Coupling between tidal mudflats and salt marshes affects marsh morphology

Abstract

It is generally assumed that coastal salt marshes are capable of adapting to moderately fast rising sea levels although local sediment availability crucially affects this capability. While there is an increasing awareness that local sediment availability is inherently related to sediment dynamics on the adjacent tidal mudflat, our current understanding of the interactions between salt marshes and tidal flats is very limited. To address this knowledge gap, we measured suspended sediment concentrations alongside hydrodynamic, morphological and sediment deposition measurements over a total period of 16weeks in a wave-exposed macro-tidal mudflat-salt marsh system on the UK east coast (Tillingham). Our results show that local sediment supply to the salt marsh is strongly linked to intertidal sediment dynamics and that the vast majority of suspended sediment deposited on the marsh originates from wind-wave induced intertidal sediment resuspension in very close vicinity (<130m) to the seaward marsh margin. Vertically the salt marsh grows at rates >5mmyr⁠−1, thereby increasing the slope of the tidal mudflat-salt marsh transition and making the salt marsh susceptible to lateral erosion. Consequently, the marsh edge retreats at a rate of approximately 0.8myr⁠−1. Our study shows that the response of coastal salt marshes to climate change is a function of the coupled tidal mudflat-salt marsh system, rather than their vertical sediment accretion rates alone. Therefore, the idea that salt marsh adaptability relies on local sediment supply needs to be expanded, incorporating the morphology and long-term evolution of the adjacent tidal mudflats.