NCS000422_SCM Inspections_20200218

WILDLANDS
ENGINEERING
MEMORANDUM
TO: Jessica Trotman, MSEH
Planning Director, Town of Black Mountain
FROM: Jacob McLean, PE
DATE: February 10, 2020
RE: Stormwater Control Measure Inspections
The Town requested that Wildlands Engineering, Inc. inspect multiple stormwater control measures
(SCMs) owned and operated by the Town in order to document their current condition and any potential
functional deficiencies and maintenance needs. All of these SCMs were constructed as grant -funded or
voluntary measures, and were not part of post -construction ordinance requirements.
Several of the SCMs were designed by Wildlands as part of a recent EPA 319 project, or were previously
inspected by Wildlands. However, two of the SCMs are sites that Wildlands had not previously visited and
for which no plans were available. Both of these sites are very simple design/layouts and inspections were
sufficient to fulfill the necessary reporting on their condition.
The following sites were inspected during January, 2020:
• Rain garden at Carver Community Center
• Rain garden at Black Mountain Public Library
• Rain garden in Town Hall Parking Lot (and rain barrels)
• Golf course bioretention / dry pond along Tomahawk Ave (adjacent to croquet court)
• Tiered bioretention cells at Town of Black Mountain Golf Course parking lot
• Stormwater conveyance and wetland cells at Lake Tomahawk Park
A short description of each site along with inspection notes, photos and recommendations is provided
within this document.
Findings
All SCMs, except the Golf course bioretention/dry pond are functioning as intended or need only minor
maintenance. Mulching and prevention of vehicular damage are notable maintenance needs in several
of the SCMs. The Golf course bioretention along Tomahawk Ave. need rehabilitation, and could benefit
from flow analysis. Wildlands is discussing the possibility of flow analysis with professors at Warren
Wilson College. Attachment 1 to this document contains general inspection and maintenance
recommendations in NC.
1

Site: Rain garden at Carver Community Center
35 °36'55.56 "N
82 °20'24.24 "W
SCM Type: Rain garden without underdrain
Classification: Non-structural / small scale
Date Constructed: 2019
SCM Description: This is a volunteer -constructed rain garden at the Carver Center with a footprint of <100
square feet (estimated). It collects rooftop runoff by routing downspouts into a shallow depression that
was excavated adjacent to the existing storm grate inlet. A berm was constructed to provide ponding
storage of approximately 1' (estimated) within the excavated depression. When the depression is full, the
berm is over -topped and excess runoff enters a grate inlet and subsequent pipe system. The pipe system
outflows to the toe of the slope in the woods 330' to the east where it flows across the buffer to the
stream. No plans are available for the SCM and it is uncertain as to the exact drainage area of rooftop
entering the practice.
Inspection Elements: Review downspout diversion connections and outlets, review SCM footprint for
erosion, check for bare soil, vegetation success.
Inspection Notes & Recommendations: The cell has areas of bare soil which could benefit from mulching.
Plants are herbaceous and were dormant at the time of inspection. Future inspections during the growing
season should document any plant mortality. No specific vegetation success criteria must be met,
however it is recommended that plants be replaced, as necessary, for aesthetic purposes. The existing soil
appears to be low in organic content and any future plant replacement should consider soil amendment
to promote successful plant growth.
2

Site: Rain garden at Black Mountain Public Library
35 °3657.01 "N
82 *1922.06"W
SCM Type: Rain garden without underdrain
Classification: Non-structural / medium scale
Date Constructed: 2019
SCM Description: This is a rain garden at the library constructed using 319 funding in 2019. It collects
primarily parking lot runoff, and possibly some rooftop runoff. It includes a shallow grassed swale that
routes flow into the west end of the constructed basin. There is a grate inlet (pre-existing) at the east end
of the basin near the rocked swale entrance. When the basin fills, water exits out the grate inlet into the
pre-existing stormwater system. The in -situ infiltration rate is such that no underdrain was installed.
Design and as -built information is available and was provided to Landy of Sky as part of 319 grant
reporting.
Inspection Elements: Review condition of mulching, particularly along cell walls which are steep, inspect
perimeter of cell and grassed and rock swale entrances to ensure flow is able to enter the cell. A primary
risk for this SCM is that of library user parking within and/or travel through the grassed area adjacent to
the basin. Review SCM footprint for erosion, bare soil, vegetation success.
Inspection Notes & Recommendations: Since construction, mulch has been replaced one time as part of
the project maintenance contract. That contract is expired, and the Town should plan on replenishing
mulch each winter, before spring, if not more frequently. Some small bare areas are present and mulch
will address these. Plants are herbaceous and some were dormant at the time of inspection, but no
obvious mortality was observed. Future inspections during the growing season should document plant
mortality, if present. Replace plantings for aesthetic purposes. There is evidence of parking along the
north edge of the basin and of wheels crossing across the corner (shown in photo). It is recommended
that the Town consider split rail fencing or other measures to reduce impacts from traffic. A gravel
shoulder may be another option, however, this could encourage use of the area for parking which is
problematic for long term aesthetics and function.
Figure 1. Rock entrance Swale. Figure 2. Apply mulch to minor bare areas.
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Figure 3. Car traffic runs over the corner of the grassed area and wheel tracks are present in the grass along the
length of the cell.
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Figure 4. If fence is installed, maintain opening for pedestrian access to educational signage.
4

