Saltmarsh vegetation alters tidal hydrodynamics of small estuaries

Saltmarshes in most estuaries in the UK and elsewhere are heavily exploited for numerous purposes including farming, fishing, and recreation. In this study, a computational model was used to investigate the impact of saltmarsh vegetation on tidal dynamics and residual currents in three distinctly different estuaries in Wales, UK, in order to understand the impacts of marsh vegetation on wider estuarine hydrodynamics. The three estuaries, Mawddach, Taf and Loughor, vary in size, tidal range, exposure, and saltmarsh coverage. Tidal constituents and residual currents were calculated using a year-long simulation of tidal dynamics. Tidal dynamics are discussed in terms of five important primary tidal constituents (M2, S2, N2, K1, O1) and two shallow water constituents (M4, MS4). The results reveal that saltmarsh vegetation reduces the amplitude of both primary and shallow water tidal constituents not only on and at the proximity of marsh platforms but also in the wider estuary, mostly confined to tidal channels and surrounding intertidal areas. Most notable changes were observed in the middle and upper estuary. Notable changes to residual current velocities were observed on marsh flat areas and in tidal channels and saltmarsh creeks which indicates that changes to marsh vegetation have the potential to alter sediment transport and hence wider estuary hydrodynamics. Our results will be useful when making decisions to restore, reclaim and realign existing saltmarshes for environmental, conservation and socioeconomic purposes, or integrate them in nature-based solutions for estuarine flood and erosion management.