How much water will flow off a 1 hectare construction site in a big storm?

The answer can be found with:

Q = CiA

Q = Flowrate

C = Coefficient of Runoff

i = intensity of the storm

A = Area of the Catchment that the rainfall will runoff of.

This formula is usually called “The Rational Method.”

Determining the Coefficient of Runoff is the trickiest part of the rational method exercise.

As you can imagine a hard surface that does not allow any rainfall to soak in will have a very high runoff coefficient, maybe 0.9 or 0.95 in a high intensity storm. Sandy soil that allows a lot of water to soak in, would have a very low runoff coefficient, maybe 0.2 if on a very flat surface. Forests also have a low runoff coefficient because a lot of the water is caught in the branches and leaves of the trees and never hits the ground and a lot is soaked up by the leaf litter, grass and other material on the forest floor. I’ve listed a few runoff coefficients below from Hydrologic Analysis and Design by Richard McCuen (1989, Prentice-Hall Publishers).

For our first example we will just assume that we had a storm of 30mm in 2 hours. A relatively common event in Sydney.

We can assume C = 0.55 as the worst case from the reporting of Runoff Coefficients from the CALM manual Urban Erosion and Sediment Control, 1992 as shown below.

Q=CiA = 0.55 x 30mm/(2 hours) x 1 hectare x 10,000 m3 / ha x (1 m / 1000mm) x (1 hour / 3600 sec)

Q = (0.55 x 30 x 1 x 10,000) / (2 x 1000 x 3600) = 0.023 m3/sec

Volume of runoff = Q x t = Flowrate x time

V = 0.023 m3/sec x 2 hours x 3600 sec / hour = 165 m3 = 165,000 Litres

Therefore we know that if we build a 165 m3 retention pond we will capture all of a 30mm/hour, 2 hour storm on our 1 hectare construction site.

How much water will flow off the one hectare site in a one day, 1 in 20 year storm.

The one in twenty year storm is often called the 5% storm, but in actual fact it is a one day in twenty year storm, or one day in (20 x 365.25) = 7305 days, so really it is the 0.014% storm.

Rainfall intensity is determined using historical data that is getting better each year. We have an excellent system in Australia developed by Dr. David Pilgrim and his team and published by the Institution of Engineers in Australian Rainfall and Runoff (ARR). Using the ARR system we can find the intensity of a storm that will last 10 minutes, an hour, 12 hours, 3 days and most increments in between. I have attached at summary of the intensities at the Homebush Olympic site that were developed from the ARR model.

From the table below the rainfall from the 20 year storm over a 24 hour period is 9.19 mm/hour. Over a 24 hour period that would be:

24 hrs / day x 9.19 mm / hour = 220 mm / day

Using the rational method again:

Q=CiA = 0.55 x 9.19 mm/hour x 1 hectare x 10,000 m3 / ha x (1 m / 1000mm) x (1 hour / 3600 sec)

Q = (0.55 x 9.19 x 1 x 10,000) / (1000 x 3600) = 0.014 m3/sec

Volume of runoff = Q x t = Flowrate x time

V = 0.014 m3/sec x 24 hours x 3600 sec / hour = 1200 m3 = 1,200,000 Litres

Therefore we know that if we build a 1200 m3 retention pond we will capture all of the 24 hour, 20 year storm on our 1 hectare construction site.

Runoff Coefficients for the Rational Formula

Land Use for less than 25 year storm C Land Use for greater than 25 year storm C Paved Parking Area, <2% slope 0.85 Paved Parking area, >6% slope 0.97 Commercial, <2% slope 0.71 Commercial, >6% slope 0.90 Streets, <2% slope 0.70 Streets, >6% slope 0.89 Industrial, <2% slope 0.67 Industrial, >6% slope 0.87 Residential 1000m2 block, loam soil <2% slope 0.22 Residential 1000m2 block, loam soil >6% slope 0.47 Pasture, sandy soil, <2% slope 0.12 Pasture, loam soil, >6% slope 0.52 Meadow, sandy soil, <2% slope 0.10 Meadow, loam soil, >6% slope 0.44 Cultivated land, sandy soil, <2% slope 0.08 Cultivated land, loam soil, >6% slope 0.34 Forest, sandy soil, <2% slope 0.05 Forest, loam soil, >6% slope 0.20

Summarised from Hydrologic Analysis and Design by Richard McCuen (1989, Prentice-Hall Publishers), page 282.

Rational Method C values for disturbed sites,

Bare packed soil, smooth = 0.25 to 0.55

Bare packed soil, rough = 0.15 to 0.45

From Urban Erosion and Sediment Control, 1992, Department of Conservation and Land Management, page 29

C Values for feedlots and irrigation areas from NSW Feedlot Manual

Feedlots = 0.80, page A6.4.1and

Irrigation areas = 0.65, page A6.4.1

Feedlot Manual The Interdepartmental Committee on Intensive Animal Industries (Feedlot Section), 1995

Storm Intensity in millimetres per hour

at Homebush Olympic Site, Sydney, New South Wales, Australia

Duration Average Storm Recurrence Interval (years) 1 2 5 10 20 50 100 5 min 98.6 126.5 161.9 180.8 207.2 241.4 267.4 10 min 75.7 97.5 126.1 141.5 162.8 190.7 211.8 15 min 63.3 81.8 106.4 119.9 138.4 162.6 181.0 30 min 45.0 58.3 76.9 87.2 101.2 119.6 133.7 60 min 30.7 40.0 53.5 61.2 71.4 85.0 95.5 3 hr 15.2 19.8 26.4 30.1 35.1 41.7 46.8 6 hr 9.7 12.6 16.7 19.0 22.2 26.4 29.6 12 hr 6.2 8.0 10.6 12.1 14.1 16.7 18.7 24 hr 4.02 5.23 6.94 7.90 9.19 10.90 12.21 72 hr 1.92 2.50 3.31 3.77 4.38 5.19 5.81

Calculated using the algebraic procedures in Chapter 2 of Australian Rainfall and Runoff (1987).