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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union

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Earth Surf. Dynam., 6, 239-255, 2018
https://doi.org/10.5194/esurf-6-239-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
28 Mar 2018
Unsupervised detection of salt marsh platforms: a topographic method
Guillaume C. H. Goodwin1, Simon M. Mudd1, and Fiona J. Clubb1,2 1School of GeoSciences, University of Edinburgh, Edinburgh, UK
2Institute of Earth and Environmental Science, University of Potsdam, Potsdam, Germany
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 labour-intensive 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.
Citation: Goodwin, G. C. H., Mudd, S. M., and Clubb, F. J.: Unsupervised detection of salt marsh platforms: a topographic method, Earth Surf. Dynam., 6, 239-255, https://doi.org/10.5194/esurf-6-239-2018, 2018.
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Short summary
Salt marshes are valuable environments that provide multiple services to coastal communities. However, their fast-paced evolution poses a challenge to monitoring campaigns due to time-consuming processing. The Topographic Identification of Platforms (TIP) method uses high-resolution topographic data to automatically detect the limits of salt marsh platforms within a landscape. The TIP method provides sufficient accuracy to monitor salt marsh change over time, facilitating coastal management.
Salt marshes are valuable environments that provide multiple services to coastal communities....
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