Abstract

Stream flooding is one of the most frequently recurring and destructive geologic hazards facing southwestern Utah. The high flood hazard results from the complex interaction of the area’s rugged topography and seasonal weather patterns that deliver moisture to the state. Two types of stream flooding typically occur in southwestern Utah: riverine floods and flash floods. Riverine floods are usually regional in nature, last for several hours or days, and have recurrence intervals of 25 to more than 100 years. They commonly result from the rapid melt of a winter snowpack or from periods of prolonged heavy rainfall. Flash floods result from thunderstorm cloudbursts. They are localized, quickly reach a maximum flow, and then quickly diminish. Recurrence intervals for flash floods are erratic, ranging from a few hours to decades or longer for a given drainage. Both types of flooding have caused extensive damage in southwestern Utah.

Three major riverine floods have affected southwestern Utah since the area was settled. They occurred in 1966, 1983, and 1984. The 1966 flood resulted from an intense three-day rainstorm that produced record peak flows on the Virgin River. The 1983 and 1984 floods occurred in response to the rapid melting of maximum-of-record and greater-than-average snowpacks respectively. Both were regional events (affecting the Virgin, Sevier, and Beaver River drainages in southwestern Utah). Two major floods in successive years, each with an estimated recurrence interval of 25 to greater than 100 years, demonstrates the unpredictable nature of riverine flooding in southwestern Utah.

Flash floods have damaged every major town in southwestern Utah. The first recorded flood was on Coal Creek in Cedar City in 1853. Since then more than 300 damaging floods have occurred in southwestern Utah. Many communities have implemented flood-control measures to reduce the flash-flood hazard; however, as communities expand into unprotected areas, new development is again subject to flash flooding.

Any new development in southwestern Utah must consider the potential for stream flooding, and through proper project planning and design, mitigate any flood hazard that may exist. In areas with a particularly high flood hazard, forgoing development entirely may represent the best development alternative.