Alabama: bias feature containing proxy-datum bias information used in shoreline change rate calculations

Bias feature creation step 1: COLLECTING SLOPE DATA FROM POINT CLOUD LIDAR DATA. The proxy-datum bias calculation is made possible by utilizing data acquired during processing for the profile method of shoreline extraction similar to the one developed by Stockdon and others (2002). The method extracts an elevation-based shoreline from point cloud lidar data using a Matlab-based approach (Matlab version R2024A). Using a coast-following reference line with 20-m-spaced cross shore profiles, lidar data are analyzed within a two-meter swath around each profile. This method is used to extract the mean high water (MHW) shoreline, as well as slope and uncertainty information at each profile. For Alabama, slope and uncertainty were averaged using the lidar dataset years 2010, 2016 and 2022, where available.

Bias feature creation step 2: CALCULATING THE PROXY-DATUM BIAS. There is a known horizontal offset between datum-based (MHW) shorelines and proxy-based (HWL) shorelines on open-ocean sandy beaches that nearly always acts in one direction (Ruggiero and List, 2009). This bias is called the proxy-datum bias (PDB or "bias"). The PDB is primarily due to wave run-up and thus is affected by the slope of the foreshore and the movement of water (waves, tides) onto the foreshore. See equation for the PDB and the PDB uncertainty in Ruggiero and List (2009). These equations require beach slope and wave data. Ideally, data collected at the time the proxy-based shoreline was collected would be used, however, for our purposes the PDB is estimated by averaging all available slope data and averaging at least 10 years of historical wave data from a nearby buoy (National Data Buoy Center) and the U.S. Wave Information Study (WIS). The resulting bias and bias uncertainty information is stored in a shapefile as points at the intersection of each profile and the coast-following reference line.

Bias feature creation step 3: TRANSECTS CAST TO CONNECT BIAS POINTS AND BASELINE. In order to be used in DSAS, data from the bias points must be connected to a shapefile that is near or coincident with the baseline. However, the bias points that result from the lidar shoreline extraction process (steps 1 and 2) do not always fall near the baseline. Transects were created for the sole purpose of connecting the bias points to the baseline. Transects were cast using DSAS v6.1 at 20-meter spacing, length= 1200 meters, smoothing value= 2000 meters. Transects were exported from DSAS and evaluated with the bias points in ArcGIS Pro v3.5.5 to ensure the transects were long enough.
This and the following process steps were performed by the same person: Meredith Kratzmann.

Bias feature creation step 4: SPLIT BASELINE. In ArcGIS Pro v3.5.5, points were established along the baseline by a distance of 20 meters using the geoprocessing tool: Data Management Tools > Sampling > Generate Points Along Lines. Then, the baseline was split at those points using the tool: Data Management Tools > Features > Split Line at Point.

Bias feature creation step 5: JOIN BIAS POINTS TO TRANSECTS. In ArcGIS Pro v3.5.5, bias points were joined to transects using the geoprocessing tool: Analysis Tools > Overlay > Spatial Join. Then, the transects that now have the bias were converted to points (midpoint) using the tool: Data Management Tools > Features > Feature Vertices To Points.

Bias feature creation step 6: CREATE BIAS FEATURE POLYLINE. In ArcGIS Pro v3.5.5, the transect bias points generated in process step 5 were joined to the baseline split into 20-meter segments (process step 4) using the geoprocessing tool: Analysis Tools > Overlay > Spatial Join. Then, the split line that now has bias data was unsplit using the tool: Data Management Tools > Features > Unsplit Line. The resultant file is the bias feature that was renamed and used in the long-term shoreline change rate calculations for Alabama.

The baseline shapefile was projected in ArcGIS Pro v3.5.5 > Geoprocessing > Data Management Tools > Projections and Transformations > Project. Parameters: Input Coordinate System - NAD_1983_2011_UTM_Zone_16N; Output Coordinate System - GCS_WGS_1984; transformation = WGS_1984_(ITRF08)_To_NAD_1983_2011.