ECONOMIC BENEFIT RISK ASSESSMENT OF CONTROLLING LAND SUBSIDENCE IN SHANGHAI

Land subsidence, a major and well-known geological hazard in Shanghai, has caused serious losses. Based on past studies, the cost and benefi t of the Shanghai's land subsidence hazard control were assessed and f orecasted by risk assessment. An economic benefit-risk assessment was taken in order to control completely land subsidence and make rational safety elevations of the flood control wall. The result of risk asses sment shows that the present 6.9-m elevation of the floodwall is not s ufficient for the standard of ''occurring only once in a thousand year s.'' After 200 years, the spring tide hazardous risk losses caused by land subsidence would amount to 49.73 million yuan per year. The propo rtion of expense in thoroughly controlling land subsidence to the econ omic benefit is 1:41.44; to construct the flood control wall to an ele vation that would prevent inundation from a flood event, to reach the standard of ''occurring only once in a thousand years,'' and to avoid tide losses, the ratio of the investment of reducing tide hazard to th e economic benefit should be 1:53.24.