Debris Flows and Hyperconcentrated Floods Along the Wasatch Front, Utah, 1983 and 1984

Abstract

The Wasatch Front north of Salt Lake City has a recurrent history of debris flows. Two distinctly different types of climatic events—summer thunderstorms and rapid spring snowmelts—have triggered debris flows. Heavy autumn rains, exceptional winter snowpacks, and cool and extended springs followed by sudden warming characterized weather during 1983 and 1984. As a result of these climatic events, large amounts of water were rapidly added to the landscape, resulting in several hundred landslides, many of which subsequently mobilized into debris flows and (or) hyperconcentrated floods. Reconnaissance during June of 1983 revealed many hillsides with recently active landslides that had moved slightly but had not mobilized, or had only partially mobilized, as debris flows. The presence of these partly-detached landslides prompted an investigation of the potential for future debris flows. Wieczorek and others (1983) developed a technique for evaluating the potential for debris flows and hyperconcentrated floods to reach the canyon mouths, based on three assumptions: 1) debris flows would occur in response to a large influx of water to the soils, either from a thunderstorm or rapid snowmelt; 2) the partly-detached landslides were the most likely sources for future debris flows; and 3) debris-flow material incorporated from channels would be proportional to that from landslide sources. The climatic and debris-flow events of 1984 were sufficiently similar to those of 1983 that we could evaluate these assumptions. Slightly fewer debris flows occurred in 1984 than in 1983, consistent with a slightly lower snow-pack water content and slightly slower rate of melting. Debris flows occurred near the rapidly receding snowline, apparently as a result of the large influx of water into the soils there. On an areal basis, partly-detached landslides identified in 1983 were statistically significant as sources for debris flows in 1984. In the few documented cases, the contribution of material scoured from channels was variable but very high and appeared significantly influenced by the geomorphic condition of the stream channel.