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Abstract

Widespread debris-flow damage occurred in northern Utah intermittently between 1912 and the spring of 1983. The worst damage in 1983 was at the mouth of Rudd Creek, a canyon that previously had not generated significant debris flows in historical time. Floodlike damage extended well outside federally mapped flood-plain boundaries. Debris basins built in the 1930s were refurbished, and new basins, at a cost about $1.1 million each, were constructed at the mouths of some canyons that produced debris flows in 1983.

Engineering analyses, assuming that the 1983 Rudd Creek debris flow was the 100-year event, used observed characteristics (debris volume, depth distribution, time to peak discharge, speed of debris advance, and duration of discharge) to construct an inflow hydrograph for use with a finite-element grid network, alluvial-fan slope values for the Rudd Creek fan, and debris-flow fluid properties. The viscosity of the fluid was varied until the 1983 debris-flow characteristics on the Rudd Creek fan were obtained. Debris production rates normalized to drainage-basin area for Wasatch Front canyons were used to estimate 100-year debris-flow volumes from other canyons in Davis County. The finite-element grid, input hydrograph shape, and debris-flow fluid properties from Rudd Canyon were used with canyon-specific fan slopes to redefine the 100-year flood plains at the other canyons. Large urbanized areas were found to be within the newly defined flood plains, and nearly all debris basins were too small to protect against the predicted 100-year sediment discharges.

Studies of the structural fabric, hydrogeology, stream channels, and landslides within canyon watersheds, and the stratigraphy and geomorphology of alluvial fans, indicate major debris flows in Davis County are rare geological events. The majority of alluvial-fan building appears to have occurred during the early Holocene when much ice-age sediment was available in the Wasatch Range. The alluvial fans are small landforms and most historical debris-flow sediment came from stream channels. Debris production and accumulation in channels is a slow, intermittent process, and channels having historical sediment discharges cannot produce large flows again until the drainages have been recharged.

Approximately $12 million were spent in 1983 in Davis County to build or refurbish debris basins; fewer than approximately $50,000 were spent on a rapid, regional assessment of debris-flow hazards along the Wasatch Front in Davis and Weber Counties and parts of Salt Lake and Box Elder Counties. Had geologic studies to understand the debris-flow processes been conducted before the debris basins were built, more emphasis would have been placed on canyons without historical debris flows, tempering the engineering approach that may have overestimated the 100-year-frequency debris-flow volumes.

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