Peak Discharge from Drainage Basin Runoff

Last modified by

on

Mar 27, 2025, 5:35:12 PM

Created by

on

Jul 8, 2016, 4:59:28 PM

Q = C \cdot I \cdot A

$Q = C * I * A$

(C) Runoff Coefficient

$(C)"Runoff Coefficient"$

(I) Average Rainfall Intensity

$(I)"Average Rainfall Intensity"$

(A) Drainage Area

$(A)"Drainage Area"$

The Math / Science

The Peak Discharge from Drainage Basin Runoff is found using the Rational Method:

$Q = C \cdot I \cdot A$ $Q=C*I*A$

where:

Q = Peak Discharge Rate
C = Runoff Coefficient
I = Average Rainfall Intensity
A = Drainage Area

Land Cover Type	Runoff Coefficient (C)
Pavement / Concrete	0.70 - 0.95
Urban Areas (high density)	0.5 - 0.70
Lawns (sandy soil, flat)	0.05 - 0.15
Lawns (clay soil, steep)	0.25 - 0.35
Agricultural Land	0.10 - 0.50
Bare Soil	0.30 - 0.60
Gravel Areas	0.30 - 0.70

Use the Rainfall Intensity Calculator if you need $I$ $I$ .

The Rational Method estimates peak discharges for drainage areas of up to about 200 acres (80 hectares) and no significant flood storage. Although the Rational Method estimates the peak discharge, it cannot estimate a flow volume or time series of flow.

Assumptions under the Rational Method: (If any one of these conditions is not true, the user should try an alternative method.)

t_c for the drainage area must be less than the duration of peak rainfall intensity
Runoff is directly proportional to rainfall intensity
Rainfall intensity is uniform throughout the storm
Rainfall is distributed evenly over the drainage area
Rational Method does not account for storage
- Available storage is assumed to be filled
Frequency of occurrence for the peak discharge is equal to the frequency of the rainfall producing that event
Minimum duration of rainfall intensity is 10 minutes
- If the time of concentration (t_c) computed is less than 10 minutes, then 10 minutes should be adopted for rainfall intensity computations

Type of snow or ice	(kg/m³)
Fresh New snow	50-70
Damp new snow	100-200
Settled snow	200-300
Depth hoar	100-300
Wind packed snow	350-400
Firn (granular)	400-830
Very wet	700-800
Glacier ice	830-917
Rain/Melt	997