A holistic study on the effects of a rural flood detention basin: Flood peaks, water quality and grass growth.

Nature-based Solutions (NbS) are widely advocated to have multiple benefits, including in flood risk reduction, water quality improvement and ecosystem health. There are, however, few empirical studies quantifying such multi-functionality. Given the ongoing pressures of flooding and poor water quality within Europe, there is an urgent need for empirical evidence to assess the potential for NbS features to address these issues. We present the first empirical results on the impacts of a nature-based flood detention basin on peak flow attenuation, water quality and pasture health. The detention basin comprised a 1.5m high soil bund constructed within a field bordering a first order tributary on a farm in the south of Ireland. Over two years, peak flows were diverted from the stream channel to the detention basin via a constructed sluice during the rising limb of high discharge. Detained waters were returned slowly to the channel via drainage pipes. During this time hydrological and quality data were collected from upstream, downstream and within the detention basin during peak flow events and pasture sward growth and health within the detention basin was assessed. These data revealed marked peak flow attenuation by < 38%, except under conditions of prolonged rainfall. The detention basin sequestered suspended solids (Removal efficiency RE 5.35%; p < 0.05) and nitrate (RE 13.37%; p < 0.001), but was a source of soluble reactive (RE -95%; p < 0.001) and particulate phosphorus (RE -83%; p < 0.001). There was no statistically significant reduction in pasture biomass, but higher Chlorophyll a/b ratio of inundated plants indicated reduced photosynthetic efficiency (30.85%; p < 0.001). Whilst capable of reducing peak flows, the mixed results on water quality and pasture health show that the operation of NbS features is complex, and that multi-functionality is not an inherent facet of NbS.