Waves, fetch, and associated shoreline change for the Point Aux Chenes and Grand Bay Estuaries in Mississippi and Alabama

Fetch length was calculated for sixteen wind directions at each shoreline location where shoreline change was estimated, using both the modern (2022) and historic (1848) shoreline data and temporally coincident offshore barrier island configuration. To generate historic fetch, the 1848 shoreline and historic mid-18th century offshore northern Gulf of Mexico (GOM) barrier island shoreline data (data for barrier islands were published in Buster and Morton, 2011) were combined to create the best available data for this timeframe. Since some fetch distances extend across the GOM (to mainland Florida, Mexico, etc.) and historic geospatial data for the entire GOM are not available, the historic data was merged with a modern shoreline data set for the remainder distant GOM shorelines (OpenStreetMap contributors, 2022), assuming the relative changes at this scale was minimal. For modern fetch, the most recent barrier island configuration from 2015 (Guy 2015; Henderson and others 2017) was combined with the modern Grand Bay and GOM shorelines available. The 2015 barrier island shoreline was checked against recent 2022 imagery and determined that, aside from some minor changes in spit lengths, it had not significantly changed position. At the same locations where shoreline change was determined, sixteen vectors (one vector every 22.5° with North at 0°) were extended from the Grand Bay shoreline to the closest land mass (barrier island or mainland) with a maximum distance of 1,600 kilometers (km) using the R package fetchR (version 2.1-0). Vectors pointing toward land were removed. The results are presented in four files. Historic and modern fetch for the entire Grand Bay region are presented as comma-separated values files (GB_fetch_his.csv and GB_fetch_mod.csv). A second analysis was completed that calculated historic and modern fetch for transects only covering the relict South Rigolets Island (SRI) and are also presented .csv files (SRI_fetch_his.csv and SRI_fetch_mod.csv). Coordinates for these four files are displayed in the North American Datum of 1983 (NAD83).

A characterization of wave power was generated using modeled, climatologically driven, wind waves (Masselink and others, 2014). Wave power was calculated from modeled wind waves for fair weather conditions using a probabilistic Delft3D model arrangement previously described and published (Jenkins and others, 2023a; Jenkins and others, 2023b). The model was forced by wind and wave climatology (Benedet and others, 2016) informed by local wind history. Characteristic wave power within Grand Bay was calculated under current conditions using the weighted sum of time-averaged waves produced by a one-year probabilistic model run (Jenkins and others, 2023a) and weighting based on the frequency of occurrence of individual climatologic conditions.

Shorelines (and original shoreline source information), transects, and shoreline change methods are published as part of this data release as Shoreline_change_analysis.zip. The GB_SLC_waves.csv file pertaining to this metadata contains rates calculated using data contained within Shoreline_change_analysis.zip. Compiled shorelines were processed using the Analyzing Moving Boundaries Using R (AMBUR) statistical package for R (version 4.0.4) to add required missing attributes and calculate shoreline change rates. More information on the AMBUR program can be obtained from Jackson (2010). The "LRR.lt" field used all available 1848-2023 shorelines. The "LRR.his" and "LRR.mod" fields used subsets of these shorelines from 1848-1957 and 1957-2023, respectively. The "LRR.lt.n3" field only used shorelines from 1848, 1957, and 2022. The "EPR.his" field only used 1848 and 1957 and "EPR.mod" field only used 1957 and 2022. The following analysis parameters were used: first intersection (if transect intersects the same shoreline more than once, then by default it selected the first intersection), confidence level 95 (confidence level for the linear regression statistics), unit label m (the units of measure for the map is for Universal Transverse Mercator and in meters), analysis type is advanced (advanced includes additional statistics for a robust linear regression), and time unit for rates is yr (utilizes years for calculating rates of shoreline change). Linear regression rate (LRR) and end point rate (EPR) were calculated as part of this study. LRR was not calculated for transects that intersected less than 3 shorelines. Coordinates are displayed in the North American Datum of 1983 Universal Transverse Mercator Zone 16 North (NAD83 UTM 16N).