Rational Formula & Peak Flow

Q = 2.78 C i A
where:
Q = Flow rate (L/sec)
C = Runoff Coefficient
I = Rainfall Intensity (mm/hr)
A = Area that rain is falling on (Ha)

Peak Flow Rate

Q =0.0L/sec
Q = 2.78 × C × I × A
Q = 2.78 × 0.9 × 102 × 0.6525
Q = 0.0 L/sec

Calculation Inputs

Time of Concentration

tc = te + ti
where: te = time of entry (minutes)
ti = time (minutes) of network flow
tc has a minimum value of 10 minutes
10 min

Rainfall Intensity (from Hirds v4)

Runoff coefficient

0.90 - Fully roofed and/or sealed developments

Catchment Area

(6,525.00 m²)

Design Notes

Time Guidelines:

te = 5 minutes for commercial or industrial areas where greater than 50% of the surface of the catchment are feeding the drain or open channel consist of roofed, asphalt, concrete, paved or metalled surfaces.
te = 7 minutes for residential areas where no overland flow path exceeds 50% of gross area.
te = 10 minutes for low density residential areas where the impervious area is 35% to 50% of gross area.

Catchment Comments & Site Plan

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