus, besides defining the
base CWPP implementation strategy, this project develops a plan to meet EISA requirements and
evaluates the extent to which the EISA compliance approach benefits receiving waters. In addition to the
permit and EISA requirements, State of North Carolina regulators have communicated several specific
requirements to be addressed in the SJAFB CWPP. It is anticipated that the BMP implementation plan that
results from this plan will address these requirements as outlined in Table 1-1.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 10

Table 1-1. Summary of Additional Requirements
No. Requirement Relation to Plan
1
Identify practices to minimize the amount of This plan provides a tool for tracking the construction
impervious surfaces (roads, parking lots, roofs, etc) and demolition of impervious area. The model results
within each watershed by minimizing the creation, demonstrate the extent to which impervious area
extension, and widening of parking lots, roads and reduction contribute to improved water quality.
associated development.
2
Identify practices preserve, protect, create, and This plan identifies areas that are well-suited for
restore ecologically sensitive areas that provide infiltration BMPs to meet regulatory objectives. The
water quality benefits and serve critical watershed model demonstrates that constructing BMPs in these
functions. These areas may include, but are not key areas will reduce pollutants and recharge natural
limited to riparian corridors, headwaters, watershed processes.
floodplains, and wetlands.
3
Implement management practices that prevent or The BMPs identified in this plan will detain, filter,
reduce thermal impacts to streams, including requiring and/or infiltrate runoff from impervious surfaces. These
vegetative buffers along waterways, and biophysical processes have been demonstrated to
disconnecting discharges to surface waters from reduce receiving water thermal impacts.
impervious surfaces such as parking lots.
4
Identifying practices that avoid development in areas The BMPs identified in this plan will detain, filter,
that are particularly susceptible to erosion and and/or infiltrate runoff from impervious surfaces. These
sediment loss. physical processes significantly reduce peak flows and
flow durations from development which are primarily
responsible for increased erosion.
5
Implement design requirements to protect trees, and The bioretention and bioswale BMPs recommended as
part of this plan include native plant species that are
other vegetation with important evapotranspiration
specifically designed to uptake nutrients and provide
qualities.
evapotranspiration within the BMPs.
6
Implement policies to protect native soils, prevent This plan specifically identifies areas with high
topsoil stripping, and prevent compaction of soils. infiltration properties to be preserved for BMP
implementation. This approach reduces BMP sizes and
costs, but also ensures a quicker path to improved
water quality by increasing BMP efficiency.
7
Implement water conservation policies that will The tool developed in this plan provides a framework for
reduce storm water and non-storm water discharges sizing, siting, and managing BMPs to meet compliance
via storm sewer. and improve water quality and reduce discharge to the
storm sewer. This framework can be adapted to include
storm water capture BMPs that could be considered for
reuse.
8
Implement policies that encourage stormwater This plan focuses on strategically placing BMPs in high-
practices close to the source of the runoff rather than efficiency areas (i.e., areas with high infiltration
downstream and lower in the watershed (LID). properties) that are immediately adjacent to runoff-
generating impervious areas.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 11

2.1 TECHNICAL APPROACH
This section provides an overview of the technical approach taken to develop the stormwater BMP
management strategy at Seymour Johnson AFB for meeting the stormwater management requirements
outlined in the EISA regulations and evaluating compliance with state MS4 permit requirements.

This project establishes a BMP watershed management strategy for meeting EISA requirements
for new development and redevelopment projects on base exceeding 5,000 square feet. Although less
specific than EISA requirements, Seymour Johnson AFB is also required to comply with the state
MS4 permit requirements. The base already has programs in place to comply with the majority of
MS4 permit requirements; however, this project provides hydrologic modeling and engineering
analysis that will support MS4 permit compliance. This project includes four distinct elements:

1. Data Review and Preparation – collect and review all existing site characteristic and
hydrologic data; review previous stormwater studies and hydrologic analyses; and collect
additional required data required for the development of a hydrologic model.

2. Model Development – input collected data to develop subdrainage delineations and hydrologic
response units; perform an impervious area assessment; and analyze 2007 and 2011 runoff
conditions on base.

3. Future Development Scenarios – using the hydrologic model, input all potential future
“FUNDED” development scenarios and determine hydrologic and water quality responses.

4. BMP Optimization and LID Toolbox – results of the future development scenario hydrologic
analysis are evaluated to determine the best storm water management approach (installation-wide
BMP management) and an LID toolbox is created to assist Seymour Johnson AFB in complying
with EISA requirements in the most cost effective manner.

Seymour Johnson AFB elected to develop an installation-wide BMP management strategy. An
installation-wide BMP management strategy would include analyzing hydrologic impact from an overall
watershed perspective and siting and sizing BMPs at the optimal locations within the watershed to
achieve the necessary numerical objectives. An installation-wide BMP strategy would allow for BMP
implementation to be precisely located maximizing cost effectiveness, but would require larger, sub-
regional BMPs to be funded, designed, and constructed likely as stand-alone water quality improvement
projects.

2.1 BMP SIZING
Any BMP tool that is developed with the express intent of meeting EISA requirements must be built to
demonstrate compliance on the basis of either Option1 or Option 2 as described in EPA’s technical
guidance document. Therefore, it is important to understand the cost and effectiveness implications
th
associated with each of those sizing methods. While Option 1 (retaining the 95percentile rainfall on-
site) is easily calculated, the conservative assumptions associated with it often cause the BMP to be sized
much larger than would be necessary than one sized by Option 2, especially on soils with low infiltration
rates. Because Option 2 is performance-based rather than prescriptive (i.e., success is measured by overall
hydrologic outcome, not by the capture of a single design storm), design engineers have the opportunity
to accurately size BMPs to match predevelopment hydrology. Option 1 is more expensive from a capital
resources perspective, and the larger BMPs take up more real estate that could otherwise be put to
productive or operational use. The disadvantage of Option 2, however, is that performing this hydrologic
matching analysis is often impractical for each individual project that might be implemented on the base.
For a BMP tool to be effective at gaining the advantage of properly sizing BMPs with Option 2, the

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 12

analysis would have to be greatly simplified and possibly include pre-processed hydrologic calculations
made at the installation scale, rather than at the individual project scale.

As highlighted above, DoD policy evaluates compliance with the EISA requirement at the installation
scale (i.e., installation boundary), rather than at the individual project scale. That strategy provides
Seymour Johnson AFB with an important opportunity to realize substantial savings because it allows the
base to place BMPs in areas where they will be the most cost-effective rather than forcing BMPs to be
placed at the project location where conditions might not necessarily be ideally suited for BMP
performance. Overall, that approach will result in smaller BMPs that meet the same EISA objective to
match predevelopment hydrology.

By combining those two advantages (i.e., Option 2 and installation-scale compliance evaluation),
Seymour Johnson AFB can substantially reduce the capital and long-term operations and maintenance
costs associated with meeting EISA. A recent study completed by EPA (USEPA 2009b) found that
implementing an optimized BMP sizing approach to meet a regulatory target at a watershed scale results
in substantially smaller BMPs than a uniform sizing strategy (similar to Option 1) that was implemented
at the project scale. Therefore, to ensure that this BMP management strategy is most cost-effective, the
primary modeling software is set up to use BMP sizing Option 2 and evaluates compliance at the
installation scale.

2.2 LID TOOLBOX OVERVIEW

Determining the optimal BMP sizes and configurations necessary to meet EISA at Seymour Johnson AFB
is only one part of the equation. For the LID toolbox to be effective, it must provide an easy-to-use
framework that facilitates the implementation of the optimized solution as new impervious area is
planned, designed, and constructed on base. As such, the toolbox has three distinct features, each of
which plays a key role in ensuring compliance with EISA:
 System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN): This
powerful GIS-based modeling platform is capable of detailed continuous simulation of hydrologic
and water quality watershed processes and BMP cost-effectiveness. The model is used
to determine the optimal BMP size and configuration necessary to meet EISA requirements.

 NC Department of Water Quality (DWQ) BMP Manual: This manual provides engineers with
the technical guidance needed to design, build and maintain effective BMPs according to
applicable stormwater regulations and water quality objectives.

 LID Toolbox: This tool provides a simplified method for selecting and determining the optimal
BMP size on the basis of any given impervious area development on any part of the base. Instead
of performing hydrologic calculations within the tool itself, the program is simply prepopulated
with the optimal results from the SUSTAIN program and performs a weighted balance to
determine BMP size.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 13

A schematic concept of the toolbox (Figure 2-1) illustrates how each of those three key components
contributes to the overall objective of implementing the cost-optimized BMP solution for EISA
compliance. While the DWQ BMP Manual provides a framework for BMP design and maintenance,
SUSTAIN uses available GIS, watershed data, and development assumptions to evaluate all the possible
BMP size combinations to identify the optimal solution.

The LID Toolbox provides a simple platform for the engineer or planner to implement the optimal
solution by determining BMP sizes for individual project requirements. If an installation based BMP
implementation approach is selected for Seymour Johnson AFB, the BMP tool would provide BMP
configurations by determining the optimal size and location of BMPs throughout the base for each
increase in impervious area on base. The output from the selected tool provides conceptual BMP design
guidance to aid design engineers and documentation to demonstrate compliance with EISA. Ultimately,
by using this design tool, planners and engineers at Seymour Johnson AFB will ensure that BMPs for
future development will be constructed at the minimum size and cost necessary to meet EISA
requirements.

Figure 2-1. Conceptual LID Toolbox Schematic

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 14

2.3 PLAN AREA

Seymour Johnson AFB is located to the southwest of the City of Goldsboro in East Central North
Carolina, which is shown in Figure 2-2. Its boundaries are defined by Stoney Creek to the west, Neuse
River to the south, and base road and air strip to the north and the east. The Stoney Creek flows into the
Neuse River at the southwest corner of the base. The Seymour Johnson AFB is located on the Atlantic
Coastal Plain and the elevations of range from 48 feet to 121 feet above sea level. The base has a total
area of 5.03 square miles. Located in a humid subtropical climate region, the annual average rainfall is
about 49.84 inches, falling relatively evenly with a slight wet season in the late summer/early fall
(www.weather.com 2007-07-11 Accessed 2011-04-20).

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Figure 2-2. Location of Seymour Johnson Air Force Base

Comprehensive Watershed Protection Plan Seymour Page 16

Johnson AFB

3.0 DATA REVIEW AND PREPARATION
To evaluate runoff conditions and simulate prospective BMP performance for current and future
conditions, the SUSTAIN model requires several key data sets to be processed and formatted for input
into the model. The following sections summarize the data sets used for the model.

3.1 SUMMARY OF DATA

The data sets required for this plan were identified as land use (current and future), imperviousness,
topography, aerial imagery, soils, groundwater, and precipitation. The majority of the information was
provided by the Seymour Johnson AFB with the exception of soils and precipitation, which were obtained
from the Natural Resources Conservation Services (NRCS) and the National Climatic Data Center
(NCDC), respectively. Table 3-1 outlines the data set names, formats, description, and source.

Table 3-1. Summary of data
Data set Type Description Source
Land use GIS Shapefile
Subdivides the base into predefined land Seymour Johnson
use categories AFB
Imperviousness GIS Shapefile Outlines building footprints, roads, Seymour Johnson
driveways, sidewalks, and airfield AFB
Topography GIS Shapefile Elevation contours at 2-foot intervals
Seymour Johnson
AFB
Stormwater GIS Shapefile Identifies stormwater structure layout, Seymour Johnson
drainage system conveyance methods, and inlet locations AFB
Aerial Imagery MrSID File Orthoimage of base and areas Seymour Johnson
immediately outside the installation AFB
Soils GIS Shapefile NRCS SSURGO
Outlines spatial extents of hydrologic soils
groups
Precipitation Time series Hourly precipitation measured from NCDC EarthInfo
01/01/2000 to 12/31/2010

3.2 LAND USE

Land use information is provided by the Seymour Johnson AFB, and the information includes both the
existing and the future land use opportunities.

3.2.1 Current Land Use

The Seymour Johnson AFB uses conventional Air Force land use categories, including airfield, family
housing, forest, governmental, instrument/communication, military, recreation, road, supply/storage,
training, and utility. The base chose the year of 2007 as the year that represents current land use in the
base. The current land use information is shown in Figure 3-1.

