SW6091006_Stormwater Management Plan_20100216

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STORMWATER MANAGEMENT PLAN
PREPARED FOR: FORT BRAGG
INDOOR RIFLE RANGE
DATE: 10-05-09
DENR-FRO
EARTH TECH, INC
701 CORPORATE CENTER DR., SUITE 475
RALEIGH, NC 27607
919-854-6200
PREPARED BY:
BRYAN M. DICK, PE, PH
CHECKED BY:
MARK KAMPRATH, PE
L
FEB 16 2010
DWQ
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PROJECT DESCRIPTION
THIS DOCUMENT IS TO SERVE AS A STORMWATER MANAGEMENT PLAN FOR THE
FORT BRAGG - INDOOR GUN RANGE PROJECT. THE PROJECT CONSISTS OF THE
ADDITION OF A SINGLE BUILDING WITH GRAVEL PARKING LOT AND ASSOCIATED
SIDEWALKS TO THE UNDEVELOPED SITE. THE COMPLETED PROJECT WILL PROVIDE FOR
EXPANSION OF BASE FACILITIES TO SUPPORT THE MISSION.
CONTACT INFORMATION
EARTH TECHI AECOM, INC.
ATTN: BRYAN M. DICK, PH
701 CORPORATE CENTER DR., SUITE 475
RALEIGH, NC 27607
TELEPHONE: [phone]
FAX: [phone]
IDENTIFY PROJECT INVOLVEMENT
THIS PROJECT IS A PART OF THE CAPE FEAR RIVER BASIN AND IS CURRENTLY
NOT SUBJECT TO ANY BUFFER RULES. THE SITE IS APPROXIMATELY 2,500 FEET FROM
THE PERSIMMON CREEK, THE NEAREST AFFECTED SURFACE WATER.
THE PROPOSED PROJECT STORMWATER PLAN WILL MEET THE REQUIREMENTS
OF RETAINING THE RUNOFF VOLUME THE 1.0 INCH RAINFALL EVENT PER THE
REQUIREMENTS BY THE STATE AS REFERENCED FROM THE DWQ STORMWATER
MANUAL, JULY 2007. ONE INFILTRATION BASINS IS USED TO TREAT THE DRAINAGE
AREA AND IT IS SIZED TO MEET THE 5-6-09 UPDATE FROM DWQ REGARDING BYPASS
AND LEVEL SPREADERS FOR INFILTRATION BASINS. THE BASIN EXCEEDS THE 2.5X
REQUIRED TREATMENT VOLUME AND DRAWS DOWN IN LESS THAN THE MAXIMUM OF 5
DAYS SET BY DWQ. THE DRAWDOWN WAS BASED ON '/2 THE KSAT VALUE REPORTED IN
THE SEALED SOILS REPORT. IN SUMMARY, OPTION 3 OF THE DWQ UPDATE ON 5-6-09
REGARDING INFILTRATION DEVICES WAS USED TO WAIVE THE REQUIREMENT FOR A
BYPASS AND A LEVEL SPREADER DEVICE. HYDROCAD MODELS BASES ON TR-55
METHODS WERE USED TO VALIDATE THE COMPLETE CAPTURE OF THE RUNOFF OF THE
I" RAIN EVENT.
STORMWATER DESIGN PARAMETERS
THE INFILTRATION BASINS ARE BASED ON THE FOLLOWING PARAMETERS OR
OBJECTIVES:
• PROVIDE 85% TSS REMOVAL
• PROTECT THE VALUE OF THE RECEIVING WATER RESOURCE
• 1.0" RAINFALL FOR BASIN STORAGE SIZING
• OVERFLOW DEVICE TO ACCOMMODATE STORMS IN EXCESS OF THE 10 YEAR
STORM
• RATIO OF IMPERVIOUS SURFACE TO THE TOTAL AREA OF THE DRAINAGE AREA
• EXCEED A MINIMUM OF 2.0' CLEARANCE FROM THE SEASONAL HIGH WATER
TABLE

