Numerical simulation of vertical marsh growth and adjustment to accelerated sea‐level rise, North Norfolk, U.K.

Abstract

In parts of North America and Europe, present and future sedimentary deficits translate into major areal losses of coastal salt marsh. Physically based simulations of medium-to long-term adjustment to accelerated sea-level rise are few, partly due to the difficulty in extrapolating imperfectly understood sedimentation parameters. This paper outlines the implementation and application of a simple one-dimensional mass balance model designed to simulate the vertical adjustment of predominantly minerogenic marsh surfaces to various combinations of sediment supply, tidal levels and regional subsidence. Two aspects of marsh growth are examined, with reference to sites on the macro-tidal north Norfolk coast, U.K.: (i) historical marsh growth under a scenario of effective (long-term) eustatic stability but slow regional subsidence; and (ii) marsh response to various non-linear eustatic rise scenarios for the next century. In contrast to more organogenic North American marshes, sedimentation rates in Norfolk are strongly time-dependent. Where the overall sediment budget is so closely linked to marsh age and relative elevation, some form of numerical simulation offers a preferred means of predicting the impact of accelerated sea-level rise. Simulations performed here show that only the most dramatic eustatic scenarios result in ecological 'drowning' and reversion to tidal flat within the conventional 2100 prediction interval. Currently favoured scenarios give rise to accretionary deficits which are clearly sustainable in the short-term, albeit at the expense of increased inundation frequency and consequent changes in the distribution of marsh flora and fauna.

A coupled geomorphic and ecological model of tidal marsh evolution

“…Our model experiments show that in the absence of vegetation the coupled response of deepening and erosion by channel expansion can cause the intertidal surfaces to degrade rapidly to completely subtidal surfaces. In contrast, previous models of vertical accretion (7,9,12) would suggest that as an intertidal platform becomes lower in elevation relative to sea level, surfaces will simply accrete faster, maintaining their morphology in response to environmental change. Analyses of vertical accretion rates alone therefore miss important 3D effects from tidal flow and erosion.…”

“…The fate of intertidal salt marshes is of societal importance and scientific interest; marshes provide highly productive habitat and serve as nursery grounds for a large number of commercially important fin and shellfish (3, 4). Additionally, marshes offer great value as buffers of coastal storms in cities such as New Orleans, which is separated from the Gulf of Mexico by marshland (5, 6).A variety of vertical accretion models have been used to address the response of tidal marshes to environmental change, including accelerated sea-level rise and reduced sediment supply (7,8). In these models, bed elevation of the marsh platform is adjusted according to a deposition rate that is proportional to water depth at high tide, a proxy for duration and frequency of inundation.…”

“…A variety of vertical accretion models have been used to address the response of tidal marshes to environmental change, including accelerated sea-level rise and reduced sediment supply (7,8). In these models, bed elevation of the marsh platform is adjusted according to a deposition rate that is proportional to water depth at high tide, a proxy for duration and frequency of inundation.…”

Salt Marsh Accretion and Storm Tide Variation: an Example from a Barrier Island in the North Sea

“…Projected acceleration in SLR, however, may outpace accretion rates in the future, if an insufficient amount of material is deposited on the marsh surface (Kirwan and Temmerman 2009;Orson et al 1985). Several investigators studied marsh response to mean sea level rise (e.g., French 1993;Kirwan and Temmerman 2009;Orson et al 1985;Reed 1995); others investigated the influence of tidal range on salt marsh accretion (e.g., Chmura et al 2001;Harrison and Bloom 1977;, but relatively little literature is available discussing the impact of non-tidal short-term sea level variations on salt marsh accretion (Bartholdy et al 2004;French 2006;Kolker et al 2009;Temmerman et al 2003b). While it is a widespread assumption that macrotidal environments are more resilient against SLR than microtidal environments , Allen (2000) and Kolker et al (2009) conclude that marsh accretion is dominated by wind-induced (short-term) sea level variations in microtidal and by SLR-induced long-term sea level variations in macrotidal environments.…”

On the shape and widening of salt marsh creeks

“…Many studies has been conducted to describe salt marsh hydrology [Redfield, 1972;Pestrong, 1965;French and $toddart, 1992], salt marsh evolution [Allen, 1997[Allen, , 1995[Allen, , 1991French, 1993;Krone, 1987], sediment deposition on salt marsh surfaces [Woolnough et al, 1995] and the Because salt marshes typically involve cohesive sediments, we then adopt a threshold shear stress condition for erosion and channel morphological equilibrium, a common approach in the literature pertaining to cohesive sediments [Mehta et al, 1989; ?archure and Mehta, 1985;Dyer, 1995]. This threshold stress condition for cohesive sediments greatly simplifies the evolution model.…”