Variably saturated flow in storm-water partial expiration trench

Storm water from impervious urban areas can adversely impact water quality and quantity. The partial exfiltration trench (PET) is a control device des igned to moderate both the quality and the quantity of urban runoff. This p aper uses a 2D numerical model to evaluate variably saturated flow profiles and residence time distributions for a PET subject to storm water loading. Parameters estimated from laboratory experiments and hydrographs measured at a prototype PET are used to calibrate the numerical model. Simulation ex periments show that flow through the PET is influenced strongly by the rate and duration of the hydraulic loading and by the type and properties of th e surrounding soil. Unless the surrounding soil is nearly saturated or high ly impermeable, propagation of the wetting front through an unlined PET occ urs as 2D variably saturated flow. Variably saturated 2D flow through the P ET is characterized by skewed residence time distributions, long mean trave l times (relative to plug flow), high exfiltration losses to the surroundin g soil, and low tracer mass recovery at the underdrain. These features of t he PET performance are beneficial for storm-water treatment because the fir st flush of runoff often contains significantly higher pollutant concentrat ions than those present in later phases of the storm.