• VOLUME IS SIZED AT GREATER THAN 2.5 THE MINIMUM REQUIRED VOLUME
• DESIGN KSAT VALUE WAS BASED ON �/z THE VALUE GIVEN AT THE LOCATION OF
THE BASIN IN THE SOILS REPORT
DISCUSSION OF TREATMENT MEASURES
THE PROPOSED TREATMENT IS TO USE A SINGLE INFILTRATION BASIN TO CAPTURE AND
TREAT THE RUNOFF FROM THE SITE FOR 10 YEAR STORM OR LESS. THE SANDY SOILS
OF THE SITE ARE SUITED TO INSTALLATION OF AN INFILTRATION BASIN AND WILL
MIMIC THE INFILTRATION OF RAINFALL FROM THE SITE THAT WOULD OCCUR PRIOR TO
THE PROPOSED PROJECT. THE BASIN MAINTAINS A CLEARANCE OF 4.8' FROM THE
SEASONAL HIGH WATER TABLE AT THE NEAREST SAMPLE LOCATION.
CONCLUSION
THIS STORMWATER MANAGEMENT PLAN REPRESENTS AN ENVIRONMENTALLY
SENSITIVE AND EFFECTIVE PLAN FOR UTILIZING THE PROPOSED STORMWATER
TREATMENTS TO ACCOMMODATE THE INCREASED RUNOFF OF THE PROPOSED SITE
DEVELOPMENT.

EARTH TECH I AECOM Vicinity Map
Gunnery Range
Ft. Bragg
Cumberland County, North Carolina
rill 0 0.5 1 2 Miles
11 1 1 1 1 1 1 1

Legend
—Streets EARTH TECH AECOM NCDWQ Classification Map
—Building Footprints Gunnery Range
—Streams Ft. Bragg
20 foot Contour Cumberland County, North Carolina
10 Foot Contour 0 500 1,OOO 2,000 Feet
1 inch = 1,000 feet

112454 Bragg Gun Range - Infiltration Basin 1
Drainge Area (Sf)
70825
Imp Area (sf)
39853
(A) Drainage Area (Ac)
1.626
0A)Imp Fraction
0.56
(RD)Rainfall Depth Per State SWP(in)
1
Step 1: Determine Runoff Volume SN V'n✓—
Simple Method IVJ
a' RV = .05 + .9(1,) = Rv (post)=.05+.9'.67
Rv (pre)=.05+.9'0
b V=3630xRDxR xA
Post V=3630'1.0'0.67'.6273
Designed By: BMD
Date: 9/28/2009
0.56
0.05
V (Post) = 3284 of
Min. Treatment Vol = 3284 cf
Treatment Vol in Ac-in 0.90 ac-in
Step 2: Determine Surface Area- Min. Required Volume for 2.5X to Not Bypass or Filter 8210 cf
(K) Infiltration Rate (ir/hr) 12.2fi 'Half the K value from the Soils Report
I I ) intended Days to Infiltrate (days) 2
a. A — V = A=3799/(2'(5.66'2)) IMin SA of Basin = 67.0 sf
2*(K *T)
Slop 3. Determine Dimensions
Volume
Storage Elevation 304.30
Elevation 301.40 Overflow Weir Elevation
Jet Elevation 301.40 TOD of Berth Elevation
Storage Depth 2.90
Surface Area Required (Avg of Top and Bottom SA)
Top Width ® Storage Elev 37.4 Top Length @ Storage Elev
Bottom Width 20 Bottom Length
4. Calculation of Volumetric Flow Rate of Irditration and Drawdown
0 from infiltration: '0 is actually the Darcian Flow Rate
of Q: SurfaceArea=3184sf (K)=2.42 in/
2.42in I ff I hr
184sf �—=O.IScfs
hr 12in 3600s
6aampleCalculatial: Q=0.I8cfs V=3184cf
V =T 3184fr' Is Ihr=2.Ihrs=.049days
Q 1 0. 18ff1 3600s
2.90
2
Target Area 0 sf
113.4 Surface Area Storage = 4241 sf
96.0 Surface Area Bottom = 1920 sf
1Avq Surface Area = 3081 sf
Volume of Basin (Depih'SA) = 8934 cf
Ezceed Required Volume by 2.72 X
Area (sf) (K) in/hr 0 (infiltration) = 0.54 cfs
1920 12.255
Drawdown Time for Design Vol
Volume c i Hours Da
8934 0.54 4.6 0.19

Drainage Area
Impervious
Post Pervious
Pre
A
Rational Method Peak Flow Calculation for Supplement
1.53 Ac C
43278 sf 0.98
23380 sf 0.15 Composite C 0.69
Qp - post 0.95
Qp - Pre 0.21