Hydrological assessment of urban Nature-Based Solutions for urban planning using Ecosystem Service toolkit applications

Ecosystem Service (ES) toolkits are increasingly used to quantify and visualize the benefits gained from Nature-based-solutions (NBS) but modules for hydrology are often absent, or if present they lack meaningful hydrological functionality or validation. This leads to gaps in the evidence base required by decision makers. To bridge the gap between such limitations and more complex hydrological models this paper presents a hydrologically based NBS model compatible with spatial ES toolkits. The approach 'Adapted Nature-based-solutions Rational Method' (ANaRM) is based on the Rational Method, widely used in hydrology. We apply this model to the city of Birmingham, England, to validate its performance and to analyse the effects of different NBS scenarios. The validated ANaRM model provided robust estimates of peak flow using design storm rainfall. It proved capable of simulating the hydrological effects of NBS such as land use change from urban to green, or installation of SuDS and ponds. Results suggest ponds are found most effective for achieving peak flow reduction in channels and are the best option for mitigating fluvial flooding downstream. Reduction in localised runoff and pluvial flooding is best achieved by converting impervious surfaces such as buildings, hardstanding and roads to green solutions such as green roofs, permeable pavements and greenspace. This paper highlights the importance of considering the spatial effects of urban NBS on hydrology, and that these can be captured with relatively simple modelling approaches such as ANaRM. Its ease of use means it suits any level of user looking to represent the flood mitigation aspect of NBS spatially and has high potential as part of any ES toolkit focused on representing the spatial effects of NBS on ecosystem services.