3.2.2 Demolition Credit

Since the year of 2007, the base demolished many existing impervious structures such as family housing
and roads. Most of the demolition activities occurred at the northeastern corner of the base, where the
family housing is concentrated. By the year of 2011, the total area of demolished imperviousness is
estimated to be about 69 acres. This demolition of impervious surfaces in the base is expected to not only
reduce total runoff volume and peak flow rate from the base, but also reduce pollutant loadings at the
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 17

same time. These hydrological and water quality benefits need to be appropriately accounted for when
developing stormwater management strategies in the base.

3.2.3 Future Land Use

Because DoD has interpreted predevelopment conditions to mean pre-project or existing conditions,
future EISA compliance will be primarily driven by the amount of new impervious area introduced
throughout the base over time. To adequately cover all possible future development outcomes, the scale
considered for the purposes of this plan assumes full build-out conditions.

Future land data includes expected future development in the urbanized portion of the base and
development in undeveloped sections. Although detailed information on future impervious area expansion
in the developed portion was not provided, it is assumed that a portion of the open space in the urbanized
areas will become impervious. Details for how that was estimated are discussed in Section 4.0.

For the undeveloped portions of the base, future development areas were confirmed with Seymour
Johnson AFB personnel. The developable areas include those undeveloped open space areas deemed
suitable for development (i.e., outside the floodplain, away from contaminated areas, outside the quantity
distance arc, and so on). The areas suitable for potential future development are also illustrated along with
existing land uses in Figure 3-1.

3.3 IMPERVIOUSNESS

Impervious information provided by Seymour Johnson AFB outlines building footprints, sidewalks,
driveways, roads, and airfield surfaces. The base has imperviousness delineations for the years of both
2007 and 2011. The impervious surface, coupled with the pervious land use categories and the soils
information, were used to develop hydrologic response units (HRUs) in the base.

3.4 TOPOGRAPHY

Topography information provided by Seymour Johnson AFB includes 2-ft interval contours over the base
installment area. The contour information was used for the subwatershed delineation and for slope
calculations.

3.5 STORMWATER DRAINAGE SYSTEM

The stormwater drainage system information was provided by Seymour Johnson AFB in a GIS shapefile
format. The database contains information for drainage basins, culverts, pipes, and inlets. The stormwater
drainage system provides information that helps with the subwatershed delineation process.

3.6 AERIAL IMAGERY

The aerial image information provided by Seymour Johnson AFB was available for the years of both
2007 and 2010, and was in an MrSID raster format. The aerial imagery provides a comparative backdrop
to the land use and impervious area information while establishing visual representation of built-out
conditions.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 18

Figure 3-1. Current land use in the Seymour Johnson AFB

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 19

3.7 SOILS

Soils information was obtained from the NRCS Soil Survey Geographic (SSURGO) database for Wayne
County. The database was developed by NRCS in cooperation with North Carolina Agricultural
Experiment Station back in 1974. The soils information helps to estimate expected infiltration rates and
sediment contributions. Figure 3-2 outlines the soils information for the area contained in the installation
limits, and the groups are defined as follows:

Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist
mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate
of water transmission.

Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of
moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture
to moderately coarse texture. These soils have a moderate rate of water transmission.

Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having
a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture.
These soils have a slow rate of water transmission.

Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These
consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that
have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.

Group B/D. Soils having a Group B categorization for drained areas and a Group D categorization for
undrained areas. Only the soils that are in their natural condition in group D are assigned to dual classes.

3.8 GROUNDWATER

Locations of monitoring wells are available from the base in a GIS shapefile format. Out of a total of 456
monitoring wells, the depth to groundwater table information is available for 71 wells. Continuous
monitoring data was not made available during the course of this plan. For purposes of BMP
implementation, the depth of 8 feet is used to separate deep groundwater (depth to groundwater table
larger than 8 feet) from shallow groundwater (depth to groundwater table smaller than 8 feet). Figure 3-3
highlights the groundwater monitoring well locations and the associated definitions of shallow or deep.
As shown in the figure, most of the monitoring wells are located near the installation boundary and along
the Stoney Creek, and the wells further inside the base are concentrated at two locations. Overall the
groundwater information was deemed to be too limited to be useful and was not included in the BMP
implementation analysis.

3.9 CLIMATE DATA

The rainfall data was obtained from the EarthInfo data CD for Seymour Johnson AFB (Station ID: 3510),
which receives the precipitation information from the NCDC. The station is at N35.3444° and W77.9647°
on the base. The hourly rainfall data for 01/01/2000 to 12/31/2010 were obtained for generating the runoff
time series from subbasins in the base. The precipitation data were prepared in an EPA Stormwater
Management Model (SWMM) format for continuous modeling. Climate data including daily high and
low temperatures, wind speed, and evaporation rate were also obtained for the runoff simulation.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 20

th th
According to the EISA requirements, the 95percentile storm must be determined and retained. The 95
percentile storm is defined as the precipitation amount that 95 percent of all rainfall events for the period
th
of record do not exceed. Using the precipitation data available, the 95percentile storm was calculated by
sorting the available data from largest recorded precipitation to smallest recorded over a 24-hour period
then eliminating values less than or equal to 0.1 inch according to technical guidelines described in EPA’s
th
technical guidance document. A statistical analysis was performed on the organized data to find the 95
th
percentile storm. For Seymour Johnson AFB, the 95percentile storm was calculated to be 1.87
th
inches. The 95percentile storm information is used to size BMPs using Option 1. The continuous time
series is used to size BMPs to match hydrology using Option 2 for the entire period of precipitation
records.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 21

USACE CONTRACT NO. W91278-10-D-0086, TASK ORDER 0025 TETRA TECH, INC.

Figure 3-2. Soils in the Seymour Johnson AFB

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 22

USACE CONTRACT NO. W91278-10-D-0086, TASK ORDER 0025 TETRA TECH, INC.

Figure 3-3. Monitored groundwater levels in the Seymour Johnson AFB
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 23

4.0 METHODOLOGY

This section describes the methodology performed to compare the two BMP sizing options for meeting
the EISA requirements. As described in Section 2.0, BMPs sized with Option 1 require a volume
th
necessary to fully capture and infiltrate all the runoff from the 95percentile rainfall event. BMPs
sized with Option 2 require a volume necessary to match future runoff patterns with the current ones.
For the purposes of this plan, both sizing approaches are presented in detail, and the BMP size
results are compared. The approach for calculating BMP sizes with Option 1 are very simple and
are briefly explained in Section 4.5. In comparison, the analyses to size BMPs with Option 2 requires
the setup and configuration of a SUSTAIN model. The model determines BMP sizing needs on the
basis of optimization of BMP designs through continuous simulation. The schematic layout of the
methodology is illustrated in Figure 4-1. As shown, the watershed characterization process involves
delineating subwatersheds, developing HRUs, and estimating existing and future impervious areas.
The relevant watershed characteristics are then used to separately estimate BMP size requirements on
the basis of the Option 1 and Option 2 approaches. Ultimately, the BMP size results are compared.
Detailed discussions about each step are included in the following sections.

Figure 4-1. Schematic for analyzing two options to meet EISA requirements

4.1 SUBWATERSHED DELINEATION

The Seymour Johnson AFB installation boundary is defined by the Stoney Creek to the west and the
Neuse River to the south. Elevations in the installation gradually decrease from the northern end to the
southern end. Extensive stormwater pipe network and open-channels exist are available for discharging
surface runoff. The runoff is discharged through multiple outfalls along the Stoney Creek and Neuse
River. The outlet for the installation is the most downstream point on the Neuse River at where the
installation boundary is crossed.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 24

The subwatershed delineation was based on the 2-ft contour and the stormwater drainage system on the
base. Subbasins were first delineated for areas that are located along and directly drain to the Stoney
Creek and Neuse River. Stormwater pipe and open channel systems were referred to while delineating
subwatershed in the rest of the installation area. A total number of 24 subwatersheds were delineated for
the installation, and the delineation is illustrated in Figure 4-2 below. The areas of the delineated
subwatersheds range from 24 acres to 329 acres, with an average size of 134 acres.

The outlet for the Seymour Johnson AFB is located at the downstream location where the Neuse River
crosses with the installation boundary. Depending on subwatershed locations, the runoff from a
subwatershed (e.g. subwatershed 19) may travel about eight miles before reaching to the outlet or may
reach the outlet through direct drainage (e.g. subwatershed 4).
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 25

USACE CONTRACT NO. W91278-10-D-0086, TASK ORDER 0025 TETRA TECH, INC.

Figure 4-2. Subbasin delineation for the Seymour Johnson AFB

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 26

4.2 HYDROLOGIC RESPONSE UNITS AT SEYMOUR JOHNSON AFB

HRUs are used in this plan to categorize different land surfaces that have distinctive runoff
characteristics. The developed HRUs can help project future land uses in a subwatershed, generalize
existing and future land use conditions, and create runoff time series for continuous simulations.

4.2.1 The HRU Concept

As defined by Flugel (1997), HRUs are, “distributed, heterogeneously structured model entities
comprising common land use and pedo-topo-geological associates generating and controlling their
homogeneous hydrological dynamics.” In other words, each HRU is a subunit that has uniform
characteristics of land use, soil, and slope. Thus, each HRU exhibits similar hydrologic responses. HRUs
are developed so that the variation of hydrological dynamics within each HRU is small when compared to
the runoff characteristics of a neighboring HRU. Collectively, HRUs retain and represent the complex and
distributed basin hydrology (Bongartz 2003).

4.2.2 Creation of HRUs at Seymour Johnson AFB

Impervious surfaces are the main source of runoff during a rainfall event. On the basis of site
investigations and the topography data, most of the impervious surfaces at Seymour Johnson AFB are on
relatively flat areas. Thus, slope is not a factor in the creation of Seymour Johnson AFB HRUs, and the
HRU development is based solely on land use and soils information.

An overlay of the land use layer, existing imperviousness layer, and the soils layer can help identify the
basic HRU categories at Seymour Johnson AFB. For example, when a building in the administrative land
use is overlaid on the soils layer, the building footprint itself would be categorized as Building, and the
pervious surfaces outside the building but still within the administrative land use parcel would be
categorized as Open Space Developed with the corresponding soil’s hydrologic soil group (HSG)
information (e.g. OS_Dev_Perv_C). For Seymour Johnson AFB, the overlay of land use layer, existing
imperviousness layer, and the soils layer create 26 HRUs in total. A summary of the 26 HRUs is shown in
Table 4-1. Among the HRUs, the recreational impervious refers to impervious surfaces at playground
areas. The Airfield is listed separately from the more general Transportation HRU in the table, mainly
because of different water quality characteristics in stormwater runoff from the two categories.

The HRU layers were created for both the year of 2007 and 2011, which are shown in Figure 4-3 and
Figure 4-4, respectively. As shown, when compared to the year of 2007, substantial amount of
imperviousness were demolished in the base, especially in the northeast corner of the family residential
area. The generated HRUs pave the base for quantifying hydrologic and water quality benefits resulting
from the demolition activities.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 27

Table 4-1. Summary of HRUs in Seymour Johnson AFB

Ground
Land use conditions HSG HRU group
B/D Agriculture_B/D
Agriculture Pervious
C Agriculture_C

Airfield Impervious -- Airfield
Building Impervious -- Building
A Forest_A
B Forest_B
B/D Forest_B/D
Forest Pervious

C Forest_C

D Forest_D

A Golf_A

B Golf_B
Golf course Pervious
B/D Golf_B/D

C Golf_C

A OS_Dev_Perv_A

B OS_Dev_Perv_B
Open space
(developed Pervious B/D OS_Dev_Perv_B/D
areas)
C OS_Dev_Perv_C

D OS_Dev_Perv_D

A OS_Perv_A

B OS_Perv_B
Open space (un
B/D OS_Perv_B/D
developed Pervious
areas)
C OS_Perv_C
D OS_Perv_D
Recreation Impervious -- RecImpv
Transport Impervious -- Transport
Water Impervious -- Water

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 28

USACE CONTRACT NO. W91278-10-D-0086, TASK ORDER 0025 TETRA TECH, INC.

Figure 4-3. The Seymour Johnson AFB HRUs for the year of 2007

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USACE CONTRACT NO. W91278-10-D-0086, TASK ORDER 0025 TETRA TECH, INC.

Figure 4-4. The Seymour Johnson AFB HRUs for the year of 2011

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 30

4.2.3 Generation of HRU Runoff Time Series

After HRUs at Seymour Johnson AFB are identified and created, each HRU is represented as a parcel into
EPA’s SWMM (Version 5.0). Long-term continuous rainfall and climate data from 01/01/1990 to
12/31/2010 are used to run the SWMM for each HRU. Appropriate soil HSG information is represented
for pervious HRUs. It was assumed that the impervious areas had a depression storage value of 0.04 inch
with the excess precipitation resulting in runoff. The resulting hourly runoff time series represent the
long-term runoff conditions from each HRU at Seymour Johnson AFB. The aggregated HRU time series
from a subwatershed represent the composite runoff conditions from a subwatershed.

