Nature-Based Solutions: Dependency of Effectiveness on Spatial Configuration

Several recent large flood events have had severe economic and social impacts. The winter 2015-16 UK floods resulted in 16,000 properties flooding and damage to critical infrastructure. It is increasingly being recognised that traditional approaches of flood defence are not sustainable due to the pressures of climate change and economic constraints. The solution to the flood risk problem in cities is no longer seen as being just on-site, and thinking is shifting upstream and to the catchment/landscape scales, known as Nature-Based Solutions or Natural Flood Management (NFM). The approach consists of measures that “Work with Natural Processes”, such as storing water in ponds, and slowing the flow in rivers. The evidence for the impacts is strong at the local scale, but the larger spatial scale impact is highly uncertain due to the cumulative impacts resulting from amplifying/mitigating effects of different interventions, controlled by spatial location and storm-track interaction.

To date, Nature-Based Solution schemes have proceeded on an opportunistic basis, without a clear design strategy (which measure and where to implement it). However, if schemes are implemented without clear understanding of their impacts, they may, at best, fail to achieve the optimum flood reduction benefit downstream, or, at worst, make flooding more severe (if implemented in inappropriate locations, when tributaries’ flows are synchronised).  

Impacts of NFM measures are spatially and temporally dependent i.e. the same intervention in two locations will have different effects on flows, and the same intervention will have different impacts during different storm events. Therefore, it is essential that when strategically designing NFM schemes for catchments, that WHERE? and WHAT? are answered together to optimise the impact, as it is possible that whilst upstream NFM may be beneficial locally it may make tributary peaks coincide and make flood magnitudes worse downstream. Here we demonstrate the importance of the spatial configuration of Nature-Based Solutions on their catchment scale effectiveness in reducing flood risk.