The following worksheet is used as an example. The calculations assume the runoff water can be removed in 3 days, with half being removed in the first day. A derivation of the minimum filter area follows. Drawdown time must be checked, and the filter size may be larger to accommodate a drawdown time.
To calculate the minimum media filter area; the catchment characteristics, filtration rate and design treatment depth are input, as shown above. No need to re-enter all the catchment data as it has already been done. For this example, the pond bank side slope is such that there is room for a 10-feet wide filter to maintain a minimum filter media depth of 2 feet. The area of the filter is calculated as 1201.58 square feet; thus, the length of the filter was set at 120 feet (1201.58/10).
Note: In this case, the EIA is that area for the runoff volume at a design treatment depth. It is not the average annual runoff coefficient. This is done as a conservative estimate because it assumes no initial abstraction for all events.