A Decision Framework for Managing the Spirit Lake and Toutle River System at Mount St. Helens (2018) / Chapter Skim
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5 The Engineered Landscape
5 The Engineered Landscape
Pages 153-194
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From page 153... ...
measures have also been implemented, but these are not included in this discussion. An assemblage of hydraulic infrastructure was planned and built in the 1980s, including provisions to maintain water levels in Spirit Lake by boring a drainage tunnel, modifications to Coldwater and Castle Lakes, changes in the North Fork Toutle River channel, the addition of the sediment retention structure (SRS)
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From page 154... ...
is addressed at the conclusion of this chapter. SPIRIT LAKE WATER LEVELS AND RISK OF CATASTROPHIC FLOODING Long-term management of Spirit Lake is of major concern because control of the lake level is instrumental in avoiding massive flooding downstream caused by breaching of the debris dam impounding the lake.
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From page 155... ...
, the 8,600-foot (2,600-m) tunnel through Harry's Ridge was ultimately selected, which drained Spirit Lake water to South Coldwater Creek and then to Coldwater Lake, which by that time had itself been stabilized with a drainage channel across a natural bedrock sill.
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From page 156... ...
It appears that the best available scientific and engineering information was used for the specialized purpose of identifying the potential failure modes of five drainage concepts: the existing tunnel, the tunnel and its possible repair, a shallow conduit, pumping, and an open channel. Protocols followed during the analysis 156
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From page 157... ...
The likelihood of a release of lake water is a function of the likelihood that the posited loading process will occur, the likelihood it will cause the outlet to fail and allow the lake level to rise, and the likelihood that an intervention to prevent breaching of the blockage will be unsuccessful. After estimating the joint probability of this chain of events for each potential failure mode, the 157
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From page 158... ...
The committee did not have direct access to the results of the PFMA; instead, it relied on the summary by Grant and others, on discussions with some of those who participated in the PFMA, and on limited unpublished materials provided by the USACE in response to specific requests for information. Alternate drainage directly to the North Fork Toutle River by means of pumping had been used before the tunnel was constructed in 1985, and it has been proposed as an alternative for the future either by constructing 158
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From page 159... ...
. Changing the outflow works at Spirit Lake -- for example, by shifting more or all the drainage now flowing into South Coldwater Creek to the North Fork Toutle River -- could change the pattern and volume of erosion and thus the amount of sediment transported to the SRS.
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From page 160... ...
TABLE 5.3 Summary of Natural Hazard Events for Managing Spirit Lake Drainage Hazard Examples Hydrologic hazards Typical wet season, atypical wet season, PMF-type extreme event Seismic hazards Shallow crustal earthquakes, deep intra-slab earthquakes, megathrust earthquake Volcanic hazards Lahars, tephra fallout, pyroclastic flows, lava flows Geomorphic hazards Landslides (shallow and deep) , channel avulsion across debris deposit, channel incision, knickpoint development, sediment transport 160
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From page 161... ...
, as summarized in Table 5.2 and as described in Chapter 4. These hazards affect human use of the landscape by influencing catastrophic and chronic flooding, sediment impacts, most ecological processes, and recreational values.
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From page 162... ...
Mount Detailed evaluation This is necessary to evaluate the St. Helens of the source zone, seismicity in this area and potential Seismic Zone potential extent of fault impact of fault rupture on the tunnel and displacement, and debris blockage.
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From page 163... ...
While it is possible -- and perhaps even likely -- that the groundwater conditions have changed little since the 1980s, and although geophysical studies have been conducted since, it is essential to verify current geotechnical conditions to serve as the basis for quantitative analysis. Uncertainties Regarding the Debris Blockage A site characterization program was undertaken in the 1980s and is d escribed in Chapter 2.
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A summary of information needs with respect to the debris blockage is shown in Table 5.5. Other Alternatives Applying traditional principles of reliability and safety (see Box 5.2)
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From page 165... ...
Geomechanics Detailed characterization of the Critical in assessing current heterogeneity of debris blockage conditions of piping (pyroclastic, deposits, debris development, debris avalanche, and underlying rock) blockage stability, and open in shear strength and erodibility.
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From page 166... ...
Despite this view, the committee heard from various interested and affected parties in attendance at the committee's information gathering sessions, including some USFS staff, that redundancy in protection was desirable. Any trade-off of safety (or other desired outcome)
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From page 167... ...
, the committee was asked "to identify possible alternatives for long-term management" of the Spirit Lake and Toutle River system. Considering other alternatives is consistent with the traditional principles of reliability and safety.
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From page 168... ...
There appears to be no engineering reason why the lake could not be drawn down to further reduce risk. It is possible, however, that reducing storage by lowering the operating lake elevation could increase flood heights in the Toutle River downstream.
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From page 169... ...
. Dry Emergency Spillway A dry emergency spillway could be constructed over the debris blockage in the event of an uncontrolled increase in lake level due to tunnel failure or major renovation, among other reasons.
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From page 170... ...
Such a channel might be created in a controlled manner to re-create the pre-1980 flow patterns from Spirit Lake to the North Fork Toutle River. A tunnel could be constructed along alignments A, B, or D, as shown in Figure 5.2.
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From page 171... ...
If designed appropriately, the flow from the second tunnel could be used to generate electric power. The second tunnel might remove nearly all risks of the identified potential failure modes in the proposed alternatives of the PFMA.
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From page 172... ...
SRS on the North Fork Toutle River (see Figure 2.9) to capture medium- to coarse-grained sediments and allow finer particles to flow downstream in suspension to the Columbia River and out to sea.
