Retrofitting to manage storm overflows- A case study in Southwest England

This study developed a simplified 1D hydraulic model to quantify field runoff entering a combined sewer network in a catchment in southwest England. This model uses the Green-Ampt model to represent the infiltration process and a reservoir routing model to represent the runoff routing. The modelled field runoff was input into the existing sewer network model as inflows in InfoWorks ICM to analyse the contributions of various components to storm overflows. The simulations revealed that storm overflow frequency is primarily driven by impermeable area surface runoff, overflow duration is mainly affected by ground infiltration, and field runoff significantly contributes to overflow volume. Nature Based Solutions (NBS) and Sustainable Drainage Systems (SuDS) were implemented within the model to evaluate their effectiveness in mitigating storm overflow through long-term simulations. Results indicated that together with an NBS pond, retrofitting 8% of impermeable surfaces with SuDS can reduce storm overflow frequency by 38%, overflow duration by 40%, and overflow volume by 77%, highlighting the potential of these interventions to minimise storm overflow impacts.