Maine Beach Mapping Program Shoreline Changes – Frequently Asked Questions

Q1. Why map Maine’s beaches?     Back

Mapping a specific feature along a beach, like the edge of the dune, over a number of years allows for the determination of shoreline change, whether it is an overall distance, or a calculated rate. This information allows scientists, beach managers, and recreational beach users, to understand how beaches along Maine’s coastline are changing. Over time, a seaward-shifting shoreline is indicative of sand accretion while a landward shifting shoreline indicates erosion. The Maine Geological Survey has been mapping beaches in Maine annually as part of the Maine Beach Mapping Program, or MBMAP, since around 2005. Additionally, in 2017, MGS began mapping the mean high water line to help in calculating the dry beach width. The dry beach width is the distance between the mean high water line and the more landward vegetation or seawall line. MGS began mapping the dry beach width to help understand how the dry beach – the space on which most people recreate and also where endangered and threatened bird species like piping plovers tend to nest – might change from year to year.

Q2. Which beaches are mapped as part of MBMAP?     Back

Beaches included in our annual survey efforts include (from south to north):

  1. Crescent and Seapoint Beaches, Kittery
  2. Long Sands and Short Sands Beaches, York
  3. Ogunquit Beach, Ogunquit
  4. Wells, Drakes Island, and Laudholm Beaches, Wells
  5. Crescent Surf, Parsons, Great Hill, Mother’s Goochs, and Colony Beaches, Kennebunk
  6. Goose Rocks Beach, Kennebunkport
  7. Fortunes Rocks and Hills Beaches, Biddeford
  8. Camp Ellis, Ferry, Bayview, and Kinney Shores Beaches, Saco
  9. Ocean Park, West Grand, and East Grand Beaches, Old Orchard Beach
  10. Pine Point, Ferry, Western, Scarborough, and Higgins Beaches, Scarborough
  11. Crescent and Kettle Cove Beaches, Cape Elizabeth
  12. Willard Beach, South Portland
  13. Indian Point, Chandler and Bennett Coves, Sandy Point, Roses Point and Hamilton Beach, Chebeague Island
  14. Lanes Island, Yarmouth
  15. Small Point (Seawall) Beach and Popham Beaches, Phippsburg
  16. Reid State Park Beach, Georgetown
  17. Pemaquid Beach, South Bristol

Q3. When are the beaches mapped?     Back

All of the beaches included in the study are generally mapped on an annual basis during the summer field season (May or June through September or October). MGS scientists try to map each beach at about the same time that it was mapped in the previous year in order to allow for similar weather conditions. Vegetation lines may also be measured to monitor changes after major events like large storms or beach nourishment projects. Although these ancillary shorelines are displayed in the viewer, they are not used in calculating shoreline change statistics. Some beaches have been mapped as part of MBMAP since 2005, but most since 2007. Although the goal is to study all MBMAP beaches each summer, some areas are missing data for certain years because of a variety of potential challenges that may not have allowed for data collection that year.

Q4. How are shorelines measured?     Back

MGS scientists use a Real Time Kinematic Global Positioning System (RTK-GPS, accurate both horizontally and vertically to a few inches) to map shoreline positions in order to calculate dune change, beach change, and dry beach width. MGS surveys the following features:

  • The seaward edge of dune vegetation (typically American Beach Grass). This is used as the proxy for dune change because it is a clear feature for field mapping and is a good indicator of inter-annual changes to the dune.
  • The approximate mean high water line (1.4 meter NAVD88 contour). This is used as the proxy for beach changes.
  • The seaward edges of seawalls or bedrock. These are mapped so that they can be used to calculate the dry beach width in areas where there are no dunes.

Surveyed data is downloaded, managed, and analyzed in a database and presented for viewing in a Geographic Information System (GIS). Shorelines are compared with previous years’ data to determine how beaches and dunes are changing.