Site: Rain garden in Town Hall Parking Lot (and rain barrels)
35 °37'11.37 "N
82 °19'9.43 "W
SCM Type: Rain garden without underdrain, Rain barrels
Classification: Non-structural / small scale
Date Constructed: 2010+/- estimated
SCM Description: This is small rain garden, with a footprint of <150 square feet (estimated), drains
approximately 5500 square feet (measured estimate) of the asphalt parking lot in the back of Town Hall.
There is no managed overflow for the rain garden — high flows presumably overtop the depression and
flow into adjacent properties. The rain garden has a river birch planted around the perimeter. No plans
or other documentation were available for the SCM. The Town Hall site also has three (3) 50-gallon rain
barrels draining downspouts on the front side of the building facing the road.
Inspection Elements: Review rain garden depression for excessive leaf litter or other deposition that
would impact storage volume. Inspect around rain garden for evidence of erosion on cell walls or going
away from the cell (from overtopping events). Inspect around rain garden for evidence that water is not
getting into the practice due to build-up of debris along pavement edge. Inspect rain barrels for
maintenance needs (Are they full? Is overflows functioning? Does water come out of the spicket?)
Inspection Notes & Recommendations: The rain garden cell appears to be functioning as intended. The
tree planted in the cell is adversely impacting the adjacent pavement through root protrusion, and there
is some evidence that this, as well as build-up along the edge of the cell prevent some stormwater from
entering the cell. There is evidence of ponding outside of the cell on the asphalt and along the curb edge.
It is recommended river stone (used as slope protection) and organic build-up be lowered or maintained
to allow water to freely drain off pavement. In the case of pavement deformation from tree roots, the
Town will need to decide at what point this constitutes an issue (it is primarily an aesthetic issue, although
it is inducing minor ponding on the pavement). Tree removal and/or pavement repair may be warranted.
For rain barrels, all were found to be full and generally not being maintained. Draw -down is an important
component of using rain barrels as water quality devices. It is recommended that rain barrels be allowed
to "leak" into adjacent gardens, beds or a shallow depression. This presents a challenge on this site since
the rain barrels are up against the building and isolated by the sidewalk from open areas that are far
enough away from the building to be considered suitable for receiving and ponding water. Rain barrels
should be either removed, or maintained on a more routine basis. During winter months, the spicket
valve may be left open to reduce risk of damage from freezing. The current siting of rain barrels is
problematic in that there is no suitable location to route overflow.
'� — Figure 1. Rain barrel at Town Hall - there are three in total.

Site: Golf course bioretention cell along Tomahawk Ave (adjacent to croquet court)
35 °37'19.59 "N
82 °19'50.28"W
SCM Type: Bioretention cell (functioning as dry pond)
Classification: Structural / medium scale
Date Constructed: 2010
SCM Description: Final plans were stamped in December, 2009, suggesting that this bioretention facility
was constructed in 2010. The facility receives flow from a residential subwatershed and the headwaters
have been developed in dense residential since its original construction.
From data provided during a 2015 evaluation of the SCM, calculations used to size the cell were based on
SCS methods using a Curve Number of 64 based on the assumptions of hydrologic group B soils and a
contributing drainage area of 1,288,929 sq. ft. = 29.6 acres. Based on the plans, the planting for the cell
was a mix of native grasses, as well as fescue, blue grass and bermuda. There was a level spreader and
turf mat proposed near the inlet of the cell — these appear to have been installed but are no longer
functioning and appear to have been damaged by high flows.
Inspection Elements: Inspect condition of pipes and drainage boxes. Review the vegetative condition of
the cell and for presence of trash/debris and erosion. Evaluate the function of the cell to provide volume
treatment. Evaluate system components including media, underdrains, emergency weir ("spillway" on
plans). A more complete list of inspection elements is provided at the end of this memo.
Inspection Notes & Recommendations: The facility was evaluated in December, 2015, by Wildlands
Engineering and a memo was prepared dated 12/21/15. That document describes the existing condition
and function of the cell. Based on follow-up evaluation in 2019 (when this SCM was being considered for
rehabilitation under the 319 grant project), and then again for the purpose of this inspection report in
January, 2020, the condition and function of the cell has remained unchanged since 2015.
Refer to the 12/21/15 report for findings and recommendations for cell rehabilitation. The Town has been
reluctant to rehabilitate the cell due to the upstream development and conditions that continue to deliver
sediment into the basin. There are open channel conveyances upstream of the SCM which deliver both
flow and sediment to the SCM. The condition of these conveyances varies, and sediment contributions
to the cell are likely from a combination of upland sources, as well as erosion within conveyances. The
original media is buried with fine sediment and organics (leaves) that enter the cell from the watershed.
Additionally, the cell does not appear to pond water up to the level of the box inlet — this is thought to be
the result of water exiting through the underdrain cleanouts. Capping these cleanouts and evaluation of
subsequent performance under storm conditions would help evaluate future options to address cell
deficiencies.
Figure 1. Sediment -laden outflow from bioretention cell, in 2015
0

