How to Calculate Drainage (Rational Method)
Rational Method: Q = C x I x A
Q = Peak flow rate (CFS)
C = Runoff coefficient (0 to 1)
I = Rainfall intensity (in/hr)
A = Drainage area (acres)
Runoff Coefficients
- Lawns (sandy): 0.10-0.20
- Lawns (clay): 0.25-0.40
- Gravel: 0.40-0.60
- Residential (mixed): 0.60-0.80
- Pavement/Roof: 0.85-0.95
French Drain Sizing
- Residential: 4" perforated pipe in 12" wide trench
- Medium loads: 6" pipe in 18" wide trench
- Length: Typically 1 ft per 5-10 GPM capacity needed