Flow exchange, energy losses and pollutant transport in a surcharging manhole linked to street profiles

Due to the increased frequency and magnitude of urban flooding events, there is a pressing need to improve the accuracy of numerical tools to better assess the hydraulic performance of new drainage systems. Nowadays, such models are inherently challenging to verify due to the difficulty of acquiring reliable data during the flood event, meaning that most models are calibrated using only an estimated measure of the extent of flooding. To address this gap, this study investigated flooding scenarios using an experimental scale facility of an urban street and manhole network, delivering a novel data-set in terms of the scale of the facility used. Several hydraulic conditions are investigated within a variety of street configurations incorporating parking slots, cars on the road and various locations of the manhole within the street. This enabled the quantification of flow exchange during hypothetical flood events for multiple cases, as well as the characterisation of energy losses, a crucial parameter that is often a source of uncertainty within numerical modelling tools. Furthermore, the experimental system was equipped with an injection system to replicate the transport of pollutants during flooding events, and this enabled the estimation of the exchange of soluble pollutants between the minor and major systems for each flooding scenario. Results obtained have confirmed the applicability of the orifice equation for the estimation of flow exchange between the two systems, showing that i) the discharge coefficients obtained (0.126–0.138) decrease as the width of the street/channel becomes narrower, ii) the surface energy loss coefficient was unaffected by all street configurations tested, iii) all tested geometries displayed significant pollutant exchange from the sewer to the street, in the range of 28–39%, demonstrating that situating the manhole closer to the edge of the street increased the mass of pollutants being exchanged to the surface and the presence of parking spaces alone did not appear to affect the mass of pollutant exchanged. These results have provided a novel series of datasets (including flowrates, flow exchange, energy losses and discharge coefficients) that could be used to calibrate and validate numerical models and be utilised as a benchmark.