Residence times of English estuaries.

The residence time of estuaries is a fundamental indicator of biogeochemical cycling, ecological productivity, pollution, and human health risk. Residence times of UK estuaries are estimated to vary from days to weeks, depending on tidal and riverine variability, as well as estuary size and shape; yet are largely unquantified for present-day and future climate conditions. This study presents the first national-scale modelling assessment of residence times across 52 English estuaries, using a consistent framework of high-resolution bathymetry and hydrodynamic modelling (Delft3D-FM). Simulations were run under present-day conditions and projected sea-level anomalies for 2050 and 2100, based on UK Climate Projections 2018 Representative Concentration Pathway 8.5 scenarios. Residence times were estimated using conservative tracer decay, across a range of river flow and tidal conditions. Results demonstrate substantial inter-estuary and intra-estuary variability, with present-day mean residence times ranging from <1 day in small, well-flushed estuaries to ∼100 days in microtidal or volumetrically large estuaries. Tidal flux, river inflow, and estuary size/shape emerged as dominant controls on flushing behaviour, allowing classification of estuary residence times as either tidally or fluvially dominated. Under projected sea-level rise (SLR) scenarios, residence times decreased in all estuaries due to the increased tidal prism and reduced intertidal storage. In four estuaries (Plymouth Sound, Portsmouth Harbour, Southampton Water, and Ore), SLR-driven changes exceeded present-day natural variability, suggesting a step-change in hydrodynamic functioning. This work provides critical evidence for climate adaptation planning in estuarine and coastal environments and a scalable framework to identify systems where future changes in residence time may compromise water quality objectives.