Abstract

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

Engineering analyses assumed that the 1983 Rudd Creek debris flow was the 100-year event. The 100-year clear-water flood from the Rudd Creek basin was increased by a bulking factor to produce the observed 1983 sediment discharge. The 100-year flood plains of other canyons in Davis County were redefined based on this bulking factor. 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, and landslides within canyon watersheds, and the stratigraphy and geomorphology of alluvial fans, indicate major debris flows in Davis County are rare geologic 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 minor, not prominent landforms, and most historical sediment came from stream channels. Debris production and accumulation in channels are slow, intermittent processes, and channels having historical sediment discharges cannot produce large-volume flows again until the drainages have been recharged with sediment.

Dilemmas arose in the local communities in the protection of health and safety of residents from flood and debris-flow hazards. Should canyons that caused damage to the community be viewed as more hazardous than those that have not produced floods in historical time? Can future debris-flow events at the mouths of canyons be modeled better by applying a bulking factor to the predicted 100-year clear-water discharges, or by evaluating the geomorphology and stratigraphy of the canyon-mouth alluvial fans? Are existing debris basins inadequate to protect the community, or are the predicted 100-year debris-flow volumes over estimated?

Approximately $12 million were spent in 1983 in Davis County to build or refurbish debris basins; less than $50,000 was spent on geologic research to understand the debris-flow processes. Had geologic studies been conducted before the debris basins were built, more emphasis would have been placed on canyons without historical debris flows, tempering the engineering approaches that may have over estimated debris-flow volumes.