Quick Answer
When sizing a first flush diverter, consider the roof catchment area, rainfall intensity, and the amount of debris or contaminants that will be washed off the roof. Calculate the required diverting volume based on a standard 1:5 to 1:10 ratio. Ensure the diverter can handle the minimum required flow rate.
Sizing Roof Catchment Area
The first step in sizing a first flush diverter is to calculate the roof catchment area. This can be done using the formula: Catchment Area (in square meters) = Roof Area (in square meters) x Runoff Coefficient (typically 0.85 for most roofs). For example, a 100 square meter roof with a 0.85 runoff coefficient would have a catchment area of 85 square meters.
Calculating Required Diverting Volume
Next, calculate the required diverting volume using a standard 1:5 to 1:10 ratio. This means that for every 5 to 10 liters of water collected, 1 liter should be diverted. For a 100 square meter roof with an average annual rainfall of 1000 mm, the required diverting volume would be approximately 2 cubic meters.
Selecting the Right Diverter Size
Finally, select a first flush diverter that can handle the minimum required flow rate and the calculated diverting volume. A good rule of thumb is to choose a diverter with a minimum flow rate of 1.5 liters per second and a diverting volume of at least 1 cubic meter. This will ensure that the diverter can handle the maximum expected rainfall intensity and divert the required volume of water.
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