Preliminary BMP Calculations

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Andra Stevens
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1 Preliminary BMP Calculations 1 Preliminary BMP Calculations 2 Ohio Department of Transportation 1

2 Preliminary BMP Calculations 3 Draw drainage basin boundary perpendicular to contours Look for conveyance (pipes, ditches, etc.) Look for drainage divides Example: 4 Ohio Department of Transportation 2

3 5 Preliminary BMP Calculations 6 Ohio Department of Transportation 3

4 BMPs must be sized for all tributary area BMPs only get credit for area within R/W Example: 7 Trib. Area = 4.0 ac R/W Area = 1.1 ac 8 Ohio Department of Transportation 4

5 Preliminary BMP Calculations 9 Depends on what you re calculating Flow rate Rational Method; coefficient of runoff Water Quality Volume Volumetric runoff coefficient Sound alike, but not the same Rational Method Q = CiA Calculate weighted coefficient 10 Ohio Department of Transportation 5

6 Depends on what you re calculating Flow rate Rational Method; coefficient of runoff Water Quality Volume Volumetric runoff coefficient Sound alike, but not the same WQ V = (P*A*Cq)/12 Cq = Runoff coefficient Cq = 0.858i i i i = impervious area divided by the total area 11 Examples: Determine the Rational Method coefficient of runoff (C) for WQ F for BMP sizing Determine the Rational Method coefficient of runoff (C) for culvert sizing Determine the runoff coefficient (Cq) for WQ V for BMP sizing 12 Ohio Department of Transportation 6

7 ODOT L&D Vol. 2, Section While all areas within ODOT right-of-way may not be covered by impervious surfaces, the area within existing ODOT right-of-way is considered impervious for the purpose of post-construction BMP design considerations. Therefore, consider all area within existing right-of-way to be impervious with a runoff coefficient of 0.90 when performing post-construction BMP calculations. 13 Coefficient of Runoff WQ F BMP Right-of-Way = 1.1 ac; C = 0.9 Woods = 2.9 ac; C = 0.3 Weighted C = [(1.1*0.9)+(2.9*0.3)] / ( ) = Ohio Department of Transportation 7

8 Coefficient of Runoff Culvert Pavement = 0.5 ac; C = 0.9 Berms 4:1 or flatter = 0.6 ac; C = 0.5 Woods = 2.9 ac; C = 0.3 Weighted C = [(0.5*0.9)+(0.6*0.5)+(2.9*0.3) / ( ) = WQ V BMP Right-of-Way = 1.1 ac; 100% Impervious Woods = 2.9 ac; 0% Impervious % Impervious (i) = (1.1+0) / ( ) = 27.5% Cq = 0.858* * * = Ohio Department of Transportation 8

9 Examples: C for WQ F for BMP sizing = C for culvert sizing = Cq for WQ V for BMP sizing = Preliminary BMP Calculations 18 Ohio Department of Transportation 9

10 85-90% Avg. Annual Runoff Treated 19 WQ V BMP Right-of-Way = 1.1 ac; 100% Impervious Woods = 2.9 ac; 0% Impervious % Impervious (i) = (1.1+0) / ( ) = 27.5% Cq = 0.858* * * = Ohio Department of Transportation 10

11 Example: WQ V = (P*A*Cq)/12 P = 0.75 in A = 4.0 ac Cq = WQ V = (0.75 in * 4.0 ac * 0.212) / 12 WQ V = ac-ft WQ V = 2,309 ft 3 21 Preliminary BMP Calculations 22 Ohio Department of Transportation 11

12 0.65 in/hr treats 85-90% of average annual flows 23 Coefficient of Runoff WQ F BMP Right-of-Way = 1.1 ac; C = 0.9 Woods = 2.9 ac; C = 0.3 Weighted C = [(1.1*0.9)+(2.9*0.3)] / ( ) = Ohio Department of Transportation 12

13 Example: Q = CiA C = i = 0.65 in/hr A = 4.0 ac Q = * 0.65 in/hr * 4.0 ac Q = cfs 25 Questions? Jon Prier, P.E. jonathan.prier@dot.ohio.gov Ohio Department of Transportation 13

DRAINAGE CALCULATIONS

DRAINAGE CALCULATIONS NW 8 th Street and NW 8 th Terrace Roadway and Drainage Improvements City of Miami Project B-30745 And NW 14 th Court Roadway and Drainage Improvements City of Miami Project B-30746