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Debris Flow, Debris Avalanche, and Flood Hazards at and Downstream from Mount Rainier, Washington Introduction

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Debris Flow, Debris Avalanche, and Flood Hazards at and Downstream from Mount Rainier, Washington Introduction U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY HYDROLOGIC U.S. GEOLOGICAL SURVEY INVESTIGATIONS ATLAS HA-729 DEBRIS FLOW, DEBRIS AVALANCHE, AND FLOOD HAZARDS AT AND DOWNSTREAM FROM MOUNT RAINIER, WASHINGTON By K.M. Scott and J.W. Vallance ABSTRACT of the mountain. Readers are encouraged to consult the comprehensive reports (Scott and others, 1992, Mount Rainier volcano has produced many large 1995) for details of the case-history selection and the debris flows and debris avalanches during the last 10,000 rationale for the case-history approach. years. These flows have periodically traveled more than Both the previous report (Scott and others, 1992) 100 kilometers from the volcano to inundate parts of and this atlas, which is a map portrayal of the hazard- the now-populated Puget Sound Lowland. Meteoro­ related conclusions in the 1992 report, conform to logical floods also have caused damage, but future the requirements and recommendations of the Wash­ effects will be partly mitigated by reservoirs. ington Growth Management Act of 1990 {Washing­ Mount Rainier presents the most severe flow risks ton (State) Administrative Code, 19901. The act of any volcano in the United States. Volcanic debris establishes standards and definitions for either man­ flows (lahars) are of two types: (1) cohesive, relatively dated or optional land-use standards in response to, high clay flows originating as debris avalanches, and among other factors, volcanic hazards. According to (2) noncohesive flows with less clay that begin most the act (chapter 365-190, p. 11), volcanic hazards commonly as meltwater surges. Three case histories "shall include areas subject to * * * debris avalanche(s), represent important subpopulations of flows with known inundation by debris flows, mudflows, or related flooding magnitudes and frequencies. The risks of each sub- resulting from volcanic activity." population may be considered for general planning This report is best applied in conjunction with and design. the recommendations in Scott and others (1992). The A regional map illustrates the extent of inunda­ purpose of map portrayal of the findings is to present tion by the case-history flows, the largest of which them in the format most useful to those concerned originated as debris avalanches and moved from Mount with the distribution of the flow risks. Those most Rainier to Puget Sound. The paleohydrologic record concerned will be planning staffs of the counties and of these past flows indicates the potential for inunda­ municipalities that include sections of the drainages tion by future flows from the volcano. A map of the of Mount Rainier. volcano and its immediate vicinity shows examples of Risk analysis is a generic term for methods that smaller debris avalanches and debris flows in the 20th support decisionmaking by quantifying consequences century. of hazardous events and their probabilities of occur­ rence (Committee on Techniques for Estimating Prob­ INTRODUCTION abilities of Extreme Roods, 1988). The risks discussed here are those of volcanic flows, which are the greatest Mount Rainier is only 70 km southeast of the volcanic risks at Mount Rainier. The goals of risk analy­ Seattle-Tacoma metropolitan area, and suburban sis are met by (1) quantifying the magnitude (volume, development is moving rapidly toward the volcano. and extent if possible) of a selected case-history flow, The first indication of the potential danger of lahars, and (2) quantifying the probability of the flow, or in the widely used Indonesian term for volcanic flows, hydrologic terms, its frequency or recurrence inter­ was evidence that past flows had inundated lowlands val, the average number of years within which the far from the volcano (Crandell, 1963; 1971). Several flow is expected to be equaled or exceeded. Thus, prehistoric debris flows as well as other volcanic phe­ the analysis is independent of economic considerations, nomena are portrayed on maps at scales of 1:250,000 which will vary with time, geography, and the pur­ (Crandell, 1973) and 1:500,000 (Crandell, 1976). poses of subsequent analyses. For example, risk analysis We subsequently re-examined the record of past for hazards planning or for design of structures such flows and recognized subpopulations differing in be­ as dams can integrate the pure data on flow magni­ havior and origin (Scott and others, 1992). Using tude (and extent, as shown on the maps) and fre­ paleohydrologic techniques, we then defined the mag­ quency with specific demographics and time horizons. nitude and frequency of the subpopulations to form For details and documentation of the record of the basis of risk analysis for decisionmaking based lahars, as well as regional geographic information, on risks of future flows. From each subpopulation, readers are referred to the reports cited above; they the most characteristic example was selected as a case may also consult Crandell and Mullineaux (1967), history suitable for extrapolation