Stormwater infiltration
In some instances, the infiltration of stormwater into the ground, at rates beyond those occurring naturally, is encouraged. This is done to reduce minor flooding and to enhance receiving water quality, by reducing the amount of surface runoff and using the soil as a filter for pollutants.
Infiltration devices include:
- trenches filled with gravel or other porous material to hold water and gradually release it to the surrounding soil,
- larger underground chambers that perform the same function,
- surface storages or depressions with permeable bases and sides,
- wells that inject water into deeper layers of soil or recharge aquifers.
They need to be located well above the local water table. Spaces within underground chambers may be empty, or may be filled with gravel, sand or a geocomposite product such as plastic gridwork. The spaces are usually lined with a permeable geofabric. Stormwater is directed into these spaces, usually after some pre-filtering to catch gross solids and sediments. It then gradually seeps into the sides and bottom of the space. While complex groundwater flow equations can be employed, a constant infiltration rate based on the hydraulic conductivity of the surrounding soil is usually assumed. This is multiplied by the surface area through which infiltration occurs to determine the overall rate of discharge from the infiltration device.
Most infiltration systems act as storages and can be designed using the continuity equation:
Storage at the end of a time period = Storage at the beginning + Inflows - Outflows.
In DRAINS such systems can be modelled as a detention basin. There is a procedure for modelling infiltration from storages using the following panel in the Detention Basin property sheet.
If this data is supplied, DRAINS will calculate an infiltration hydrograph, which can be viewed among the results.
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