Calculation of Design Rainfall Intensity and Peak Storm Water Runoff Rate - S.I. units (for specified return period and storm duration) Inputs Drainage Area, A =
25
ha
Runoff Coefficient, C =
0.35
Design Return Period =
15
years
Design Storm Duration, d =
22
min
( = time of concentration )
Data from IDF (intensity-duration-freque (intensity- duration-frequency) ncy) graph or table for the design location: Input
Input
Calculated
i, mm/hr
d, min
1/i, hr/mm
187
10
0.01
153
15
0.01
105
30
0.01
Calculation of equation constants ( a & b ) using linear regression:
slope =
0.000207
= 1/a
a = 1/slope =
intercept =
0.0033
4840
= b/a
b = a*intercept =
16.2
Calculation of Design Rainfall Intensity, i, using the equation: i = a/( d + b ) : (using the value for storm duratin, d, specified above)
Design Rainfall Intensity, i =
126.7
mm/hrÂ
Calculation of Design Peak Storm Water Runoff Rate, Q, using the equation: Q = CiA
