Coastal Changes and Impacts

Shoreline Definition in Emergent Wetland Environments

Coastal shoreline mapping is a particularly complex issue because of the dynamic nature of water levels at the land-water interface, the various tidal vertical datums in use, and the spatial scale of the shoreline delineation. In addition, the definition of a shoreline varies depending on whether the shoreline will be used for nautical charts and navigation, delineating the legal federal and state boundaries in the United States, or conducting scientific analyses such as shoreline change or ecological studies.

During the 20th century, aerial photo or high-resolution satellite imagery interpretation was typically used to define the shoreline based on a visible feature such as the wrack line, which represented the mean high water (MHW) line. At the end of the 20th century, new methods to derive shorelines from remotely sensed data, such as light detection and ranging (lidar), were developed that are more objective and consistent than previous methods.

Although the wetland shoreline derived from the lidar point cloud classification is very detailed compared to existing shoreline products or wetland maps, the lidar point classification is typically not conducted at a fine enough scale to support mapping small, isolated interior water bodies. These water bodies are typically mapped using image processing techniques applied to high-resolution, small footprint imagery, or by hand-digitizing from aerial photos.

Therefore, techniques and methods are being developed to efficiently extract land/water masks from a combination of variables derived from lidar point clouds and digital elevation models (DEMs). These techniques will be applied to pre- and post-Hurricane Sandy lidar point clouds to detect changes in the amount and configuration of open water in a small pilot area in coastal New Jersey.

The results of this study will provide a detailed lidar-derived shoreline for coastal New Jersey dominated by a complex configuration of emergent wetlands and open water. The data will provide consistent, comprehensive wetland shoreline that is important for hydrodynamic models, shoreline change detection analyses, and could serve a base layer for wetland classification maps. The wetland shoreline will also be used to identify the land/water interface, a critical step in developing integrated topobathymetric elevation models.

NGS 2011 NJ shoreline definition emergent wetlands
NOAA National Geodetic Survey 2011 lidar point cloud (displayed by RGB image values) for a wetland area near Great Egg Harbor Inlet and Margate City, New Jersey. Image created by Cindy Thatcher.

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