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

Two types of debris flow have occurred periodically throughout postglacial time at Mount Rainier (1) cohesive debris flows, containing at least 3 to 5 percent clay, which have flowed untransformed more than 100 km from the volcano; and (2) noncohesive debris flows, containing less than 3 to 5 percent clay, which commonly transform down stream to more dilute flows, passing through the range of hyperconcentrated flow. The noncohesive debris flows form most commonly by bulking of a flood surge with volcaniclastic and morainal sediment, and during formation they also pass through the range of hyperconcentration.

Three subgroups of a mixed population of lahars and glacially related debris flows were studied over time intervals related to their frequency, and from this spectrum of case histories an example of each was selected for consideration in flow-hazard analysis. Case I, a large cohesive lahar formed by mobilization of a deep-seated landslide, is capable of inundating parts of the Puget Sound lowland or the Cowlitz River valley. It is suitable for consideration in hazards planning in lowland areas. Case II is a noncohesive lahar of intermediate size and sufficient frequency that it may be applicable to the design of some structures such as dams and power plants around the volcano. Case III originates as a debris avalanche of a typical size observed at Mount Rainier, and which probably will mobilize to form a lahar. It poses risk primarily to local areas within Mount Rainier National Park.

The maximum lahar is typified by the largest flow in the postglacial history of Mount Rainier. It is a statistical outlier of the group of large cohesive lahars. The smallest and most frequent flows are dominated by glacial-outburst floods that bulk to debris flows and provide behavioral models of the larger noncohesive debris flows. They commonly attenuate rapidly at the base of the volcano through the rapid debulking of sediment, yielding hyperconcentrated streamflow and secondary debris flow by any of three types of transformations. Their inundation potential can be assessed and hazard zones can be established based on the level of erosion of tephra set W, on dendrochronology, and on numerous historic flows.

Each of the five major river systems draining the volcano has a record of lahars. Although the records indicate that flows differ in size and frequency among river systems, the risk of future lahars may not correlate highly with the record in each individual system. Rather, extrapolation of the entire volcano's past history to the future is more appropriate, with the risk dispersed among the individual drainages. Sites of future instability of the type producing major areas of collapse and the large cohesive lahars cannot be forecast. This is likewise true of the sites where intensified geothermal activity will produce noncohesive lahars by melting of ice and snow, and the places where renewed explosive volcanism will greatly increase the risks of all types of flows. However, the types of flows that will recur, as well as their approximate sizes and probabilities, can be forecast for the volcano as a whole. This is the most essential element of flow-hazard analysis.