Modified Rational Method
PRE DEVELOPMENT
Land Use Area (m2) Runoff C Impervious
Total Site Area
POST DEVELOPMENT
Land Use Area (m2) Runoff C impervious
Total Site Area
INTENSITY-DURATION-FREQUENCY (IDF) CALCULATOR: I = A/(T+B)^D
Return Period (Years) 2 5 10 25 50 100
A
B
D
CALCULATED FLOW BASED ON RATIONAL METHOD: Q (M3/S) = 2.78*C*I*A
Modified C, C* = a*C + b
IDF TABLE
Return Period (Years) 2 5 10 25 50 100
A
B
D
10 min
15 min
30 min
1 hr
2 hr
6 hr
12 hr
24 hr
PRE DEVELOPMENT
Project Area, A (ha)
Runoff Coefficient, C
Time of Concentration, T (min)
Return Period (Years) 2 5 10 25 50 100
Modified C ratio, a
Modified C intercept, b
Modified C
I (mm/hr)
Q (litres/sec)
Q (m3/sec)
POST DEVELOPMENT
Project Area, A (ha)
Runoff Coefficient, C
Time of Concentration, T (min)
Return Period (Years) 2 5 10 25 50 100
Modified C ratio, a
Modified C intercept, b
Modified C
I (mm/hr)
Q (litres/sec)
Q (m3/sec)
ALLOWED SCENARIO
Project Area, A (ha)
Runoff Coefficient, C
Time of Concentration, T (min)
Return Period (Years) 2 5 10 25 50 100
Modified C ratio, a
Modified C intercept, b
Modified C
I (mm/hr)
Q (litres/sec)
Q (m3/sec)
CALCULATED REQUIRED STORAGE BASED ON MODIFIED RATIONAL METHOD
Design Return Period (year)
IDF Parameter
A
B
D
Site Parameters
Post development C
Maximum Allowable Q (m3/s)
Area (ha)
Time of concentration, T (min)
Modified Intensity ratio, Ix= a*I
T (min) I (mm/hr) Ix (mm/hr) Peak Discharge, Q (m3/s) Trapezoidal Area Volume (m3) Triangular Area Volume (m3) Storage Volume (m3)
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
Max. Required Storage (m3)
...

Canada, Ontario.

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