Flexible debris-flow nets for post wildfire debris mitigation in the western United States

Abstract

Wildfires are a continual threat in the western United States. Post-fire debris flows annually cause millions of dollars in damage and often result in loss of life. Rapid post-fire response is essential to prevent additional hazards in terms of debris-flow damages. Flexible systems utilizing high-strength steel ring nets have proven to be reliable and cost effective. These systems can be installed rapidly to minimize or eliminate the dangers caused by post-fire debris flows. Wildfires in the western United States generally occur during the dry season in late summer and fall. Although monsoonal storms can cause debris flows in the summer months, seasonal rain storm events occur in fall and winter, often resulting in devastating debris flows. In the Rocky Mountain states debris flows occur during the summer monsoon season. In both areas, storm cells can remain stationary over mountain peaks for hours dropping large amounts of rain in a very short time. The resulting runoff and erosion can cause damaging debris flows miles away from the rain event. Debris impacts differ significantly from rockfall impacts. The debris nets must withstand both a surge in pressure on impact, and a static load once the flow has dissipated. Tested and engineered, flexible nets can be rapidly deployed in strategic locations to lessen or eliminate the threat. Compared to large, rigid structures and debris basins, these nets are cost-effective, rapidly constructed, environmentally friendly, and approval by regulatory agencies can relatively quick. This study focuses on current mitigation and protection practices using flexible debris-flow nets as developed in Switzerland and used in the United States. Case studies of projects in Colorado, New Mexico, and California detailing site investigations, engineering, and construction of flexible debris-flow nets are described. Of special interest are the steps and protocols taken as a result of debris flows following the Thomas Fire in California.