Harnessing Nature: The Future of Sustainable Storm Overflow Management
Storm overflows have become one of the most pressing challenges for water companies, communities, and regulators. Originally designed as emergency measures in Victorian sewerage systems, these combined sewage overflows (CSOs) increasingly operate beyond their intended purpose, discharging untreated sewage into rivers, lakes, and coastal waters with alarming frequency. Nature-based Solutions (NbS) offer innovative and cost-effective approaches to address this issue at its source by reducing the volume of surface water entering combined sewer networks in the first place.
The Growing Problem of Storm Overflows
The UK’s combined sewerage system, a legacy of Victorian engineering, collects both domestic wastewater and surface runoff in the same pipes. During heavy rainfall, these systems can become overwhelmed, triggering storm overflows that release untreated sewage into waterways to prevent flooding of homes and businesses. Climate change, increased urbanisation, and aging infrastructure have exacerbated this issue. According to the Environment Agency, in 2021, 90% of storm overflows discharged at least once, with 5% discharging more than 100 times. This has resulted in significant ecological damage and public health concerns, sparking outrage from communities, environmental groups, and regulators.
The Case for Nature-based Solutions
Nature-based Solutions represent a paradigm shift in water management philosophy. Instead of relying solely on conventional grey infrastructure to transport and treat stormwater, NbS work with natural processes to manage water at source, mimicking natural water cycles and delivering multiple benefits beyond just flood prevention. DEFRA’s Storm Overflows Discharge Reduction Plan explicitly acknowledges this approach: “Rainwater should be treated as a resource to be valued for the benefit of people and the environment, not mixed with sewage or other contaminants”, and “Rainwater should be discharged back to the environment as close as possible to where it lands or channelled to a close watercourse without first mixing it with sewage.”
Nature-based Solutions align perfectly with this philosophy by:
- Reducing the volume of surface water entering combined sewers.
- Slowing the flow of stormwater.
- Filtering pollutants naturally.
- Creating wildlife habitats and enhancing biodiversity.
- Improving urban environments and community wellbeing.
- Sequestering carbon and building climate resilience.
Legislative, Regulatory and Other Drivers
There is no standalone legislation that explicitly mandates water companies to use Nature-based Solutions or green infrastructure for managing storm overflows. Instead, NbS approaches are positioned as preferred strategies within the water industry’s broader regulatory framework, with water companies facing strengthening policy signals and financial incentives for their adoption. While government policies, regulatory guidance, and the price review process increasingly encourage these sustainable approaches, they remain recommended rather than legally required solutions within the complex landscape of water industry regulation.
The water industry is facing mounting pressure to adopt nature-based approaches from multiple directions:
Environment Act 2021
The Environment Act 2021 does not specifically mandate NbS but does require water companies to “secure a progressive reduction in the adverse impacts of discharges from storm overflows.”Nature-based solutions provide a sustainable pathway to achieving this requirement while delivering broader environmental benefits that align with the Act’s biodiversity net gain objectives.
DEFRA’s Storm Overflows Discharge Reduction Plan
DEFRA’s Storm Overflows Discharge Reduction Plan (updated September 2023) provides strong direction rather than explicit requirements. It states, “We expect water companies to prioritise a natural capital approach, considering carbon reduction and biodiversity net gain, as well as catchment-level and nature-based solutions in their planning.” This clear directive puts NbS at the heart of future storm overflow management strategies, also encouraging water companies to“prevent additional rainwater from entering the combined sewer network and remove existing rainwater connections where it is the best value solution.” This provides a direct mandate for implementing sustainable drainage solutions that divert rainwater from combined sewers.
Ofwat’s PR24 Framework
The most explicit regulatory encouragement comes through Ofwat’s PR24 price review framework, where the regulator has committed to “actively support companies in the use of green infrastructure where it is the most appropriate and best value approach.” This effectively creates financial incentives for NbS approaches through the regulatory settlement process. There is increased emphasis on environmental performance and natural capital from Ofwat, the regulator has explicitly stated that it will “Challenge, support and enable water companies to meet the targets set out in the Plan” and “Actively support companies in the use of green infrastructure where it is the most appropriate and best value approach”. Ofwat’s focus on long-term resilience and environmental outcomes incentivises water companies to look beyond traditional infrastructure solutions toward more sustainable approaches like NbS.