to other drainages Mullineaux (1974), and Hoblitt and others (1987). The actual distributions of four case-history flows areal projection of a steep slope in the horizontal in the greater Mount Rainier area, as well as the plane of a map, the inundation boundary is sharply extrapolations of three case-history flows to other defined. However, field interpretation of local topog­ drainages of the volcano, are outlined on sheet 1 raphy, which may include road and rail embankments (1:100,000 scale). The three most likely case-history that post-date the topographic bases, as well as com­ flows are extrapolated by placing the same cross- mon sense, will be necessary additional elements in sectional areas of flow at the same channel distances using these maps. from the mountain. Assumption of similar channel hydraulic characteristics for the conveyance of large DRAINAGE SYSTEM OF MOUNT RAINIER debris flows is a reasonable and practical approach. The five major drainages on Mount Rainier the White, Five major river systems drain Mount Rainier: the Cowlitz, Nisqually, Puyallup, and Carbon Rivers are White River on the northeast, the Cowlitz River on similar in overall topography and channel configura­ the southeast, the Nisqually River on the south, the tion. Slopes and longitudinal profiles are likewise similar, Puyallup River on the west, and the Carbon River on as are the proportion of forested and cleared areas. the north (sheet 1). Only the Cowlitz does not drain Consequently, channel roughness is similar at the scale to Puget Sound across the Puget Sound Lowland; of the largest, most dangerous flows, which reach tens rather, it drains to the Pacific Ocean by way of the and even hundreds of meters up valley sides. It is not Columbia River. Three of the river systems contain possible to model the flows because of the unknown reservoirs that could either mitigate or aggravate the nature of the input hydrographs, that is, the size and downstream effects of large lahars originating on the shape of the appropriate flow waves at their points volcano. Reservoir effects are summarized in a later of origin. Paleohydrologic studies of past flows yield section and are described in more detail in Scott and the most meaningful estimates of the sizes of the flows, others (1992). as well as their frequency, dynamics, and extent. Thus, sheet 1 illustrates the types, probabilities, FLOW TYPES and risks of the most dangerous types of debris ava­ lanches and debris flows and their distal runout phases Debris flows, slurries of sediment and water that at and downstream from Mount Rainier. Risks from look and behave much like flowing concrete, have debris flows and other sediment-laden flow types are repeatedly traveled from Mount Rainier to Puget Sound. greater than those from streamflow (water) floods About 60 percent or more of the volume of a debris because of impact force, unpredictability, and other flow consists of sediment; the remainder is water. Flow factors described by Scott and others (1992). The rea­ deposits consist of coarse clasts dispersed in a fine­ sons for concern and an appropriate response to the grained matrix of sand (0.0625 to 2.0 mm), silt (0.004 risks of debris avalanches and debris flows are described to 0.0625 mm), and clay (finer than 0.004 mm). All by Crandell and Mullineaux (1975) and Crandell and debris flows are commonly known as mudflows, but others (1979, 1983). scientists confine that term to types rich in mud (silt- Modern debris avalanches and debris flows at and clay-size sediment). Mount Rainier are shown on sheet 2 (1:50,000 scale). The largest debris flows at Mount Rainier began These flows of low magnitude and high frequency are as debris avalanches that originated as huge volcanic of concern only on and in the immediate vicinity of landslides known as sector collapses. Debris avalanches the volcano. Although areas of risk are more wide­ are high-velocity, unsorted debris flows (Schuster and spread than the locations shown, the risk is portrayed Crandell, 1984) that can be either wet or dry; the by using actual historical flows as examples. Past flows presence of water is not essential to their movement. have recurred from the same general locations, but Debris avalanches at Mount Rainier probably have other areas of similar topography may also yield future contained abundant water, suggested by their rapid flows. Many past and potential flow sources are the mobilization to debris flow, commonly on the flanks now-destabilized side slopes of valleys previously filled of the volcano shortly after initiation. Many fragile with ice before Neoglacial recession began in the early blocks in the avalanches disaggregated during move­ 1800's (see Scott and others, 1992). ment to contribute to the relatively high clay matrix of the downstream debris flows. The transformation process is scale-dependent that is, large debris ava­ HOW TO INTERPRET THE INUNDATION AREAS lanches at Mount Rainier commonly transformed to SHOWN ON THE MAP SHEETS debris flows directly during movement; small avalanches Each flow boundary shown on sheet 1 encloses produced small secondary debris flows by surficial an area that would be inundated by a flow at a selected slumping of their dewatering deposits.
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