The effectiveness of Temporary Storage Areas for Natural Flood Management: Empirical evidence from a lowland catchment, UK

Temporary storage areas (TSAs) are a nature-based solution for attenuating flood peaks through the temporary detention of floodwaters in small (up to 10,000 m3) storage ponds on hillslopes or floodplains. Despite their increasing prevalence as part of Natural Flood Management (NFM) schemes in the UK, empirical evidence demonstrating their capability to mitigate flooding at catchment scales is limited. Addressing this evidence gap is a key priority for informing future flood risk management policies.In this study, we intensively monitored a prominent NFM scheme in the Littlestock Brook, a lowland rural sub-catchment (6.4 km2) of the River Evenlode in England. Ten TSAs providing a combined 25,000 m3 of flood storage were implemented between 2018 and 2020 to protect a flood-prone settlement. Measurements of river discharge (5 min), TSA stored volume (5 min), and precipitation (10 min) enabled the filling and drainage dynamics of individual TSAs to be quantified. The monitoring period (2019-2021) captured several notable storm events, including one with an estimated return period of 1 in 37 years. To quantify the aggregated impact of multiple TSAs on flood hydrographs at the catchment scale, observed TSA inflows and river discharge were used within a time-of-travel based hydrograph reconstruction approach to enable the estimation of downstream discharge in the absence of TSAs. Comparison of observed (with TSAs) and reconstructed (without TSAs) hydrographs indicate a 23% reduction in peak discharge for a 1 in 16-year return period storm. Furthermore, analysis of individual TSAs revealed substantial variation in storage utilisation and drainage during and after storms. These results provide quantitative evidence of how TSAs function both individually and in combination. The potential effectiveness of TSAs as a sustainable Natural Flood Management intervention will be discussed.