Infiltration trenches receive runoff in a shallow excavated trench that has been backfilled with aggregate to form a below-grade reservoir.
Water then enters the underlying subsoil according to the infiltration rate of the native soils. Infiltration is effectively a means to reduce stormwater quantity and peak flows, and to provide local recharge of groundwater.
Ideally pre-treatment would be provided prior to infiltration to prevent clogging of the device. Infiltration and soakage trenches are often encountered at the end of a treatment train, or where they receive relatively clean roof runoff with appropriate systems in place to divert the first flush of contaminants.
There may also be specific constraints for these practices where slope stability is potentially affected by groundwater (making geotechnical approval necessary) and localised mounding of groundwater, which may affect extractive bores, structural soils or foundations (requiring civil engineering assessment).