20181245 Ver 2_Design Calcs 9_11_20_20200911

STILLWELL ENGINEERING, P.A.
TINSEL TOWN DEV.–
FERGUSON STREET
STORM WATER CALCULATIONS

William Stillwell, P.E.
9/11/2020

STILLWELL ENGINEERING, PA
PO BOX 838
SYLVA, NC 28779
NC FIRM LIC# C-2996
PH: 828.586.6066

[e-mail]

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STORM WATER MANAGEMENT CALCULATIONS

A. BUILT UPON AREA (B.U.A.)
a. PARCEL #1:
· TOTAL PROPOSED PARCEL ACREAGE: 3.0 ACRES
· DEVELOPABLE PROPERTY EXCLUDING REMAINING STREAMS AND BUFFER ZONE: 2.31 ACRES
· RESIDENTIAL DEV. ADDED B.U.A.: 0.37 ACRES
· ACCESS ROAD ADDED B.U.A.: .14 ACRES
· TOTAL ADDED B.U.A.: 0.51 ACRES
· 2.31 AC / 0.50 AC = 22% B.U.A. - LOW DENSITY DEVELOPMENT

b. PARCEL #2:
· TOTAL PARCEL DEVELOPABLE ACREAGE: 3.23 ACRES
· POST-CONSTRUCTION B.U.A.: 2.35 ACRES
· 2.35 AC / 3.23 AC = 73 % B.U.A. - HIGH DENSITY DEVELOPMENT REQUIRED TO TREAT RUNOFF
FROM 1-INCH STORM EVENT FOR PROPOSED TRACTOR SUPPLY COMPANY SITE ONLY. INCORPORATE
BIORETENTION AREA FOR CONTROL AND TREATMENT:

1. REQUIRED VOLUME:

a. PRE-EXISTING IMPERVIOUS AREA:
GRAVEL SURFACE: 16,282 SF
STRUCTURES AND SLABS: 4,963 SF
TOTAL PRE-EXISTING IMPERVIOUS AREA: 21,245 SF = 0.49 ACRES

b. POST-CONSTRUCTION BUILT UPON AREA:2.35 ACRES

c. TOTAL NET BUILT UPON AREA ADDITION: 2.35 ACRES - 0.49 ACRES = 1.86 ACRES

d. BIORETENTION AREA VOLUME REQUIRED:
IMPERVIOUS FRACTION (Ia): 1.86 AC / 3.23 AC = 0.58
Rv = RUNOFF COEFFICIENT = 0.05 + 0.9 X Ia = 0.57
DESIGN VOLUME: 3,630 X Rv X Rd X A = 3,630 X 0..57 X 1 IN X 3.23 AC = 6,683 CF
PROVIDED BY BIORETENTION AREA: 6,703 CF

e. BIORETENTION AREA POND ELEVATIONS:
· PROPOSED FLOOR ELEV: 2,145.00'
· OUTLET ELEVATION: 2,145.00' + 1' = 2,146.00'

