The fate of infiltrated stormwater from infiltration basins to the stream: quantifying the impact of the urban karst

Urbanization leads to severe alterations to the flow regime of receiving waters, including increased frequency and magnitude of storm flows as well as reduced baseflows. Infiltration basins are among the most widely applied stormwater control measures worldwide, in part for their ability to intercept stormwater runoff and allow it to infiltrate into the ground, with the assumption that this will recharge groundwater and thus help in restoring clean, filtered baseflows to receiving waters. Recent research has highlighted that in fact, the fate of infiltrated stormwater is highly uncertain, particularly because of likely interactions with underground infrastructure—e.g. sewer pipes, telecommunication cables, etc. These infrastructures are typically surrounded by highly permeable material which has the potential to substantially alter the way infiltrated stormwater moves through the subsurface (a phenomenon known as the urban karst).

This study aimed to predict and generalize the impact of the urban karst on infiltrated stormwater as it can provide a preferential flowpath and thus may prevent infiltrated stormwater from reaching receiving waters or may short circuit subsurface storages that can increase routing time delays and thus baseflow. In doing so, a modelling study using HYDRUS-3D was undertaken. In addition, a novel approach to generalize the results was proposed based on groundwater level and the hydraulic conductivities of soil and gravel/sand. We predicted that the impact of the urban karst on infiltrated stormwater increases whit higher groundwater levels, and greater contrasts between the hydraulic conductivity of regional soil and gravel. The HYDRUS results for a wide range of scenarios are compared with the generalization, which captures the impact of Urban Karst well.

It is important to consider the impact of the urban karst where one of the goals of building infiltration basins is to recharge the baseflow of the stream downslope. This suggests that decision on basin location is important where urban infrastructure is located between potential infiltration basin sites and downslope stream. The impact of the urban karst should be investigated at each specific site before implementing infiltration systems and this study works towards simplified representations of impact for design.