HAZARD DESCRIPTION... 1 LOCATION... 2 EXTENT... 4 HISTORICAL OCCURRENCES...

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Emily Williamson
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1 L A N D S L I D E HAZARD DESCRIPTION... 1 LOCATION... 2 EXTENT... 4 HISTORICAL OCCURRENCES... 5 SIGNIFICANT PAST EVENTS... 6 PROBABILITY OF FUTURE EVENTS... 6 VULNERABILITY AND IMPACT... 6 HAZARD DESCRIPTION A landslide is the process that results in the downward and outward movement of slope-forming materials (NYS Geological Survey). Natural rocks, soil, artificial fill, or any combinations of these materials move by falling, toppling, sliding, spreading, or flowing. Landslides are caused by one or a combination of the following factors: change in slope of the terrain, increased load on the land, shocks and vibrations, change in water content, groundwater movement, frost action, weathering of rock, and removing or changing the type of vegetation covering slopes. Landslide hazard areas are where the land has characteristics that contribute to the risk of downhill movement of material, such as the following: A slope greater than 33 percent A history of landslide activity or movement during the last 10,000 years Stream or wave activity, which has caused erosion, undercut a bank or cut into a bank to cause the surrounding land to be unstable The presence or potential for snow avalanches The presence of an alluvial fan, indicating vulnerability to the flow of debris or sediments The presence of impermeable soils, such as silt or clay, which are mixed with granular soils such as sand and gravel. Other natural hazards, such as earthquakes, heavy rain, floods, or wildfires, can trigger landslides. The frequency at which landslides occur is often related to the frequency at which the area experiences these other hazards. The geology, vegetation, and amount of predicted precipitation of an area are all used in the predication of landslides. S T R A T E G I E S T O M I T I G A T E T H E I M P A C T S O F N A T U R A L D I S A S T E R S O N H U M A N L I F E A N D P R O P E R T Y

2 LOCATION The entire continental U.S. has historical evidence of landslides, with 36 states having moderate to highly severe landslide hazards. The growth of urban and recreational areas into hillsides is leading to a greater percentage of the population being threatened by landslides each year. Figure 13-1 shows the potential for landslides in the U.S. Figure Landslide Potential for the U.S. 1 The potential for landslides exists throughout New York State, including Otsego County. Scientific and historical data indicate that some areas of northern and eastern New York State have substantial landslide risk. However, New York is not categorized as a state with a serious landslide threat. According to USGS and NYSGS, around 80% of the State has a low susceptibility landslide hazard risk rating. Generally, the highest potential for landslides is located along major rivers and lake valleys that were previously glacial lakes resulting in glacial lake deposits (glacial lake clays), and usually areas associated with steeper slopes. A good example in New York State is along the Hudson and Mohawk River Valley. 1 Source: USGS, Circle indicates the approximate location of Otsego County. Page 2 Otsego County Hazard Mitigation Plan Update

3 Figure 13-2 illustrates the landslides susceptibility overview map of New York State; Otsego County has a low landslide incidence. Figure Landslide Susceptibility in New York State 2 Figure 13-3 shows the location of landslide incidences in New York State. The steeper areas are mostly found near drainage ways or along the north facing Helderberg and Onondaga limestone escarpments. The escarpments rise 600 feet above and to the south of the Mohawk Valley. 2 Source: NYS HMP, Otsego County located inside black circle. Otsego County Hazard Mitigation Plan Update Page 3

4 Figure Location of Landslides in New York State 3 EXTENT Soil properties, topographic slope, and historical incidence contribute to the potential for landslide activity. The landslide hazard is usually represented by landslide incidence and /or susceptibility. Landslide incidence is the number of landslides that have occurred in a given geographic area. High incidence means greater than 15-percent of the area has experience a landslide; medium incidence means that 1.5 to 15-percent of an area has been involved; and low incidence means that less than 1.5- percent of an area has been involved. Landslide susceptibility is defined as the degree of response of geologic formations to natural or artificial cutting, to loading of slopes, or to unusually high precipitation. It can be assumed that unusually high precipitation or changes in existing conditions can initiate landslides in areas where rocks and soils have experienced numerous landslides in the past. Only potentially affected areas are identified by landslide susceptibility, not a time frame for when a landslide might occur. The same percentages that are used for landslide incidence are used for landslide susceptibility (high= 15+%, medium %, low 0-1.5%). Figure 13-4 contains the landslide incidence and susceptibility of the northeastern U.S., identifying areas that have the potential for landslides. These areas are determined by looking at factors that contribute to 3 Source: NYS HMP, 2011 Page 4 Otsego County Hazard Mitigation Plan Update

5 causing landslides, such as steep slopes, weak geologic areas that lose strength when saturated, and poorly drained rock or soil, with the past distribution of landslides. Figure Landslide Incidence and Susceptibility in the Northeast U.S. 4 Landslide Incidence Low (less than 1.5% of area involved) Moderate (1.5%-15% of area involved) High (greater than 15% of area involved) Landslide Susceptibility/Incidence Moderate susceptibility/low incidence High susceptibility/low incidence High susceptibility/moderate incidence Most of New York State s soils consist of dense glacial till which holds up strong to landslides. However, certain types of soil are found throughout the State that has a higher risk of landslide susceptibility and incidence. Otsego County has an overall low landslide incidence. HISTORICAL OCCURRENCES According to the New York State Hazard Mitigation Plan, between 1837 and 2007 a total of 326 landslides were reported to have occurred in New York State. Of those reported in the state, Otsego County experienced seven landslides. No other information has been provided regarding these seven incidences. However, since 2006, no records of landslides in Otsego County have been reported in the SHELDUS or NCDC Databases. 4 Source: USGS, The black circle indicates the approximate location of Otsego County. Otsego County Hazard Mitigation Plan Update Page 5

6 SIGNIFICANT PAST EVENTS While the threat of a landslide event in the County is considered to be low on the susceptibility scale, descriptive data on historic events is scant. It has been documented that seven landslide events have occurred in the County since 1837, however, further research into County archives will be necessary to describe past events. PROBABILITY OF FUTURE EVENTS As indicated in the State s hazard mitigation plan, and given the history of landslide occurrences in New York State, it is certain that a landslide will occur again in the future. Therefore, the probability of future landslides in New York State is considered high. However, the probability of occurrence for landslides in Otsego County is considered unlikely, meaning an event is probable in the next ten years. VULNERABILITY AND IMPACT Vulnerability to landslides is a combination of location, soil type, geology, type of human activity, use, and frequency of events. The impact of landslides on humans and buildings can be reduced by avoiding, prohibiting, or imposing restrictions on hazard-zone activity. Governments can reduce landslide effects through land use policies and regulations. Individuals can reduce their exposure to hazards by educating themselves on past hazard history of the site and by making inquiries to planning and engineering departments of local governments. Due to the low level of risk for landslides occurring in Otsego County and previous damages, impact for landslides is Limited meaning less than 10 percent of buildings will suffer damages from landslides. However, the potential for a more severe event is possible but not to exceed the limits of being located in a low landside incidence corridor. No events have been reported to the NCDC and SHELDUS databases over the past 62 years (1950 to 2012). Annualized loss is neglible for landslide damage; approximately $0 of damage occurs annually based on available data. Frequency of return of a landslide event can be assumed to be less than one every 10 years. Page 6 Otsego County Hazard Mitigation Plan Update

Landslides & Mudflows

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