Urban flood modelling and sensitivity analysis: water exchange between the ground surface and the underground sewer network

Most 1D-2D urban drainage models simplify the water exchange process between the ground surface and the underground sewer network. They assume that surface runoff enters directly into the sewer network without modelling the initial overland flow. Moreover, surface flooding is only seen as the result of sewer network overflows, neglecting that it can also occur due to limitations of street inlets draining capacity. In fact, it is a common practice to ignore street inlets and to assume that water exchange occurs exclusively through manholes, which in reality, can have water transfers (sewer overflow) only after the displacement of their covers. These simplifications, do not allow to determine which water exchange processes have a greater impact in the occurrence of surface flooding.

This study developed a more realistic model representation of the urban drainage system (including street inlets and initial overland flow) and carried out a thorough sensitivity analysis of the parameters controlling water exchange processes. A combination of Latin-Hypercube (LH) and One-factor-At-a-Time (OAT) sampling techniques were used to measure global and local sensitivities. Brederode neighborhood in Antwerp (Belgium), a flat area that frequently suffers from pluvial flooding, was used as a study case. Results show that the inclusion of street inlets reduces the calculated total surface flood volume in simulations with design storms ranging from low to high return periods (T5-T100). It was also found that parameters controlling surface drainage are the most sensitive, with the street inlets clogging coefficient obtaining the highest sensitivity index value. Parameters controlling reverse flow showed almost null sensitivity.

Given that the draining processes are most sensitive (particularly street inlets clogging) to the occurrence of surface flooding, urban drainage models should explicitly include manholes and street inlets in their configuration. Moreover, it is recommended to apply rainfall directly on 2D mesh elements representing streets and open areas (for runoff produced on rooftops, use sub-catchment units). In this way, models can account for initial overland flow and properly assess the street inlets' drainage capacity.