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Landslide Susceptibility Mapping in Maine
Landslides are one of the most common geologic hazards in Maine (Figure 1), causing damage in both rural and urban areas of the state. In many of Maine's landslide susceptible areas, factors affecting slope condition such as construction, seismic activity, or increased soil moisture may cause movement or may reactivate prior landslides. Until recently, only a few landslides have caused extensive property damage in Maine. Two landslides of note include one in Gorham, Maine in 1983 (Novak, 1987a) (Figure 2), and another in Rockland, Maine in 1996 (Berry and others, 1996) (Figure 3).
During the spring of 2005, 2006, and 2007, landslides severely damaged residential properties in the towns of Sanford, Wells, Brunswick, and Cumberland (Figure 4, Figure 5, Figure 6), and a major landslide occurred in Greenbush along the Penobscot River destroying parts of a major roadway (Figure 7). All of these landslides occurred after heavy rains and led to considerable concern among state and local authorities, and the general populace, about the stability of slopes in these areas and elsewhere in Maine.
Initial investigation of these slides showed that they were not unique geologic events in the locations where they occurred. Many nearby areas exhibited evidence of earlier landslides. What they all had in common was that they occurred in areas underlain by a glaciomarine clay called the Presumpscot Formation, and usually occurred in areas with steep slopes. The Presumpscot Formation is a widespread blanket of glaciomarine silt, clay, and sand that covers much of coastal Maine and inland lowlands, and has proven to be highly susceptible to slope failure. Due to the increased landslide occurrences, the Maine Geological Survey (MGS) produced a series of Landslide Susceptibility Maps for areas in Maine underlain by glaciomarine deposits, and in particular, the marine clay of the Presumpscot Formation.
Risk Factor Analysis
The Landslide Susceptibility Maps were created using Risk Factor Analysis based on the following principles:
Simply put, all available data is collected for risk factors that are located within the area of study. Next, all landslide locations within this same area are systematically mapped. Using a Geographic Information System (GIS), mapped landslides are compared to each risk factor and examined to determine which risk factors are the most statistically significant causes of landslides. Once the analysis is complete, statistically significant risk factors are mapped, and zones of landslide susceptibility are created, ranging from areas of no risk factors (lowest landslide potential) to areas where there are 3 or more risk factors present (highest landslide potential).
Landslide risk factors are shown in Table 1. The following risk factors were used in this study:
Figures 8-16 illustrate the process of using Risk Factor Analysis to assemble the landslide susceptibility map. The area surrounding the town of Eliot, Maine is used as an example.
Step one: Prepare a map that combines the permanent risk factors of surficial geology (Figure 8), slope steepness (Figure 9), slope aspect (Figure 10), slope curvature (Figure 11), and any other data available on a DEM (digital elevation model) base (Figure 12, Figure 13). These datasets would be available as GIS data layers, easily plotted on the map.
Step Two: Overlay the landslide inventory map (Figure 14) onto the map of combined risk factors. This will identify areas where past landslide occurred and where current risk factors are located.
Step Three: Run a statistical analysis on the risk factors versus the landslide inventory, and determine which risk factors are statistically significant to landslide susceptibility. Select the risk factors of the highest statistical significance to use for your Landslide Susceptibility zonation: areas with one (1) risk factor; two (2) risk factors; etc. (Figure 15).
Step Four: Produce a landslide susceptibility map layer delineating these risk factors zones for your area of study (Figure 16).
The Final Map
This final Landslide Susceptibility Map ranks the degree to which parts of the study area are prone to future landslides, based on the risk factors that produced past landslides. A landslide susceptibility map does not provide information on how often landslides occur, as in a landslide hazard map, or the extent of damages and injuries that might be anticipated from a future landslide event, as in a landslide risk map.
Limitations of the maps
Applicability of the maps
Novak, Irwin D., 1987, Geology of the September 1983 landslide at Gorham, Maine; in Andrews, David W., Thompson, Woodrow B., Sandford, Thomas C., and Novak, Irwin D. (editors), Geologic and geotechnical characteristics of the Presumpscot Formation, Maine's glaciomarine 'clay': unpublished proceedings of a symposium sponsored by the Maine Geological Survey, Morrison Geotechnical Engineering, University of Maine, and University of Southern Maine, March 20, 1987, 14 p.
Berry, Henry N., IV, Dickson, Stephen M., Kelley, Joseph T., Locke, Daniel B., Marvinney, Robert G., Thompson, Woodrow B., Weddle, Thomas K., Reynolds, Richard T., and Belknap, Daniel F., 1996, The April 1996 Rockland landslide: Maine Geological Survey, Open-File Report 96-18, 55 p. and plate.
Text and photos by Michael E. Foley.
Originally published on the web as the November 2010 Site of the Month.
Last updated on December 7, 2010
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