Site: Tiered bioretention (rain garden) at Black Mountain Golf Course parking lot
35 *3733.34 "N
82 °20'3.23 "W
SCM Type: Bioretention with underdrain, grassed swale and riparian buffer plantings
Classification: Structural but simple / medium scale
Date Constructed: 2019
SCM Description: This is a bioretention basin with four cells treating runoff from the golf course parking
lot constructed with 319 funding in 2019. The SCM has a grassed swale that is intended to capture water
from the upper lot near the pro shop. Other runoff enters directly into the cells, and due to the asphalt
berm strip in the parking lot, a large percentage of the runoff enters the 3rd cell and bypasses the upper
two cells. An underdrain system is present which outlets onto the creek terrace slope adjacent to the
primary overflow pipe (from the storm box in the 4t" (lowest) cell). The first cell is grassed and the
remaining cells are planted with herbaceous species and mulched in the bottom and grassed on the
slopes.
Inspection Elements: Review edge conditions in parking lot. When edges build up with grass/sediment,
this stops water from entering the grass swale causing it to run across the parking lot instead. Evaluate
condition of mulching. Inspect grassed weirs connecting cells for rilling / preferential flow paths. Look for
rutting or other signs of inappropriate vehicular access. Mowing during wet periods could produce
undesirable rutting as well. Review SCM footprint for erosion, bare soil, vegetation success. Review
condition of grassed swale (erosion of invert or slopes), as well as connecting pipe under golf cart path.
Inspection Notes & Recommendations: The parking lot edge near the uppermost part of the swale is not
allowing water off the pavement — the grassed edge needs to be periodically knocked down (see photo),
or some more permanent solution installed such as shown in the screenshot below. One very minor area
of rilling was noted between cell 344 on the downhill weir slope. This should be plugged with sod and
observed to ensure that approach works. The site was also visited on 2/7/20, after a 2-3" rain event in the
prior days. The ponds were holding water which is expected for a period of 24-48 hours following rain.
No additional issues were observed at that time.
Figure 1. View of all four tiered cells
from low end.
Figure 2. Riparian plantings installed
along streamside of cells.
7

Figure 3. Grate inlet for high flows that
overtop the cell storage volume. Sand in
cell is from sand storage on parking lot.
Figure 5. Minor rilling on weir slope
between cells. Plug with sod and
monitor.
Figure 4. Grassed swale routes flows
from upper parking lot to cells.
Figure 6. Grass build-up along parking
lot edge prevents runoff from leaving
pavement into swale. The edge
needs to be knocked down
periodically to address this condition.

Figure 7. Depicts the location of grass build-up shown in Figure 6 photo. If a more permanent solution
is desired, installing a grate inlet at the location shown would be an effective way to avoid the
consistent maintenance of the parking lot shoulder. It is also recommended to consider eliminating
the asphalt berm that starts at this location and routes water further out into the parking lot. Both the
grass build-up and the asphalt berm result in bypass of the upper two cells in the 4-tier system.
0