4.2.3.1 Water Quality Time Series

In addition to the analysis of hydrologic runoff conditions, which are needed for addressing EISA
regulations, water quality runoff conditions from the Seymour Johnson AFB are also desirable for a more
comprehensive assessment of the stormwater management practices. The water quality information will
help quantify and compare treatment benefits from different BMP implementation scenarios, and that will
lead to informed decision-making in the TMDL implementation process.

Water quality time series were developed for the Seymour Johnson AFB for nutrients (TN and TP) and
sediment (TSS). Pollutant loading rates reported in previous regional studies were referred to when
developing the water quality time series for the base. In a study that helps the North Carolina Department
of Environment and Natural Resources (NC DENR) to develop the Stormwater Rule for the Tar-Pamlico
River Basin, Hunt and Lucas (2003) provided specific Event Mean Concentrations (EMCs) for a set of
land covers for both TN and TP. The EMCs presented in Table 4-2 were reported in the coastal plain
version of the Stormwater Rule export calculations, and were used for developing nutrient time series in
the Seymour Johnson AFB.

Table 4-2. Loading rates for nutrients (adapted from Hunt and Lucas 2003)

Land use TN (mg/L) TP (mg/L)

Transportation 2.6 0.19

Buildings 1.95 0.11

Airfield 2.6 0.19

Agriculture 4.23 1.23

Forest 0.95 0.14

Golf 1.42 0.28

Open Space_Developed 1.42 0.28

Open Space_Undeveloped 0.95 0.14

The sediment yields and percentiles of TSS concentrations for over 100 watersheds across North Caronia
(including the coastal plain region) were reported by Simmons (1993). The analysis included percentage
of each drainage area in general land use categories including cropland, pasture, forest, and urban. In a
more recent study by Tetra Tech (2012) in a North Carolina piedmont watershed, the TSS loadings of the
composite urban land use was disaggregated into impervious and pervious loadings. The TSS loadings
from both studies are summarized in below.

Table 4-3. Loading rates for TSS (adapted from Simmons 1993 and Tetra Tech 2012)

Land use TSS (lbs/ac/yr)

702
Transportation (roads, small

parking lots, sidewalks, etc)

Buildings (residential) 702

Buildings (commercial) 429

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 31

Airfield 429
Agriculture 116
Forest 38
Golf 390
Open Space_Developed 390
Open Space_Undeveloped 77

The nutrient and sediment loading rates in Table 4-2 and Table 4-3 were represented into the SWMM
model that was used for the hydrologic runoff time series generation. Continuous simulation was carried
out for each HRU, and the composite water quantity and quality time series were created. The time series
were later used for the long-term simulations.

It is worth noting that neither the hydrologic nor the water quality time series was calibrated to actual
runoff conditions at Seymour Johnson AFB. This was mainly due to the lack of monitored hydrologic or
water quality data at the base. However, since the focus of this plan was to compare the changes in
runoff conditions before and after BMP implementation, the un-calibrated time series was deemed
sufficient for carrying out such analyses. When monitored data become available in the future, the
hydrologic and water quality time series can always be calibrated through the SWMM model and then be
used to update the BMP implementation analysis.

4.3 IMPERVIOUS AREA ASSESSMENT

Impervious surfaces are the main sources of runoff generation during an event. The increase of
impervious surfaces from existing conditions to the future conditions determines the corresponding peak
and total volume increase and, thus, the required BMP volume to treat the increased runoff. While the
existing imperviousness at Seymour Johnson AFB is relatively straightforward to calculate, the future
imperviousness estimation involves research into new developments, redevelopment, and infill.
Estimating existing and future imperviousness at Seymour Johnson AFB is described in the subsections
below.

4.3.1 Current Imperviousness

Seymour Johnson AFB provided GIS shapefiles of all developed and undeveloped areas of the base. The
majority of the base development has occurred on the north side of the airfield. The impervious area
information outlines the buildings, roads, sidewalks, driveways, and runway for the extents of the
installation. The area attributes of the impervious GIS information were intersected with the base
subwatershed delineations to provide the total current impervious area per subwatershed. Table 4-4 shows
the current impervious area in each subwatershed. The base-wide percent impervious for Seymour
Johnson AFB is 26.4 percent.

4.3.2 Future Imperviousness

Two types of future developments are expected at Seymour Johnson AFB. The first type is the
development in open space areas, and the second type is the redevelopment and infill that occurs in
existing developed areas. The estimation procedure of each type of future imperviousness is described in
the sections below.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 32

4.3.2.1 Open Space Development

Seymour Johnson AFB identified undeveloped, open space areas where future development can occur.
The following steps were used to establish an estimate of increase in impervious area for open space
development.

1. Calculate percentages of current impervious and pervious HRUs in the developed portion of the
base. The developed portion of the base refers to the section of the installation that is highly
developed and is concentrated on the north side of the runway. The runway is not included in the
calculation of percentages. The three impervious HRUs (Buildings, Recreation Impervious, and
Transportation) have percent impervious of 6.7, 0.6, and 15.5 percent, respectively.
2. Calculate existing HRU percentages in the identified developable areas of the base. The areas of
each HRU in each developable area in each subwatershed are tabulated to summarize the existing
conditions within the area to be developed.
3. Apply the impervious HRU percentage compositions from Step 1 to the developable areas in Step
2. The impervious HRU percentages from the current developed area (Step 1) are applied to each
developable area in each subwatershed (Step 2). The remaining pervious percentage is composed
of the original open space HRUs with the same percentage of composition as the predevelopment
conditions.

4.3.2.2 Re-Development

Within the developed portion of the base, additional small-scale projects (e.g. extension of an existing
parking lot) can occur. No information exists about where such projects might occur or how large those
projects could be. The strategy to account for the small-scale projects is a three-step process.

1. Delineate typical parcels using edge of road intersections and fence lines then define each
parcel’s land use. Parcels are created by the road segment intersections and obvious fence lines to
create multiple parcel polygons. The parcel information is overlaid with the land use to create
multiple parcels with an associated land use.
th
2. Calculate impervious percentage for each identified parcel. Identify the 75percentile percent
imperviousness for each land use type. The HRU areas are tabulated for each parcel and divided
into similar land use categories. The imperviousness percentage is calculated for each parcel and
grouped by land use category. Once the percent impervious of each parcel is grouped by land use,
th
the 75percentile percent impervious can be calculated for each land use category.
th th
3. Apply the 75percentile impervious percentage to all parcels of the same land use type. The 75
percentile percent impervious is assumed to be the maximum built out condition for each parcel.
th
Parcels with less than the 75percentile impervious percentage in each land use category are
th
increased up to the 75percentile percent impervious of that land use category. Parcels with
th
percent impervious values greater than the 75percentile were unchanged.

4.3.3 Summary of Existing and Future Imperviousness

The results of the change in HRU areas from the two estimating methods were combined to create future
expected HRU areas on the basis of current development per each subwatershed. The demarcated
developable areas took precedent over the parcel land use estimates where overlap occurred. Table 4-4
shows the current impervious area and the estimated future impervious area (from futures project layer) in
the 24 subwatersheds at the base. As shown in the table, development occurs in 16 out of the 24
subwatersheds on the base. The total future increase of imperviousness is about 167 acres, or 17 percent,
in Seymour Johnson AFB as compared to the existing land use conditions
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 33

Table 4-4. Summary of existing and future imperviousness in Seymour Johnson AFB subwatersheds

Existing Future Increased Predicted

impervious area impervious impervious area percent
SubbasinID
(acres) area (acres) (acres) change
3.48
1 3.48 - 0.00%
6.84
2 6.86 - 0.00%
26.21
3 26.26 - 0.00%
21.81
4 49.24 27.38 55.61%
30.54
5 31.28 0.67 2.15%
4.59
6 4.60 - 0.00%
2.60
7 2.61 - 0.00%
1.98
8 1.98 - 0.00%
105.12
9 136.04 30.70 22.57%
71.33
10 72.64 1.16 1.59%
48.54
11 49.18 0.54 1.09%
106.21
12 117.88 11.45 9.71%
22.44
13 23.36 0.87 3.74%
41.16
14 51.61 10.37 20.09%
87.75
15 106.52 18.59 17.45%
11.53
16 21.36 9.81 45.92%
13.74
17 14.22 0.45 3.16%
44.56
18 46.86 2.21 4.71%
28.21
19 28.27 - 0.00%
59.42
20 59.54 - 0.00%
21.82
21 32.36 10.49 32.42%
19.59
22 27.19 7.56 27.79%
52.82
23 60.87 7.94 13.04%
15.83
24 40.47 24.60 60.79%
TOTAL 848.10 1,014.67 166.57 19.64%

4.4 EXISTING AND FUTURE RUNOFF CONDITIONS

After the HRU time series are generated and the existing and future imperviousness assessed, runoff
conditions from existing and future subwatershed runoff conditions can be assessed. Aggregated runoff
conditions were analyzed for the existing (2007), the year of 2011, and the future land uses, and the
results are summarized in Table 4-5 below.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 34

Table 4-5. Summary of runoff conditions for different land use scenarios in the Seymour Johnson AFB
Annual average
Total
Pollutant loadings (lbs/yr)
Land use scenario
runoff volume
Impervious
TSS TN TP
(10^6 ft3/yr)
area (acres)

Existing (2007) 848.1 243.38 115,290 25,165 2,276
With demolition (2011) 778.8 233.39 111,160 24,155 2,235
Future 1014.7 267.84 130,528 28,379 2,459
Diff. between 2007 69.3 9.99 4,130 1,010 41
and 2011 (-8.2%) (-4.1%) (-3.5%) (-4.0%) (-1.8%)
166.6 24.5 15,238 3,214 183
Diff. between 2007
and future (19.6%) (10.1%) (13.2%) (12.8%) (8.0%)

As shown in the table, the demolition of impervious areas between 2007 and 2011 is about 69 acres. That
resulted in approximately 4 percent reduction in annual average runoff volume, 3.5 percent in TSS load, 4
percent in TN load, and 1.8 percent in TP load. When the installation becomes fully built out in the
future, the net increase of impervious surfaces from the existing (2007) conditions is about
167 acres. The increase of imperviousness causes deteriorations in hydrologic and water quality
conditions, and the corresponding increases in annual average runoff volume, TSS load, TN load,
and TP load are 10.1 percent, 13.2 percent, 12.8 percent, and 8 percent, respectively.

4.5 ANALYSIS FOR OPTION 1

th
Option 1 for meeting EISA regulations require that runoff from the 95percentile storm be fully
th
contained. The BMP volume needed is estimated by the increased imperviousness multiplied with the 95
percentile storm depth.

th
The analysis of long-term rainfall data for Seymour Johnson AFB identified the 95percentile
storm depth as 1.87 inches. The impervious surface is assumed to have no depression storages in
Option 1 analysis. The effective BMP volume needed for treating a certain amount of increased
imperviousness in a subwatershed was calculated as the area of the increased imperviousness
multiplied by the storm depth of 1.87 inches. As for the BMP designs, it was assumed that the BMP
has a uniform depth of 4 feet with an effective depth of 1.75 feet. That is to be consistent with the
Option 2 BMP designs so that the two options could be compared. The actual BMP volume required is
the effective BMP volume divided by the effective porosity (0.4375). For further details on the
assumed BMP design configuration, see Section 4.6.1.1.

The total required BMP volume for all subwatersheds within one outlet is the total BMP volume needed
in that outlet. The total BMP volume required at each outlet under Option 1 will be compared to those
from Option 2 analysis, which is introduced below.

4.6 ANALYSIS FOR OPTION 2

th
The SUSTAIN model provides a continuous simulation alternative to the 95percentile storm approach
under Option 1. During the continuous simulation, long-term rainfall characteristics, evapotranspiration
from BMPs, and detention effects from BMPs are all accounted for. In addition, the optimization
capability of the SUSTAIN framework allows for identifying the most cost-effective BMP
implementation schemes for meeting the watershed-wide control target. Runoff from subwatersheds are
represented as HRU time series in the SUSTAIN model. The BMPs for the SUSTAIN setup, drainage
network, and the SUSTAIN framework at Seymour Johnson AFB are discussed in this section.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 35

4.6.1 BMPs for SUSTAIN Analysis

Infiltration types of BMPs such as bioretention are effective measures to restore post-development runoff
conditions to the predevelopment level. The SUSTAN model can help quantify the hydrologic benefits
from BMP implementations. As subwatershed runoff is routed through an infiltration BMP, porous spaces
in BMPs provide total volume and peak flow reduction control. The degree of hydrologic benefits varies
as the size of the BMPs change. Although rainwater reuse type BMPs can also provide volume and peak
flow reduction, they were not considered as an option in this plan.

