MANAGING NATIONAL FOREST LANDS IN DEBRIS FLOW-PRONE TERRAIN

National Forest geologists in the Pacific Northwest have long recognized that debris flows occur more frequently on managed lands than in adjacent unmanaged areas, observing that debris flows are initiated by shallow-rapid slope failures in steep zero-order basins. Forest Service geologists have analyzed terrain susceptibility to shallow-rapid failure for more than twenty-five years, using simple slope gradient/aspect models and, later, more complex methods such as LISA and SHALSTAB. The results of these analyses become the basis for assessing the risk of damage to dwellings, other private and public structures, and important downslope resources from landslides and debris flows. Benda’s Debris Flow Runout model has aided this effort.

Landslide susceptibility and risk analyses are used in Forest Service land management planning to reduce or prevent timber harvest and road construction in critical areas. Watershed Analyses incorporate debris flow history—occurrence rates under natural and managed conditions, and volume estimates—into sediment budgets. Debris flows are recognized as having potentially beneficial effects on fish habitat in some streams, adding complexity and nutrients by delivering large wood and sediment. Forests operating under the Northwest Forest Plan have made watershed restoration a top priority, aimed at achieving Aquatic Conservation Strategy goals. Roads Analyses identify opportunities to decommission roads by removing stream-crossing culverts and fill materials on mid-slope and valley bottom roads, allowing naturally occurring debris flows to deliver large wood and sediment to higher-order streams without interruption.