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<br />Chapter 4 <br />EVALUATING AND COMMUNICATING THE HAZARD <br /> <br />METHODS OF LANDSLIDE <br />EVALUATION AND ANALYSIS <br /> <br />Recognition of the presence of active or potential slope <br />movement, and the type and causes of the movement, <br />is essential to landslide mitigation. Recognition depends <br />on an accurate evaluation of the geology, hydrogeology, <br />landforms, and interrelated factors such as environmen- <br />tal conditions and human activities. Only trained profes- <br />sionals should conduct such evaluations. However, <br />because local governments may need to contract for <br />such services, they should be aware of the techniques <br />available and their advantages and limitations. <br />Teclmiques for recognizing the presence or potential <br />development of landslides include: <br />. map analysis <br />. analysis of aerial photograpby and imagery <br />. field reconnaissance <br />. aerial reconnaissance <br />. drilling <br />. geophysical studies <br />. computerized landslide terrain analysis <br />. instrumentation <br />Summaries of these techniques are presented below, <br />and more detailed discussions are included in Appendix <br />1. <br /> <br />Map Analysis <br />Map analysis is usually one of the first steps in a land- <br />slide investigation. Maps that can be used include <br />geologic, topographic, soils, and geomorphic. Using <br />knowledge of geologic materials and processes, a <br />trained person can obtain a general idea of landslide <br />susceptibility from such maps. <br /> <br />Analysis of Aerial Photography and <br />Imagery <br />Aerial photograpby is a rapid and valuable technique for <br />identifying landslides, because it provides a three- <br />dimensional overview of the terrain and indicates human <br />activities as well as much geologic information. In addi- <br />tion, the availability of many types of aerial imagery <br />(sate1lite, infrared, radar, etc.) make this a very versatile <br />technique. <br /> <br />Field Reconnaissance <br />Many of the more subtle signs of slope movement can- <br />not be identified on maps or photographs. Indeed, if an <br />area is heavily forested or has been urbanized, even <br />major features may not be evident. Furthermore, land- <br />slide features change over time on an active slide. Thus. <br />field reconnaissance is necessary to verify or detect <br />many landslide features. <br /> <br />. Aerial Reconnaissance <br />Low-level f1ights in helicopters or small aircraft can be <br />used to obtain a rapid and direct overview of a site. <br /> <br />Drilling <br /> <br />At most sites. dri1Iing is necessary to determine the <br />depth to the slip surface and thus the thickness and <br />geometry of the landslide mass, the water table level, <br />and the amount of disruption of the landslide materials. <br />It can also provide samples for age-dating and testing <br />the engineering properties of landslide materials. Fmally, <br />dri1Iing is needed for installation of some monitoring in- <br />struments and hydrologic observation wells. <br /> <br />Geophysical Studies <br />Geopbysical techniques (the study of changes in the <br />earth's gravitational and electrical fields, or measure- <br />ment of induced seismic behavior) can be used to deter- <br />mine some subsurface characteristics such as the depth <br />to bedrock. zones of saturation, and sometimes the <br />ground-water table. In some instances these methods <br />can be used in lieu of drilling. Monitoring of natural <br />acoustic emissions from moving soil or rock has also <br />been used in landslide studies. <br /> <br />'~~, <br /> <br /> <br />Computerized Landslide Terrain Analysis <br /> <br />In recent years computer modeling of landslides has <br />been used to determine the voJtune of a landslide mass <br />and changes in surface expression and cross section <br />over time. This information is useful in calculating the <br />potential for stream blockage, cost of landslide removal <br />(based on cubic yards), and type and mechanism of <br />nwvement. Very promising methods are being <br />developed to utilize digital elevation models (DEMs) for <br />rapid evaluation of areas for susceptibi1ity to land- <br />slide/debris-flow events (FIlson. 1987; Ellen and Mark. <br /> <br />21 <br />