4.6.1.1 BMP Designs

The sectional view of the representative bioretention unit used in the SUSTAIN model is shown in Figure
4-5. As shown, the bioretention unit consists of a media layer and a sand or stone positive drainage layer
underneath. In the SUSTAIN setup, a 6-inch ponding area is also added on top of the media layer.
Detailed design parameters for the infiltration system are shown in Table 4-6. The values selected for
modeling offer conservative results for the modeling approach and are intended to be representative of the
average field implemented BMPs. Note that for the purposes of modeling, all BMPs are assumed to have
vertical side walls. If designed and constructed BMPs have sloped sidewalls, which is likely, design
engineers must adjust sizing parameters appropriately.

Figure 4-5. Sectional view of a typical bioretention unit

Table 4-6. Design parameters for the bioretention BMP

Value

(in)
Component Design parameters
Ponding area Depth 6
Depth 36
Media layer
Porosity 0.35*

Depth 6
Sand/Stone layer
Porosity 0.4*

*no units

4.6.1.2 BMP Costs

Unit cost is a critical component when optimizing BMP designs through continuous simulation. Capital
costs of a BMP consist of the land cost, engineering planning and design costs, construction cost, and the
costs for environmental mitigation. Construction cost is often used to represent the capital cost for
planning level analyses because the land cost, engineering costs, and the costs for environmental
mitigation are site specific (Sample et al. 2003). BMP construction costs have been reported separately in
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 36

several previous studies (USEPA 1999; Wossink and Hunt 2003; Weiss et al. 2005; CWP 2007). In a
more recent study by Barr Engineering (2011), BMP construction cost values from the previous studies
were summarized, adjusted for regional factors, and then translated to the 2010 dollars. In the updated
BMP construction costs, the unit cost for an infiltration type of BMP was estimated to be $11 per cubic
feet, and this was used for calculating the BMP construction costs at the Seymour Johnson AFB.

In this plan, the cost mainly serves for relative comparisons among different BMP configurations in the
subwatersheds. The estimated total costs for BMP implementations are relative costs, and the actual costs
to implement those BMPs would likely be higher to accommodate for existing site conditions.

4.6.2 Drainage Network

In the SUSTAIN setup for Seymour Johnson AFB, BMPs are placed for individual subwatersheds.
Runoff from new impervious surfaces is routed to the BMPs, and runoff from the rest of the subwatershed
is routed to the subwatershed outlet. Overflow from the BMPs is also routed to the subwatershed outlet.
Subwatershed runoff is connected through actual pipes and open channels and is eventually routed to the
installation outlet. The schematic of the SUSTAIN routing network is illustrated in Figure 4-6 below. The
actual stormwater manhole elevation, pipe length, and diameter information are represented in the routing
network. The ArcGIS 3D Analyst was used to derive representative cross-sectional information along the
Stoney Creek and Neuse River. This detailed representation of the flow routing network was necessary
for Seymour Johnson AFB, on which the surface runoff has a relatively long flow path. The actual
channel dimensions help to realistically account for the delay effects of runoff traveling from a headwater
subwatershed (e.g. subwatershed 19) versus runoff from a downstream subwatershed (e.g. subwatershed
4).

Figure 4-6. The SUSTAIN model routing network for Seymour Johnson AFB
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 37

4.6.3 Option 2 SUSTAIN Analysis

With the completion of SUSTAIN model setup for Seymour Johnson AFB, the optimization process was
carried out to identify the cost-effective BMP sizing schemes to maintain the total runoff volume at the
predevelopment level. During the optimization process, BMP dimensions become decision variables, and
the optimizer searches through different BMP sizing schemes to identify the optimal BMP site layout for
maintaining the post-development runoff at the pre-development level. The matching among different
runoff conditions are measured through the annual average runoff volume, and the assessment point for
the optimization analysis was the installation outlet.

After the identification of the optimal BMP site layout at the Seymour Johnson AFB, peak flow and water
quality benefits from the optimal solution were also calculated. For quantification of peak flow benefits,
peak flow values at various exceedance percentiles during wet periods were used to compare the near-
optimal BMP configuration performance to those from the existing conditions. Runoff from future
without BMP condition was also included in this analysis to highlight the BMP benefits. A varying
degree of exceedance percentiles were used to ensure that the whole spectrum of BMP performance was
evaluated. For quantification of water quality benefits, the total nutrient and sediment loads from the
future with BMP land use condition was compared with those from the existing land use condition.
Results from these analyses are presented next in Section 5.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 38

5.0 COMPARISON OF BMP SIZING APPROACHES

This section presents and compares the two options for determining necessary BMP volumes to meet
th
EISA requirements at Seymour Johnson AFB. Option 1 fully infiltrates runoff from the 95percentile
storm from the newly developed impervious surfaces. Option 2 uses the SUSTAIN program to identify
near-optimal BMP sizes through continuous simulation. Within Option 2, three different variations were
analyzed: 1. To treat the new imperviousness only, 2. To tap into existing imperviousness, and 3. To tap
into existing imperviousness and to implement BMPs in different phases. Results from the Option 1
approach and the three variations of the Option 2 approach are compared side-by-side in this section.

5.1 OPTION 1 RESULTS

The Option 1 approach is to fully capture the runoff volume from the increased imperviousness during the
th
95percentile storm. The BMP volume in each subwatershed that has future developments was calculated,
and the total BMP volume required for the entire base was aggregated. Table 5-1 shows the required BMP
th
volume, area, and cost needed to meet the 95percentile storm retention requirement for EISA at Seymour
Johnson AFB. Note that the BMP area ratio represents the unit area of BMP required for every unit area of
new impervious area built.
Table 5-1. BMP size needed for EISA Option 1 at Seymour Johnson AFB

BMP volume
Increased
Analysis Total BMP cost
required
imperviousness BMP area ratio
scenario (million $)
3
(ft)
(ac)
9% 28.43
Option 1
166.6 2,584,908

5.2 OPTION 2A: TREATING NEW IMPERVIOUSNESS ONLY

In the initial setup of the SUSTAIN model for Seymour Johnson AFB, runoff from the new impervious
surfaces were routed to respective BMPs located in the same subwatershed, and the optimization was
carried out to identify the most cost-effective solutions that can maintain the post-development runoff at
the pre-development level. The optimization process resulted in a tradeoff relationship between the total
BMP cost and the annual average total runoff volume, which is illustrated in Figure 5-1 below. Each point
in the figure represents a BMP sizing alternative evaluated by the optimizer during the optimization
process. As the search of cost-effective BMP implementation strategies evolves, a stable tradeoff front is
formed between the total cost and the annual average total runoff volume. The near-optimal solution that
most approximates the existing annual average runoff volume is identified to have a total cost of $23.11
million and an annual average runoff volume of 243.41 million cubic feet, as highlighted in Figure 5-1.
Overall the figure demonstrates that as the total cost (BMP sizes) increases, the annual average runoff
volume decreases, and vice versa. The tradeoff results also show the minimum and maximum annual
runoff volume values, which correspond to maximum and minimum BMP sizes on the installation.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 39

Figure 5-1. The tradeoff relationship and the near-optimal solution identified for treating new imperviousness
at the Seymour Johnson AFB

The power of optimization is well demonstrated through the tradeoff relationship in Figure 5-1. When a
line parallel to the x-axis is drawn at a certain annual average runoff volume level on the y-axis, the points
crossed by the line are possible BMP sizing alternatives that will result in a common level of treatment.
But the cost of these alternatives will vary significantly as indicated by their x-axis values. The same
observation is also true when selecting a certain total cost (e.g. fixed budget), a line drawn from the x-axis
that is parallel to the y-axis will cross many BMP sizing alternatives that have drastically different annual
average runoff volumes. At a selected annual average runoff volume level, the first point being crossed by
the line parallel to the x-axis has the minimum cost, indicating the first point being the most cost-effective
alternative for maintaining the pre-development runoff conditions. A comparison of annual total runoff
volume and water quality performances among various land use scenarios following Option 2a is
summarized in Table 5-2 below.

Table 5-2. Comparison of annual average runoff volume and water quality performances among different
land use scenarios in Option 2a for complying with EISA regulations at Seymour Johnson AFB
Annual average
Total
Pollutant loadings (lbs/yr)
Land use scenario
runoff volume
Impervious
TSS TN TP
(10^6 ft3/yr)
area (acres)

Existing (2007) 848.1 243.38 115,290 25,165 2,276
Future 1014.7 267.84 130,528 28,379 2,459
Optimal solution 1014.7 243.42 115,789 25,346 2,257
166.6 24.5 15,238 3,214 183
Diff. between 2007
(19.6%) (10.1%) (13.2%) (12.8%) (8.0%)
and future
Diff. between 2007 166.6 0.04 499 181 -19
and optimal solution (19.6%) (0.05%) (0.43%) (0.72%) (-0.83%)

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 40

As shown in the table, the optimal solution has a close match in both annual average runoff volume and
the water quality performances with the existing runoff conditions, with all differences are smaller than 1
percent. The optimal solution even results in less annual TP load than the existing land use.

Peak flow performances of the optimal solution was also quantified and compared to those from the
existing and future land uses. Wet weather peak flow values at exceedance percentiles of 0, 5, 25, 50, 75,
and 100 percent were estimated for the existing, future, and future with optimal BMP land use conditions.
Comparisons of peak flow statistics for these three conditions are summarized in Table 5-3 below.

Table 5-3. Comparison of peak flow statistics between existing, future, and future with optimal BMP land use
conditions following Option 2a at Seymour Johnson AFB

Exceedance Future with
Existing (2007) Future

percentiles optimal BMP

0% 3,821.77 3,847.54 3,838.18

5% 28.94 33.29 28.80

25% 4.19 4.81 4.19

50% 1.02 1.10 1.02

75% 0.20 0.21 0.20

100% 0.00 0.00 0.00

As shown in the table, the optimal BMP site layout results in peak flows that closely match those from the
existing land use condition. Except for flow rate at the 0 percent exceedance level, all the future with
optimal BMP land use condition has peak flow rates equal to or even smaller than the existing land use.
These comparisons highlight the BMP treatment benefits in treating smaller events through infiltration
and provide additional ecological benefits (e.g., groundwater recharge).

In addition to the hydrologic and water quality benefits from the optimal BMP site layout, the most
important benefit from employing the optimization technique in the analysis was the cost savings. A
comparison in total BMP costs between Option 1 and Option 2a approaches of complying with the EISA
regulations is summarized in Table 5-4 below, along with the BMP volume requirement and the BMP
area to impervious surface area ratio. As shown in the table, the overall cost savings from implementing
the Option 2a BMP site layout is $5.32 million.

Table 5-4. Comparison of total BMP costs between Option 1 and Option 2a of complying with the EISA
regulations at Seymour Johnson AFB

BMP volume BMP cost
Analysis Total BMP cost
required BMP area ratio savings (million
scenario (million $)
3
(ft)
$)
9% 28.43 -
Option 1
2,584,908
7% 23.11 5.32
Option 2a
2,101,331

The implementation of Option 2a optimization results is relatively straight forward: for every one acre
2
of new imperviousness, build 3,049 ft(7%) of BMPs to achieve the compliance. The BMPs are
located closely to the new developments and the two are in the same subwatershed.

5.3 OPTION 2B: TAPPING INTO (RETROFITTING) EXISTING
IMPERVIOUSNESS

Building on the substantial savings from Option 2a, in which BMPs were implemented to treat runoff
from the increased impervious areas, alternative BMP implementation schemes were investigated in
search for further cost savings. As soil infiltration rates in the 16 subwatersheds that have future
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 41

developments vary from high to low, one alternative scheme analyzed was to tap into (or retrofit) existing
imperviousness in subwatersheds with high infiltration rates, and to comply with runoff regulations
applicable to new developments through treatment of existing impervious surfaces. That is, when one
acre of new development occurs in a subwatershed (regardless of location), BMPs will be built
to treat one acre of existing imperviousness in a subwatershed with high infiltration rates. This
retrofitting of existing imperviousness will start with subwatersheds with the highest infiltration rate and
move down to the ones with lower infiltration rates. The process will take into account of both the
available imperviousness that can be retrofitted and also the pervious spaces that are available for BMP
implementation.

