What approaches exist to evaluate the effectiveness of UK-relevant natural flood management measures? A systematic map.

Background: This systematic map principally sought to understand the different forms of effectiveness that existing studies evaluate in relation to Natural Flood Management (NFM) in the UK with a supplementary question of whether studies engaged with climate change and future flood risk. NFM measures seek to protect, enhance, emulate, or restore the natural function of rivers as part of approaches to flood risk management (FRM). While there is agreement in both academic and practice/policy literature that NFM should be part of a holistic FRM strategy to address current and future flood risk, the specifics of how to expand the application of and consistently implement NFM successfully in practice are less well known. A core focus of this study is on how the effectiveness of NFM measures is evaluated in different studies based on approaches drawn from the Environmental Impact Assessment (EIA) literature: procedural, substantive, transactive and normative. The systematic map also examines how studies account for climate change, which is a crucial issue given the connections between NFM and climate change adaptation and resilience.

Methods: We searched 13 bibliographic databases, Google scholar as a web-based search engine, and 21 organisational sites. Articles were screened by title, abstract, and full text based on defined eligibility criteria. Checks were performed for consistency amongst reviewers. Forms of effectiveness were coded on the basis of the included studies in the systematic map. The quantity and characteristics of the available evidence are summarised with the frequencies of effectiveness forms for each NFM measure are presented in heat maps.

Review findings: A total of 216 articles reported eligible studies that were coded as part of the systematic map. Overall, the systematic map shows that the majority of studies considered at least one approach to effectiveness; however, very few studies considered multiple forms of effectiveness. The systematic map also demonstrates that climate change is considered systematically by around one-quarter of studies although many studies make claims about NFM's effectiveness in the face of future climatic change.

Conclusions: NFM can be effective in several different ways owing to their multiple benefits; however, there are evidence gaps around understanding these different forms of effectiveness. This is particularly marked for studies considering transactive and normative effectiveness. Interdisciplinary studies are more likely to consider multiple forms of effectiveness. This systematic map also found that whilst 75% of studies mention future climate change in their studies, only 24.1% contain a systematic consideration of the issue through, for example, using climate change projections. NFM is also at risk of climate change (e.g. through drought) and therefore it is imperative that study designs seek to incorporate consideration of effectiveness under future climate change. Policymakers should be made aware of the lack of understanding of how NFM measures perform under future climate change.