A Probabilistic Approach to Landslide Hazard Mapping in Cincinnati, Ohio, with Applications for Economic Evaluation

The economic value of geologic hazards research and data can be estimated only if the information is used in ways that are associated with specific economic consequences, such as a decision whether or not to mitigate against a potential landslide hazard at a specific location. The economic benefits of a decision to mitigate are measured by the expected losses that can be avoided if mitigation is undertaken, after the costs of mitigation activities are subtracted. To evaluate the spatial distribution of expected losses in a Cincinnati, Ohio, study area, we used regional geologic and topographic information to establish a regression equation that estimates the probability of landslide occurrence in 100-m square units. The distribution of different landslide probabilities can be mapped. Probability estimates provide a numerical measure of the relative hazard potential for each subunit of a community and permit systematic estimation of expected losses where property values are known. By postulating a sequence of community decisions about where to impose regulations that require individuals to incur the expense of mitigation activities, a forecast of the economic consequences of decisions made with and without specific kinds of regional information can be quantitatively estimated. The probabilistic assessment of landslide susceptibility provides an essential tool for economic evaluation of community-imposed requirements for landslide hazard mitigation. A set of hypothetical strategies for community mitigation that impose the provisions of the Uniform Building Code (UBC) Chapter 70 (International Conference of Building Officials, 1979), were applied to a study area in Hamilton County (Cincinnati), Ohio. One strategy requires mitigation throughout the entire area. This strategy would yield annualized gross benefits (losses avoided) of $4.9 million at a cost of $5.0 million for mitigation activities—an annualized negative net benefit (net loss) of $0.1 million. In other words, the uncritical application of the UBC Chapter 70, grading code provisions to the entire study area would probably not be cost-effective. Cost-effective alternative strategies might require mitigation only in selected parts of the area. For example, if the areas that require mitigation are selected by using regional topographic information (i.e., slope), the best strategy requires mitigation where slopes are steeper than 8° and provides annualized gross benefits of $3.1 million at a cost of $1.7 million—an annualized positive net benefit of $1.4 million. If the areas are selected by using both slope and regional geologic information (i.e., the regional distribution of surficial materials having different shear strengths), the optimum strategy requires mitigation where slopes are steeper than 14° or where materials have shear strengths (tan ϕ r ′ ) less than 0.49. This strategy provides annualized gross benefits of $3.1 million at a cost of $1.4 million—an annualized positive net benefit of $ 1.7 million. The use of regional geologic information in addition to the slope data, therefore, yields to the community an incremental improvement (annualized marginal net benefit) of $0.3 million annually, an annual return that is greater than the one-time cost of acquiring the information.