A desktop analysis was carried out on the Seymour Johnson AFB to identify existing imperviousness that
can be retrofitted and the nearby pervious spaces (e.g. HSG A and B soils) for BMP implementation. It
was a planning-level analysis that accounted for the feasibility of routing impervious runoff to potential
BMP sites. For example, if the existing imperviousness was located at a downstream location and was
close to the subwatershed outlet and the HSG A or B soil in the subwatershed was located in upstream in
the headwater area, then the existing imperviousness was considered not suitable for retrofitting. The
analysis was limited to the subwatersheds that were expected to experience further developments in the
future land use scenario. The analysis stopped when the identified existing imperviousness exceeded the
projected total future development in the base, or 167 acres. A summary of the identified impervious and
pervious surfaces in the 9 subwatersheds is shown in Table 5-5 below. The results of the desktop analysis
are also illustrated in Figure 5-2.

Table 5-5. Summary of identified impervious surfaces for tapping and corresponding pervious spaces in
subwatersheds with high infiltration rates in Seymour Johnson AFB
Average Nearby available
Existing imperviousness
Subwatershed infiltration pervious areas
available for tapping
rates
ID
(acres)
HSG A HSG B
(in/hr)

0.378
9 9.3 - 2.7
0.981
11 6.6 9.7 -
1.152
12 69.2 26.7 3.8
0.966
13 9.8 0.7 2.1
1.052
14 11.7 2.0 1.4
0.875
15 35.0 0.2 8.0
1.100
21 20.2 5.0 0.3
0.601
23 5.9 - 1.5
0.726
24 1.8 - 0.6
-
Total 169.5 44.3 20.5
As shown in the table, the desktop analysis identified a total of 169.5 acres of existing imperviousness
that can be retrofitted. The area ratios between the available pervious spaces to the identified existing
imperviousness are always 30 percent or above, indicating enough pervious space is always available for
implementing BMPs.
The identified existing imperviousness and nearby pervious spaces provide the basis for an optimization
analysis on tapping into existing impervious surfaces. In the optimization setup, runoff from the identified
existing impervious surfaces in the high infiltration subwatersheds were routed to the nearby pervious
surfaces, where the sizes of potential BMPs was optimized to achieve the most cost-effective control. The
optimization target was the pre-development annual average runoff from those areas that would
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 42

experience future developments, which was about 4.51 million cubic feet. Because this optimization
analysis was carried out only on selected subwatersheds on the installation, no actual channel network
was used in the optimization setup and dummy connections were used for the flow routing.
Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 43

Figure 5-2. Existing imperviousness that can be tapped into and nearby available pervious spaces for potential BMP implementation

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 44

The optimization analysis for retrofitting existing imperviousness was carried out using the SUSTAIN
model, and the resulting tradeoff relationship between total BMP costs and the annual average runoff
volume is shown in Figure 5-3 below, with the near-optimal solution highlighted. As shown in the figure,
a similar relationship between the BMP size and treatment benefits as previously shown in the Option 2a
results is observed, in which the annual average runoff decreases as the total BMP cost (BMP size)
increases, and vice versa. The near-optimal solution that approximates the pre-development annual
average runoff volume has a total BMP cost of $18.07 million.

Figure 5-3. Tradeoff between total BMP costs and the annual average runoff volume for tapping into existing
imperviousness in Seymour Johnson AFB

Runoff conditions from the pre-development pervious area, the identified existing impervious area, and
the identified existing impervious area with BMP land use scenarios are summarized in Table 5-6 below.
As for the 166.6 acres of pervious land that may be developed in the future, the table shows that
the pre- development annual average runoff is about 4.51 million cubic feet. The annual average
runoff volume from the corresponding existing impervious surfaces is more than four times higher at
20.19 million cubic feet. Similarly, the annual average TSS, TN, and TP loads are increased by 658
percent, 1080 percent, and 315 percent, respectively. The identified near-optimal solution is able to
maintain the pre- development runoff conditions. As for water quality performances, the near-optimal
BMP site layout causes substantial reductions of annual average pollutant loads from the tapped
existing impervious surfaces runoff. The resulting TSS annual average load is about 19 percent
more than the pre-

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 45

development level, and the TN load is about 89 percent more than the pre-development level. The
resulting annual average TP load, meanwhile, is about 32 percent less than the pre-development level.

Table 5-6. Comparison of annual average runoff volume and water quality performances among different
land use scenarios in Option 2b for complying with EISA regulations at Seymour Johnson AFB
Annual average
Land use scenario
Pollutant loadings (lbs/yr)
runoff volume
TSS TN TP
(10^6 ft3/yr)

Pre-development
4.51 2,063 324 65
pervious
24.7 15,631 3,823 270
Existing impervious
Optimal solution 4.51 2,468 613 44
20.19 13,568 3,499 205
Diff. between pre-dev.
and existing imperv. (448%) (658%) (1080%) (315%)
Diff. between pre-dev. 0 405 289 -21
and optimal solution (0%) (19.6%) (89.2%) (-32.3%)

Wet weather peak flow performances of the near-optimal BMP solution was also checked against those of
the pre-development and the tapped existing impervious land use scenarios. The results of the comparison
are summarized in Table 5-7 below. As shown in the table, runoff does not occur in the pre-development
land use for almost 95 percent of the time. In comparison, the existing imperviousness has a highest peak
flow rate (0 percent exceedance percentile) of about 10 percent higher, and runoff is generated for
exceedance percentiles larger than 75 percent, indicating higher and more frequent runoff. The near-
optimal BMP implementation scheme, as shown in the fourth column in the table, results in peak flow
performances that are similar to the pre-development levels. The highest peak flow rate is about 5 percent
more than that of the pre-development land use, and the flow rate at the 5 percent exceedance level is
even lower than the pre-development value. This again demonstrates the exceptional treatment benefits
provided by the near-optimal solution.

Table 5-7. Comparison of peak flow statistics between existing, future, and future with optimal BMP land use
conditions following Option 2b at Seymour Johnson AFB

Existing impv.
Exceedance Pre- Existing

with optimal
percentiles development imperviousness

BMP

436.31 481.94 456.34
0%

0.41 35.22 0.01
5%

0 5.13 0
25%

0 0.68 0
50%

0 0.08 0
75%

0 0 0
100%

Total BMP volume, BMP area to impervious area ratios, and the total BMP costs for complying with the
ESIA regulations through Option 1, Option 2a (treating new imperviousness), and Option 2b (tapping into
existing imperviousness at high infiltration subwatersheds) are compared against each other in Table 5-8.
As shown in the table, Option 2b has the lowest total BMP cost of $18.07 million. The cost is equal to

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 46

about $10.36 million savings as compared to that of the Option 1 approach, which sizes BMPs uniformly
to treat 1.87 inches of impervious surface runoff. As previously discussed, these cost savings are achieved
by taking advantage of the variations of soil infiltration rates in the base, and by strategically placing the
BMPs to meet the same compliance targets.

The implementation of Option 2b is slightly different from that of Option 2a, in that the BMPs and the
new developments are not necessarily located in the same subwatershed. During the implementation
process, one acre of new developments might occur in a low infiltration subwatershed. According to
Option 2b, the corresponding BMP will be placed at the high infiltration subwatersheds to treat its
existing imperviousness. The BMP area to impervious area ratio in this case is 6 percent.

Table 5-8. Comparison of total BMP costs between Option 1, Option 2a, and Option 2b of complying with the
EISA regulations at Seymour Johnson AFB

BMP volume BMP cost
Analysis Total BMP cost
required BMP area ratio savings (million
scenario (million $)
3
(ft)
$)
9% 28.43 -
Option 1
2,584,908
7% 23.11 5.32
Option 2a
2,101,331
6% 18.07 10.36
Option 2b
1,642,648

5.4 OPTION 2C: PHASED IMPLEMENTATION OF TAPPING INTO
(RETROFITTING) EXISTING IMPERVIOUSNESS

Numerous studies have shown that as BMPs are implemented to treat runoff from impervious surfaces,
the return in hydrologic and water quality benefits per unit increase in BMP size tends to decrease. That
is, BMPs are more cost-effective when the BMP to impervious area size ratio is relatively small, and the
cost-effectiveness gradually decreases (levels off) as the BMP to impervious area ratio increases.
Recognition of this dynamic relationship can be very useful for institutions such as Seymour Johnson
AFB, especially when considering the fact that not all planned future developments may eventually get
built.

An investigation was carried out on Seymour Johnson AFB to quantify the change in runoff treatment
benefits as the size of implemented BMP size increases. The investigation was based on the optimal BMP
3
size from Option 2b, which has a total BMP volume of 1,642,648 ftas previously shown in Table 5-8.
During the analysis, the implemented BMP size was increased by 10 percent at each step, and the
corresponding runoff volume reduction benefits (estimated as percentage to the maximum control) was
estimated using the SUSTAIN model. The BMPs were set up to treat runoff from all of the identified
existing imperviousness at each size step. When the implemented BMP size was increased to be equal to
3
the optimal BMP size (100 percent) at 1,642,648 ft, the corresponding runoff volume would achieve the
control target of 20.19 million cubic feet per year and resulting in runoff conditions that match the pre-
development runoff of 4.51 million cubic feet per year.

Results of the phase implementation analysis are illustrated in Figure 5-4. The red line in the figure
represents the accumulative percentages of retained runoff volume as opposed to the control target of
20.19 million cubic feet per year. The blue line represents the step increase of retained runoff volumes
that are expressed in percentages. Both lines share the same x-axis that shows the implemented BMP size
as percentages to the optimal BMP size. As shown, cumulative percentages of retained runoff volume

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 47

increase as the BMP size increases at each size step. The trend of step increase in runoff volume retention,
at the same time, tends to level off as the BMP size increases. For example, when BMPs are implemented
3
as the first 10 percent of the optimal BMP size (or 164,265 ft), the corresponding increase in runoff
retention is about 30 percent. When BMPs are implemented as the last 10 percent of the optimal BMP
size, the same sizes of BMPs can only result in less than 5 percent of runoff retention.

The phased implemented analysis demonstrates that the trend of diminishing returns in runoff volume
retention as BMP sizes increase is also applicable to the Seymour Johnson AFB. As compared to the
relatively linear relationships between new imperviousness and BMP sizes in implementing Option 2a
and Option 2b approaches, the leveling off effects in Option 2c analysis could be accounted for through a
practical phasing scheme.

Figure 5-4. Accumulative and step increase in percentages of runoff volume being retained as the BMPs are
implemented in a 10 percent size step

The 10 percent step increase of BMP size as shown in Figure 5-4 is aggregated to three major phases: 30
percent, 60 percent, and 100 percent, and the corresponding BMP costs and treatment benefits are
compared to those from Option 2b. Results from the comparison are summarized in Table 5-9 below. The
treatment benefits are expressed in acres of impervious surfaces that the BMPs can treat. The
imperviousness can be treated through Option 2c is estimated assuming a linear relationship between

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BMP size and impervious surfaces at the three ratios, so as to the effects of Option 2b and Option 2c can
be compared side-by-side.

Table 5-9. Comparison of the BMP cost and treatment benefits between Option 2b and the three phase
implementation scheme for Option 2c
Ratio between Corresponding new imperviousness
implemented BMP Total BMP costs that can be implemented (ac)
(million $)
size to optimal
Option 2b Option 2c
BMP size
49.5 (30%) 95.7(58%)
30%
5.42
99 (60%) 137 (83%)
60%
10.84
167 (100%) 167 (100%)
100%
18.07

As shown in Table 5-9, at the BMP implementation level of 30 percent of the optimal BMP size, Option
2b can treat impervious surfaces of 49.5 acres, which is also 30 percent of the new developments in the
base, whereas Option 2c can treat up to 95.7 acres of new imperviousness based on the runoff retention
benefits shown in Figure 5-4. This results in a 28 percent difference in the area of imperviousness
treatment. When the BMP implementation level is increased to 60 percent, Option 2b can accommodate
up to 99 acres of new imperviousness, which is also 60 percent of the future imperviousness at the base.
Option 2c, at the same time, can accommodate up to 137 acres of new imperviousness, which is equal to a
23 percent increase in the area that can be treated. The two approaches eventually meet the same target,
where the total BMP size and the total imperviousness being treated are the same. The results of this
analysis are also illustrated through Figure 5-5.