Q5. Why are the seaward edge of vegetation and the mean high water contour used to represent the dune and beach shorelines?     Back

Beach shapes in Maine change seasonally. In the summer, calm waves and weather typically allows the beach to build, resulting in a wide berm, stable dunes, and under periods of stability, seaward growth of dune grass. During the winter, waves and winds can erode beaches, removing the berm and sometimes eroding the dune and removing dune vegetation. Dominant vegetation extent is generally a good indicator of how the dune may be changing from year to year.

The mean high water (MHW) is the average of the high tides along southern Maine’s beaches and is approximated by the 1.4 meter contour (NAVD88). Keeping track of this contour allows for analysis of how the beach is changing from year to year.

Relative changes in the vegetation line and the MHW from year to year can help to indicate whether a system has undergone a period of erosion, accretion, or stability since it was last mapped.

Also, by comparing the distance between the vegetation line or seawall and the MHW at a beach, we can calculate what is called the dry beach width. This is the area of the beach where most people recreate, and where most birds nest. It is also a good indicator of the buffering capacity of the beach to storm events.

Q6. What are limitations of using the vegetation or MHW to represent the shorelines?     Back

MGS scientists try to consistently map the seaward edge of dominant dune vegetation and the mean high water line. Limitations of using these features to define the dune and beach shorelines include:

  • Influence of engineered shorelines. Engineered shoreline areas (e.g., shorelines comprised of rip-rap or bulkheads) are generally not monitored. Technically, the shoreline can’t “move” past the wall unless the wall is damaged. So these areas aren’t necessarily mapped unless they have dune grass in front of the walls. Along most of these features, the MHW line is mapped as much as possible.
  • Human influence. In many areas, municipalities or homeowners plant dune grass to help stabilize the dune. If a dune is built and grass is planted (and the edge of vegetation moves as part of this effort), it will be reflected in the collected data, which would show that the shoreline had built seaward, or accreted. Also, shorelines that may been eroding may receive artificial beach nourishment if a harbor is dredged and the sand placed on or near the beach. As long as the beach was nourished before the time we surveyed it, this would be included in the collected data.
  • Other influences. It is possible that dune grass can die off due to other reasons than erosion, which may cause a shoreline to appear that it has “eroded”. Also, occasionally there are areas of private property, protective fencing (for endangered species like the Piping Plover), or other features that preclude data collection along the beach. Other times gaps in the shoreline may be due to a poor signal that does not allow the RTK-GPS to collect accurate data. Fortunately, small gaps in the data for a particular year tend not to seriously hinder the process of identifying large-scale shifts in the shorelines.

Q7. How are shoreline changes calculated?     Back

Although a visual inspection of the shifting dune or beach can give some insight into large-scale geomorphological changes, statistical calculations offer a more in-depth look at how Maine’s beaches are changing. Analyzing shoreline positions from a number of years allows for the calculation of different shoreline change statistics. This is done in a GIS following methods outlined in the U.S. Geological Survey’s Digital Shoreline Analysis System (DSAS). DSAS is used to cast perpendicular transects at 10 meter intervals (or less in some locations) from a reference “baseline” along a shoreline. The baseline is simply a line that is roughly parallel to a shoreline that is drawn landward of the landwardmost shoreline. At each cast transect, intersections along the transect with each mapped shoreline from a number of different years are used to calculate statistics based on the distance of each mapped shoreline from the reference baseline. This routine is used to calculate dune change and beach change rates. Positive values indicate that the dune or beach accreted, or grew seaward over time, while negative values indicate that the dune or beach eroded, or moved landward over time.

We also calculate what is known as the mean dry beach width. This is simply the mean (for all years data is available) distance between the mean high water line (mapped as the 1.4 m contour along the beach) and the vegetation or wall line along the beach. We also provide the dry beach width change (positive or negative) from the previous survey year.

