Detecting the Morphology of Prograding and Retreating Marsh Margins—Example of a Mega-Tidal Bay

Abstract

Retreat and progradation make the edges of salt marsh platforms their most active features. If we have a single topographic snapshot of a marsh, is it possible to tell if some areas have retreated or prograded recently or if they are likely to do so in the future? We explore these questions by characterising marsh edge topography in mega-tidal Moricambe Bay (UK) in 2009, 2013 and 2017. We first map outlines of marsh platform edges based on lidar data and from these we generate transverse topographic profiles of the marsh edge 10 m long and 20 m apart. By associating profiles with individual retreat or progradation events, we find that they produce distinct profiles when grouped by change event, regardless of 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 dip landward as retreat reveals older platforms. Furthermore, vertical accretion of the marsh edge is controlled by elevation rather than its lateral motion, suggesting an even distribution of deposition that would allow bay infilling were it not limited by the migration of creeks. While we demonstrate that marsh edges can be quantified with currently available DTMs, oblique observations are crucial to fully describe scarps and better inform their sensitivity to wave and current erosion.Remote Sens. 2020, 12, 13 2 of 26 constriction of salt marsh habitat [29][30][31], as well as the mutual interaction between wave impact, retreat processes and the morphology of retreating marsh margins [32][33][34]. While marsh retreat is demonstrably linked to nearby channel deepening in a macro-tidal setting [35,36], the action of tidal currents on marsh margins remains poorly understood relative to wave action.Likewise, remote observation of salt marsh margins are scarce in the literature, in contrast with the wealth of documentation on the use of light detection and ranging (lidar) and hyperspectral data to characterise marsh platform elevation and vegetation [37][38][39][40]. This knowledge gap hampers our understanding of present coastal mobility in general but also our predictions of the future retreat or advance (which we refer to as progradation) of salt marshes. The mobility of marsh edges is often studied through the determination of wave-or current-generated stresses rather than direct observation of marsh edges. This lack of observation data prevents us from contextualising results on the influence of scarp topography on wave action [32].The paucity of data on marsh edge topography may be due to technical difficulties: in many micro-tidal systems and some meso-tidal systems the foot of the marsh scarp is rarely exposed [41] and few sites have as good topo-bathymetric data as the repeatedly studied Venice Lagoon in Italy [42] and Plum Island in Massachussets, USA [43], both of which are the object of long-term monitoring campaigns. Moreover, the spatial resolution of airborne lidar images is usually in the range of 1-...

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