EXTREME PRECIPITATION AND THE LANDSLIDE MULTI-HAZARD CONUNDRUM

Historic yearly, daily, and hourly precipitation data as well as extreme storm, flood, and temperature events were correlated with landslide, slope and retaining wall failures in the Northeast, Northwest, Utah, Colorado, and Minnesota. Primary triggering mechanisms including earthquakes, floods, and man-made development activity (blasting, excavation, fill placement, erosion, and changes in drainage) were analyzed along with associated secondary hazards such as continued slope failures, mudslides, diverted or blocked streams, damaged roads-rail lines, damaged utilities-structures, and threats to public safety.

Results indicate that extreme precipitation events are increasing in frequency and intensity with each passing year. Extended periods of rainfall as well as very intense short-duration precipitation events were observed to trigger increased failures. Extreme temperature freeze-thaw cycles and winter rainfall events also significantly increased failure risk. A site specific precipitation, temperature, and storm evaluation methodology is proposed to assess impacts of extreme weather on landslide-slope failure risk.

A four step landslide-slope hazard assessment methodology is presented that incorporates: (1) an evaluation of the primary triggers from other hazards (earthquakes, volcanoes, floods, fire); (2) assessment of key risk factors (past failure history, bedrock type, slope steepness, and hydrology); (3) an evaluation of human activities (site development, drainage changes, construction activities); and (4) and the increasing impacts of extreme weather (precipitation, temperature, flash flooding, erosion).

Five failure case histories are presented which illustrate how a four-step hazard assessment could have evaluated the risk of landslide-slope failures, identified potential mitigation/prevention measures and averted the devastating impacts on public safety and property damage. The primary triggering mechanisms and role of extreme precipitation events of the 2006 Kilbuck landslide, 2013 Eastern Pennsylvania landfill cap failure, 2014 Oso Washington landslide, 2014 Pittsburgh Mount Washington slide and 2014 Baltimore Maryland railroad retaining wall failure are highlighted.