Site: Bottom of event lawn at Lake Tomahawk Park
35 °37'13.56 "N
82 *1945. 63 "W
SCM Type: Grassed swales and wetland depressions
Classification: Non-structural / medium scale
Date Constructed: 2019
SCM Description: This is a system of swales and shallow depressions that route stormwater runoff from
the stormwater pipe outlet at the west side of the tennis courts along Laurel Circle Drive through the open
area at the bottom of the event lawn and into the Lake Tomahawk forebay upstream of the pedestrian
bridge. The SCM was constructed with 319 funding in 2019 and treats the road and parking lot, as well as
the lower half of Laurel Ave that drains from the west into a road inlet above the courts.
Flow from the end of the pipe rapidly fills a shallow depression next to the courts — an existing grate inlet
is present but does not drain quickly enough to outlet water. As a result, stormwater flows across the
eastern edge of the parking lot and onto the gravel access between the tennis courts and the grassed field
(and picnic shelter). This gravel access was raised to prevent water from continuing towards the Lake
(where it was causing erosion of natural surface trails). The raised portion of the road is intended to force
water off the access road and into the grassed area to the west where it is routed through a swale and
series of depressions into the forebay. A grassed swale was also constructed adjacent to the picnic shelter
to route rooftop runoff away from the building and into the lower portion of the SCM.
Inspection Elements: Review for ponding in areas other than swales and treatment depressions. Observe
the condition of vegetation and pipes. Ensure that water off the parking lot is being forced off the gravel
access road into the grassed area. Look for necessary aesthetic enhancements in order to keep the cells
looking good. Indicate where there are maintenance activities or uses that are damaging the cell — because
the area stays wet for prolonged periods of time, caution must be used when mowing. No vehicles should
enter the vicinity of the ponding and should preferably remain on gravel access road and off the grass.
Inspection Notes & Recommendations: The swales and depressions are functioning as intended. A vehicle
has driven across the grass, rutting out the area at the head of the Swale. This area needs to be regraded
to remove the rutting.
ek
Figure 1. View of shallow depressions
along swale.
Figure 2. Herbaceous wetland
plantings in depression.
10

Figure 3. A vehicle has driven across the
grass and rutted out the head of the
swale. Further rutting may allow water
to escape the intended pathway and go
straight to the Lake as it previous did.
Figure 5. Swale adjacent to picnic
shelter routes rooftop runoff towards
the music stage.
Figure 4. View of swale and
depression SCM from the head of the
swale / edge of gravel access.
Figure 6. A pipe was installed to route water
from the swale next to the picnic shelter into
the primary SCM swale. The purpose was
to facilitate ease of access to the music
stage.
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Attachment 1.
Inspection and Regular Maintenance Recommendations for Stormwater Control Measures
Primary site -specific inspection considerations were identified within the inspection summaries provided.
The following guidelines are recommended for broad long-term maintenance and operation.
• Do not place snow, mulch, yard waste, or any other material in cells.
• Heavy equipment should NEVER be driven over cells.
• Prevent sediment from entering the cells.
The bioretention cell should be inspected routinely and after large storm events. Wildlands suggests
establishing a form -based or map -based inspection protocol that allows records (photos, notes, etc.) to
be established during field visits and maintained in a geodatabase for future reference. In addition, the
NC Stormwater Manual recommends the following inspection and maintenance activities:
BMP element
Potential problems
Remediation
The entire
Trash/debris is present.
Remove the trash/debris.
BMP
The perimeter
of the
Areas of bare soil and/or
Regrade the soil if necessary to remove the gully, and then
bioretention
erosive gullies have formed.
plant a ground cover and water until it is established.
cell
Provide lime and cone -time fertilizer application.
The pipe is clogged
Unclog the pipe. Dispose of the sediment off -site.
The pipe is cracked or
Replace the pipe.
otherwise damaged
o
The inlet
device:
Erosion is occurring near inlet,
Regrade the inlet if necessary to smooth it over, replace turf
pipe,
level spreader,
around turf reinforcement
matting or apply riprap to avoid future problems with
matting
erosion.
turf
reinforcement
Level spreader is clogged or
Remove sediment and clogged stone and replace with clean
covered in sediment
stone.
matting
If sediment accumulation in
Search for the source of the sediment and remedy the
the level spreader continues to
problem if possible. Consider retrofitting pretreatment
be a problem
(forebay) to the design to help with sediment management
The
Best professional practices
bioretention
show that pruning is needed to
Prune according to best professional practices.
cell:
maintain optimal plant health.
vegetation
Determine the source of the problem: soils, hydrology,
Plants are dead, diseased or
disease, etc. Remedy the problem and replace plants.
dying.
Provide a one-time fertilizer application to establish the
ground cover if a soil test indicates it is necessary.
The
Determine the extent of the clogging - remove and replace
bioretention
Soils are clogged with
either just the top layers or the entire media as needed.
cell: soils
sediment.
Dispose of the spoil in an appropriate off -site location.
Search for the source of the sediment and remedy the
problem if possible.
An annual soil test shows that
Dolomitic lime shall be applied as recommended per the soil
pH has dropped or heavy
test and toxic soils shall be removed, disposed of properly
metals have accumulated in
and replaced with new planting media.
the soil media.
12

The
underdrain
Clogging has occurred.
Wash out the underdrain system.
system
Clogging has occurred.
Clean out the drop inlet. Dispose of the sediment off -site.
The drop inlet
The drop inlet is damaged
Repair or replace the drop inlet.
The receiving
Erosion or other signs of
Contact the NC Division of Water Quality 401 Oversight Unit
water
damage have occurred at the
at 919-733-1786.
outlet.
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