A GIS-based tool for identifying areas for nature-based solutions to aid water companies in a green first approach to reduce combined sewer overflows

Combined sewer spills are a significant environmental concern in the United Kingdom and other countries where rain and wastewater are not separated during storms. In recent years, the pollutant impacts of sewer spills on water bodies have received increasing scrutiny. Public awareness and monitoring efforts have consequently grown due to extensive media coverage, highlighting the scale of the issue. The UK Storm Overflows Discharge Reduction Plan sets strict targets for water companies to reduce the number of spills per year. Traditionally, the water industry has relied on grey infrastructure, such as storm tanks, to mitigate storm overflow impacts. However, there is growing recognition of the benefits of green infrastructure, particularly nature-based solutions. While urban-focused Sustainable Drainage Systems (SuDS) and end-of-pipe treatments are common, permeable runoff contributions to combined sewer networks are often overlooked, representing a gap in current approaches. Permeable areas, like parkland or agricultural fields, are often large and, although permeable, overland flow can still occur and reach the sewer networks. Therefore, it is crucial to identify permeable areas that have potential to contribute to surface runoff entering the combined sewer and ideally, put in place solutions to mitigate this risk.

To address this, a Geographic Information System (GIS) tool set within ArcGIS Pro environment, and an equivalent R-based open-source tool has been developed to support water companies in identifying permeable areas with potential for nature-based solutions to reduce overland flow into combined sewer systems. The tool uses spatial layers as inputs and incorporates multiple options to allow for customisation, whilst also accounting for barriers (e.g. hedges and walls) that may interrupt overland flow. Overall, these features help address the topographic complexities of the urban fringe. The final output from the toolbox provides a layer that consists of any potential permeable areas that could drain to the inlets for the combined sewer network and consequently contribute to combined sewer overflows.

The output from the tool identifies permeable areas draining to the combined sewer, as well as areas for potential sites for nature-based solutions. This geospatial data can be further evaluated through desktop and field surveys to confirm a locations suitability for nature-based solutions. Integration with UK Industry standard hydraulic models such as InfoWorks, enables a comprehensive assessment of the potential benefits of solutions. The tool has currently been applied in the 1,800 km² Tamar catchment, UK, demonstrating a pathway for the water industry to spatially prioritise green infrastructure, reduce storm overflows, and transition away from conventional grey solutions, aligning with a green first approach. And in turn, provide additional benefits such as ecosystem services, as well as being less costly than grey infrastructure solutions.