Mean high water (MHW) shorelines along the coast of California used to calculated shoreline change from 1998 to 2016

High-accuracy LiDAR data were downloaded from NOAA Digital Coast in the form of digital elevation models (DEMs) for both pre-El Nino (2015) and post-El Nino (2016), as well as 1998/2002 for perspective. Due to sparse LiDAR data coverage in southern California, SoCal LiDAR from UC San Diego Library Collections was used in this region.

The operational MHW line was extracted using the ArcGIS smoothed contour method with the smoothing tolerance parameter set to 30 m alongshore. The NAVD88 datum was used for the MHW elevation. The mean high water (MHW) shoreline elevation in each analysis region (Northern, Central, and Southern California) maintained consistency with that of the National Assessment of Shoreline Change.

The smoothed contour line was then quality controlled to remove artifacts, as well as remove any contour tool interpretation of human-made infrastructure (such as jetties, piers, and sea walls), using satellite imagery available in ArcGIS.

Using the uncertainty values reported in the DEM metadata, the DEM vertical uncertainty was converted to horizontal uncertainty using the slope of the beach at 50-meter points along the MHW line, using the following steps: Slope maps were created from the DEMS within ArcMap using the 3D Analyst Raster Surface Slope tool. Next, the MHW lines were converted to 50-meter-spaced points, at which the slope value of the DEM was extracted. The average of these values for North, Central, and Southern California was used to convert the vertical uncertainty of the DEM to a horizontal uncertainty. Finally, the horizontal uncertainty and converted vertical uncertainty were summed in quadrature to calculate a final uncertainty value associated with each shoreline.