Skip Maine state header navigation

Agencies | Online Services | Help

Skip All Navigation

Home > Explore! > Geologic Hazards > Landslides > Landslide Susceptibility

Landslide Susceptibility Mapping in Maine

landslide
Figure 1

Introduction

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).

Gorham 1983 landslide
Figure 2		 Rockland 1996 landslide
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.

Cumberland 2006 landslide
Figure 4		 Wells 2005 landslide
Figure 5		 Sanford 2006 landslide
Figure 6		 Greenbush 2006 landslide
Figure 7

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:

  1. It is likely that landslides will occur where they have occurred in the past.
  2. Landslides are likely to occur in similar geological, geomorphological, and hydrological conditions as they have in the past.

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:

  • Geomorphic Risk Factors
    • Slope: The steeper the slope, the larger the shear stress on the materials and the more susceptible the slope is to failure
    • Curvature (concave): Concave topography will concentrate groundwater flow, raising pore pressures and reducing shear strength of the soil.
    • Slope aspect: Repeated freeze/thaw cycles reduce the shear strength of the shallow soil material, increasing the likelihood of shallow soil slumps and creep.
    • Relief/slope height: As the thickness of the potential landslide block increases, the shear stress on the lower section of block increases, making the block (slope) more susceptible to failure. Therefore, thicker sections of surficial materials will be more susceptible to landslides.
  • Soil properties
    • Surficial geologic materials: Cohesive materials such as clays are prone to landslides along planes of weakness in the sediment. Less cohesive materials (sands) may slump if slopes oversteepen or ground water pore pressure increases and reduces internal friction.

Map Assembly

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.

surficial geology layer
Figure 8		 slope layer
Figure 9		 slope aspect layer
Figure 10		 slope curvature
Figure 11		 relief layer
Figure 12		 digital elevation model layer
Figure 13

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).

known landslide polygons
Figure 14		 grouped risks
Figure 15		 landslide susceptibility zoneation
Figure 16

The Final Map

final map
Figure 17		 After performing the Risk Factor Analysis and delineating the landslide susceptibility zones, the datasets are combined to create a final Landslide Susceptibility Map (Figure 17). These maps consist of a shaded relief base map overlain by surficial geology, landslide locations, and zones of relative landslide susceptibility.

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

  • Town-level landslide mapping is required to validate the susceptibility maps and provide credibility to local and State officials.
  • Landslide susceptibility maps indicate where landslides are likely to occur, not whether or when a landslide will occur.
  • The maps are based on landslide sites mapped below the glacial marine limit. Extension to other geologic settings would require additional study.
    • 18% of landslides occur on glacial till
  • Additional statistical analysis and incorporation of additional risk factors may improve the usefulness of the susceptibility maps. Data for additional risk factors must be available throughout the study area.

Applicability of the maps

  • The maps identify where landslides have already occurred and where future landslides may occur.
  • Public education.
  • The effects of future landslides may be mitigated by adopting appropriate building codes or land use policies in landslide prone areas.

Conclusions

  • Utilizing input and reviews of maps from local and state officials will enhance future map products.
  • Acquisition of LIDAR data will enhance quality and accuracy of maps.
  • Updating Maine's landslide inventory database will increase the accuracy and utilization of the maps.
  • Landslide susceptibility mapping is an ongoing process and will continually evolve into a more useful tool for landslide hazard mitigation.

References

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