Identifying Salt Marsh Shorelines from Remotely Sensed Elevation Data and Imagery

Abstract

Salt marshes are valuable ecosystems that are vulnerable to lateral erosion, submergence, and internal disintegration due to sea level rise, storms, and sediment deficits. Because many salt marshes are losing area in response to these factors, it is important to monitor their lateral extent at high resolution over multiple timescales. In this study we describe two methods to calculate the location of the salt marsh shoreline. The marsh edge from elevation data (MEED) method uses remotely sensed elevation data to calculate an objective proxy for the shoreline of a salt marsh. This proxy is the abrupt change in elevation that usually characterizes the seaward edge of a salt marsh, designated the "marsh scarp." It is detected as the maximum slope along a cross-shore transect between mean high water and mean tide level. The method was tested using lidar topobathymetric and photogrammetric elevation data from Massachusetts, USA. The other method to calculate the salt marsh shoreline is the marsh edge by image processing (MEIP) method which finds the unvegetated/vegetated line. This method applies image classification techniques to multispectral imagery and elevation datasets for edge detection. The method was tested using aerial imagery and coastal elevation data from the Plum Island Estuary in Massachusetts, USA. Both methods calculate a line that closely follows the edge of vegetation seen in imagery. The two methods were compared to each other using high resolution unmanned aircraft systems (UAS) data, and to a heads-up digitized shoreline. The root-mean-square deviation was 0.6 meters between the two methods, and less than 0.43 meters from the digitized shoreline. The MEIP method was also applied to a lower resolution dataset to investigate the effect of horizontal resolution on the results. Both methods provide an accurate, efficient, and objective way to track salt marsh shorelines with spatially intensive data over large spatial scales, which is necessary to evaluate geomorphic change and wetland vulnerability.

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

“…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.…”

“…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.…”

Are Elevation and Open‐Water Conversion of Salt Marshes Connected?

“…The UVVR was determined using 1-m horizontal resolution aerial imagery from the National Agricultural Imagery Program (NAIP) and 1-m horizontal resolution CoNED elevation data, with the Marsh Edge from Image Processing method (Farris et al, 2019). Briefly, four bands (red, green, blue, near infrared) from 8-bit NAIP imagery and the elevation data set were grouped into 32 classes with unsupervised classification (minimum class size of 5,000 cells).…”

Estuarine Mapping and Eco-Geomorphological Characterization for Potential Application in Conservation and Management: Three Study Cases along the Iberian Coast

“…In recent years, numerous studies worldwide have focused on the morphological and ecological changes that estuaries have undergone [5,[14][15][16][17][18]. However, these studies have rarely merged both perspectives [19].…”