Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan

Assessment of data gaps that would improve EHA assessments

The largest data gap that exists in Maine is in the ability to construct EHA determinations in areas where seawalls have been in place since the 1940s. As listed in the table below, 30 to 40% of Maine beach shorelines have seawalls that preclude horizontal measurements of erosion rates based on mapping the leading edge of vegetation as a shoreline proxy. These engineered dunes are almost always seaward of structures that may, at some time, become threatened by erosion. It seems logical that seawalls were first built due to the threat of erosion to buildings in the dunes. Given that most shoreline data indicates the natural trend is for the dunes to erode and shoreline to recede inland, the erosion threat to these developed areas are probably larger today than it has been in the past.

In his study, Nelson (1979) examined 30 miles (49 km) of beach both with and without seawalls. Of those miles, 31% had active seawalls where no erosion or accretion could be measured. An additional 8% of shorelines examined had seawalls, yet these areas experienced accretion in front of the walls. Another 31% had no seawalls and experienced shoreline recession (erosion) while 17% of natural dunes experienced accretion. The remaining 12% of the natural shorelines showed stability.

Shoreline Environment	Condition	Amount	FIRM Affected
Natural beach and dune	Eroding	31%	Yes
Natural beach and dune	Accreting	17%	Yes
Natural beach and dune	Stable	12%	No
Seawall, active	No change	31%	Yes
Seawall, inactive	Accreting	8%	Yes

In a recent analysis by the Maine Geological Survey (Slovinsky and Dickson, 2003), a method of shoreline change and topographic analysis confirmed that there are a number of dune areas where structures in Saco Bay could be threatened by erosion in the next century. This analysis is based on only two sets of historical air photographs to determine shoreline change through the end point method. Consequently it needs to be strengthened and made more accurate through the use of additional air photos and a linear regression analysis. In future studies, the analysis should be performed in a geographic information system (GIS) and not in a graphics software program. Nevertheless, the approach demonstrated that there is a significant need to continue with erosion rate calculations and sediment budget analyses because there are properties at risk.

In 2003 digital high-resolution aerial photographs of southern Maine were acquired. These images of the beaches will become available as a georeferenced mosaic for use in a GIS. This new "base" will be ideal for registering and comparing historical air photos for future EHA analysis. This new data set will provide a contemporary shoreline (the previous high quality air photo used by MGS was 1995) and data gap of 8 years will be filled shortly.

In 2003 or 2004 NOAA plans to acquire another LIDAR survey of southern Maine beaches for MGS to use. Flight planning is currently in progress and the State of Maine will receive the data once it is processed. This data will improve EHA determinations in two ways. First, it will allow calculation of topographic changes that have occurred to the beaches and dunes over a 4 year period (since the September 2000 LIDAR flight). This comparison will allow estimation of the horizontal movement of various elevations of the beach such as the mean high water (MHW) line or the mean sea level (MSL) line as well as various elevations of the seaward slope of the frontal dune. The data will allow a comparison of the vertical elevation of the beach seaward of the seawalls. These data, in turn might be linked to beach profile data collected by volunteers around during the year so seasonal variation in elevation can be compared to LIDAR elevation changes.

Second, the LIDAR data will be closely spaced in time to the new set of digital air photographs of the coast so features such as new structures and changes to engineering can be visually compared and interpreted. This pair of data will enable beach profiles to be constructed and the elevation of the land and engineering structures to be compared to the FHA elevations along the beach front. These parameters, along with distances of the shoreline proxies to habitable structures will allow a better GIS estimate of what is at risk due to erosion in various time frames (30-, 60-, 100-year periods).

Another data gap that needs to be filled is to scan and orthorectify historical air photos. Until the historical photos can be placed in an earth coordinate system with a high degree of accuracy, the measurement and calculation of shoreline change rates in Maine will have errors that exceed the real change in many locations. Currently, the digital data needed for precise shoreline change mapping is not available.

Figure 10

Lastly, the Maine Geological Survey has explored new methods of estimating how much shoreline recession would have occurred in a particular location if seawalls had not been built. The approach is new and holds promise to overcome some of the difficulties present with historical photo analysis along a shoreline with seawalls. Using LIDAR topographic profiles from a natural dune system that most closely represents the beach in a particular area, the profile can be matched throughout its mid- to low-tide levels to those in adjacent beaches with seawalls. By matching the lower portion of beach profiles, the offset in the upper profiles can be examined. This offset can project the landward position of the frontal dune in an area that has been fixed due to seawalls and development. This concept is illustrated with data from Wells Beach in Figure 10. The apparent offset is on the order of 50 feet (15 m) and represents what erosion would have taken place without the coastal engineering structure. This offset can be used, along with the age of the seawall, to determine an erosion rate. Since most seawalls in Maine were built in the 1940s and 1950s, the time frame for calculating this proxy for shoreline erosion is on the order of half a century, or about equivalent to the period for which aerial photographs have been available.

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

Last updated on February 8, 2006.

Contents

Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan

Assessment of data gaps that would improve EHA assessments