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4 Natural Hazards
Pages 127-152

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From page 127...
... Following the 1980 eruption, the initial and pressing concern was the possibility that Spirit Lake might breach its debris blockage and create a cataclysmic, sediment-laden flood in the Toutle River valley and the Cowlitz River plain. This possibility was much in the minds of cognizant agencies and of affected parties.
From page 128...
... These three could result in or exacerbate any issues related to the management of sediment and water transport in the Spirit Lake and Toutle River system; thus, their probabilities and impacts need to be thoroughly understood to make wise management decisions. Two other natural hazards are considered in the Cowlitz County Hazards Mitigation Plan: landslides and wildfire (Cowlitz County Department of Emergency Management, 2013)
From page 129...
... The chapter concludes with a brief discussion related to ongoing monitoring. METEOROLOGICAL INPUT AND CHRONIC FLOODING As has been discussed earlier in this report, the Toutle River valley and lower Cowlitz River plain are subject to both chronic and catastrophic flooding.
From page 130...
... The post-eruption condition of the Toutle River valley has another important effect on flood hazard in the settled parts of the Toutle River and lower Cowlitz River valleys, namely, as a source of sediment that can settle in the Cowlitz channel and reduce the flood conveyance capacity of the engineered levee system between Castle Rock and the Columbia River confluence (see Figure 4.2)
From page 131...
... Effect of Changing Climate Changing global climate adds uncertainty in assessments of flood hazards in this region. An increase in the frequency of flood-generating weather events is likely.
From page 132...
... . Impacts of Chronic Floods The area potentially at risk from chronic flooding extends north from the confluence of the Columbia and Cowlitz Rivers and east along the Toutle River to the confluence of the North Fork Toutle and Green R ­ ivers.
From page 133...
... a of Floodinga (miles) b Populationc Castle Rock City of Castle Rock 118 0.0085 1.5  2,140 Lexington Lexington Flood 167 0.006 2.7 NA Control Zone District of Cowlitz County Kelso Cowlitz County 143 0.007 5.7 11,925 Consolidated Diking Improvement District No.
From page 134...
... Consideration of volcanic activity in a decision framework has two main components: (1) defining hazards that affect the Spirit Lake and Toutle River system resulting in downstream impacts and (2)
From page 135...
... . Some might reach and directly introduce material into Spirit Lake, but assuming no major topographic changes in the area, most would likely contribute to increased sediment flux into the Toutle River and the SRS if flow deposits are remobilized in tributaries of the North Fork Toutle River by alluvial or fluvial processes.
From page 136...
... • Debris avalanches in the Toutle River valley are unlikely until the volcano rebuilds itself from the 1980 sector collapse. Their probability, then, depends upon the time frame of interest.
From page 137...
... Adequacy of Existing Information Disruptive eruption scenarios can be defined by the eruption processes accompanying any event that could disrupt the Spirit Lake and Toutle River system. For example, an eruption that produces enough tephra fallout could, as described above, block the Spirit Lake tunnel and ultimately cause breaching of the debris blockage if the tunnel could not be cleared before water levels rise to dangerous levels.
From page 138...
... Impact capable Distance >5 miles. Impact capable process within or near crater of reaching Spirit Lake of reaching Spirit Lake and beyond Geologic Record P/yr Geologic Record P/yr Geologic Record P/yr Airfall Multiple explosive 0.01 Multiple explosive 0.01 Multiple explosive eruptions 0.01 tephra/ash eruptions that eruptions that deposited that deposited tephra fallout deposited tephra tephra fallout several several inches to more than fallout several inches to more than 3 3 feet thick inches to more feet thick than 3 feet thick Lava dome Pre-1980, 0.01 A lava dome is unlikely
From page 139...
... 1980 (1) 1980 Mudflow ~40 in Toutle River 0.1 ~40 in Toutle River valley 0.1 ~40 in Toutle River valley in 0.01 valley in past in past 50ka past 50ka 50ka a Estimates based on stratigraphic evidenced and post-1980 monitoring of Mount St.
From page 140...
