The foundation for EVA is a 50-year planning window denoting an estimated shoreline position in 50 years. The “window” was developed using multiple datasets to achieve the most accurate prediction of shoreline position within the framework of a two dimensional simplistic spatial model. The window was based on erosion rates calculated from a comparison of historic shoreline maps developed by the Maryland Geological Survey (MGS) (http://www.mgs.md.gov/coastal/maps/schangevect.html). These rates of shoreline change were evaluated at 20m intervals along tidal shoreline in Maryland utilizing available current and historic digital maps and imagery. Based on transects cast perpendicular to the shore, across the reference shorelines, rates of shoreline change were computed employing the Digital Shoreline Analysis System (DSAS). Using these results, a classification was developed by MGS to characterize shoreline change.
Erosion Classification
- High erosion > 8 ft/yr
- Moderate erosion 4-8 ft/yr
- Low erosion rate 2-4 ft/yr
- Slight erosion rate 0-2 ft/yr
- No change
- Accretion > 0.01 ft/yr
- Protected erosion control structures in place
- No Data no historic data available to compute change
- Unknown available data unreliable
This study used the median value for each class when calculating the window. To fill certain data gaps, the original survey data from MGS was updated to reflect current status of shoreline protection and improve on the shoreline segments previously classified as “unknown” or “no data”. The Virginia Institute of Marine Science’s Maryland Shoreline Inventory (http://ccrm.vims.edu/gis_data_maps/shoreline_inventories/index.html) was used to provide updated conditions of shoreline protection and stability based on field surveys conducted within the past 5 years. In revising the original dataset the structures delineated in the Shoreline Inventory took precedence over any “protected” status classified in by MGS. This was considered reasonable since the Shoreline Inventory of structures was collected on the ground, while the MGS datasets interpreted moderately high resolution vertical imagery where delineation of structures is very difficult.
The Shoreline Inventory also provided data to correct shoreline segments classified as “No Data” or “Unknown” in the MGS product. Qualitative attributes associated with erosion at the bank were used (e.g. low erosion, high erosion, and undercut). Low erosion segments were classified as “no change”. Since most of the segments occurred in low fetch environments, high erosion and undercut segments were classified as “slight”, indicative of erosion rates between 0-2 ft/year. Where there were no inventory conditions for erosion, the shoreline was recoded as “no data”
The planning window constitutes an approximation of land loss in 50-years based on a simplistic spatial model, and presumes loss of resources and infrastructure within the window of shoreline retreat. Using the revised erosion rates the shoreline is shifted landward (or seaward in the case of accretion) by the median annual rate (ft/year) multiplied by 50 years. The process assumes no change in management of the shoreline in the next 50 years (i.e. no new shoreline protection structures to slow or halt erosion rates). The calculations also do not consider upland slope, rates of sea level rise, or catastrophic events as a factor when constructing the window.