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From page 173... ...
. By 2007, 4 to 5 million tons of sand had been transported from the Toutle River valley into the Cowlitz River (Biedenharn Group, 2010)
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From page 174... ...
The strategies adopted for reducing impacts of this sediment load on downstream channel stability and flood conveyance capacity have included temporary check dam structures to capture a portion of the sediment within the North Fork Toutle River valley, raising and strengthening engineered flood levees along the lower Cowlitz River, and dredging the lower Cowlitz River to maintain navigation and flood conveyance capacity as necessary. Sediment Budget and Management of the SRS The flow of sediments into the SRS since 1980 is shown in Figure 5.3.
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From page 175... ...
. The USACE commissioned a new study of the post-eruption sediment budget of the Toutle-Cowlitz river system with projections of sediment loads 175
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From page 176... ...
; they further estimated that after the SRS filled to the spillway elevation in 1998, approximately 80% of the sediment supply to the Toutle River mouth consisted of output from the SRS, with the South Fork Toutle River catchment providing another 13%. The study also concluded that between 2008 and 2035, with no changes made to the SRS, the most likely sediment load at the Toutle River mouth was 173 million tons -- but with a range from about half to two times that value and a more likely uncertainty range of about onethird (Biedenharn Group, 2010)
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From page 177... ...
The USACE is considering four alternatives for future flood risk management in the Cowlitz River sediment plain in the wake of the spillway rise in 2012. These are described in detail in USACE (2014)
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From page 178... ...
The USACE (2014) indicates that the preferred alternative would have major adverse effects on fish in the upper North Fork Toutle River area.
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From page 179... ...
The current levels of authorized flood risk reduction in the lower Cowlitz River, for which the levee system is sized, are listed in Table 4.1. These are provided by levee projects at Castle Rock, Lexington, Kelso, and Longview, the locations of which are shown in Figure 5.4.
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From page 180... ...
SOURCE: USACE, 2010b. alternatives2 for flood-risk reduction in the lower Cowlitz River flood plain that are not considered in current planning and that are not nec essarily mutually exclusive.
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LOPs for Longview and Kelso remain above authorized until 2035, but those of Castle Rock and Lexington depend on the 2014 Plan. LOPs for lower Cowlitz decrease into the future due to settlement accumulation if the actions adopted under the 2014 Sediment Management Plan are not enacted.
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From page 182... ...
The Lexington and Castle Rock levees are particularly vulnerable to the flooding impacts associated with sediment loads. Since these are areas where losses are lowest from a regional perspective, however, the analysis of viability of nonstructural alternatives, such as relocating these communities, may be inadequate.
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From page 183... ...
FIGURE 5.6 Longitudinal top of levee and Safe Water Level for Longview levee.
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From page 184... ...
Information needs with respect to flood risk reduction in the lower Cowlitz River are more modest than are those with respect to other aspects of the management of the system. The main uncertainties that require b etter information are those related to the potential for nonstructural strategies of a flood risk reduction and those related to the rate of future sediment accumulation in the lower Cowlitz.
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From page 185... ...
in combination with structural solutions might provide optimal alternatives to management. Evaluation of The USACE has yet to The principal uncertainties have future sediment forecast how much future to do with the volume and timing accumulation sediment buildup would of future sediment releases over reduce flood risk protection the SRS and the consequent in the Cowlitz River, amount of bed elevation in the although interim actions Cowlitz River channel, especially have been undertaken.
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From page 186... ...
. Operational risks in a complex system such as the engineered landscape of the Spirit Lake and Toutle River region may exceed risks due to natural hazards, engineering design, or other factors that more commonly appear in risk assessments (Regan, 2010)
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Even though precautions are taken to prevent such an operational risk, the risk still exists. The possibility of a dramatic and quick rise in lake levels during tunnel closure has led to an operational bias of avoiding tunnel closure more than was necessary.
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From page 188... ...
. Given that the Spirit Lake and Toutle River system is an engineered system, it needs to be managed and operated by a cognizant authority.
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From page 189... ...
Operational risks as discussed above are also important to infrastructure risk management. A PRA provides the platform for integrating the many hazards affecting the Spirit Lake and Toutle River system and for combining those hazards and their potential consequences within a formal decision protocol as discussed in Chapters 6-8.
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From page 190... ...
of load events occurring against the system. "Threat" is a term commonly associated with man-made or malicious actions, while "hazard" is a term commonly associated with natural processes, as in the case of the Spirit Lake and Toutle River system.
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From page 191... ...
Adequacy of the Current Risk Analysis The decision framework outlined in Chapters 6-8 is predicated on a quantitative assessment of the hazards, vulnerabilities, and consequences attending infrastructure decisions and the management of risk in the Spirit Lake and Toutle River system. This is not now available in a comprehensive form.
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From page 192... ...
Event Tree Representations as Information for the Decision Framework Event trees are now commonly used to represent the components of a PRA for civil infrastructure systems, particularly for dam and levee safety (USBR and USACE, 2012)
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From page 193... ...
. This approach can be carried forward to include vulnerabilities and consequences of any type in the Spirit Lake and Toutle River system.
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From page 194... ...
Assessing the consequences of future natural hazards requires integration of our understanding of potential hazards, as discussed above, and engineering aspects of the debris blockage, the Spirit Lake tunnel or other drainage scheme, and the SRS. The consequences downstream can be estimated in measures deemed important through the decision framework (see Chapter 6)
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