SW6191001_SW Narrative_20191028

DRAINAGE DESIGN NARRATIVE & CALCULATIONS
Yard Storage Inc Request
September 2019
Project Description
The purpose of the project is to construct gravel parking area for recreation vehicles. Approximately 1.7
acres will be disturbed during this construction period. The site is 2.2 acres located in Buckhorn
Township, Harnett.
Site Description
The site has rolling topography with slopes generally 2 to 7%. Slopes steepen to 10% in the northeast
portion of the property where the proposed stormwater basin will be located. The site at present is
predominantly stripped off of vegetation, rest of the areas are covered with short grass and woody
vegetation at north boundary. There is no evidence of significant erosion under present site conditions.
Adjacent Property
Land use in the vicinity is residential. The land immediately to the north and east has been developed for
residential use. Area to the north is predominantly undeveloped. The existing ditch along the gravel
drive will receive the overflow from the infiltration basin. Sediment control measure will be taken to
protect the existing ditch and the adjacent residential properties.
Soils
The subsurface is sandy loam, 10 inches thick. The subsoil consists of clay, 45 inches thick. Below 55
inches is a layer of loam. Soil morphological characteristics indicating a seasonal high water table
(SWHT) were not observed within 120 inches below surface. Infiltration rates (permeability) range from
0.2 in/hr in the clay layer to as high as 2 in/hr in the loamy sand.
Proposed SCM
Infiltration system is proposed to capture stormwater runoff from all disturbed areas. The infiltration
system proposed to utilize Ecorain system. Volume in excess of the Ecorain tank capacity will outlet to a
drain pipe and pop-up emitter which overflows to the existing road ditch.

Stormwater Calculations
Total Area = 96,333 sq.ft = 2.21 acre
Impervious Area = 72,000 sq.ft = 1.65 acre
Impervious fraction, IA = 1.65 / 2.21 = 0.747
Runoff Volume Calculation:
Rv=0.05+0.91A
Where: Rv = Runoff coefficient (unitless)
IA = Impervious fraction (unitless)
Rv = 0.05 + 0.9(0.747)
Rv = 0.722
DV = 3630 * Rp * Rv* A
Where: DV = Design volume (cu.ft)
RD = Design storm depth (in)
A = Drainage area (ac)
RD= 1in
DV=3630*1*0.722*2.21
DV = 5,792.10 cu.ft
Infiltration System
Pre-treatment: Minimum 6 feet wide grass verge around the basin
Ecorain Tank Footprint Calculations
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Calculated Volume, Vdesign: : 5792.1 : cu ft
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Final volume to be retained by the ecorain tank (with 10%
contingency): 6371.3 cu ft
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Ecorain tank volume per module,
2.25'Lx1.34'Wx7.12'D: 20.78 : cu ft
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Total number of Ecorain tank modules: 307 : ea
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Ecorain Dimension (with 1-foot sand on all sides):
Length: 41 ft
Width: 25 ft
Minimum Total Depth: 7.3 ; ft
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Specifications and Maintenance
Ecorain Tank
Proposed infiltration system utilizes Ecorain retention structures. 307-units of Ecorain ET-1505 with
dimensions 1.34'W x 2.251 x 7.12'D is proposed. Refer to manufacturer's guide for tank installation and
maintenance.
Overflow Emitter
Volume in excess of the storage volume will overflow through a 6" drain pipe and a 6" NDS pop-up
emitter. Pop-up emitter outlets to the existing road ditch. Refer to manufacturer's guide for tank
installation and maintenance.