The Environment Agency published this week (18th May 2026) new official guidance introducing the Surface Water Investment Model (SWIM) approach. This framework is specifically designed to modernise how funding is secured for widespread sustainable drainage systems (SuDS) across England, establishing a more structured method for evaluating decentralised flood resilience.
What the SWIM approach is
The SWIM approach is a strategic appraisal methodology created to assess the cumulative benefits of multiple small-scale SuDS features distributed over a broad geographic area. Historically, standard funding models have favoured large, single-site engineering assets because the individual impact of a single raingarden, permeable pavement, or small attenuation basin can be difficult to quantify in a traditional business case. The SWIM framework solves this problem by grouping these distributed interventions together, allowing practitioners to model and measure their collective impact on reducing surface water flood risk.
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Who the guidance is for
This guidance is primarily aimed at Risk Management Authorities (RMAs) responsible for managing local flood risks. This includes lead local flood authorities, district councils, unitary authorities, and internal drainage boards. It is also an essential reference tool for civil engineering consultants, environmental planners, and catchment managers who are tasked with designing regional flood alleviation schemes and bidding for central government funding.
What it should be used for
The core purpose of the SWIM approach is to help local authorities build robust, evidence-based business cases to secure Flood and Coastal Erosion Risk Management Grant in Aid (FCERM GiA) funding. Alongside the guidance, the Environment Agency has released a dedicated SuDS programme business case template. RMAs must use this specific template when developing any surface water management project that relies on the SWIM methodology.
By applying this approach, local authorities can successfully justify public investment in green infrastructure. It transitions surface water management away from isolated, reactive fixes and enables the strategic, proactive rollout of catchment-wide urban greening that protects communities from the increasing threat of intense rainfall events.
[Main image credit: Jun Huang / shutterstock.com]