Modelling the environmental impact of combined urban drainage systems with a lumped hydrological approach

The management of combined urban drainage systems is a complex task, as it requires detailed knowledge about precipitation regime, hydrological features of the catchment, hydraulic characteristics of the drainage network, and information about the water use by the served inhabitants. Heavy semi-distributed hydrological and physically based hydraulic models are used for network conduits design. However, in many management problems, the knowledge of the hydraulic flow characteristics in all the conduits is not required, and the uncertainty of the available information hampers the use of complex hydrological models. Hence, simple models with few parameters and small computational effort may be preferable, especially for management and planning problems requiring the execution of many simulations.

In this study, a novel approach is proposed for the definition of effective lumped simplified models of urban drainage systems, the parameters of which can be estimated directly from cartographic information. For several case studies, the hydrographs predicted by lumped simplified models result close to those obtained with semi-distributed models in SWMM. The results show that robust relationships linking lumped model parameters with morphological and topological characteristics of the urban catchment can be established (Farina et al., 2023).

The proposed lumped modelling approach is applied to carry out a sensitivity analysis of the effects of parameters characterizing climate, urban catchment, and overflow discharge device, on several indicators of the environmental impact of combined sewer overflows (CSO) (Farina et al., 2024). In fact, pollution from CSO is still not satisfactorily addressed by current management practices and regulations, usually setting a dilution threshold for the discharged overflow, and enforcing limitations to the number of overflow activations per year. The sensitivity analysis indicates that the percentage of impervious surface of the catchment is the most influent parameter on all the indicators, and its reduction can effectively contain the yearly discharged pollutant mass. The overflow activation threshold, instead, results the second least influent parameter, suggesting that its regulation alone would not be a suitable strategy to control CSO pollution. The results also indicate that neither sustainable urban drainage practices, nor interventions on the CSO device, significantly affect the frequency of the overflows, which is indeed controlled by the local precipitation regime. Furthermore, the yearly discharged pollutant mass and the mean concentration of pollutants in the overflow result independent on the overflow activation frequency. Hence, the regulation of this latter does not seem a suitable means to reduce the environmental impact of combined urban drainage systems.

References.

Farina, A., Di Nardo, A., Gargano, R., van der Werf J.A. & Greco, R. (2023). A simplified approach for the hydrological simulation of urban drainage systems with SWMM. Journal of Hydrology, 623, 129757.

Farina, A., Gargano, R., & Greco, R. (2024). Effects of urban catchment characteristics on combined sewer overflows. Environmental Research, 244, 117945.