David S. Bowles

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Proceedings of a Specialty Conference

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During 1982, 1983, and 1984, abnormally wet conditions in Utah triggered flash floods, landslides, and debris flows. Pore pressures built in hillside soils below melting snows and during prolonged periods of rainfall until the mass suddenly gave way, sometimes as a landslide and other times as a non-Newtonian debris flow that moved rapidly long distances down mountain slopes until finally stiffened by moisture loss or velocity loss because of flatter gradients. Also, runoff from heavy rainfall bursts picked up weathered and other loose material that accumulated on land surfaces over long dry periods . The sediment laden waters flowed out of mountain canyons onto lowlands where they deposited their loads, filled channels and c logged culverts, and then spread over the land surface to infiltrate, except as intercepted and diverted by streets, storm sewers, and irrigation canals. These were in turn often over topped to cause flooding in areas with no natural hazard. Snow melt runoff continued over extended periods, keeping stream flows too high to be contained within the clogged streams, and causing water to flow down streets for weeks disrupting traffic and inundating low-lying property. In closed basins, the waters eventually drain into a terminal lake where rising waters gradually inundated large areas. This complex of interrelated phenomena created a hazard situation that is greatest at the toe of the mountain slopes and concentrates where mountain canyons drain onto alluvial fans and the water spreads in a pattern that varies substantially from storm to storm. These hillside areas are prime res identical site s and command a high pr ice in the market. Development that should not be located in high hazard areas is reasonable a little further down slope where the risk is less. Quantitative methods are needed for mapping flood, debris, and landslide risks in these basin margin areas so that objective decisions can be made on where to locate and how to landscape and design buildings. Monitoring programs and warning systems are needed to track emerging hazards, emergency plans, and get people to respond. During two spring months of 1983, Utah sustained direct damages from landslides and debris floods in excess of 250 million dollars. Public official.s and residents were prepared for water flooding. However, neither the scientific community nor the agencies responsible for dealing with emergency situations were prepared for the widespread 1andslides and devastating debris flows. At least 92 significant landslides along a 30-mile length of the Wasatch Front Mountains sent torrents of water and debris down on the residential areas below. Along the Wasatch Plateau, more than 1000 landslides occurred. Additional massive landslides in Spanish Fork Canyon, Utah County, created Thistle Lake, and in 12-Hile Canyon, Sanpete County, dammed a river and sent a 30-foot high flash flood surge down the canyon. These devastating floods, landslides and debris flows were so extensive that 22 of Utah's 28 counties were declared national disaster areas .


This work made publicly available electronically on April 3, 2012.