Evaluation of SuDS device-scale simplified modelling methods

As Sustainable Drainage Systems (SuDS) / Low Impact Development (LID) methods are increasingly utilised to manage stormwater in urban areas, it becomes crucial for drainage engineers to accurately represent their hydrological and hydraulic impacts within drainage modelling tools. However, not all tools include explicit SuDS modelling capabilities or their utilisation may be considered too computationally expensive in some practical contexts. This paper examines the potential for representing two common SuDS devices – a green roof and a bioretention cell – using generic hydrological/hydraulic model components. Various approaches, including initial losses, detention storage, catchment area disconnection, and reassignment to permeable surfaces, were compared against outputs from the SWMM LID module. The main findings indicate that all approximation approaches have limitations. Methods involving complete disconnection or transferring catchment areas to pervious surfaces failed to accurately simulate the hydrological dynamics at the device scale. In contrast, two approaches based on initial losses and detention storage showed greater potential, with continuous losses based on evapotranspiration (ET) providing more realistic responses compared with a daily fixed recharge depth. In scenarios where bioretention drainage is controlled via an orifice, these two approaches produced outflow profiles similar to those of SWMM’s LID module in continuous simulation (both NSE > 0.8). However, neither model performed acceptably in scenarios where detention effects were dominated by substrate percolation.