High Platform Elevations Highlight the Role of Storms and Spring Tides in Salt Marsh Evolution

Guillaume C H Goodwin,Simon M Mudd

doi:10.3389/fenvs.2019.00062

Abstract

We combine sea level records and repeat lidar surveys at 8 sites in the United Kingdom and the United States to explore controls on marsh accretion. We compare marsh elevations relative to sea level as well as lidar-derived marsh accretion rates to simple 0-dimensional settling simulations in order to explore constraints on suspended sediment concentration and particle size. We find that the marsh platforms examined occupy a narrow range of elevations in the upper tidal frame, situated between Mean High Tide $MHT$ and the Observed Highest High Tide $OHHT$. Under sinusoidal tidal forcing, common in marsh accretion models, marshes at these elevations are never inundated, highlighting the inadequacy of sinusoidal forcing in numerical models of salt marshes. Forcing the model with year-long tidal records, deposition rates follow hyperbolic contour lines when expressed as a function of sediment concentration and median grain size. We also observe that when using a median sediment grain size $D_{50} = 50 \mu m$ and sediment concentrations derived from satellite data, modeled deposition rates are much lower than when using field data. We find that the deposition of coarse, concentrated sediment is necessary for platforms in the upper tidal frame to withstand sea level rise, suggesting a strong dependance on infrequent high-deposition events. This is particularly true for marshes that are very high in the tidal frame, making accretion increasingly storm-driven as marsh platforms gain elevation. Finally, we reflect on the capacity of marshes to regenerate after erosion events within a context of changing sediment supply conditions.

Highlights

The issue of salt marsh elevation change is one that preoccupies coastal geomorphologists and land managers alike
In all of the sites, irrespective of measurement error, the major part of the marsh platform lies within the upper tidal frame, defined as the range of elevations between MHT and OHHT
We test a 0-dimensional settling model to estimate elevation change on real salt marsh platforms, and compare these results with accretion fluxes derived from DEM surveys taken at least four years apart

Summary

Introduction

The issue of salt marsh elevation change is one that preoccupies coastal geomorphologists and land managers alike. Flooding frequency in turn determines salinity, which influences the type and productivity of the plant communities on the marsh (Pennings et al, 2003; Silvestri et al, 2005; Belliard et al, 2017), and underpins the functioning of the entire ecosystem. Decreased sediment supply due to anthropogenic activities is set to accentuate the pressure of sea level rise on coastal wetlands, in deltaic systems (Syvitski et al, 2009). Factors that influence marsh growth are less favorable than in the past (Kirwan et al, 2011), and so Sediment Supply and Settling Rates determining if salt marshes will maintain their elevation within the tidal frame is an intensively studied research question(Crosby et al, 2016; Kirwan et al, 2016; Lerberg, 2016)

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