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CHAPTER 12. LANDSLIDE <br />12-5 <br />12.2.3 Severity <br />Landslides destroy property and infrastructure and can take the lives of people. Slope failures in the United <br />States result in an average of 25 lives lost per year and an annual cost to society of about $1.5 billion. <br />According to SHELDUS, the 1965, 1972, and 2009 storms caused $15 million in property damage due to <br />landslides and other combined hazards. <br />12.2.4 Warning Time <br />Mass movements can occur suddenly or slowly. The velocity of movement may range from a slow creep <br />of inches per year to many feet per second, depending on slope angle, material and water content. Some <br />methods used to monitor mass movements can provide an idea of the type of movement and the amount of <br />time prior to failure. It is also possible to determine what areas are at risk during general time periods. <br />Assessing the geology, vegetation and amount of predicted precipitation for an area can help in these <br />predictions. However, there is no practical warning system for individual landslides. The current standard <br />operating procedure is to monitor situations on a case-by-case basis, and respond after the event has <br />occurred. Generally accepted warning signs for landslide activity include: <br />• Springs, seeps, or saturated ground in areas that have not typically been wet before <br />• New cracks or unusual bulges in the ground, street pavements or sidewalks <br />• Soil moving away from foundations <br />• Ancillary structures such as decks and patios tilting and/or moving relative to the main house <br />• Tilting or cracking of concrete floors and foundations <br />• Broken water lines and other underground utilities <br />• Leaning telephone poles, trees, retaining walls or fences <br />• Offset fence lines <br />• Sunken or down-dropped road beds <br />• Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content) <br />• Sudden decrease in creek water levels though rain is still falling or just recently stopped <br />• Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb <br />• A faint rumbling sound that increases in volume as the landslide nears <br />• Unusual sounds, such as trees cracking or boulders knocking together <br />12.3. SECONDARY HAZARDS <br />Landslides can cause several types of secondary effects, such as blocking access to roads, which can isolate <br />residents and businesses and delay commercial, public and private transportation. This could result in <br />economic losses for businesses. Other potential problems resulting from landslides are power and <br />communication failures. Vegetation or poles on slopes can be knocked over, resulting in possible losses to <br />power and communication lines. Landslides also have the potential of destabilizing the foundation of <br />structures, which may result in monetary loss for residents. They also can damage rivers or streams, <br />potentially harming water quality, fisheries and spawning habitat.