Sponge Measures for Natural Flood Management in Agricultural Landscapes: Water Retention Effectiveness, Co/Dis-Benefits, and Hydro-Geomorphic Change in Nature-based Solutions in the Upper Thames, UK

Floods and droughts pose significant threats to both human communities and natural landscapes. The EU Horizon SpongeScapes project (www.spongescapes.eu 2023-2027) aims to enhance landscape resilience against these hydrometeorological extremes by exploring "landscape sponge functions" – the natural ability of landscapes to absorb, store, and gradually release water. This project includes research in various “sponge measures” (i.e., Nature-based Solutions (NbS) and/or hybrid interventions) across European sites with varying climates, geographies, and soil conditions, to address three main research questions: (1) what is the longer-term effectiveness of sponge measures (and what indicators/metrics are more adequate to monitor change); (2) what is the overall effect of all sponge measures in a catchment (i.e. of sponge strategies); and (3) what are the main co-benefits and tradeoffs of sponge measures and strategies?Here we will present findings from one of the SpongeScapes sites, in an agricultural sub-catchment of the Thames basin where research has been ongoing since 2017. The Littlestock Brook Natural Flood Management (NFM) site includes several NbS measures including woody leaky dams connecting floodplain and field corner bund storage areas, and regenerative agriculture practices, that provide resilience to hydro-climatic extremes of floods and droughts to soil and fluvial systems.Results are based on baseline and ongoing field monitoring, including analyses based on hydrological (surface water levels and soil hydraulic properties) and survey data (airborne Lidar and ground topo-bathymetric campaigns) for the agricultural fields, floodplain and storage areas. Longevity of interventions will be discussed. Since installed over 5 years ago, several surface water storage measures have been colonised by vegetation providing co-benefits (plant and macroinvertebrate recent re-survey results will be presented). While also gradually infilled by fluvial and/or agricultural field sediment (geomorphic change results will be presented), or degraded, such as some woody leaky dams.We will discuss longer-term water retention effectiveness, monitoring/maintenance needs and potential co-benefits, dis-benefits/tradeoffs or unintended consequences. We will frame these findings in the context of a recently developed sponge measure monitoring framework, and identify research priorities within the wider project towards achieving more climate resilient landscapes.