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 productivity and even survival of salt marsh vegetation depends on the topographic evolution of marsh platforms. Quantifying marsh platform topography is vital for improving the management of these valuable landscapes. Determining platform boundaries currently relies on supervised classification methods requiring near-infrared data 5 to detect vegetation, or demands labor-intensive field surveys and digitization. We propose a novel, unsupervised method to reproducibly isolate saltmarsh scarps and platforms from a DEM, referred to as Topographic Identification of Platforms (TIP).Field observations and numerical models show that saltmarshes mature into sub-horizontal platforms delineated by sub-vertical 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 saltmarshes in 10England with varying tidal ranges and geometries, for which topographic platforms were manually distinguished from tidal flats. Agreement between manual and unsupervised classification exceeds 94% for DEM resolutions of 1 m, with all but one sites 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 digitized platforms, and have similar elevation distributions. We also find that our method allows the accurate detection of local bloc failures as small as 3 times the DEM resolution. Detailed inspection 15 reveals that although tidal creeks were digitized as part of the marsh platform, unsupervised classification categorizes them as part of the tidal flat, causing an increase in false negatives and overall platform perimeter. This suggests our method would have increased accuracy if used in combination with existing creek detection algorithms. Fallen blocs and high tidal flat portions, associated with potential pioneer zones, may also be areas of discordance between our method and supervised mapping.Although pioneer zones prove difficult to classify using a topographic method, it also suggests that these transition areas 20 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 possibleand sufficient to monitor and analyze topographic evolution.

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, ).…”