Debris Flow Hazard Mitigation for Colluvium-Filled Swales

Abstract

Debris flows originating in colluvial deposits within hillside swales cause significant property damage and loss of life in steep, soil-mantled environments and will continue to do so for as long as mitigation measures are not adopted for both new and existing development. Colluvium-filled swales constitute mappable debris flow source areas and should be identified routinely as potential hazards in geotechnical feasibility and design-level investigations. Toward this end, we propose a swale classification system that incorporates the state of activity, slope, and depth of colluvial fill within a swale. Recognition and siting considerations allow for debris flow hazard mitigation by avoiding locations downslope of colluvium-filled swales, along low-order channelways, or on debris fans. Alternative mitigations, particularly appropriate for areas of existing development, include removal of colluvial deposits, subdrain installation, or downslope intervention to reduce the impact of a debris flow.