Unsupervised detection of salt marsh platforms: a topographic method

Abstract

Abstract. Salt marshes filter pollutants, protect coastlines against storm surges, and sequester carbon, yet are under threat from sea level rise and anthropogenic modification. The sustained existence of the salt marsh ecosystem depends on the topographic evolution of marsh platforms. Quantifying marsh platform topography is vital for improving the management of these valuable landscapes. The determination of platform boundaries currently relies on supervised classification methods requiring near-infrared data to detect vegetation, or demands labourintensive field surveys and digitisation. We propose a novel, unsupervised method to reproducibly isolate salt marsh scarps and platforms from a digital elevation model (DEM), referred to as 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, we identify scarps as lines of local maxima on a slope raster, then fill landmasses from the scarps upward, thus isolating mature marsh platforms. We test the TIP method using lidar-derived DEMs 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 94 % for DEM resolutions of 1 m, with all but one site maintaining an accuracy superior to 90 % for resolutions up to 3 m. For resolutions of 1 m, platforms detected with the TIP method are comparable in surface area to digitised platforms and have similar elevation distributions. We also find that our method allows for the accurate detection of local block failures as small as 3 times the DEM resolution. Detailed inspection reveals that although tidal creeks were digitised as part of the marsh platform, unsupervised classification categorises them as part of the tidal flat, causing an increase in false negatives and overall platform perimeter. This suggests our method may benefit from combination with existing creek detection algorithms. Fallen blocks and high tidal flat portions, associated with potential pioneer zones, can also lead to differences between our method and supervised mapping. Although pioneer zones prove difficult to classify using a topographic method, we suggest that these transition areas should be considered when analysing erosion and accretion processes, particularly in the case of incipient marsh platforms. Ultimately, we have shown that unsupervised classification of marsh platforms from high-resolution topography is possible and sufficient to monitor and analyse topographic evolution.

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

“…In this contribution, we examine the morphological properties of both prograding and retreating salt marsh margins in Moricambe Bay, a sheltered mega-tidal bay for which topographic data are available at a grid step of 1 m and a vertical accuracy ranging from 3 to 7 cm. We use the TIP method [53] to determine the location of salt marsh margins for 3 surveys in 2009, 2013 and 2017. We then design and use a new algorithm to generate 10 m long topographic profiles, regularly spaced every 20 m along each margin.…”

“…For each of the DTMs, we isolate marsh platforms using the Topographic Identification of Platforms (TIP) method [53]. The TIP method uses a high-resolution DTM in raster format (e.g., from lidar data) to classify pixels as "marsh platform" or "tidal flat" within an area of interest.…”

Salt Marsh Loss Affects Tides and the Sediment Budget in Shallow Bays

“…In recent years salt marshes have been the focus of many restoration plans built on the concept of nature‐based solutions for flood defenses (e.g., Temmerman et al, ), which aim to use vegetated surfaces to reduce the impact of storms on coastlines. The storm protection function of these ecosystems has been estimated up to 5 million USD per square kilometer in the United States (Costanza et al, ) and 786 million GBP per year for the U.K. marshes (Foster et al, ; Goodwin et al, ; U.K. National Ecosystem assessment, 2011; Xiaorong et al, ). Salt marshes are thought to be relatively stable along the vertical direction, because inorganic matter accumulation and organic mass production allow the marsh to keep pace with sea level; however, salt marshes are seldom in equilibrium along the horizontal direction, and continuously expand or contract in response to external forcing such as wind waves and sediment inputs (e.g., Carniello et al, ; Fagherazzi et al, ; Leonardi & Fagherazzi, ; Leonardi, Defne, et al, ; Marani et al, ; Schwimmer, ; Schwimmer & Pizzuto, ).…”

“…In recent years salt marshes have been the focus of many restoration plans built on the concept of nature-based solutions for flood defenses (e.g., Temmerman et al, 2013), which aim to use vegetated surfaces to reduce the impact of storms on coastlines. The storm protection function of these ecosystems has been estimated up to 5 million USD per square kilometer in the United States (Costanza et al, 2008) and 786 million GBP per year for the U.K. marshes (Foster et al, 2013;Goodwin et al, 2018;U.K. National Ecosystem assessment, 2011;Xiaorong et al, 2018).…”

Identifying Salt Marsh Shorelines from Remotely Sensed Elevation Data and Imagery

“…This feature is also highlighted in Figure 4 of [57], which shows cross-shore profiles in areas that have undergone erosion. Goodwin et al [73] used scarps to differentiate marsh platforms from tidal flats. They developed a new method to identify scarps in a DEM that they call topographic identification of platforms (TIP).…”