High-resolution topography: tools and analysis of the life and death of salt marshes

Abstract

Salt marshes are grassy platforms that develop on sheltered coasts with high sediment supply. They may be found on sub-tropical shores where they often coexist with mangrove swamps, or in temperate climates where they might front brackish and fresh wetlands. These landscapes filter pollutants, protect coastlines against storm surges, and sequester carbon at high rates, making salt marshes some of the most valuable ecosystems on Earth. However, their survival is jeopardised by imbalance between formative and destructive processes: salt marshes rely heavily on external sources of sediment, and the poor sediment supply may prevent them from recovering from wave-driven erosion or from matching accelerating sea level rise. The sustained existence of a salt marsh ecosystem depends strongly on its topographic evolution. Hence, quantifying marsh platform topography is vital to improve coastal management, and the current development of high-resolution topographic data acquisition techniques presents geomorphologists with important opportunities to achieve this objective.

This thesis addresses the need for topographic analysis tools specific to the morphology of salt marshes and explores a selection of potential uses for these tools. First, I propose a novel, unsupervised method to reproducibly isolate salt marsh scarps and platforms from a Digital Terrain Model (DTM). This method takes the form of a multiple routing algorithms grouped under a single programme referred to as the Topographic Identification of Platforms (TIP). Field observations and numerical models show that salt marshes mature into subhorizontal platforms delineated by subvertical scarps. Based on this premise, the programme identifies scarps as lines of local maxima on a slope raster, then fills the DEM from the scarps upward, thus isolating mature marsh platform objects. I then test the TIP method using lidar-derived DTMs from six salt marshes in England with varying tidal ranges and geometries, for which topographic platforms were manually isolated from tidal flats. Agreement between manual and unsupervised classification exceeds 90 $\%$ for resolutions up to 3m. I also find that our method allows for the accurate detection of local block failures as small as 3 times the DTM resolution. Ultimately, I show that unsupervised classification of marsh platforms from high-resolution topography is possible and sufficient to monitor and analyse topographic evolution over time. The relevance of such monitoring is however dependant on the frequency and time-span of data acquisition, a point which I discuss further in the conclusive chapter.

Second, I use the TIP method to extract the distribution of elevations of multiple marsh platforms in the United Kingdom and the United States. I compare marsh elevations relative to current sea level and run simple 0-dimensional settling simulations in order to explore constraints on suspended sediment concentration and particle size. These experiments set a basis for comparison with observed accretion rates from field sources, as lidar-derived accretion rates are found to be inaccurate. I find that the marsh platforms examined occupy a narrow range of elevations in the upper tidal frame, situated between Mean High Tide and the Observed Highest High Tide. At these elevations, accretion models using sinusoidal tidal forcing do not allow these platforms to be inundated nor experience deposition. However, when forced with year-long tidal records, I find not inconsiderable deposition rates that follow hyperbolic contour lines when expressed as a function of sediment concentration and median grain size. I find that the deposition of coarse, concentrated sediment is necessary for platforms in the upper tidal frame to immediately match sea level rise, suggesting a strong dependance on infrequent high-deposition events for short-term accretion. 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, I reflect on the capacity of marshes to regenerate after erosion events within a context of changing sediment supply conditions and how this may affect the long-term, dynamic equilibrium of marsh platforms.

Finally, I add a module to the TIP method to determine the topographic signature of retreat and progradation on the edges of salt marsh platforms in mega-tidal Moricambe Bay (UK) in 2009, 2013 and 2017. I first describe the TIP method, and from the outlines it determines I generate transverse topographic profiles of the marsh edge 10m long and 20m apart. Profiles are grouped into categories depending on whether they experienced erosion or accretion in the 2009-2013 or 2013-2017 periods respectively, and I find that profiles belonging to the same retreat or progradation event have distinctly similar morphologies, regardless of the event magnitude. Progradation profiles have a shallow scarp and low relief that decreases with event magnitude, facilitating more progradation. Conversely, steep-scarped, high-relief retreat profiles that dip away from levees as retreat reveals older platforms. Furthermore, vertical accretion of the marsh edge is found to be primarily controlled by elevation in the study site, suggesting an even distribution of deposition that would allow bay infilling were it not limited by the migration of creeks. The scope of this research within future research on marsh margins is further discussed in the conclusive chapter.