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The Cincinnati, Ohio Study In 1985, the U.S. Geological Survey, in cooper- ation with the Federal Emergency Managament Agency, conducted a geologic/economic develop- ment study in the Cincinnati, Ohio area. This study developed a systematic approach to quantitative forecasting of probable landslide activity. Landslide probabilities derived from a reproducible procedure were combined with property value data to forecast the potential economic losses in scenarios for proposed development and to quantitatively identify the potential benefits of mitigation activities. The study area was divided into 14,255 grid cells of 100-square meters each. Informa- tion calculated for each cell included: probabil- ity oflandslide occurrence, economic loss in the event of a landslide, cost of mitigation, and economic benefit of mitigation. This informa- tion was used to develop a mitigation strategy. In areas where both slope and shear strength information were available, the optimum strat- egy required mitigation in those cells with slopes steeper than 14 degrees or where mater- ials had effective residual stress friction angles ofless than 26 degrees. This strategy yielded $1.7 million in estimated annualized net bene- fits for the community. In areas where only slope information was used, the best strategy required mitigation in those cells where slopes were greater than 8 degrees. This yielded an estimated annualized net benefit of $1.4 mil- lion. Therefore, using regional geologic inform- ation in addition to slope information resulted . in an additional $300,000 net benefit. The Cincinnati study cost only $20,000 to prepare (Bernknopf et a!., 1985). The Benefits of Mitigation in Japan Japan has what is considered by many to be the world's most comprehensive landslide loss reduction program. In 1958, the Japanese gov- ernment enacted strong legislation that provid- ed for land-use planning and the construction of check dams, drainage systems, and other physical controls to prevent landslides. The success of the program is indicated by the dramatic reduction in losses over time (Figure 4). In 1938, 130,000 homes were destroyed and more than 500 lives' were lost due to landslides 6 in the Kobe area. However, since the Japanese program went into effect, losses have decreased dramatically. In 1976-0ne of Japan's worst years for landsliding-only 2000 homes were destroyed with fewer than 125 lives lost (Schuster and Fleming, 1986). NUMBER OF DEAD OR MISSING ITOP BARI o JULY 1938 JULY 1945 SEPT. 1947 JULY 1951 r.n JUNE 1953 ~ JULY 1953 e AUG. 1953 en SEPT. 1958 " Z AUG. 1959 ii: LiI JUNE 1961 " C) SEPT. 1966 ii: to- JULY 1967 I ~ JULY 1967 ~ JULY1972 ~ AUG. 1972 .. ...J JULY 1974 ~ AUG. 1975 cr: AUG. 1975 ~ SEPT. 1976 MAY 1978 OCT.'t978 AUG. 1979 AUG. 1981 o o 0 2 0 0 0 0 0 0 8 g g ~ ~ ~ g ~ ~ ~ g ~ g ~ ~ ,....., ;J' C~ta 1 ~ r = ~ ... r . . r- r I I r 130,000 15,141 9,754 g g g g g 8 g g g 0 g g Q 0 0 0 0 0 0 0 0 800 ~ N M ~ ~ ~ ~ ~ ~ Q ~ ~ NUMBER OF HOUSES DESTROYED OR BADLY DAMAGED I BOTTOM BAR I Figure 4. Losses due to major landslide dIsasters (mainly debris flows) in Japan from 1938-1981. All of these landslides were caused by heavy rainfall, most commOnly related to typhoons, and many were assoc- iated with catastrophic flooding (data from Ministry of Construction, Japan, 1983). Planning as a Means of Loss Reduction The extent and severity of the landslide hazard in a particular area will determine the need for a landslide hazard mitigation plan.