Brownfield Runoff Rates and Volumes
Drainage design can be very complicated and specialist modelling codes will commonly be used for the detailed drainage design for all but the simple development cases. At the outline planning stage, and particularly when considering current brownfield runoff rates, detailed drainage design will not be feasible.
Brownfield runoff rates for developments can be modelled in exactly the same way as for the post development case by setting the urban area to be equal to current brownfield impervious area and proceeding as for the post development case.
If you wish to include the influence of sewer or drain capacities on the brownfield runoff from current urban areas sites you can undertake this using the functionality in the software in conjunction with the guidance on use provided for the urbanised catchment case. You might wish to consider this if part of the current, brownfield impervious surface is drained to sewer and potentially exported from the downstream catchment. To achieve this you would set the area of the sewer system to the area of impervious surface that is positively drained. The sewer system will effectively remove runoff from the catchment up to the sewer capacity and is described in detail within the Urban Model section. It is stressed that this functionality should only be used for outline design and should be confirmed using site specific information wherever possible.