Geological Survey Professional Paper 1547 Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier Washington

SUMMARY OF FLOW ORIGINS AND TRANSFORMATIONS

In order to rank the flows according to magnitude and frequency, the preceding discussion focused on the relative sizes of the various flow types and the evidence for their ages. To assess risk, the size and frequency of flows must be known, but other factors, such as the probability of a warning (Costa, 1985), are also important. Table 5 extracts the general flow origins that can be recognized, as well as the transformations that occur with each type. For simplicity, the formation of a secondary debris flow from the surface or interstices of a primary debris flow is treated as a transformation (type C in table 5), but the original concept of flow transformations invoked a fundamental change in flow behavior (Fisher, 1983). Because the change is from debris flow to debris flow, albeit accompanied by a change in texture, there is no change in rheology.

Unlike cohesive debris flows, noncohesive flows undergo the complete transformation of the entire flood wave to hyperconcentrated streamflow, which then evolves to normal streamflow with sediment content below hyper-concentration. Both these distal transformations involve fundamental changes in flow behavior and grain interaction (Scott, 1988b, table 9; Pierson and Costa, 1987). Each also involves the progressive loss of sediment, which, combined with the commonly more peaked flood wave of the noncohesive flows, can produce greater attenuation. Conversely, during the formative transformations to debris flow, as sediment bulks into the flow, the flow wave may be amplified in volume many times. These differences between cohesive and noncohesive debris flows are tendencies, not laws of behavior. For example, a cohesive flow can be more peaked, and can lose sediment rapidly by deposition on a wide flood plain. However, the overall tendencies are consistent with flow type.

Three types of flow transformations are represented in figures 2, 18, and 20. At Mount Rainier, the first (fig. 2) is the complete, progressive transformation of the entire flood wave (Scott, 1988b, fig. 37). The second (fig. 18) is the repeated, successive deposition of the flow front as a series of lobes, until only the hyperconcentrated tail of the flow remains to flow downstream. The third (fig. 20) is the creation of a secondary debris flow by dewatering and slumping of the surface of a debris avalanche, or by drainage of the matrix from coarse, clast-supported debris flow deposits. Avalanche dewatering has produced significant lahars elsewhere, but is not known to have produced any but small flows at Mount Rainier. Although large cohesive lahars have occurred in the post-Y time period, we found no upstream debris avalanches that corresponded in size and age and from which they could have been derived secondarily.