... Individual seismic events produce multiple hazards processes such as strong ground shaking, which can result in earthquake-induced liquefaction and slope instability. These may or may not affect the Spirit Lake and Toutle River system in a way that causes problems downstream.
From page 141...
... Earthquakes generated in the north-south trending Mount St. Helens Seismic Zone shown in Figure 2.4 have produced numerous earthquakes of smaller magnitude than have the above two sources, and they can produce shaking very near to Spirit Lake and the SRS and hence potentially be quite damaging to elements such as the diversion tunnel and the debris blockage.
From page 142...
... • Ground shaking -- Ground motions and associated accelerations can damage features such as the tunnel and tunnel portals, the debris blockage, and the SRS. This is usually expressed as a horizontal spectral acceleration associated with a given seismic scenario.
From page 143...
... Site-specific seismic hazard analysis would be required to provide estimates of anticipated levels of ground shaking at sites with features of interest and of potential fault rupture where fault zones exist. PSHA studies are a standard part of the seismic evaluation of structures such as the Spirit Lake tunnel, the debris blockage, or the SRS.
From page 144...
... Analyses are needed to evaluate the potential lateral spreading of the liquefied sediments, their impacts on the SRS, and their potential flow over the SRS. It should be noted that much of the characterization of the region and the design and construction of infrastructure to manage the Spirit Lake and Toutle River region (e.g., the Spirit Lake tunnel and the SRS)
From page 145...
... The integrity of the debris blockage is vulnerable to rise in lake level, volcanic eruption, and seismic shaking. If the debris blockage were breached as a result of failure of the engineered lake-level control system; piping due to a critical hydraulic head gradient in the groundwater; or erosion working headward up the debris barrier from the remnant drainage channel system of the North Fork Toutle River valley north of the volcano, it would represent a primary source of potential catastrophic flooding downstream.
From page 146...
... Reinforced outlet channels were constructed for the impoundments at Coldwater Creek and Castle Creek in the early 1980s. Some detailed studies have since been made of mechanisms for the breaching of the Spirit Lake debris blockage (Swift and Kresch, 1983; Sager and Chambers, 1986)
From page 147...
... The calculated boundaries of the mudflow throughout the Toutle River and Cowlitz River valleys as far as Castle Rock are consistent with those mapped by Scott and Janda (1982) , although the peak discharge of the calculated flood in the middle Toutle River valley (2.55 million cfs)
From page 148...
... Understanding the North Fork Toutle River valley groundwater flow regime since the eruption would provide the needed scientific basis for assessing potential failure mechanisms for debris dam, surficial erosion processes, and water balance in Spirit Lake. It would also provide hydrogeologic information necessary to consider alternatives, for example, related to the control of Spirit Lake water levels.
From page 149...
... The topography of the North Fork Toutle River valley has evolved considerably since the eruption and subsequent downstream delivery of sediment. Construc tion of the Sediment Retention Structure and its operation and filling over the past few decades has potentially changed the way lahars might behave.
From page 150...
... The weaknesses in current information have to do with the fragility of the debris blockage and its corresponding vulnerability to seismic hazards, the current state of groundwater conditions, and details of current geotechnical conditions. ONGOING MONITORING Since the 1980 eruption much has been learned about the character of the natural hazards existing in the Spirit Lake and Toutle River system.
From page 151...
... Monitoring capabilities and data collection programs need to be reexamined and updated and analytic capabilities need to be reevaluated in light of the information needs of all interested and affected parties. Those key physical variables that impact decision making need to be identified through a process in which those with management authority engage each other and other interested and affected parties in the region (as will be discussed in greater detail in Chapters 6-8)
From page 152...
... Current characterization of the Spirit Lake debris blockage; the behavior and location of groundwater in the blockage; current and future meteorological trends; a quantified characterization of the types of risks posed by volcanic activities on Spirit Lake levels and elsewhere in the region; and the seismic response to local and regional seismic activity of the debris blockage and other parts of the system are all examples of factors that may affect the understanding of risk associated with management alternatives that might be considered. For the purposes of planning and decision making, the large body of knowledge on natural hazards needs to be integrated in a consistent way.


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