Figure 5-5. Comparison of allowable new imperviousness between Option 2b (project-by-project) and Option

2c (phased) as in a three-step implementation scheme.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 49

The imperviousness that can be accommodated through Option 2b and Option 2c are represented as the
solid line and the dotted line in Figure 5-5, respectively. As shown, the allowed imperviousness through
Option 2c is always higher than that through Option 2b, and the two lines meet at the same end point of
167 acres. The differences in allowable imperviousness between the two approaches at the 30 percent and
60 percent levels of BMP implementation show that while both approaches eventually achieve the same
goal, Option 2c could bring significant cost savings over Option 2b in early stages of the implementation
process.

Following Option 2c of complying with the EISA regulations, BMPs will be built in three main batches in
subwatersheds with high infiltration to accommodate runoff from new developments. In the first batch,
3
BMPs will be built with the size of 492,794 ft(30 percent) and will treat all the identified existing
imperviousness in those subwatersheds. With this BMP size, the allowable new development in the base
is 95.7 acres. As soon as the new developments exceed 95.7 acres, the BMP sizes will be increased in a
3
second batch, which has an additional BMP volume of 492,794 ft. In correspondence to the additional
BMP volume, a total of 41.3 acres of new imperviousness can be built in the base, resulting in the total
imperviousness allowed to be 137 acres. As soon as the new developments exceed 137 acres, the final
3
batch of BMPs will be built with the volume of 657,060 ft. The final batch of BMPs will be able to
accommodate all additional developments in the base up to the 167 acre limit.

5.5 SUMMARY

A total of four EISA compliance options were investigated and compared to each other at the Seymour
th
Johnson AFB. While the Option 1 approach of retaining runoff from the 95percentile of 24-hour of
rainfall was easy to implement, the approach was not as cost-effective as the three variations following
the Option 2 approach of continuous simulations. Among the Option 2 approach variations investigated,
the Option 2a of treating the new imperviousness at where the development occurs was able to achieve
about $5.3 million of cost savings over the Option 1 approach. The cost savings were increased to about
$10.4 million when existing imperviousness in high infiltration subwatersheds were tapped into in Option
2b. If the BMPs identified through Option 2b were implemented through a phased approach, as
demonstrated in Option 2c, cost savings at the initial stage of the implementation were also
substantial.

Results from the compliance alternative analysis are very informative for developing a LID toolbox that
can bridge the analysis results with BMP implementations on the ground. Detailed discussions on the
development of the LID toolbox are provided next in Section 6.

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6.0 IMPLEMENTATION STRATEGY
The implementation strategy of tapping into (retrofitting) existing imperviousness at high
infiltration subwatersheds and implementing BMPs in three major phases have demonstrated
most substantial cost- savings over the other three alternatives at Seymour Johnson AFB. The
strategy is thus selected for developing the LID Sizing Toolbox for the base, and the developed
toolbox is presented in this section. Example applications of the LID Sizing Toolbox are provided in
Appendix A and Appendix B.

6.1 OVERVIEW OF TOOLBOX CAPABILITIES

The LID Sizing Toolbox for Seymour Johnson AFB needs to have several major capabilities. First of all,
the tool needs to be able to account for the demolition credit. As previously discussed, land uses at
the year of 2007 are set as the existing conditions for the base. A total of 69.3 acres of imperviousness
were demolished on the base between 2007 and 2011, creating a demolition credit of 69.3 acres. Thus,
the tool needs to ensure that no BMP implementation is needed before the demolition credit is
exhausted for the base.

Second, the LID Sizing Toolbox needs to be able to track developments on the base. As the tool to
assist the overall stormwater management at the base, the tool has to be able to record each and every
BMP implementation activity on the base. This is important not only for retaining records of
compliance, but also for accurately following the phased implementation strategy selected for the base.

Third, the LID Sizing Toolbox has to be able to execute the phased implementation strategy for the
base, which involves both tapping into the existing imperviousness at high infiltration watersheds
and implementing BMPs at different phases. As shown in previous analyses, there are three major
phases in the implementation strategy, in which the BMPs are implemented at 30%, 60%, and 100%
levels. The BMP to new impervious area ratio as well as the new impervious area to the existing
imperviousness to be tapped at each phase are different. In reality, the BMPs are often not
implemented through a batch mode of building BMPs for the whole phase at once, but rather through
individual projects. This means that the LID Sizing Toolbox needs to be able to: 1.Retain the ratio
between the BMP area and the new imperviousness area at each major phase, and 2. Retain the ratio
between the new imperviousness area and the existing imperviousness to be tapped at each major phase.
In addition, the toolbox will need to be able to automatically go through the subwatersheds list from high
infiltration to low infiltration during the assignment of existing imperviousness to be tapped.

Besides the above major capabilities, the toolbox is also expected to provide a user-friendly interface
within Microsoft Excel, to have graphic display of the base in displaying subwatershed characteristics, to
be flexible in allowing user-specified BMP designs, and to generate documents that facilitate the
compliance approval process.

6.2 LID SIZING TOOLBOX DEVELOPMENT
An LID Sizing Toolbox was developed in the Microsoft Excel 2010 environment to help implement the
sizing strategy for Seymour Johnson AFB. The interfaces of the toolbox and their functionalities in
connection with the toolbox capability requirements are introduced in the following subsections.

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6.2.1 Main Interface

The main interface for the toolbox is shown in Figure 6-1. As shown, the main interface is where the user
inputs project metadata information and proposed impervious development area.

In the projection information section, the user can specify information including project number, fiscal
year, project budget, a short description of the project, and most importantly, the imperviousness footprint
of the project in square foot. There is a “Confirm” button next to the imperviousness footprint input. By
clicking the “Confirm” button, the user-supplied inputs are verified, and a confirmation window will pop
up to direct the user to choose the subwatershed where the new development will occur. There is also a
“Check current level of development” button that allows the user to check the log of current
developments and associated BMP implementations in the base.

Figure 6-1. Main interface of the LID Sizing Toolbox for Seymour Johnson AFB.

After the user verifies the project information input on the main interface, a click of the “Select Project
Subwatershed” button will bring the user to the page of “Subwatershed Characteristics.”

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6.2.2 Subwatershed Characteristics Interface

The interface for “Subwatershed Characteristics” is shown in Figure 6-2. As shown, the interface has a
map for the Seymour Johnson AFB with subwatershed delineations. On the right-hand of the page, there
is a “Subwatershed Characteristics” section, in which basic information of subwatershed total area, 2007
(existing condition) imperviousness, 2011 (after demolition) imperviousness, maximum of increase in
imperviousness, average infiltration rate, slope, etc. is listed upon user-specification of subwatershed
name. The user-specified subwaterhsed name is where the proposed development will occur. A figure is
also provided quantifying the availability of existing impervious area to tap and route to new BMPs.

Figure 6-2. Interface for the presentation of subwatershed characteristics.

Two buttons are provided on the “Subwatershed Characteristics” page, one is the “Back to Main Menu”
button that takes a user back to the main interface, and the other is the “Select and Develop BMPs” button
that leads the user to the “BMP Implementation” page.

6.2.3 Interface for BMP Implementation

The interface for BMP implementation is shown in Figure 6-3 below. The interface automatically imports
the user-specified subwatershed name in which future development occurs and the imperviousness foot
print entered through the main interface. The analysis results of subwatershed to implement BMP, and the
existing imperviousness to be tapped are automatically populated. The subwatershed(s) to implement
BMP is automatically expanded from high infiltration subwatersheds to include low infiltration
subwatersheds as the tapping continues. The subwatersheds displayed in parentheses represent
subwatersheds in which available impervious area has been exhausted within the given phase and no
longer presents an opportunity for BMP implementation. In larger projects, impervious areas may need to
be tapped in multiple subwatersheds; the order in which BMPs are implemented in various subwatersheds
is arbitrary as long as the total recommended tapped impervious area is routed to new BMPs of
proportional size during each project.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 53

The user cans next select their preferred BMP from the dropdown list based on the recommended
subwatersheds and extent of tapped impervious. Available BMP types include bioretention, sand filters,
infiltration basins, infiltration trenches, and permeable pavement. After a BMP type is selected, the user
can decide to apply the predefined BMP parameters assumed in the Toolbox or click on the “1.Custom
BMP Design (Optional)” button to customize the BMP configuration. If the predefined configuration is
preferred, the “2.Calculate BMP Surface Area” button is clicked to perform the BMP sizing calculation,
in which the phasing of developments is automatically incorporated. That is, as the phase of development
increases from 1 to 3, the internal BMP area to new development ratio, and the ratio between new
development and the tapped existing imperviousness are automatically adjusted based on Section 5
results.

Figure 6-3. Interface for performing the BMP implementation analysis.

A click on the “1.Custom BMP Design (Optional)” button brings the user to the interface for specifying
localized BMP designs, which is introduced in the next subsection. A click on the “3.Update BMP

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Tracking Log” button will bring the user to the summary page of “3.Update BMP Tracking Log,” which
will be introduced in subsection 6.2.4. A click on the “4.Certificate of Applicability” button will bring the
user to the summary page of “4.Certificate of Applicability,” which will be introduced in subsection 6.2.6.

The “BMP Implementation” page also provides two buttons for internal navigation: the “Back to Main
Menu” button leads the user back to the main interface, and the “Back to Subwatershed Characteristics”
button allows the user to go back to the subwatershed characteristics page.

6.2.4 Interface for BMP Design Specification

The interface for BMP Design Specifications is shown in Figure 6-4. As shown, the interface has built-in
designs of layers for different types of BMPs. The “Customized Depth (inches)” column is where the user
can enter localized BMP designs, while observing the recommended ranges of the design parameters. The
default maximum effective depth for the BMPs are 21 inches, and a warning message will be given when
the user-specified designs result in effective depths that are larger than 21 inches.

Figure 6-4. Interface for user-specified BMP design inputs.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 55

The interface provides buttons that link to BMP design cut sheets that follow the user specifications. One
example bioretention design cut sheet is shown in Figure 6-5. As shown, the cut sheet includes a cross-
section view of the BMP, along with specifications for each layer. The cut sheet is in a printer-ready
format that base engineers can use for preliminary site designs.

On each BMP design cut sheet, there is a button that allows the user to navigate back to the BMP Design
Specification interface. On the BMP Design Specification interface, there is also a “Back to Select and
Develop BMPs” button that leads the user back to the “BMP Implementation” interface.

Figure 6-5. Example bioswale design cut sheet.

6.2.5 Interface for BMP Tracking Log

The tracking section includes a record of project inputs and outputs. When a new impervious
development is added (through clicking the “Confirm” button), a new line of record is added to the
tracking section. For each new development, the tracking section automatically assigns a sequential
number, the project area is converted into acres, the current phase of development, the current date, and a
user-input of personnel name. The current phase of development is automatically assigned based on the
current total area of development, and this is where the demolition credit is accommodated and the phased
approach is initiated. When the total area of developments is less than 69.3 acres, each individual
development is assigned with the phase name of “Phase 0,” during which no new BMP will be

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 56

implemented because demolition compensation credit is used to offset new development. When the total
area of developments is larger than 69.3 acres and less than 165 acres (95.7 acres of net increase from
existing condition), individual developments are assigned with the phase name of “Phase I.” When the
total area of developments is larger than 165 acres and less than 206.3 acres (137 acres of net increase
from existing conditions), individual developments are assigned with the phase name of “Phase II.” When
the total area of developments is larger than 206.3 acres and less than 236.3 acres (167 acres of net
increase from existing conditions), individual developments are assigned with the phase name of “Phase
III.” When the total area of development is larger than 236.3 acres, developments are assigned with the
phase name of “Maximum development exceeded,” in which no BMP sizing will be performed because
the development exceeds the built-out level of imperviousness in the base.

There is also a “Delete” button in the tracking section of the main interface. Through the “Delete” button,
users can delete experimental BMP sizing analysis records. The deletion is carried out through first
checking the individual record(s) to be deleted and then clicking the “Delete” button. The resulting phase
assignment is also updated. Note that input of projects that span two phases may result in an error in the
Toobox – in such a situation, the individual project should be divided into two projects such that
development does not span two phases.

Figure 6-6: Tracking log interface.

