Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan

Erosion mapping methods and Maine data sets

Historical Shoreline Change Analysis

Map Analysis

Maps and nautical charts can be examined or superimposed to compare changes to the shoreline. Early charts and maps of Maine are available in digital form from the National Ocean Service and at the Osher Map Library of the University of Southern Maine in Portland. In Maine this type of analysis is used to understand large-scale changes to the shoreline such as the closure of a tidal inlet (Little River, Scarborough) or the presence of a large tidal delta (Saco River) in the last two centuries. The large-scale shoreline morphology mapped in early charts can be useful in generating coastal sediment budgets and examining conditions prior to human influence. These early maps and charts are not useful for exact shoreline change rate calculations because of the level of geographic control results in errors that exceed the absolute amount of shoreline change in many Maine locations.

Air Photo Analysis

Erosion can be detected by comparing sequential shoreline positions in vertical air photos. This approach uses various analytical methods and equipment to superimpose shorelines on a map and then to calculate an erosion rate. One way is to compare the earliest and most recent pairs of photos. This approach uses the longest span of years to determine an erosion or accretion rate. This "end point" method can be a reliable proxy for predicting shoreline change in locations that have a steady, chronic erosion problem (e.g. Higgins Beach, Scarborough).

Another method uses a time series of shoreline positions taken from many air photos. At any particular location, the horizontal position over time is used to calculate a linear regression or "best fit" to the data. This approach is favorable to reduce the influence of erosion and accretion cycles that may exist on a beach (e.g. Popham Beach, Phippsburg or Goose Rocks Beach, Kennebunkport).

Shoreline Proxies

Erosion rate measurements (using either the end point or regression method) rely on one of several proxies for a shoreline position. The optimal proxy is the seaward edge of dune vegetation since it fluctuates gradually over a year. Alternatively, the position of the high-tide line or wet-dry beach line can be estimated in some air photos, but this position changes daily and seasonally.

In Maine, the fortnightly variation of the tides creates water elevation differences of over one foot in the level of "high tide." The spring-neap tidal height differences result in a horizontal shift of the wet-dry beach over 10 feet in just two weeks. Consequently, to use historical air photos, the monthly variation in the tides must be known as well as the slope of the beach in the photos in order to interpret the shoreline proxy of the "high-tide" line or the wet-dry line.

Errors

There are horizontal errors introduced by the process of digitizing any shoreline proxy and in calculating an erosion rate. In areas where the long-term erosion rate is slow (perhaps less than 1 foot in 2 years), the absolute distance eroded may not exceed the analytical errors that propagate through mathematical calculation of an erosion rate. Cumulative errors may exceed the amount of shoreline change (Crowell et al. 1991). In some locations, low erosion rates cannot be discerned from short-term temporal variation of the beach.

Maine Studies

In the early 1990s, the Maine Geological Survey used the end point method to compare 1953 and 1991 shorelines along most of the large beaches. This study used the leading edge of dune vegetation as a shoreline proxy. In areas without natural dunes, seawalls were digitized. A novel approach using an analytical stereoplotter was used that reduced horizontal errors and resulted in a geographic information system (GIS) map with both shorelines displayed (Duffy and Dickson, 1995). The results are only useful in areas without seawalls, but they do provide data on the natural rate of dune erosion or accretion over a period of 38 years.

One drawback of this data set is that the 1991 photos were taken about 3 weeks after a major storm (October 1991 "Perfect" Storm). Some scientists have suggested that the most robust shoreline change analysis should avoid using photographs taken after storms. At the time of the Maine study, there were no more recent or better quality photos to use than the 1991 set so, despite the influence of the storm on, the dune line was mapped for comparison.

A series of air photos and geomorphology from early nautical charts was used by Bruce Nelson (1979) to examine historical shoreline change of Maine beaches. His work only exists as small-scale maps from his thesis; the original large-scale maps no longer exist. Nelson used a Zoom-Transfer Scope in order to map sequential shorelines. Despite the passage of twenty-four years, his analysis and results are still very useful for understanding shoreline change. Nelson concluded that most natural beaches have a shoreline retreat rate of about 1 foot per year. As in the MGS study mentioned above (Duffy and Dickson, 1995), there are areas where seawalls are present in all the old photographs so horizontal erosion could not be measured.

Beach Profile Monitoring

Since 1999 Maine has had teams of volunteers profiling several Maine beaches (Heinze, et al., 2002; Hill et al., 2002). Using the Emery method once a month, these teams record elevation changes to the beaches. Some of these measurements are in natural settings and others are seaward of seawalls or adjacent to jetties. Results of these surveys continue to be analyzed. In the first two years of data it was determined that there is a large annual elevation change in many beaches, including those with seawalls. Over two years, however, the profiles did not show an equilibrium or stable condition (State of Maine Beach Profiling Project, 2003). A considerable difference in the volume of sand was found on the beaches from one summer to another.

The implications from the topographic analyses of this data set are important to floodplain mapping. Shore-normal beach profiles used in wave runup models could generate different results depending on the year or season that the profile was made. In some locations the vertical change through a year can be as much as a meter. This profile variability has a bearing on the certainty of selecting an appropriate coastal flood profile and hence on accurately projecting flood hazard areas in the dunes. From what is understood of the seasonal changes to the beach, flood hazard areas based on a topographic profile of August elevations underestimate flood hazards in February, a time when flooding is most likely to take place. The most valuable aspect of this beach profile data set for floodplain mapping is in understanding site-specific seasonality in beach elevations.

Topographic Change Analysis

Figure 5

High-resolution topographic data for southern Maine is available from the National Ocean Service. A 2000 LIDAR data is the first in what may be repeated surveys every few years (Figure 5). MGS proposed to NOS that the southern Maine coast be resurveyed in the next year and NOS is currently planning flight options to accommodate this request. When this second survey is completed, and others are made in the future, this 3-D approach to analyzing shoreline change will be far superior to that using 2-D historical air photos. Together, the two methods will better define the rate and character of shoreline recession in both the long- and short-term.

Repeated LIDAR surveys have been used to measure shoreline change and to calculate erosion rates. Since the data have a vertical (as well as horizontal) component, equal elevation contours can be delineated and changes at a particular elevation mapped. This allows several shoreline proxies, such as mean sea level (MSL) and mean high water (MHW), can be extracted from the data. Temporal changes to dune ridge elevations can also be analyzed so trends relative to floodplain elevations can also be computed (e.g. is the frontal dune ridge getting lower over time; will the dune ridge remain above the V-Zone FHA in the next 10 years?).

Georeferenced, three-dimensional data have been used by MGS to examine beach and dune profiles in relation to flood elevations from FIRMs (Slovinsky and Dickson, 2003). This study indicates areas of vulnerability to flooding, and when combined with historical shoreline change rates, indicates which areas are most likely to need mitigation for combined flood and erosion hazards. This work also serves as a good example of how shorelines can be ranked and compared to determine where both flooding and erosion increase the priority for revising FIRMs.

Shoreline Change and Remapping FIRMs

In order to do a sophisticated erosion analysis and to generate accurate erosion rates it is essential to understand the temporal and seasonal changes of shoreline positions taken from historic air photographs or recent detailed topographic analyses. Sound erosion rates must be generated with a minimum error in order to distinguish areas that are eroding slowly from those that are not. Most importantly, accurate erosion rates are necessary to predict when FIRMs need to be remapped.

Introduction Erosion processes Methods and data Suitability Assessment Spatial analysis Bluff erosion Obsolescence Conclusions References

Last updated on February 8, 2006.

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