Great Lakes Nearshore Relative Exposure Index 2006 to 2014

"Wind fetch calculations were performed for each of the Great Lakes with wind data averaged for the open-water months (April-Nov.15) across the year range 2006-2014. Wind data was derived from offshore meteorological buoys operated by the NOAA's National Data Buoy Center (https://www.ndbc.noaa.gov/data/historical/stdmet/) and the Department of Fisheries and Oceans Canada (http://www.meds-sdmm.dfo-mpo.gc.ca/isdm-gdsi/waves-vagues/index-eng.htm). Wind fetches in this model were calculated using scripts designed by David Finlayson, U. S. Geological Survey, Pacific Science Center, while he was a Ph.D. student at the University of Washington. This method calculates effective fetch using the recommended procedure of the Shore Protection Manual (USACOE 1984). This method spreads nine radials around the desired wind direction at 3-degree increments. The resultant wind fetch is the arithmetic mean of these nine radial measurements. This model was last updated by the Upper Midwest Environmental Sciences Center of the U.S. Geological Survey to operate using ArcGIS (10.x). The model can be downloaded here: https://www.umesc.usgs.gov/management/dss/wind_fetch_wave_models_2012update.html. Input land/water rasters were derived from the Great Lakes Aquatic Habitat Framework (GLAHF) spatial framework zones. The wind data was summarized in 36 directions, starting at 0 degrees and proceeding in 10 degree increments (e.g. 0, 10,...,350). The frequency which wind blew and the average wind speed along each direction was calculated across the input years. Fetch calculations were performed for each of the 10 degree increments assigning the middle bin value (e.g., 5,15,...,355). The final wind-fetch outputs are the result of two different methods. The first is the directional frequency method. This method produces a layer where each grid cell is the distance to shore along each input direction (5, 15, 25, etc.) weighted by the percent frequency that the wind blew from that direction and summed, resulting in a continuous layer with the value of meters. The second is the Relative Exposure Index (REI) method, after Paul Keddy (1982), which was developed to establish the total effect of wave exposure on shoreline vegetation. The REI method produces a layer where each grid cell is the distance to shore along each input direction, weighted by the frequency that the wind blew from that direction, scaled by the average wind velocity along that direction, and summed. Fetch is measured in meters, percent frequency is unitless and wind speed is measured in m/s; therefore the units for the REI method equate to m2/s. The REI method is intended however, to produce a measure related to the size of the waves arriving on shore integrated over a particular time period and as such the units are not relevant. Therefore, for the REI method the values are treated as dimensionless. For each lake, multiple offshore buoys were included in the analysis. To generate a final composite which incorporated wind data from all the selected buoys, the first step was to create a lake-wide wind-weighted fetch layer for each individual buoy. This was achieved by re-classing the land-water raster to use as a cost surface, where water grid cells were assigned a value of 1 and land grid cells were assigned a value of 1,000,000. Then, the cost allocation tool was run with the buoy points and the cost surface as inputs. This produces a layer that can then inform which water grid cell data should be allocated to which buoy. Finally, the GLAHF nearshore and coastal margin delineations were used as an extraction mask from the final lake-wide wind-weighted fetch composite maps."