Contents

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

Suitability of Maine data for Erosion Hazard Area (EHA) determinations and Erosion Rate Analysis

Previous Work

Maine data on shoreline change is variable from one geographic area to another. This variability is due to the availability of historical data and its application by scientists for specific studies. Work to date has shown that shoreline change occurs along all of Maine's beaches at different rates. The average shoreline recession rate for natural dune areas has been calculated at 1 foot per year (Nelson, 1979) using historical data from air photos at many locations.

Of course, those areas with no change at a seawall may have had significant change to the elevation of the beach. In fact it is quite likely that the beach lost sand and became lower during the decades while the seawall was active. Consequently, it is misleading to assume that "no change" represents a static shore-normal profile and thus a static condition for estimating wave runup and coastal flood profiles.

Recent analysis of Saco Bay by Slovinsky and Dickson (2003) serves to illustrate the potential for examining EHAs using historical air photos and LIDAR data. By projecting shoreline change onto beach profiles generated from LIDAR topography, volume estimates of sand loss or gain were generated to calculate sand budgets for discrete shoreline segments and littoral cells. This information is then useful in understanding the cause(s) of shoreline change and to understand if the sediment budget for a region is in balance. The budget approach also allows comparison of human activities, such as dredging, to be compared to natural rates of sand movement.

A Maine Geological Survey investigation of regional sediment budgets over the last 50 to 100 years has found a profound influence on sand loss and accumulation due to human activity. Over the time frame of air photos commonly used to determine EHAs, there have been larger changes to the shoreline from anthropogenic factors than from natural ones. Consequently, the first data needed for interpreting shoreline change (and if it will be sustained) to project future EHAs is to construct and interpret a sediment budget based on historical data. Two examples serve to illustrate the profound impact of humans on shoreline change.

Wells Embayment

The sand budget for Wells Embayment (Ogunquit, Wells, Kennebunk, and Kennebunkport) is very revealing of the influence of human activity on shoreline change. In terms of sand volumes, about 15% of the coastal barrier sand is on land (see Figure 6, Figure 7, and Table 1). About 4.2 million cubic yards of sand is in the frontal dune and 9.0 million cubic yards are in the back dunes. An enormous 75 million cubic yards are in the submerged shoreface seaward of the beaches.

Figure 6

Figure 7

Table 1

Analysis of dredging records and shoreline change indicate that human activity has resulted in the accelerated movement of up to 2 million cubic yards of sand in the Wells Embayment in the just last 40 years. This human influence is approximately equivalent to moving half of the frontal dune ridge in the entire bay. Consequently, there is no doubt that coastal engineering and dredging practices must be considered in projecting EHAs in this region.

Saco Bay

The sand budget for Saco Bay is very different from that in Wells Embayment (Figure 8, Figure 9, and Table 2). In Saco Bay the frontal dune contains about 3.3 million cubic yards of sand while the back dune has 16.2 million cubic yards. Offshore, the shoreface contains 73 million cubic yards. As in the Wells Embayment, over 75% of the sand in the system is offshore. The most impressive change to the bay has come from a northward transport of sand from the Saco River ebb-tidal delta to the Scarborough ebb-tidal delta and Pine Point. In the century from 1859 to 1955, over 4 million cubic yards of sand moved from the southern to the northern end of Saco Bay (Kelley et al., 1995). The rate of transport north accelerated 300% after the federal jetties were constructed at the mouth of the Saco River. In other words, the human influence on the shorelines and sand budget of Saco Bay overwhelmed the natural processes and thus EHAs in the bay are dominantly anthropogenic.

Figure 8

Figure 9

Table 2

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

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Last updated on February 8, 2006.

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