DEBRIS FLOWS - BASIC MORPHOLOGIES AND PROCESSES

One approach to explaining debris flow morphologies and processes is to separate the flow behavior into initiation, transport, and deposition segments. Flows are generally initiated by either focused sediment influx by landsliding, or by more diffuse erosion such as sheetwash, rill formation and gullying. The type of initiating mechanism depends on soil moisture levels, vegetative cover, and sediment sources and availability. During transport, channel scour and sediment bulking can dramatically increase the volume of the flow and peak discharge levels. Discharge levels are also influenced by channel shape, slope, and tortuosity. These same factors control flow velocity. Deposition takes the form of channel levees and runout fans, both of which depend on slope, the opportunity for debris to thin and slow down, as well as dynamic fluid properties of the debris.

Some aspects of debris flow behavior are problematic and difficult to predict. For example, some flows run out in very long, levee-bounded fingers rather than in short distributary fans. Flows have widely varying intensity of channel scour and magnitude of sediment bulking. The time frame required to accumulate colluvium and slope wash in old debris flow channels to the degree that debris flow is again possible is important, yet unknown. Likewise, the influence of revegetation in burned or otherwise disturbed land is unquantified. Finally, the level of rainfall or antecedent moisture required to initiate flows is unpredictable. Yet, rather than limiting our understanding of debris flow mechanisms, exploration of these questions helps us develop a model for the combinations of events that produce debris flows. This presentation will discuss these issues in light of ongoing research measuring debris bulking by channel scour, the influence of diffuse erosion, multiple flow events in a single canyon, and the design and effectiveness of various mitigation measures.