Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

3. Depth of flow (d) at WQF (3” – 5” with Manning’s n = 0.20) d = ‘y 1 ’ ft.(Description on how to calculate d is explained below)A = by + Zy 2 (eqn 1)R = A/P = [ ( by + Zy 2 ) / (b + 2y√ (1 + Z 2 ))] (eqn 2)Q = (1.49/n) (by + Zy 2 ) [ ( by + Zy 2 ) / (b + 2y√ (1 + Z 2 ))] 2/3 (s) 1/2 (eqn 3)Using Eqn. 3:0.486 = (1.49/.2) (5y + 3y 2 ) [(5y + 3y 2 ) / (5 + 2y√10)] 2/3 (.01) 1/2Trial & ErrorY.35 => 7.45(2.1175)(.44169)(.1) = .696.29 => 7.45(1.7023)(.3959)(.1) = .502.28 => 7.45(1.6352)(.3878)(.1) = .472.282 => 7.45(1.64857)(.38945)(.1) = .478.285 ft. => 7.45(1.668675)(.39186)(.1) = .487 cfsy 1 = Depth of WQF = 0.285 ft.4. Design Slopea. s = 1% minimum without underdrains; 4% maximum s = 1%without grade controls5. Design Flow Velocity (Manning’s n = 0.20) V = 0.29 ft/secVelocity = Q/A = (using eqn. 3&1) = 0.486cfs / 1.67ft. = 0.29 ft/sec.6. Contact time (t c = 7 minutes minimum) t c = 7 minutes7. Design LengthL = (t c ) x (flow velocity) x 60L = 7min x (.291ft/sec) x 60 = 122.22 ftL = 122.32 ft