CONSIDERING ENGINEERING GEOLOGY INPUT FOR PROBABILISTIC FLOOD HAZARD ASSESSMENTS

Probabilistic risk assessments were developed in the 1970s as consistent approaches to assessing protection of public health by nuclear-facility safety measures. Risk-informed initiatives resulted in characterization of processes that produce extreme events (hazards) independently from detrimental effects of such events on people or environment from facility damage (risks), as well as quantifying uncertainties. For example, large dams are designed to perform without uncontrolled reservoir release under seismic motion with 1/10,000 annual frequency. Geologic inputs for seismic hazards include ground motion sources and site response. Probabilistic flood hazards analyses are emerging in response to uncertainty about the effects of climate change, aging flood control structures, and acceptance of probabilistic seismic hazard analyses. Geologic inputs for flood hazard have focused on paleoflood hydrology from slackwater deposits and boulder bars. Procedures are available for calculating probable maximum floods, produced from the most severe combination of meteorological and hydrologic conditions, but not for assessing annual frequencies of such events. Flood routing, the domain of hydrologists and hydraulic engineers, typically stipulates channel stability. What if channels erode during extreme floods, watershed slopes are susceptible to landslides, or landslides reduce channel cross sections? Hydrologists and hydraulic engineers evaluate flood flow and water elevation effects at facilities, whereas engineering geologists need to assess slope response and mobilization of debris under extreme precipitation. Keeping slope assessments consistent with probabilistic approaches is challenging. A real location allows a hypothetical example to illustrate selected aspects of the geological approach and to utilize results of some available tools.