Rational Method Peak Runoff

Q = C i A (US: Q_{cfs} = C i A_{ac})

Peak runoff Q equals runoff coefficient C times rainfall intensity i (for duration = tc) times drainage area A. Use consistent units. C from Handbook land-cover tables. Intensity from IDF curves at t = time of concentration.

$Q$ (cfs or m³/s): Peak runoff
$C$: Runoff coefficient
$i$ (in/h or mm/h): Intensity
$A$ (ac or ha): Area

Step-by-step solved example

C = 0.70, i = 3.0 in/h, A = 4.0 ac. Find Q in cfs (US rational).

Watershed as tributary area: Q = CiA with consistent units.

1. US units
In US customary, Q (cfs) = C i A with i in in/h and A in acres.
2. Compute
Q = 0.70×3.0×4.0 = 8.4 cfs.
$Q = 8.4 cfs$

Answer: Q = 8.4 cfs

10 practice questions

0/10 correct

1.The rational method estimates

2.Rainfall duration used for i should equal

3.C = 0.9, i = 2 in/h, A = 1 ac. Q =

4.A paved parking lot C compared with a lawn is

5.The method is best for drainage areas

6.If tc increases, IDF intensity i typically

7.Composite C for mixed land use is

8.SI form needs a unit conversion because

9.C has units of

10.Kirpich tc is a function of