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

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https://doi.org/10.5194/esurf-2017-60
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
01 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).
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, UK
2Department of Earth Sciences and St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN, USA
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 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 England 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 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 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 analyze 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. Discuss., https://doi.org/10.5194/esurf-2017-60, in review, 2017.
Guillaume C. H. Goodwin et al.
Guillaume C. H. Goodwin et al.

Model code and software

LSDMarshPlatform
G. C. H. Goodwin, S. M. Mudd, and F. J. Clubb
https://doi.org/10.5281/zenodo.1007788
Guillaume C. H. Goodwin et al.

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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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