Q8. What shoreline change statistics are calculated?     Back

For each updated survey year, the beach change, dune change, and dry beach width, shoreline change statistics are calculated for each transect shown in the viewer. These statistics can be viewed by simply making the layer of interest active and then clicking on any transect. Note that some transects might be of different lengths. For example, if a dune is relatively stable and hasn’t changed much, all of the surveyed vegetation lines might be close together and not need a long transect to intersect each shoreline when calculating statistics. At the same time, the beach might have grown far seaward, so for calculating beach change or dry beach width, a longer transect might be needed to calculate statistics.

For viewing Dune Change data, the important attribute is LRR_ft, the linear regression rate, as described above, for dune change in feet per year, for all years measured. A positive value represents a rate of dune growth, while a negative value represents a rate of dune loss.

For Beach Change, the most important attribute is LRR_ft, or the linear regression rate in feet per year. A positive value represents a rate of beach growth, while a negative value represents a rate of beach loss for all years measured.

For Dry Beach Width, the most important attribute is DBW_Mean_ft. This is the average dry beach width in feet for all years measured.

For Dry Beach Width Change, the important attribute is DBW_Change_ft. This is the change in the dry beach width from the previous survey year. A negative DBW_Change value means that the dry beach narrowed from the previous year, while a positive value means that the dry beach increased in width from the previous year. It's important to note that dry beach width can increase because of a widening beach (the mean high water contour moves seaward), or if the dune has been eroded landward (the vegetation line moves landward), or a combination of the two.

Overall statistics that are computed for all transects (as applicable, given available data) include:

  • Beach Name: Name of the surveyed beach.
  • TransectID: The ID of the transect. This includes the baseline ID; for example, 1-1, is baseline 1, transect 1.
  • Analysis Date Range: the years of collected shoreline data that were analyzed to create the below statistics.
  • LRR_ft: Linear Regression Rate, in feet/year. The rate of shoreline change calculated by the least-squares regression best fit between all shoreline positions. The distance from the baseline, in meters, is plotted against the shoreline date, and slope of the line that provides the best fit is the LRR. This calculation needs a minimum of three surveyed shorelines to work. Positive or negative values. Positive indicates accretion and negative indicates erosion.
  • LRR: Linear Regression Rate, in meters per year.
  • LR2: R-squared statistic, or coefficient of determination. The percentage of variance in the data explained by a regression, or in this case, the LRR value. It is a dimensionless index that ranges from 1.0 (a perfect fit, with the best fit line explaining all variation) to 0.0 (a bad fit, with the best fit line explaining little to no variation) and measures how successfully the best fit line (LRR) accounts for variation in the data. Positive, dimensionless value.
  • LCI95_ft: Standard error of the slope at the 95% confidence interval, in feet per year. Calculated by multiplying the standard error, or standard deviation, of the slope by the two-tailed test statistic at the user-specified confidence percentage. For example if a reported LRR is 1.34 ft/yr and a calculated LCI95 is 0.50, the band of confidence around the LRR is +/- 0.50. In other words, you can be 95% confident that the true rate of change is between 0.84 and 1.84 ft/yr.
  • LCI95: The LCI95, in meters per year.
  • EPR_ft: End Point Rate, in feet per year. The rate of shoreline change calculated by using the distance between the oldest and most recent shorelines at each transect (or the NSM), and dividing by the time elapsed (in decimal years). Excludes shorelines from all other years. Positive or negative values. Positive indicates accretion and negative indicates erosion.
  • EPR: End Point Rate, in meters per year.
  • NSM: Net Shoreline Movement, in meters per year. The distance between the oldest and most recent shoreline locations at each transect. Positive or negative values. Positive indicates seaward movement of the shoreline (accretion) while negative indicates landward movement of the shoreline (erosion).
  • SCE: Shoreline Change Envelope, in meters: the distance between the oldest and most recent shoreline locations at each transect. Always positive values.

For more information on all of these statistics, please see the DSAS Manual.

Q9. How is shoreline change information shared?      Back

Besides providing the viewer on this page, MGS provides shoreline change information for MBMAP monitored beaches as part of the biannual State of Maine’s Beaches report.

Last updated on September 9, 2024