Nature-Based Flood and Drought Resilience: The Evidence Gap POSTnote 768 Just Flagged

The briefing, by Ben Cutting and Jonathan Wentworth, looks at how nature-based solutions can address flood and drought risk at catchment scale. Nature-based flood and drought resilience is a growing area of research and policy interest for tackling these challenges. It is balanced, technically careful and direct about what the field does not yet know. The catch-22 it sets out is worth understanding before nature-based solutions show up in the next pre-application discussion, Section 106 negotiation or LLFA drainage review.

What the briefing covers

The briefing covers flood and drought resilience together, treating them as the same hydrological problem at opposite ends of the year. Climate change is projected to bring “a possible threefold increase in days of extreme heat” alongside “a fourfold increase in high-impact rain events” in the UK, which means the same catchment may be too dry one summer and too wet the next. Peatland restoration, woodland creation, soil and land management, leaky barriers, floodplain reconnection and field-corner bunds are presented as interventions that, in theory, work for both.

The policy backdrop is supportive. The 2026 Land Use Framework for England, the 2025 National Framework for Water Resources, five new Regional Water Planning Groups, and the 2026 revision of HM Treasury’s Green Book (which now requires natural environment impacts to be considered from the outset of project proposals) all push in the same direction. The briefing does not dispute that direction. It asks whether the evidence base yet supports the scale of deployment now being asked for.

The evidence the briefing assembles

The case studies the briefing assembles do show that nature-based solutions can deliver measurable reductions in flood peaks. Field-corner bunds in the Eddleston Water catchment in Scotland reduced peak discharges “by up to 55%” across modelled storm events. The Holnicote project in Exmoor reported peak reductions “of up to 38% in major events.” On the River Otter, two beaver families built 28 dams that slowed and backed up nearly two kilometres of river. In Norfolk, the Wendling Beck partnership has restored around 800 hectares of arable land to mixed habitat. Catchment-scale programmes in the Scottish Borders, Severn Valley, Holnicote and Eden are doing similar work at greater scale.

Read in isolation, the numbers look encouraging. The qualifiers around them are doing a lot of work.

The catch-22

The Eddleston dataset is, on the briefing’s own account, almost the only long-term, real-world UK nature-based solutions dataset cited. It covers a single small catchment. Most other studies the briefing references monitor short windows or rely heavily on theoretical modelling. POST describes current NbS funding as fragmented, short-term and sector-specific. Without sustained funding, no scheme can be monitored long enough to produce the kind of multi-decade, multi-catchment evidence the Environment Agency, water companies and infrastructure operators would need to commit at scale.

That is the catch-22. Proving catchment-scale nature-based solutions work needs long-term, large-catchment trials. Funding long-term, large-catchment trials needs funders prepared to commit before that long-term, large-catchment evidence exists. The briefing also flags a related distributional problem: market-based instruments for funding NbS can disproportionately benefit affluent areas, concentrating monitoring data in the catchments that least need new schemes. The headline issue is the evidence loop itself. It is not unique to nature-based solutions, but it is acute here because the alternative, engineered defences with a well-understood performance envelope, is sitting right next to it on the shelf.

Implications for planning applications

The practical effect is felt in pre-application discussions, in Section 106 negotiations, and in the increasingly common request for nature-based or “green SuDS” elements as part of a planning betterment offer.

First, NbS performance figures should be quoted with care. A 38% reduction in flood peak from Holnicote is a real result for that scheme on those storms. It is not a generic factor that can be applied site-unseen. Lead Local Flood Authorities and the Environment Agency are alive to this. A flood risk assessment or drainage strategy that leans heavily on borrowed performance data without site-specific justification will draw a request for clarification at best, and an objection at worst.

Second, climate change allowances still apply. The Environment Agency’s standard uplifts are calibrated to rainfall and river-flow projections, not to the assumed performance of a nature-based feature whose monitoring history is short. Where NbS forms part of a scheme’s flood mitigation, residual risk analysis should account for the NbS not performing as modelled.

Third, nature-based solutions rarely live alone. Every catchment-scale programme cited in the briefing combines uplands, floodplains and channel works with conventional drainage and engineered defences. The briefing itself is explicit that NbS should be portrayed “not as a cheaper alternative but as part of a catchment-wide approach.” That framing matters at planning. A drainage strategy that presents NbS as a substitute for source control or attenuation will be tested harder than one that presents it as additive.

Where the policy levers are

The briefing points at three things that would loosen the catch-22.

The first is standardised monitoring, with shared methodologies and accessible data, so that the next decade of small-catchment trials accumulates into something a regulator can rely on. The second is integrated, multi-year funding that pays for installation, monitoring and maintenance, not for installation alone. The 2026 Green Book revision and the Regional Water Planning Groups are early signals in that direction. Neither yet closes the funding gap.

The third, which the briefing keeps returning to, is honesty in framing. Nature-based solutions are most defensible when described as a complement to engineered defences, with a confidence interval and a maintenance plan, rather than as a cheaper or greener alternative. That is also how they are most likely to survive contact with planning policy, statutory consultees and a flood-risk-aware insurance market.

What this means at site level