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Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan
Summary of existing data on the severity of bluff erosion and how it might affect FIRMs
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.
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).
Last updated on February 8, 2006.
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