A Practical Integrated 1D – 2D Hydrodynamic Modelling Approach for Evaluating the Surface Water Management Impact of Distributed Retrofit Sustainable Drainage Systems at the Concept Stage

ABSTRACT There is increasing interest among decision makers in retrofitting large numbers of sustainable drainage systems (SuDS) across urban areas to help deal with the growing issue of pluvial flooding. For the practitioner an important early stage in the development of a retrofit SuDS scenario is the optioneering process, when it is desirable to evaluate the flood risk benefit of the SuDS scenario for the first time. However, the modelling options currently available are unsuited to the task. Some require specific design information that is not available at the early stages of SuDS projects, while others ignore the complexity of the urban topography or the sub‐surface drainage system. In this paper we used a dynamically coupled integrated 1D–2D hydrodynamic model to evaluate the flood risk benefit of a distributed retrofit SuDS scenario developed for an urban neighbourhood in the UK. The 675 SuDS that comprise the SuDS scenario were conceptualised through a multidisciplinary optioneering process and represented in the model by modifying the 2D surface and parameters. The model successfully represented pluvial flooding in the study area and provided a comprehensive means of comparing a distributed retrofit SuDS scenario to a Baseline scenario. The 2D surface was essential for representing the heterogeneous distribution of the simulated flooding, while the 1D sub‐surface drainage model demonstrated the non‐linearity of the drainage rate, which included a period of surcharge. The SuDS scenario delivered meaningful reductions in the depth of flooding, but the impact of the SuDS diminished as storm severity increased. Comparison of the model with the UK Environment Agency Risk of Flooding from Surface Water map suggests that it reproduced the spatial patterns of flood risk at the study site and highlighted the importance of the 1D representation of the drainage system. This study demonstrates the potential of a dynamically coupled integrated 1D–2D hydrodynamic model to produce spatially explicit evaluations of plausible retrofit SuDS scenarios developed by practitioners.