WISER Document from EA
The Water Industry Strategic Environmental Requirements (WISER) document from the Environment Agency provides guidance that water companies should consider natural capital and ecosystem service benefits in their planning, again creating soft rather than hard requirements.
Climate Change Act 2008
The UK’s legally binding commitment to reach net-zero by 2050 under the Climate Change Act provides another powerful incentive for nature-based approaches. Traditional wastewater infrastructure is carbon-intensive both in construction and operation. As the Storm Overflows Discharge Reduction Plan acknowledges: “Traditional solutions to reduce discharges, such as increasing storage capacity, are carbon intensive. The costs and benefits of such interventions must be considered in decision-making.” Nature-based solutions not only require less embodied carbon to implement but can actively sequester carbon through vegetation growth, helping water companies reduce their carbon footprint.
Community and Local Authority Pressure
Communities and local authorities increasingly demand action on storm overflows, with greater emphasis on sustainable solutions that deliver multiple benefits. The Storm Overflows Reduction Plan notes that nearly 22,000 responses were submitted during the public consultation, highlighting the public interest in this issue. Local authorities, with their own commitments to climate resilience and biodiversity net gain, are natural partners for implementing catchment-wide NbS approaches. Their support can be crucial for gaining planning permission and community buy-in for ambitious NbS schemes.
Overcoming Barriers to Implementation
Despite the clear benefits and policy support, several barriers still hinder the widespread adoption of Nature-based Solutions for managing storm overflows:
Land Availability and Ownership
Many NbS approaches require more land than conventional infrastructure, particularly in urban areas where space is limited and expensive. Addressing this challenge requires:
- Hybrid designs that maximise functionality in smaller spaces.
- Co-location of drainage features with other urban amenities.
- Collaboration with landowners and local authorities to secure land for strategically located interventions.
Technical Uncertainty and Risk Perception
Water companies, bound by strict regulatory requirements, may be hesitant to rely on NbS approaches without extensive performance data. The Storm Overflows Discharge Reduction Plan acknowledges this, noting: “We are aware that green infrastructure enhancements often have longer delivery timelines than traditional concrete solutions and may therefore be seen as riskier investments by water companies.” To overcome this, the plan specifically states that “the Environment Agency and Ofwat will work to ensure assessment processes promote and incentivise the use of nature-based solution in favour of more carbon intensive alternatives.”
Funding and Valuation Challenges
Traditional cost-benefit analyses often fail to fully capture the multiple benefits of nature-based approaches. The water industry can address this by:
- Adopting natural capital accounting principles.
- Developing shared funding models with other beneficiaries (local authorities, developers, etc.).
- Using Ofwat’s framework for valuing the broader benefits of NbS in PR24 business plans.
Conclusion: A Nature-Led Future for Storm Overflow Management
Nature-based Solutions represent a paradigm shift in how we manage storm overflows—addressing the root cause by managing rainwater where it falls rather than merely treating the symptoms through end-of-pipe interventions.
The legislative and regulatory landscape now actively encourages this approach, with DEFRA’s Storm Overflows Discharge Reduction Plan providing a clear mandate. Early adopters are already demonstrating that well-designed NbS can significantly reduce CSO discharges while delivering additional benefits for biodiversity, climate resilience, and community wellbeing.
As water companies develop their plans to meet the ambitious targets in the Storm Overflows Discharge Reduction Plan, integrating nature-based approaches will be essential—not just for compliance, but for delivering truly sustainable, multi-benefit solutions that address both current challenges and future pressures from climate change and population growth.
By embracing nature as a partner rather than an afterthought, the water industry can transform the challenge of storm overflows into an opportunity to create more resilient, biodiverse, and liveable environments that reconnect communities with their local waterways.
