Designing Sustainable Landscapes: Salt Marsh Ditching Metric


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Environmental Sciences | Sustainability


The majority of salt marsh ditches in the Northeast have been ditched, both to facilitate harvest of salt marsh hay and to control mosquitoes (Smith and Niles 2016). Ditching changes the hydrology and flows of sediment and nutrients of marshes in ways that are not well understand, though ditched marshes may have altered invertebrate and shorebird communities, and may be less resilient to sea level rise (LeMay 2007). Marshes with intensive ditching (ca. 10 m spacing) appear to be most strongly affected (Vincent et al. 2013). The salt marsh ditching metric is an element of the ecological integrity analysis of the Designing Sustainable Landscapes (DSL) project (see technical document on integrity, McGarigal et al 2017). Consisting of a composite of 21 stressor and resiliency metrics, the index of ecological integrity (IEI) assesses the relative intactness and resiliency to environmental change of ecological systems throughout the northeast. As a stressor metric, salt marsh ditching provides an index of the relative intensity of ditching in salt marshes. Metric values range from 0 (no effect from nearby ditches) to 1 (severe effect). The metric is based on a custom image analysis process that identifies most ditches in salt marshes throughout the northeast from 1 m LiDAR (Light Detection And Ranging)-based DEMs (Digital Elevation Models). The algorithm, uses a kernel to identify local depressions that could be ditches. It then uses a morphological skeletonize algorithm to draw a 1 m-wide line through the middle of depressions, and then uses an original approach, “clockfacing,” to find linear features in these centerlines of depressions and connect disconnected sections. These potential ditches are converted from raster to linear features, and long, fairly straight sections are tagged as ditches. These linear ditches are converted back to a 1 m raster, then to a 30 m raster, with a value indicating ditch density in each cell. Finally, the ditching metric itself measures the intensity of ditches in the neighborhood of each salt marsh cell using a kernel estimator. Funding for this project was provided by the North Atlantic Landscape Conservation Cooperative and Department of the Interior Project #24, Decision Support for Hurricane Sandy Restoration and Future Conservation to Increase Resiliency of Tidal Wetland Habitats and Species in the Face of Storms and Sea Level Rise.


Designing Sustainable Landscapes: Salt Marsh Ditching Metric