THE POTENTIAL INFLUENCE OF STAIR-STEP MORPHOLOGY ON THE INITIATION OF BULKING DEBRIS FLOWS IN SOUTHWESTERN IDAHO, USA

Hillside hollows which produced bulking debris flows in southwestern Idaho in 2003-2005 consistently show a stair-step morphology with vertical risers and planar, roughly horizontal treads. We propose a process model of step formation by plunging flow which increases sediment detachment and transport, thereby encouraging the formation of bulking debris flows.

Bulking debris flows in our study area initiated on steep, unforested, south-facing hillsides sparsely vegetated with grasses and forbs, or on recently burned hillsides. Debris flows were triggered by summer thunderstorms falling on soils with low infiltration capacities associated with extremely dry soil or recent wildfire and producing Horton overland flow.

We propose that small surface irregularities increase turbulence, flow depth, and local slope, enhancing erosion by overland flow in the immediate vicinity of the irregularity and initiating the incision of steps. Flowing water erodes the steep upper face of these irregularities until it forms a vertical riser at which point there is little further erosion because of the lack of a component of the flow's force normal to the riser. The risers are often fluted and show other sedimentologic evidence of overland flow. The lack of antecedent moisture and pore pressure makes failure of these steps during erosion less likely. Once vertical risers have formed, the maximum erosional force of the plunging flow will be exerted on the tread, where sediment is mobilized. Highly turbulent flow coming off the riser increases the capacity of the fluid to suspend particles and transport the eroded sediment over the tread. The erosion of the tread initiates a positive feedback cycle of increased riser height and increased erosive force that is stopped only by reaching bedrock or the end of the overland flow event. As the flow decreases on the falling limb of the hydrograph, suspended material is deposited on the treads creating a slightly sloped, planar surface. The process model is in agreement with theoretical calculations and field evidence.

The proposed step formation process appears to be dependent on poorly vegetated hillsides with low infiltration capacities both of which may be responsive to changes in climate and the management of forest fires and grazing.