Quantifying the impact of the urban karst on infiltrated stormwater

Urbanization alters the flow regime of streams, including increasing the frequency and magnitude of storm flows, along with reducing baseflows. An increasingly common management strategy is stormwater infiltration, which is thought to reduce surface runoff and recharge groundwater and thus restore lost baseflows to streams. Recent research has pointed to considerable uncertainty on the fate of infiltrated stormwater, particularly due to the presence of human-made underground infrastructure – e.g. sewer and water supply pipes, telecommunication cables. Such infrastructure is commonly housed in trenches partially filled with highly permeable material and this can cause urban karst like flow conditions. We used a dynamic subsurface flow model (HYDRUS-3D) to predict the impact of the urban karst on the fate of infiltrated stormwater. The model was constructed with the presence of a sewer pipe situated between an infiltration basin and a stream. The model predicted that the impact of the urban karst on infiltrated stormwater increases with higher groundwater levels, and greater contrast between hydraulic conductivity of regional soil and gravel which surrounds the sewer pipe. Results suggest that it is important to consider the impact of the urban karst in cases where the goal of stormwater infiltration is baseflow restoration.