Natural England commissioned this desk based study to source and use current data and historical data to measure and evaluate estuarine function over time for the Severn Estuary and Solway Firth. The aim was to collect quantitatively robust data, which will permit rigorous statistical analysis and support robust condition assessment judgements.
Morphological equilibrium in the Severn Estuary and Solway Firth was analysed using Regime Theory, which defines empirical relationships between estuary tidal prism and cross-sectional area. The results for the Severn Estuary and Solway Firth are generally similar. Both estuaries are under-sized compared to their predicted forms in their inner parts; the observed channels are narrower than predicted for the present-day tidal regimes. This means that to obtain an equilibrium form they have to widen from their current forms. They should erode by loss of intertidal habitat because in both estuaries the high water mark is constrained by coastal defences which do not allow it to migrate landwards. In contrast, both outer estuaries are over-sized compared to their predicted forms; the observed channels are wider than predicted for the present-day tidal regimes. Here, they should accrete and develop further intertidal habitat by natural processes. The central parts of the estuaries are tending towards equilibrium whereby their observed and predicted widths are similar.