B. DRAINAGE DISCHARGE OUTFALL PROTECTION FROM 10 YR PEAK FLOW STORM.

1. DISCHARGE OUTFALL - FES-14

a. TREATED STORM WATER DISCHARGE WILL BE DIRECTLY INTO THE REMAINING REMAINING
POND AREA, 1-FOOT ABOVE ELEVATION OF NORMAL WATER LEVEL OF POND. ORIGINAL CALCULATIONS
INDICATE A FLUCTUATION OF APPROXIMATELY 0.36 INCHES FOR THE 10 YR / 24 HOUR STORM
b. 10 YR STORM INTENSITY WITH TIME OF CONCENTRATION OF 5 MINUTES IS 6.65 IN/HR (I) PER
NOAA STATION "CULLOWHEE"
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c. STORM WATER FLOW, Q = C*I*A WHERE C = COMPOSITE RUNOFF COEFFICIENT (0.95 -
ASPHALT/ROOF/CONCRETE); I = 6.65 IN/HR; A = 2.35 ACRES
d. Q = 0.95 * 6.65 IN/HR * 2.35 AC = 14.85 CUBIC FEET PER SECOND (CFS)
e. DISCHARGE VELOCITY:
· OUTFALL PIPE DIA: 24", MATERIAL: HDPE, n=.013
· PIPE SLOPE: 2.73%
· FOR PIPE FLOWING HALF FULL - Q = (1.49/.013) *1.57 * .5^(2/3) * .0273^1/2 = 18.73 CFS
· USING CRITERIA FOR 1/2 FULL FLOW, OUTLET VELOCITY = 18.73 CFS/1.57 SF = 11.9 FT/S
· PER FIGURES 8.06C AND 8.06D OF CHAPTER 8 OF THE NCDEQ EROSION AND SEDIMENT
CONTROL PLANNING DESIGN MANUAL, REQUIRED STONE SIZE IS CLASS I RIP RAP TO BE INSTALLED
BETWEEN OUTFALL AND NORMAL WATER LEVEL.

2. DISCHARGE OUTFALL FES-12
a. FLOW IS FROM A ROADWAY CULVERT CROSSING FERGUSON ROAD.
b. ESTIMATE FLOW FROM 10 YR STORM EVENT WITH TIME OF CONCENTRATION OF 5 MIN, I = 6.65
IN/HR FOR FERGUSON ROAD DRAINAGE INTO CULVERT DITCH
c. TOTAL PAVED AREA DRAINING TO CULVERT: 0.25 ACRES
d. ESTIMATED FLOW = Q = 0.95 * 6.65 IN/HR * 0.25 AC = 1.58 CFS
e. DESIGN OUTLET FLOW PROTECTION FOR VELOCITY RESULTING FROM 1/2 FULL PIPE FLOW.
e. DISCHARGE VELOCITY:
· OUTFALL PIPE DIA: 18", MATERIAL: HDPE, n=.013
· PIPE SLOPE: 1.13%
· FOR PIPE FLOWING HALF FULL - Q = (1.49/.013) * 0.883 SF * .375^(2/3) * .0113^1/2 = 5.60 CFS
(4.0 CFS IN EXCESS OF ESTIMATED FLOW FROM ROAD CULVERT)
· USING CRITERIA FOR 1/2 FULL FLOW, OUTLET VELOCITY = 5.6 CFS/0.883 SF = 6.34 FT/S
· PER FIGURES 8.06C AND 8.06D OF CHAPTER 8 OF THE NCDEQ EROSION AND SEDIMENT
CONTROL PLANNING DESIGN MANUAL, USE CLASS A RIP RAP WITH APRON LENGTH OF 4 * 1.5FT = 6FT

3. FOREBAY OUTFALL FES-1
a. DISCHARGE VELOCITY: USING CIRCULAR CHANNEL RATIOS: d/Dfull = 0.32 RESULTING IN v/Vfull
= 0.75. Vfull = 21.54 FT/S, v = 16.16 FT/S
b. PER FIGURES 8.06C AND 8.06D OF CHAPTER 8 OF THE NCDEQ EROSION AND SEDIMENT
CONTROL PLANNING DESIGN MANUAL, USE CLASS 1 RIP RAP 24" DEEP WITH APRON LENGTH OF 8 * 2 FT
= 16FT

4. EMERGENCY SPILLWAY
a. DESIGN FOR 25 YR STORM INTENSITY WITH TOC OF 5 MIN IS 7.62" (I) PER NOAA STATION
"CULLOWHEE"
b. FLOW Q = 0.95 * 7.62 IN/HR * 2.35 AC = 17.0 CUBIC FEET PER SECOND (CFS)
c. OPEN CHANNEL FLOW FOR AVERAGE SLOPE 11 FOOT BOTTOM SPILLWAY WITH CLASS 2 RIP RAP
LINER AND FLOW DEPTH OF .34':
Q = (1.49/.078) * 4.068 * .311 * .28^.5 = 17.17 CFS