Identifying opportunity areas for catchment-based interventions to reduce runoff contributions from permeable surfaces to combined sewer overflows (CSOs)

Combined sewer overflows (CSOs) are relief valves built into urban combined sewer systems to prevent sewer flooding during extreme rainfall events. In recent years, it has been recognised that they are spilling far too frequently, causing significant pollution events on a regular basis. In England and Wales, the Storm Overflows Discharge Reduction Plan has set strict targets and requires huge changes in the water industry to reduce the environmental impacts of CSOs and the frequency of spills.

Green infrastructure and Nature-based Solutions (NbS) are increasingly recognized as valuable tools for mitigating CSO spills while offering additional environmental benefits compared to traditional grey infrastructure or conventional (typically capital intensive) engineering solutions. Whilst much of the focus has been on reducing impermeable inputs to sewers through urban SuDS (Sustainable Drainage Systems) or using wetlands to treat CSO spills, much less attention has been given to the potential for catchment-based solutions or NbS to reduce rural or green space runoff entering the combined sewers on the urban fringe. There is therefore a major knowledge gap in understanding where surface water from permeable surfaces could be entering the combined sewers and consequently where NbS could be applied to mitigate the problem of CSOs.

This study presents a workflow for identifying opportunity areas for catchment-based interventions or NbS to reduce rural or green space runoff contributions to combined sewers. The methodology involves GIS-based (Geographical Information System) topographic analysis to delineate sub-catchments draining towards sections of impermeable surfaces (e.g. roads) that connect to combined sewers. As a proof of concept, this approach was applied to wastewater catchments in South West England. A key data source for our analysis is Impermeable Area Survey (IAS) or Contributing Area Survey (CAS) data that define where impermeable surfaces drain to (e.g. watercourses, soakaways or combined sewers). The level of opportunity within a wastewater catchment is highly dependent on the presence of road drains that connect to combined sewers being adjacent to and downslope of rural/green spaces.

The geospatial analysis results identify areas with topographic connectivity to the combined sewers. Next, desk and field-based surveys of potential opportunity areas can indicate runoff potential and whether there is true connectivity or whether there are barriers not represented in the digital elevation model (DEM).  Following the workflow, hydraulic modelling quantifies runoff contributions to CSO spill counts and volumes and the potential for field runoff mitigation through NbS. The level of potential opportunity varies greatly between wastewater catchments. In some cases, field runoff could be making a notable contribution to the volume and/or number of CSO spills. In one case study, modelling indicates that NbS could achieve approximately 45% to 80% reduction in spill volume and 30% to 60% reduction in spill duration depending on the field infiltration rate.

This approach has the potential to be used by water and sewerage companies to strategically identify opportunities to reduce rural and green space surface runoff inputs to combined sewers, through catchment-based solutions or NbS, ultimately helping to meet spill reduction targets and enhance environmental outcomes.