RECENT FIRE-RELATED DEBRIS FLOWS IN NORTHERN UTAH – CAUSES, IMPACTS, RISK REDUCTION, AND LESSONS LEARNED

During the recent drought in northern Utah, 26 fire-related debris flows have occurred in 7 wildfire areas. These debris flows occurred with little warning and some traveled into subdivisions, damaging houses and property and depositing sediment. Homeowners in these subdivisions had little time to react. The study of these debris flows provides insight into how the flows are generated, their impacts, and effective risk-reduction measures.

Wildfires greatly increase the debris-flow hazard due to the loss of vegetation and the formation of water-repellent soils on steep slopes, which result in decreased rainfall infiltration and increased surface-water runoff. Runoff concentrates in drainage-basin channels, eroding and entraining loose, unconsolidated sediment to form debris flows. The flows increase in volume until they reach canyon mouths where they spread out and deposit sediment on alluvial fans. Brief, intense thunderstorm rainfall of 0.5 inch or less has triggered debris flows in burn areas. These rainfall events have average recurrence intervals of two years or less, showing that the storms are not unusual events. Debris flows caused damage in subdivisions by moving and partially burying vehicles, breaking through exterior house walls, and flowing into houses through broken basement windows and exterior doors.

The debris-flow hazard is drastically increased following a fire and communities without protective structures are at substantial risk. Some communities had to rely on less effective protection measures of thunderstorm rainfall warnings and subdivision evacuation until protective structures were constructed. Debris basins and debris-flow deflection dikes with sediment runout areas are the most common types of large-scale risk-reduction structures. The increased debris-flow hazard from a burned drainage will persist until the drainage basin recovers to pre-burn conditions. However, the fire-related debris-flow hazard will reoccur in the future when large portions of drainage basins are again burned by wildfire. Recent studies by the U.S. Forest Service indicate that interior chaparral vegetation of mixed shrub and conifer association have natural fire recurrence times of 40 to 60 years for stand-replacement fires.