When an area is exposed to more than one hazard, a multiple hazard map (MHM) helps the planning team to analyze all of them for vulnerability and risk. By facilitating the interpretation of hazard information, it increases the likelihood that the information will be used in the decision-making process. In either the planning of new development projects or the incorporation of hazard reduction techniques into existing developments, the MHM can play a role of great value.
BENEFITS OF MULTIPLE HAZARD MAPPING
The main purpose of MHM is to gather together in one map the different hazard-related information for a study area to convey a composite picture of the natural hazards of varying magnitude, frequency, and area of effect. A MHM may also be referred to as a "composite," "synthesized," and "overlay" hazard map. One area may suffer the presence of a number of natural hazards. (Figure 6-1 is a tabulation of natural phenomena that can be considered for presentation on such maps). Using individual maps to convey information on each hazard can be cumbersome and confusing for planners and decision-makers because of their number and their possible differences in area covered, scales, and detail.
Many natural hazards can be caused by the same natural event. The inducing or triggering mechanism which can interconnect several hazards can more easily be seen through the use of a MHM. Characteristics of the natural phenomenon and its trigger mechanisms are synthesized from different sources and placed on a single map.
Additionally, the effects and impact of a single hazard event, as in the case of volcanoes and earthquakes, include different types of impacts, each having different severities and each affecting different locations.
The MHM is an excellent tool to create an awareness in mitigating multiple hazards. It becomes a comprehensive analytical tool for assessing vulnerability and risk, especially when combined with the mapping of critical facilities.
The adoption of a multiple hazard mitigation strategy also has several implications in emergency preparedness planning. For example, it provides a more equitable basis for allocating disaster planning funds; stimulates the use of more efficient, integrated emergency preparedness response and recovery procedures; and promotes the creation of cooperative agreements to involve all relevant agencies and interested groups: It must be emphasized that the MHM will not meet the site-specific and hazard-specific needs of project engineering design activities.
The effective use of natural hazard information to avoid damage or to reduce loss requires a considerable effort on the part of both the producers and the users of the information. Unless the scientific and engineering information is translated for the layman, the effective user community is limited to other scientists and engineers. If the users do not become proficient in interpreting and applying technical information, the information is likely to be misused or even neglected in the development planning process. Studies by Kockelman (1975, 1976, 1979) on the use of earth-science information by city, county, and regional planners and decision-makers in the San Francisco Bay region of the United States show that the most effective use of hazard information is achieved when maps clearly depict the likelihood of occurrence, location, and severity. Furthermore, hazard reduction was more likely when agencies had scientists or engineers on their staffs. Their skills permitted a broader use of the technical information, and the agencies were able to make interpretations of the information for their own purposes.