Contents

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

Summary of existing data on the severity of bluff erosion and how it might affect FIRMs

Figure 19

The "rocky" coast of Maine is much less stable than has been previously thought. Maine Geological Survey and University of Maine studies of coastal bluff processes and evolution have shown that bluffs are present along over half of the Maine coast (Dickson, 2001b; Figure 19). Research has resulted in a series of maps that identify the location and severity of bluff erosion (Kelley and Dickson, 2000). Bluffs are classified using geomorphology and shoreline type. Examples of bluffs in Casco Bay can be found on maps by Bryant (2002a, b). Development occurs on bluffs that are eroding and, depending on the proximity of the bluff top to the structure, there can be buildings at risk of damage from the gradual loss of sediment from the bluff face. In Casco Bay, there are bluffs on the large islands such as Great Diamond and Peaks Islands where there are many ocean-front homes.

In addition to the chronic erosion that is common on the face of many bluffs (Figure 20), there is another process of internal mass movement of the land that places some structures at risk. Landslides, formed by the internal failure of sediments, tend to occur in areas where the sediments are (a) made of clay, (b) the clay thickness exceeds 20 feet, and (c) there is water saturating the ground. Factors that lead to landslides are described on the MGS Coastal Landslides map series (Dickson, 2001c) and include the slope of the bluff face, types of vegetation, depth to bedrock, sediment type, ground water level, weathering, and earthquakes. Land use that affects vegetation, surface or groundwater levels, and buildings that load the ground all can contribute to increase or decrease landslide risk.

Figure 20

Figure 21

Shoreline engineering is sometimes used to stabilize the shoreline at the base of bluffs. This effort has been successful in some locations and not in others. In order to reduce the risk of landslides, bluffs are sometimes graded to a lower slope in order to relive the earth load within the sediments. The MGS map products provide an indication of which areas are vulnerable to bluff erosion and landslides. The risk to property from landslides is very real and the timing of failure is difficult to predict (Figure 21).

As with the beaches, bluff erosion is only a significant factor in making FIRMs obsolete if the rate of bluff erosion is rapid and there has been a significant amount of time between the last map and the current date. In this study, bluff erosion rates of about 0.5 feet per year were assumed for the City of Portland. The FIRMs for Portland are only 5 years old, so the absolute amount of land loss is only a few feet since the maps were made. By comparison, bluff erosion in the City of Portland is less of an issue for map modernization that beach erosion in some other southern Maine communities. Elsewhere in Maine, bluff erosion rates have been calculated at a few locations and are as high as 3 .5 feet per year (Smith, 1990).

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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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