6.2.6 Interface for Certificate of Applicability

When the user clicks on the “4.Certificate of Applicability” button on the “BMP Implementation”
interface, a certificate of applicability is generated. The certificate of applicability summarizes all the
user-supplied information about the project (project number, fiscal year, description imperviousness
footprint, etc.), the subwatershed where the development occurs, the subwatershed where the BMP is
built, the size of the BMP, the existing imperviousness to be tapped, and the BMP cross-sectional design.
All of the information is provided in a printer-ready format, which will substantially help with both the
documentation purposes and the compliance approval process.

The “Certificate of Applicability” interface consists of two pages, which are shown in Figure 6-6 and
Figure 6-7. Each page has a “Back” button through which the user is led back to the “BMP
Implementation” interface.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 57

Figure 6-7. Example Certificate of Compliance page 1.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 58

Figure 6-8. Example Certificate of Compliance page 2.

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7.0 REFERENCES
Barr Engineering. 2011. Best Management Practices Construction Costs, Maintenance Costs, and Land
Requirements. Prepared for: Minnesota Pollution Control Agency. Prepared by: Barr Engineering,
Minneapolis, MN.

Center for Watershed Protection (CWP). 2007. Manual 3: Urban stormwater retrofit practices, Version
1.0. Appendix E, Derivation of Unit Costs for Stormwater Retrofits and New Stormwater Treatment
Construction. Center for Watershed Protection, Ellicott City, MD.

Department of Defense (DoD). 2010. Memorandum, DoD Policy on Implementing Section 438 of the
Energy Independence and Security Act (EISA). Office of the Under Secretary of Defense (DoD),
Washington, DC.

Hunt, W.F., and A. Lucas. 2003. Development of a Nutrient Export Model for New Developments in the
Tar-Pamlico River Basin. Biological & Agricultural Engineering, North Carolina State University.
Prepared for The North Carolina Department of Environment & Natural Resources, Tar-Pamlico
Stormwater Group.

North Carolina Division of Environment and Natural Resources (NCDENR) Division of Water Quality.
2003. Assessment Report: Biological Impairment in the Stoney Creek Watershed. Neuse River
Basin, Wayne County, NC.

NCDENR. 2009. North Carolina Stormwater BMP Manual.

NCDENR. 2011. Seymour Johnson AFB NPDES Stormwater Discharge Permit. Division of Water
Quality, Raleigh, NC.

Sample, D.J., J.P. Heaney, L.T. Wright, C.Y. Fan, F.H. Lai, and R.F. Field. 2003. Costs of best
management practices and associated land for urban stormwater control. Journal of Water Resources
Planning and Management 129(1):59–68.

Simmons, C.E. 1993. Sediment characteristics of North Carolina streams, 1970-79. U.S. Geological
Survey Water-Supply Paper 2364.

Tetra Tech. 2012. Goose Creek and Crooked Creek Watersheds: Model Development and Calibration.
Prepared for Centralina Council of Governments and North Carolina Division of Water Quality.

UFC (Unified Facilities Criteria). 2010. Unified Facilities Criteria (UFC): Low Impact Development.
Department of Defense, Washington, DC.

URS. 2006. Storm Sewer Drainage System Study. Seymour Johnson AFB. Contract No. FA4890-04D-
005-BW04.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 60

U.S. Department of Agriculture, Soil Conservation Service (USDA-SCS). 1986. Technical Release #55,
Urban Hydrology. U.S. Department of Agriculture, Soil Conservation Service, Washington, DC.

U.S. Environmental Protection Agency (USEPA). 1999. Preliminary data summary of urban stormwater
best management practices. EPA-821-R-99-012, Office of Water, Washington, D.C.

USEPA. 2009a. Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal
Projects under Section 438 of the Energy Independence and Security Act. U.S. Environmental
Protection Agency, Washington, DC.

USEPA. 2009b. Optimal Stormwater Management Plan Alternatives: A Demonstration Project in Three
Upper Charles River Communities. U.S. Environmental Protection Agency, Boston, MA.

Virginia Department of Conservation and Recreation (VADCR). 1999. Virginia Stormwater Management
Handbook, Volume II. Division of Soil and Water Conservation. Richmond, VA.

Weiss, P.T., J. S. Gulliver, and A. J. Erickson. 2005. The Cost and Effectiveness of Stormwater
Management Practices. Minnesota Department of Transportation Report 2005-23.

Wossink, A., and W. Hunt. 2003. The Economics of Structural Stormwater BMPs in North Carolina,
UNC-WRRI-2003-344.

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APPENDIX A: EXAMPLE APPLICATION OF THE LID TOOLBOX AT A
BASE-WIDE SCALE
The following example demonstrates use of the LID Toolbox for facility-wide master planning efforts at
Seymour Johnson Air Force Base.

Disclaimer: All example project details are hypothetical and are presented only for demonstration
purposes. Spatial representation of development, impervious areas, and BMP sizes are not necessarily
to scale and are provided only to illustrate the concepts.

PROJECT 1, STEP 1: DEFINE PROJECT IMPERVIOUS AREA

Suppose that 69.3-ac of new impervious development are proposed onsite for an Air Force museum
complex (Figure A1). Project details are input to the Main Menu screen and the “Confirm” button is
clicked to read the data into the Toolbox (Figure A2).

Figure A1: Proposed project 1 development

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Figure A2: Input Project 1 metadata and impervious footprint

PROJECT 1, STEP 2: SELECT PROJECT SUBWATERSHED

For project tracking purposes, the project subwatershed is selected on the next screen (Figure A3).
Subwatershed hydrologic details are reported for reference. The “Select and Develop BMPs” button is
clicked to advance to the next screen.

Figure A3: Select project subwatershed from dropdown list.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 63

PROJECT 1, STEP 3: SELECT AND DEVELOP BMPS

This development project takes advantage of the “demolition credit” provided as a result of historic
removal of impervious surfaces at Seymour Johnson Air Force Base; therefore, no BMPs are required for
this project (Figure A4). The “Update BMP Tracking Log” button is clicked to print this project to the
tracking log.

Figure A4: Toolbox notice when demolition credit is applied.

PROJECT 1, STEP 4: CONFIRM PROJECT TRACKING LOG

The project tracking log screen automatically tracks BMP projects input into the LID Toolbox (Figure
A5). Phase and cumulative impervious area are tabulated for the user.

Figure A5. Project tracking log during the demolition compensation phase

PROJECT 2, STEP 1: DEFINE PROJECT IMPERVIOUS AREA

The next set of hypothetical projects consist of constructing a new research center, training facilities, and
barracks at multiple locations throughout the site (Figure A6). Because the Toolbox is used for planning
purposes, these projects are combined into one “aggregate” project totaling 95.7 acres (Figure A7).

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 64

Figure A6. Hypothetical development during Project 2.

Figure A7: Input Project 2 metadata and impervious footprint

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PROJECT 2, STEP 2: SELECT PROJECT SUBWATERSHED

Because the hydrology of the entire base was considered during background modeling efforts, BMPs need
not be constructed in the same subwatershed where development is occurring. For this reason, selection of
the project subwatershed does not affect model output – it is only for tracking purposes. Because Project 2
is divided among three subwatersheds, the subwatershed with the most development was arbitrarily
selected (Figure A8).

Figure A8: Select project subwatershed from dropdown list.

PROJECT 2, STEP 3: SELECT AND DEVELOP BMPS

Now that the demolition credit (69.3 ac) has been exhausted by Project 1, BMPs are required to mitigate
the hydrologic impacts of increased impervious area. This screen recommends subwatersheds to
implement BMPs and corresponding total impervious area to tap into and route to new BMPs
(Figure A9). As projects are added incrementally, the Toolbox suggests subwatersheds based on
descending average infiltration rates (estimated from hydrologic soil group). Using this strategy,
BMPs are first placed on highly permeable soils.

The impervious area to tap and route to BMPs in each subwatershed is based on planning-level spatial
analyses of the base. As available impervious area in each subwatershed is exhausted, the Toolbox
advances to the next subwatershed (from right to left in the list shown in Figure A9. Subwatersheds with
no remaining available imperviousness are shown in parentheses. Impervious areas can be tapped in any
combination of the listed subwatersheds as long as the total tapped area equals the “Existing
Imperviousness to be tapped (ac)” output by the model.

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Figure A9: Select and develop BMPs for Project 2

Once the subwatersheds and extent of tapped imperviousness are known, a conceptual BMP template can
be selected to best treat the given drainage areas. Predefined BMP designs can be selected from the
dropdown menu or custom configurations can be input by clicking on the “1. Custom BMP Design
(Optional)” button (Figure A10). Gravel and media depths can be adjusted on the custom BMP layout
screen to meet site constraints. BMP cutsheets are provided for reference.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 67

Figure A10. Customize BMP Design (optional)

PROJECT 2, STEP 4: CONFIRM PROJECT TRACKING LOG

Once a predefined or custom BMP configuration is chosen, the required BMP footprint for the given
design is output. This BMP footprint should be distributed proportionally among the suggested
subwatersheds based on the impervious area to be tapped. Note that BMPs should be sited and
implemented only on hydrologic soil group A and B soils to comply with the assumptions of the
Toolbox. By clicking the “3. Update BMP Tracking Log” button, the BMP details are printed to the log
on the next screen (Figure A11). The log indicates that this project falls into Phase 1.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 68

Figure A11. Project tracking log during Phase 1

PROJECT 2, STEP 5: PRINT CERTIFICATE OF APPLICABILITY FOR RECORDS

A Certificate of Applicability is generated when the “4. Print Certificate” button is clicked (Figure A12);
this certificate summarizes relevant project information and provides example BMP dimensions for
compliance with the proposed development.

Figure A12: Example Certificate of Applicability

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PROJECT 2, STEP 6: CONCEPTUAL DESIGN

The Toolbox presents recommendations to support planning efforts. In this example, the Toolbox
recommends an impervious surface totaling 167 ac needs to be tapped and BMPs implemented
throughout the suggested subwatersheds to compensate for the 95.7 ac of hypothetical imperviousness
proposed in this Project 2. Figure A13 shows a representation of the impervious area tapped and
associated BMPs implemented at the completion of Phase 1. Figure A14 shows the base-wide cost
savings associated with tapping into and treating impervious areas using a phased strategy instead of
using a proportional, project-by-project approach.

Figure A13: Conceptual plan showing impervious area tapped and BMP locations during Phase 1 (not to
scale)

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58% of total new development,
30% of total BMP area,
Allows 28% smaller BMP for given
new impervious ($5.1M savings)

Figure A14. Base-wide cost savings during Phase 1

PROJECT 3, STEP 1: DEFINE PROJECT IMPERVIOUS AREA

The next hypothetical project consists of constructing a new runway totaling 41.3 ac of new impervious
surface (Figure A15 and A16).

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Figure A15. Hypothetical development during Project 3.

Figure A16: Input Project 2 metadata and impervious footprint

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PROJECT 3, STEP 2: SELECT PROJECT SUBWATERSHED

The subwatershed in which development occurs is selected for tracking purposes.

PROJECT 3, STEP 3: SELECT AND DEVELOP BMPS

Phase 2 commences because, at the completion of this project, a total of 206.3 acres of new impervious
will have been constructed at the base. The Toolbox will therefore reset the list of available
subwatersheds with impervious areas to tap. As new projects are added in Phase 2, the Toolbox steps
through the list of subwatersheds and recommends new BMPs and associated impervious drainage areas
within each subwatershed (Figure A17).

From a practical perspective, these new BMPs will typically represent expansion of existing BMPs
constructed during Phase 1. At the completion of Phase 2, the entire tapped impervious area within each
suggested subwatershed (totaling 167 ac) will be draining to BMPs that are sized 60% of the size required
to mitigate the hydrology from the entire 167 ac of tappable impervious drainage area.

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Figure A17: Select and develop BMPs for Project 2

PROJECT 3, STEP 4: CONFIRM PROJECT TRACKING LOG

By clicking the “3. Update BMP Tracking Log” button, the BMP details are printed to the log on the next
screen (Figure A18).

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Figure A18. Project tracking log during Phase 2

PROJECT 3, STEP 5: PRINT CERTIFICATE OF APPLICABILITY FOR RECORDS

A Certificate of Applicability is generated when the “4. Print Certificate” button is clicked; this certificate
summarizes relevant project information and provides example BMP dimensions for compliance with the
proposed development.

PROJECT 3, STEP 6: CONCEPTUAL DESIGN

Cost effective implementation of the suggested new Phase 2 BMPs will typically occur by expanding the
size of Phase 1 BMPs. This strategy is represented in Figure A19 by expanding the footprint of the
existing BMPs. The tapped impervious area at the end of Phase 2 remains the same as at the end of Phase
1. Figure A20 shows the base-wide cost savings associated with tapping into and treating impervious
areas using a phased strategy instead of using a proportional, project-by-project approach.

