DEBRIS-FLOW CHARACTERISTICS AND MITIGATION IN SMALL DRAINAGES

Small drainage basins consist of steep (>25-30 degree) upper slopes where unstable soils may exist, entrenched channels and steep(>10 degrees)alluial fans. When upper basin soils and bedrock conditions suggest local or widespread instability, vegetation cover is suitable, and saturated soils are possible, debris flows are possible. When channels and alluvial fans include unsorted rock and debris deposits too large to have been transported by water flooding, debris flows may be active and structural mitigation may be needed.

In the western United States debris-flow mitigation usually takes place on the alluvial fans where private property is located. Mitigation usually consists of: (a) avoidance, (b) berming and channelization (on steep slopes), (c) debris-storage basins, (d) and elevated or reinforced structures. Basins must be sufficiently large to contain the design event. Impact design must consider the velocity and bulk density of the flows, the kinetic energy head and climbing height of flows upon impact, and the point impact of boulders.