FLOOD06042 - Laserfiche WebLink

Home Browse Search

Chapter 3 VULNERABILITY AND IMPACT VULNERABILITY Vulnerability is the susceptibility or exposure to in- jury or loss from a hazard. People, structures, com- munity infrastructure (transportation systems, water supply, communications, and electricity), and social systems are all potentially vulnerable. Colorado's vulnerabi1ity to the landslide hazard is largely a consequence of the increasing expansion of commercial and residential development onto steep or unstable terrain that is prone to landsliding. Before resources are invested in hazard mitigation measures, the social and economic costs and impacts associated with Iandsliding need to be determined and put into perspective. The first step in determining the overall vulnerability of the state to landslide hazard is the identification of communities, areas, and facilities at risk (for Colorado see chapter 6). POTENTIAL IMPACTS OF LANDSLIDES ON PEOPLE AND PROPERTY Once the vulnerability of various communities, areas, and facilities to Iandsliding has been determined, site- specific evaluations of the potential impacts of Iandsliding should be performed. Impact is the effect of the occur. rence of the hazard on people and the infrastructure. Significance is a quantification of the major social and economic elements impacted. Table 1 reflects the types of impacts that can result from Iandsliding and their con- sequences in estimated order of increasing significance. INTERRELATIONSHIP OF LANDSLlDING WITH OTHER NATURAL HAZARDS (THE MULTIHAZARD CONCEPT) Natural hazards often occur simultaneously or, in some cases, one hazard triggers another. For example, an ear- thquake may trigger a landslide, which in turn may block a valley causing upstream flooding. Different hazards may also occur at the same time as the result of a com- mon cause. For example, heavy precipitation or rapid snowmelt can cause debris flows and flooding to occur in the same area. The simultaneous or sequential occurrence of interac- tive hazards may produce cumulative effects that differ sigllificalltiy from those expected from any single one of the component hazards. Landslidin9 as Related to Dam Safety The safety of a dam can be severely compromised by the occurrence of landsliding upstream from the dam, or on slopes bordering the dam's reservoir or abutments. Possible'impacts include 1) the formation of wave surges that can overtop the dam, 2) increased sedimentation, and 3) dam failure. Flood surges can be generated either by the sudden detachment of large masses of earth into the reservoir, or by the formation and subsequent failure of a landslide dam across a tributary stream channel. Waves formed by such failures can overtop the dam and cause downstream flooding without actually causing dam failure. landslidiog into upstream areas or reservoirs can greatly increase the amount of sediment that is deposited in the reservoir, ultimately reducing storage capacity. This increases the likelihood that the dam will be overtopped during periods of excessive runoff, caus- ing downstream flooding. Sedimentation can also damage pumps and water intake valves. Actua1 dam failure could be caused by lands1iding at or near the abutments or in the emb:m1rmPllts of earthen dams. Rapid changes in the water level of reservoirs can also trigger landslides. When the water level in the reservoir is lowered, the subsequent loss of support provided by the water and increased seepage pressure can initiate sliding. Alternately, the increase in saturation caused by rising water can trigger landslides on slopes bordering the reservoir. Landsliding as Related to Flooding Landsliding and flooding are closely allied because both are related to precipitation, runoff, and ground satura- tion. In addition, debris flows usually occur in small, steep stream channels and are often mistaken for floods. In fact, these events frequently occur simultaneously in the same area and in some cases grade into each other. Landslides and debris flows can cause flooding by forming landslide dams that block valleys and stream channels, allowing large amounts of water to back-up. This causes backwater flooding and, if the dam breaks, 17