Modified Rational Method Calculator

Enter runoff coefficient, drainage area, time of concentration, IDF data, and an allowable release rate to estimate peak discharge and preliminary detention storage across storm durations with the Modified Rational Method.

Input Parameters

Watershed Parameters

Enter watershed characteristics

Runoff Coefficient (C)

Rational method C value (0 to 1)

Drainage Area

acres

Contributing watershed area

Time of Concentration

min

Watershed Tc in minutes

Return Period

Design storm frequency

Detention Design

Specify allowable release rate

Allowable Release Rate

cfs

Maximum permitted outflow (e.g., pre-development peak)

Leave blank to calculate peak discharge only without detention sizing.

IDF Curve Data

Intensity-Duration-Frequency relationship

IDF Input Method

IDF Equation: i = A / (t + B)^C

where i = intensity (in/hr), t = duration (min)

Coefficient A

Numerator constant

Coefficient B

Time shift constant

Coefficient C

Exponent

Modified Rational Method

The Modified Rational Method extends the standard Rational Method to estimate runoff volume and size detention facilities. It analyzes multiple storm durations to find the critical storm that requires the most storage.

Key concepts:

Peak Discharge: Q = C * i * A (Rational Formula)
Critical Duration: Storm length producing maximum storage requirement
Storage: Difference between inflow and allowable outflow volumes

The method is suitable for preliminary sizing of small detention facilities (typically < 200 acres).

Typical Runoff Coefficients

Land Use	C Value
Asphalt/concrete	0.90 - 0.95
Roofs	0.85 - 0.95
Commercial areas	0.70 - 0.95
Residential (1/4 acre lots)	0.40 - 0.55
Residential (1 acre lots)	0.30 - 0.45
Parks/open space	0.10 - 0.25

Source: FHWA HEC-22 (2009), Table 3-1.

For educational purposes only. Not a substitute for professional engineering judgment.