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Figure A19: Conceptual plan showing impervious area tapped and BMP locations during Phase 2 (not to
scale)

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82% of total new development,
60% of total BMP area,
Allows 22% smaller BMP for given
new impervious ($4.2M savings)

Figure A20. Base-wide cost savings during Phase 1

PROJECT 4, STEP 1: DEFINE PROJECT IMPERVIOUS AREA

The next hypothetical project consists of constructing a new aircraft hangar totaling 30 ac of new
impervious surface (Figure A21 and 22).

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Figure A21. Hypothetical development during Project 4.

Figure A22: Input Project 2 metadata and impervious footprint

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PROJECT 4, STEP 2: SELECT PROJECT SUBWATERSHED

The subwatershed in which development occurs is selected for tracking purposes.

PROJECT 4, STEP 3: SELECT AND DEVELOP BMPS

Phase 3 commences because, at the completion of this project, a total of 236.3 acres of new impervious
will have been constructed at the base. The Toolbox will therefore reset the list of available
subwatersheds with impervious areas to tap. As new projects are added in Phase 3, the Toolbox steps
through the list of subwatersheds and recommends new BMPs and associated impervious drainage areas
within each subwatershed (Figure A23).

As was recommended during Phase 2, these new Phase 3 BMPs will typically represent expansion of
existing BMPs constructed during Phase 1 and 2. At the completion of Phase 3, the entire tapped
impervious area within each suggested subwatershed (totaling 167 ac) will be draining to BMPs that are
sized 100% of the size required to mitigate the hydrology from the entire 167 ac of tappable impervious
drainage area.

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Figure A23: Select and develop BMPs for Project 2

PROJECT 4, STEP 4: CONFIRM PROJECT TRACKING LOG

By clicking the “3. Update BMP Tracking Log” button, the BMP details are printed to the log on the next
screen (Figure A24).

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Figure A24. Project tracking log during Phase 3

PROJECT 4, STEP 5: PRINT CERTIFICATE OF APPLICABILITY FOR RECORDS

A Certificate of Applicability is generated when the “4. Print Certificate” button is clicked; this certificate
summarizes relevant project information and provides example BMP dimensions for compliance with the
proposed development.

PROJECT 4, STEP 6: CONCEPTUAL DESIGN

Phase 3 implementation strategy is represented in Figure A25 by expanding the footprint of the existing
BMPs. In some situations, new BMPs would have to be constructed where previous BMPs have reached
their maximum size (as determined on a site-by-site basis). Figure A26 shows a graphical representation
of the base-wide implementation strategy.

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Figure A25: Conceptual plan showing impervious area tapped and BMP locations at the end of Phase 3 (not
to scale)

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Figure A26: Graphical representation of base-wide implementation strategy

PROJECT 5, STEP 1: DEFINE PROJECT IMPERVIOUS

If additional impervious surface is input to the Toolbox in excess of 236.3 ac, the Toolbox reports an error
warning the user that the maximum allowable development has been exceeded (Figure A27 and A28). At
this point, the Toolbox is no long a useful decision support tool.

Figure A27: Define project metadata and impervious footprint

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Figure A28: Error message when total developed impervious exceeds 236.3 ac

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 84

APPENDIX B: EXAMPLE LID TOOLBOX SITE-SCALE PROJECT
In the preceding example Toolbox application (Appendix A), the Toolbox was demonstrated at the base-
wide scale. This example will present how the concepts of phased tapping of excess impervious areas can
be applied to develop a site-scale conceptual plan in subwatershed 15.

Disclaimer: All example project details are hypothetical and are presented only for demonstration
purposes. Conceptual plans are based on planning-level assumptions and field verification should be
performed.

STEP 1: IDENTIFY IMPERVIOUS AREAS TO TAP AND AVAILABLE BMP AREA

Using the Toolbox, the recommended impervious area to tap and BMP sizes are reported for each
subwatershed and phase. Once this information is reported for a given project, the user will have to
identify specific impervious areas to tap within each subwatershed. In this example, a parking lot along
Cannon Avenue was identified as one possible location to retrofit (Figure B1). Such as retrofit could be
performed individually to offset a single new development, or could be incorporated as one component of
a subwatershed-scale plan (as presented in Appendix A).

Identifying impervious area to tap is an iterative process because the designer must ensure that tapped
area can be drained to a BMP for treatment. For this reason it is beneficial to simultaneously identify
impervious areas to tap and associated space for BMP implementation.

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 85

1.94 ac impervious

drainage area

0.185 ac available open

space for BMPs

Figure B1: Identifying impervious area to tap and available BMP areas for a parking lot.

STEP 2: GENERATE BMP IMPLEMENTATION SCHEDULE

Once the total tapped drainage area is known, the sizing ratios used in the Toolbox can be used to size
BMPs for each development project (if implementing as part of a larger master plan). The ratios between
new development, tapped impervious area, and new BMP footprint remain constant throughout each
phase of development; these ratios, as shown in Table B2, can be used to create a conceptual BMP
implementation schedule. . If implementing BMPs to offset an individual development project, the
Toolbox will output the specific tapped impervious and BMP areas. An example Certificate of
Applicability for Phase 1 of this project is shown in Figure B2 to demonstrate how the model output can
be translated to a conceptual plan on a project-by-project basis.

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Table B1: Example phased BMP schedule for parking lot
New BMP :

Tapped Drainage Area : Tapped New
Phased BMP
Phase Tapped Drainage New Impervious Area Drainage Impervious
Footprint (sf)
Area Ratio) Ratio Area (acre) Offset (ac)

1 0.018 1.745 1.11 1521

1.94
4.044
2 0.018 0.48 1521

3 0.024 5.567 0.35 2028
Total 1.94 5070

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 87

Figure B2: Certificate of Applicability for parking lot example

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 88

STEP 3: CREATE BMP CONCEPTUAL DESIGNS

Once the implementation schedule is generated, conceptual designs can be developed for each phase for
project. Retrofits should focus on use of existing drainage elements, such as use of curb cuts and existing
catch basins to control surface ponding (Figure B3). Creative designs may be necessary to route the
desired impervious drainage area to locations where BMPs are proposed. An example conceptual design
is shown in Figure B4 and the associated potential cost savings are illustrated graphically in Figure B5.

Figure B3: Using curb cuts and existing catch basins to control surface ponding in a parking lot bioretention

retrofit

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 89

Figure B4: Conceptual plan for phased BMP implementation

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 90

Figure B5: Graphical representation of site-scale implementation strategy

Comprehensive Watershed Protection Plan Seymour Johnson AFB Page 91

Only
2007 DATA
Existing Paved 2014 CEI Current 2011 2007
TETRA TECH TABULATIONS
2014 DATA -
Rec. Athletic Airfield Canopy/ Structures Parking Vehicle Total ACRES IMPERVIOUS IMPERVIOUS
Sub-BasinRec. TrailMathRoadPoolAthletic CourtsAirfield SurfacesCanopy/PavilionPlaygroundsExisting StructuresSlabsParking LotsSidewalksAddn' SidewalksMathVehicle DrivewaysSub-basin TotalAcres

CIP3.0
TrailRoadPoolCourtsSurfacesPavilionPlaygrounds(Buildings )SHEDSSlabsLotsSidewalksDrivewaysIMPERVIOUS ACRESACRES
1042671.411883131415684.9149779185358.8192320201714159619.14123.7
TOTAL ACRES3.58140.040.172.15344.360.680.0798.880.526.13142.1021.1817.36777.22778.8848.1
2077537.9755157.46832429150836.6921314013072330347.13317.6

3067733.371053734.3221182.22262311081.3108595069261143875.22326.3
40122402.9847009.973615479.594420196111004945.45223.1
5062605.551266956.382750.715 Credit 816 70.8869.3 08321331960.64530.6

6092545.619968.386379280.878608480200274.86774.6
7056275.166704.330641400.7706167007408113458.26342.6
8019333.1726611054.005653655.18171943086251.36042.0

90286549.818513219494.7561293344419.934660256655420.397110518154.8019774.009586944589270.856105.4
100121584.114672733200.75830797660.9748561123509.70971336933.0273165.1365215873113411.7271.5

110274993.11754009.91310922.732220063513.79324800127332118852.54148.6
120194538.73952142.692424230141.25616423207525.2371102905471.986827359.93471504635994.867106.4
131808.94375426.8311168202.8763581977225433.28473352324134.83471484892400.041412000.2077144972518.003822.3
APPENDIX C: IMPERVIOUS SURFACE ACRE CALCULATIONS (SJAFB GIS DATA)
146510.889519532.6685517535.73144226146274.58849144289889.25825340616735.96384163020954.2118104771.06513501744778.21140.1
150679902.372226643391270.6056164045601947268.7339101191678061.6632097266714.519333572.597714173816997.987.6
160287298.87335222561223.9987316121474.80491975927.0366135.1839141508908.744111.7

17618.53341855.6002233070.3181811266207095.3965349265570.12544174164.3573821.786531968601131.223213.8
180555369.3120309022858815531.58939414125298.41097803317340.34886701.74242337194766344.7
190372009.5628018877601585.91032065416341.406681707.0331162161217329.46627.9

201201.73893605.2167899299.610465297341148528.3667244.92865157933358.715166793.582438162561065.65458.8
211414.77844244.33521503272433107563.9781113778555616.53343095028532965589.838322.2
223512.656110537.9683275054.320095443640426638.9266124.23739418050.867640254.33873939839682.775319.3
234008.903712026.7111576334.9104283694603517233.22511807950627.28741380994387.576921937.885646172298083.02152.8
2417048.146751144.4401137472.653242081407488602.3373277251178.473821101081166642420.85514.7
Total area from Tt calcs (ac):2.49143.950.175.60343.530.932.31143.386.00141.7318.8413.2525.9436944430.76848.1
Measurements in sq ft.
Math was performed on two fields Addn'l sidewalks
Notes:
1. Tt used 1 ft as the cell size during the impervious polygon tabulation.
2. A summary of the "Road_area" layer shows that the total area for the layer is 144.1 acres, which is less than the total area of 147.44 acres shown in the base calculation.
3. A summary of the "Existing_Structures" layer shows that the total area for the layer is 143.38 acres, which is less than the total area of 144.74 acres shown in the base calculation.

Delete
SelectProject Subwatershed
Confirm Version 1.0
for Seymour Johnson Air Force BaseLow Impact Development Toolbox

APPENDIX D: TETRA TCHTOOLBOX PROJECT TRACKING SOFTWARE
Information of new development projects:Tracking of BMP implementations
Base:Seymour Johnson AFB Area
No.UnitDescriptionDateYearReviewerFundedCostBasinEISA / MS4Project #PhaseContact
VKAG053006R1
New Hospital Clinic (2013),
0.6166
Project No.:
1Acres26858.2 sf. 2/5/2015FY2013Ronnie WilsonYes$53,391,02423YesVKAG053001CENM
916th Tanker Parking Apron
6.6
FY:2017
2AcresExpansion3/23/2015FY2015Ronnie WilsonYes$9,800,00012YesVKAG089003CENM
Description:CONSTRUCT FAMCAMP EXPANSION Construct FamCamp
0.7142
3AcresExpansion3/30/2015FY2015Ronnie WilsonYes$270,00024YesVKAG134007CENM
Total7.9308AcresDemolition Compensation
Project
31110 SF
Impervious
Footprint
Area:
Energy Independence Security Act (EISA) 2007 Section 438 requirements apply to Federal projects that construct facilities with
a footprint greater than 5,000 gross square feet (0.115 acres), or expand the footprint of existing facilities by more than 5,000
gross square feet (0.115 acres). The project footprint consists of all horizontal hard surfaces and disturbed areas associated
with the project development, including both building area and pavements (such as roads, parking, and sidewalks). These
requirements do not apply to internal renovations, maintenance, or resurfacing of existing pavements. MS4 requires
postconstruction requirements for development or redevelopment projects (including transportation projects at SJAFB) > 1 acre.

This BMP sizing tool uses the year of 2007 as the existing conditions. Between the year of 2007 to 2011, a total of 69.3 acres of
imperviousness were demolished. When 69.3 acres have been exhausted (Phase I), based on the BMP tracking, SJAFB will
recalculate the amounts of impervious surfaces for that current year and issue certifications to contractors if BMP measures or
retrofits are required. Tracking will be completed on funded projects annotated and coded in ACES.