How to calculate storm water runoff

Written by eric moll
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How to calculate storm water runoff
Compare expected rainfall to water needs to determine what size cistern you need. (MihailUlianikov/iStock/Getty Images)

Runoff is equal to the amount of rain which falls during a storm, minus the water which is absorbed by soil or plants. Being able to predict the amount of runoff which occurs during a rainfall event is important for a variety of different projects. City planners use the information to determine the readiness of drainage systems and waste water treatment facilities to deal with big storms. On a more domestic level, knowing the amount of runoff on your property is important whether you're installing a roof cistern or simply reshaping your garden's topography to collect water for your plants.

Measuring your property

Measure all of the impermeable surfaces on your property, using measuring tape to determine the length and width and then multiplying them. Impermeable surfaces include roofs, concrete, cement -- basically any surface which won't absorb much water. A permeable surface generally means soil of some sort. For our example, we will say we have only one impermeable surface, a large roof of 400 square metres (4,305 square feet).

Measure the total area of your property. If your property is too large for this to be feasible, consider using the distance function on Google Earth or a similar program to estimate the total area. Let's say that our example property is 1 hectare (2.47 acres).

Divide the area of impermeable surfaces by the total area of your property, then multiply the result by 100 to get the percentage of your property which is impermeable. Make sure both your numbers are in the same unit, either hectares or square metres. There are 10,000 square metres (107,639 square feet) per hectare. In our example, multiply 1 hectares by 10,000 to get 10,000 square metres. Then we divide 400 by 10,000 and multiply by 100 -- showing that we have 4% impermeable surfaces.

Calculating runoff: shortcut method

Determine the number of centimetres of rainfall during the storm. This can be done using a simple rain gauge, or looking up meteorological data for a specific storm or for an average storm. Divide by 100 to get the rainfall in metres. For our example, we will use a 2.5 cm storm, or 0.025 m.

Calculate the volumetric runoff coefficient, Rv. The equation for this is Rv = 0.05 + 0.009(I) where I is the percentage you calculated in section one, step three. For our example, Rv is equal to 0.009 x 4 + 0.05, or 0.086.

Calculate the runoff, R, using the equation R=P x Rv x A. P is equal to the rainfall in metres, Rv is the volumetric runoff coefficient from the previous step, and A is the total area of your property. In our example, we multiply 0.086 by 0.025 by 10,000 for a total runoff of 21.5 cubic metres of water.

Calculating runoff: advanced method

Perform a more detailed analysis of the different surfaces on your property. In this more advanced, more precise method, you must differentiate between flat roofs/unpaved car parks, tilted roofs/paved car parks, sandy soils, silty soils, clayey soils and other impermeable surfaces. Measure the area of each type.

Using the table linked in the "resources" section, determine the individual volumetric runoff coefficients of each land type.

Calculate a weighted volumetric runoff coefficient for the whole property. Multiply the individual Rv of each land type by its respective area, then add all of these together. Divide this number by the total area to get the weighted Rv value.

Perform the calculation from section 2, step 3, using the weighted Rv value instead of the shortcut version.

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