SW5180501_Narrative_20180523

1 | P a g e

STORMWATER TREATMENT AND MANAGEMENT
Siler Solar, LLC
302 Siler Business Drive
Town of Siler City, Chatham County, NC 27344
May 14, 2018

I. Narrative
The Siler Solar, LLC project site is a proposed solar power generation facility located on a 53.4-acre parcel in Chatham
County, North Carolina. The existing site area consists of mildly sloping terrain ranging from 5% to 11% from north to
south, north to west and north to southwest . The existing site is densely vegetated with grasslands and contains
woodlands along the perimeter. The site area is part of the Cape Fear River Basin and overland stormwater runoff
discharges to Bloor Run Creek, located west/southwest of the project.

The proposed solar project improvements will encompass Limit of Disturbance Area of 44 acres and will consist of
gravel access roads, concrete equipment pads, steel pile foundations to support the solar panels, and a perimeter
chain link fence. The project site will be accessed from the northwest dead-end, truncated paved segment of Siler
Business Drive, which will be improved with a paved cul-de-sac and two concrete driveway aprons constructed to the
north and south. Gravel access roads will lead from the new driveways to double-swing gates, and provide continuous
onsite vehicle circulation and access to equipment pads.

A simplified hydrologic and hydraulic (H&H) analysis was performed for a single 44-acre project drainage watershed
area (e.g., disturbance area) for evaluating the calculated peak runoff for the existing and proposed project conditions.
The Rational Method was used to calculate the peak run off for the 10-year storm event, which is summarized in the
tables below (see Calculations included with the Stornmwater Permit submittal package). The slight increase in
calculated post-construction runoff is attributed to the increase in impervious area from the solar project improvements
(gravel roads, equipment pads, etc.). It should be noted that two major proposed impervious features – i.e., paved
cul-de-sac and concrete driveways, may be considered as “off-site” areas since they are physically located outside
the fenced perimeter and overland drainage from this subwatershed area will sheet flow to the south/southeast
direction. Also, it is reasonable to estimate that runoff from these impervious areas may be already accounted for in
the existing roadside drainage ditch design for Siler Business Drive.

2 | P a g e

II. Construction
Prior to beginning construction activities, site preparation shall be performed – i.e., tree clearing, brush cutting and
debris removal. Onsite construction will start with minor grading (cutting/filling) within the northwest array field area;
roadway excavation, and equipment pad subgrade preparation ; then perimeter chain link fence installation, gravel
road placement and compaction, concrete equipment pad forming and pouring; and steel pile installation. Utility trench
excavation and backfill will also be performed for installation of the solar system conduit/wiring runs. Siler Business
Drive will be completed with a paved cul-de-sac and concrete driveways leading to the two site access gates.

Temporary Best Management Practices (BMPs)
Prior to land disturbing activities, temporary BMPs will be installed to perform site preparation work and construction
activities as specified on the approved Erosion & Sedimentation Control Plans (CHATH -2018-115; 5/11/18). The
project “Construction Sequence” is outlined sheet C-002; installation of Temporary BMPs is depicted on plan sheets
EC-700 through EC-702; and BMP details are provided on sheets EC-703 and EC-704. Temporary BMPs must be
maintained and repaired/replaced as necessary throughout the duration of project construction.

Temporary Construction Entrance/Exit: a temporary construction entrance/exit will be provided at the access driveway
locations to prevent sediment from leaving the site on construction vehicles (e.g., track out). Street sweeping shall be
conducted on Siler Business Drive at the end of the work day (see Detail A, EC-703).

Silt Fencing: silt fencing will be installed along the site perimeter to capture any sediment that does not outfall into the
temporary basins from leaving the site. Silt fence traps will also be installed at strategic downstream locations to allow
flow-through drainage and sediment deposit cleanup (see Details B & I, EC-703).

Temporary Diversion Ditches: diversion ditches have been designed to route stormwater from the calculated 10-year,
24-hour rainfall event to the temporary skimmer and sediment basins; and check dams will be used to deter sediment
from migrating downstream. Ditches shall be inspected and cleaned weekly and after each significant rainfall event
(0.5-inch or greater); and repaired as necessary. Reference Temporary Diversion Ditch(V-Shaped) Calculations
summary table on sheet EC-700. (See Detail G, EC-703).

Temporary Skimmer and Sediment Basins: a total of six skimmer basins (Detail C, EC-703) and one sediment basin
(Detail L, EC-704) have been designed for the onsite delineated subwatershed areas (see Temporary Basin
Calculations table on sheet EC-700). Basins will include Baffles (Porous/Coir) and Rock Chute Slope Drains per
plans. At a minimum, temporary basin maintenance shall consist of the following procedures:
1. Conduct weekly inspections and after each significant rainfall event (≥ 0.5-inch); repair immediately.
2. Remove sediment and restore the basin(s) to original dimensions when it accumulates up to one-half the
design depth. Place removed sediment in an area with sediment controls.
3. Check the embankment, spillways, and outlet for erosion damage; and inspect the embankment for piping
and settlement. Make all necessary repairs immediately.
4. Remove trash and other debris from the riser and pool area.

3 | P a g e

III. Post-Construction
The site will be stabilized and reseeded within 15 working days or 90 calendar days after completion of the
construction activity related to the installation of the solar panels, racking and associated equipment. Upon
establishment of permanent ground cover to satisfy NCDEQ and NPDES stormwater coverage requirements, a final
inspection shall be requested from the Raleigh Regional Office. Then the project Notice of Termination (NOT) may
be filed, which will release the project from General Stormwater Permit coverage for construction.

Then removal of temporary measures may be performed (silt fence, diversion ditches, rock chute slope drains,
skimmer and sediment basins) to eliminate point discharges and establish sheet flow whe re possible. Diversion
ditches shall be backfilled and seeded; skimmer and sediment basins shall be deconstructed, re-graded and seeded
to blend with adjacent natural grade as much as possible. Any disturbed project areas observed to have non-sufficient
ground cover shall be re-seeded as specified on the Permanent Seeding Notes on plan sheet C-002.

Permanent Vegetative Buffer
A permanent 20-ft wide vegetative buffer (Type A) per Town of Siler City requirements will be planted along the
project’s south boundary and on the north frontage of Siler Business Drive, including around the new cul -de-sac.
Natural trees and vegetation will remain in-place to serve as a “buffer” along the north and west project boundaries.
The vegetative buffer will provide visual screening from adjacent properties, serve as a barrier to slow down overland
drainage sheet flow and stop potential erosion and sediment transport.

Although the project site is located within a Phase 2 Area (Tipped County) inside Siler City, which requires a Post-
Construction State Stormwater Permit, no permanent stormwater treatment measures are anticipated due to the
relatively small increase in calculated peak runoff (e.g., +6.5 cfs). Once the solar site is fully vegetated within the
disturbed areas, it is reasonable to estimate that overland drainage will closely resemble the existing pre-project
condition – especially since drainage patterns will remain unchanged.

Solar Facility Operations and Maintenance
Once project construction has been completed and the solar plant is operational, the Project Owner will conduct
regularly scheduled O&M activities. Maintenance personnel will perform grass mowing; debris and trash removal;
panel washing/cleaning; onsite road repairs; and electrical equipment testing. The solar facility gates will be equipped
with a Knox Box to provide access to fire department and/or emergency services personnel.