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. Whilst there is an increasing awareness that the 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 16 weeks 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 windwave induced intertidal sediment resuspension in very close vicinity (< 130 m) to the seaward marsh margin. Vertically the salt marsh grows at rates >5 mm yr-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 0.8 m yr-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.

Salt Marsh Dynamics in a Period of Accelerated Sea Level Rise

“…In this case, a spatial model is deemed necessary. Another problem is to account for temporal variability, e.g., high suspended sediment concentrations during energetic events (Schuerch et al, 2019), which leads to episodic vertical accretion (Goodwin & Mudd, 2019; Reed, 1989). Models for marsh evolution can simulate temporal variability, even though they often face the problem of satisfying mass conservation.…”

Seasonal Variation in Sediment Delivery Across the Bay‐Marsh Interface of an Estuarine Salt Marsh

“…The influence of waves and storms on deposition near marsh edges has been studied previously based on long-term accretion measurements (Bartholdy et al, 2004;Temmerman et al, 2003;Schuerch et al, 2012) and short-term sediment accumulation (Reed, 1989;Ma et al, 2018). More recently, Ma et al (2018) and Schuerch et al (2019) showed that SSC in the shallows and deposition on the marsh are directly related to wave energy. Duvall et al (2019) found that SSC in bay shallows increases with wave energy but that the effect on sediment supply can be limited if large waves occur at times when the marsh is not inundated.…”

Divergence of Sediment Fluxes Triggered by Sea‐Level Rise Will Reshape Coastal Bays

“…Without accounting for the sediment import from the ocean, we cannot close the sediment budget for the entire system (Hopkinson et al, 2018). In many systems (e.g., Virginia, USA, and island of Sylt, Germany), it was found that intense storms can resuspend sediments along the shelf and transport them to shallow bays and salt marshes (Castagno et al, 2018;Lacy et al, 2020;Schuerch et al, 2012). The Labrador Current, Warm Core Rings, and the Gulf Stream can also affect the along shelf transport and therefore the marine fluxes of sediment in Plum Island Sound (Townsend et al, 2015;Zhang et al, 2016).…”

“…Determining whether costal bays are stable and in equilibrium under SLR requires the quantification of sediment fluxes, and in particular the exchange of material between different geomorphic units like tidal flats and salt marshes (Duvall et al, 2019; French, 2006; Lacy et al, 2020). Sediment inputs are required to accrete tidal flats and salt marshes in addition to organic matter contribution and maintain constant water depths in a period of accelerated SLR (Horton et al, 2018; Schuerch et al, 2019).…”