Long-term and short-term shoreline change rates for Nantucket, Massachusetts calculated with and without the proxy-datum bias using the Digital Shoreline Analysis System version 5.0

Explanation of the methods used to delineate shoreline features that are a part of this update for the Massachusetts Office of Coastal Zone Management Shoreline Change Project: This data release computes both long- and short-term rates of shoreline change, and depending on the available data may include shorelines referenced to mean high water (MHW) or a proxy-based high water line (HWL). The new data recently published as part of this update are from Himmelstoss and others (2018), these shorelines were combined with data from previous assessments most recently compiled in Smith and others (2013). Shoreline data were merged into a single feature class for a region after all input files were projected to UTM zone 19N coordinates and confirmed to have the same attribute names needed for use in DSAS. MHW shorelines are datum-based and in the case of the shorelines used here, were extracted from lidar data using a profile method, whenever possible, or a contour method. Profile open-ocean coast: datum-based mean high water (MHW) shoreline. The elevation of MHW was obtained from Weber and others (2005). These data have an associated uncertainty table that provides the horizontal uncertainty associated with the shoreline, a proxy-datum bias value describing the unidirectional horizontal offset between the MHW shoreline and the historical proxy-based high water line (HWL) shorelines, and the uncertainty associated with the calculated proxy-datum bias value. For more information about the proxy-datum bias, see Ruggiero and List (2009). These shorelines are polyline-M shapefiles. Profile not open-ocean: datum-based mean high water (MHW) shoreline. Since Weber and others (2005) only covers open-ocean coast, all MHW elevations for these data come from NOAA's vdatum (version 3.8; https://vdatum.noaa.gov/). These data have an associated uncertainty table that provides the horizontal uncertainty associated with the shoreline. These shorelines are polyline-M shapefiles. Contour method: this method is used along sections of the coast that were too crenulated for the profile method. The elevation of MHW was used from Weber and others, 2005 when available. In areas not covered by Weber and others (2005), NOAA's vdatum (version 3.8; https://vdatum.noaa.gov/) is used to determine MHW. Once this value is determined, the contour line of that value is extracted from the DEM surface in the area of interest. These shorelines are polyline shapefiles. Most of the historical shorelines from Smith and others (2013) are proxy-based historical shorelines. Vector shorelines digitized from georegistered T-sheets using standard editing tools in ArcMap provide a proxy-based high water line (HWL) feature that is not tidally-referenced. Individually these shorelines are stored as polyline shapefiles. In previous analyses these data were published as a merged file with profile method data extracted from lidar. Therefore the published data are all polyline-M, but the historic HWL shorelines contain no linear referencing. Visually identified HWL-type proxy shorelines are virtually never coincident with datum-based MHW-type shorelines. In fact, HWL shorelines are almost universally estimated to be higher (landward) on the beach profile than MHW shorelines. Not accounting for this offset will cause shoreline change rates to be biased toward slower shoreline retreat, progradation rather than retreat, or faster progradation than in reality (for the typical case where datum-based MHW shorelines are more recent data than the proxy-based HWL shoreline dates). The Digital Shoreline Analysis System software used to compute rates has an option to indicate when proxy-based and datum-based shorelines are present and uses linear referencing to retrieve the information on bias and uncertainty stored in the DBF table associated with the profile method shorelines and correct for the proxy-datum bias offset. This process step and all subsequent process steps were performed by the same person: Emily A. Himmelstoss.

Transect features generated in a personal geodatabase using DSAS v5.0.20180213.1225. Parameters Used: baseline layer=Nantucket_north_baseline, baseline group field=NULL, transect spacing=50 meters, search distance=1000 meters, land direction=right, shoreline intersection=seaward, File produced = Nantucket_north_transects. For additional details on these parameters, please see the DSAS help file distributed with the DSAS software, or visit the USGS website at: https://woodshole.er.usgs.gov/project-pages/DSAS/

Transect features generated in a personal geodatabase using DSAS v5.0.20180213.1225. Parameters Used: baseline layer=Nantucket_south_baseline, baseline group field=NULL, transect spacing=50 meters, search distance=1000 meters, land direction=right, shoreline intersection=seaward, File produced = Nantucket_south_transects.

Some transects were manually edited for length, moved, or deleted in an edit session using standard editing tools in ArcMap v10.5.1

Shoreline intersects and rate calculations performed for long-term rates with bias (using the southern baseline). Parameters Used: shoreline layer=Nantucket_LT_shorelines, transect layer = Nantucket_south_transects, shoreline date field=DSAS_date, shoreline uncertainty field name=Uncy, the default accuracy=4.4 meters, shoreline intersection=seaward, stats calculations=[LRR], shoreline threshold=3, confidence interval=90%. Files produced = Nantucket_PDB_LT_rates_20180406_150209, Nantucket_PDB_LT_intersect_20180406_150209.

Shoreline intersects and rate calculations performed for long-term rates without bias (using the northern baseline). Parameters Used: shoreline layer=Nantucket_LT_shorelines, transect layer = Nantucket_north_transects, shoreline date field=DSAS_date, shoreline uncertainty field name=Uncy, the default accuracy=4.4 meters, shoreline intersection=seaward, stats calculations=[LRR], shoreline threshold=3, confidence interval=90%. Files produced = Nantucket_NB_LT_rates_20180406_134610, Nantucket_NB_LT_intersect_20180406_134610.

Shoreline intersects and rate calculations performed for short-term rates with bias (using the southern baseline). Parameters Used: shoreline layer=Nantucket_ST_shorelines, transect layer = Nantucket_south_transects, shoreline date field=DSAS_date, shoreline uncertainty field name=Uncy, the default accuracy=4.4 meters, shoreline intersection=seaward, stats calculations=[LRR], shoreline threshold=3, confidence interval=90%. Files produced = Nantucket_PDB_ST_rates_20180406_143249, Nantucket_PDB_ST_intersect_20180406_143249.

Shoreline intersects and rate calculations performed for short-term rates without bias (using the northern baseline). Parameters Used: shoreline layer=Nantucket_ST_shorelines, transect layer = Nantucket_north_transects, shoreline date field=DSAS_date, shoreline uncertainty field name=Uncy, the default accuracy=4.4 meters, shoreline intersection=seaward, stats calculations=[LRR], shoreline threshold=3, confidence interval=90%. Files produced = Nantucket_NB_ST_rates_20180406_140128, Nantucket_NB_ST_intersect_20180406_140128.

The rate feature classes were exported to shapefiles in ArcMap v10.5 by right-clicking the transect layer > data > export data.

The exported rate shapefiles were projected in Esri's ArcToolbox (v10.5) > Data Management Tools > Projections and Transformations > Project. Parameters: input projection - UTM zone 19N (WGS84); output projection - geographic coordinates (WGS84); transformation = none.