ML20133F656
| ML20133F656 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 12/05/1983 |
| From: | Trammell C NRC |
| To: | Ballard R, Fliegel M, Gonzales R NRC |
| Shared Package | |
| ML19264D658 | List:
|
| References | |
| FOIA-85-353 TAC-49636, NUDOCS 8508080375 | |
| Download: ML20133F656 (14) | |
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- 1fr For Responding to the Long-Term Threat Created by the Eruption of Mount St. Helens, Washington l
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November 3, 1983 M
MEMORANDUM THRU '2HE dECRETAMI Uc A n s, h.s.'l N }
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THE SECRETARY OF DEFENSE
? NOV 1983 Gf FOR THE PRESIDENT
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SUBJECT:
Mount St. Helens - Solutions to Long-Term Problems f
Pursuant to the request contained in your May 18, 1982, memorandum, this report presents a comprehensive plan for responding to the long-term threat created by the 1980 eruption of Mount St. Helens.
This report consists of two parts:
(1)
This memorandum, which discusses the alternatives and indicates a course for implementation, and (2) a Corps of Engineers analysis of alternatives.
The pr'oblems presented as the result of the 1980 eruption of Mount St. Helens are unlike any others ex-
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perienced in the United States.
In the three and one-half years since the eruption, the Federal Government has expended in excess of one-third of a billion dol-lars in order to minimize damage and property losses in those areas adversely affected by extraordinary conditions created as a result of the eruption.
We believe the time has arrived to -program long-term strategies to minimize the continued threats to people and property presented by the conditions created by the eruption.
It must be recognized that it is impos-sible to predict accurately what natural phenomena may still occur at Mount St. Helens.
Nonetheless, every t
reasonable effort has to be made to provide protection l
against such unknowns.
Further, the amount of sedi-ment movement and the timing of that movement are critical in evalua ting long-term solutions.
Our state of knowledge does not permit exact determinations con-cerning these
- items, and it is therefore important that a range of assumptions be evaluated.
Continued close cooperation among Fed e ral,
State and local agencies as well as continued.close professional moni-toring of the erosion process will be necessary so that adjustments can be made to any solutions which will be programmed.
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T. 1 The long-range problems resulting from the Mount St. Helens eruption may be separated into two general
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categories ~:
First, those associated with a debris dam blocking the outlet for Spirit Lake in the upper reaches of the Toutle River.
If the debris, dam ware to give way, a disastrous flood could result in the areas below.
The United States Geological Survey has estimated, under a worst case scenario, that failure of the debris dam could cause as much as S2.5 billion in loss of property in the areas below, to say nothing of the potential loss of life.
The second long-range problem is concerned with the very large amount of sediment deposited in the Toutle River watershed.
This sediment has not sta-bilized and continues to be transported downstream creating flood threats along the lower reaches of the Toutle and Cowlitz Rivers.
Continuing deposition of a portion of this sediment in the Columbia River also adds to the cost of the maintenance of the necessary navigation channels of the Columbia River.
Spirit Lake Alternatives since the declaration of an emergency. by the President last year, temporary pumps and pipelines have been installed and operated at Spirit Lake in order to keep the lake level from rising and thereby posing an overtopping threat.
A permanent solution to this threat should have top priority -- not only to reduce the hazard of overtopping but to eliminate the costly temporary pumping operation currently in place.
The Army Engineers examined six alternatives to pe r-manently stabilize the lake level and to minimize future threats of dam overtopping and failure.
Two alternatives envisioned retaining the Spirit Lake waters within the Toutle River watershed by means of a pipeline or surface channel through the debris dam to a safe distance downstream.
Three alternatives en-visioned the construction of a tunnel from Spirit Lake into an adjacent watershed or into the North Toutle drainage some distance below the debris dam.
A sixth alternative envisioned a permanent pumping facility.
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Under any of the alternatives, the level of the
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lake to be maintained is a critical determination.
The Army Engineers believe an appropriate level is elevation 3,440 feet or approximately 20 feet below the level currently being maintained.
Based 'upon the data available it. appears that this is a safe level.
However, development of additional' information as a result of the public hearing process and interagency review which will follow in the near future, could e
j lead to a different determination.
Diversion of Spirit Lake water out of the Toutle River watershed should be avoided unless there are compelling reasons for it.
Reasons for avoiding such diversions. include (1) the required construction time, (2) resultant diminution of low flows within the Toutle River stream system, (3) unknown effect on the fisheries within the Toutle and adjacent wa tershed s, (4) impact on downstream environmental conditions and water rights within the Toutle and Cowlitz watersheds, and (5) uncertainty of tunnel construction through areas for which there is not detailed geological information.-
is believed a safe diversion structure throu,gh It the debris embankment can be constructed.
While some concern has been expressed regarding safety of the
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conveyance alternatives through the debris embankment in the event of potential future eruptions of Mount St. Helens or other unforeseen events, we believe that emergencies can be accommodated if the need arises.
A good example is. placement of the present temporary pumping installation into operation in less than 60 days during the late fall of 1982.
In summary, with respect to a long-term solution to the Spirit Lake problem, the Corps of Engineers should proceed with public hearings, interagency re-views, and the further planning necessary to implement one of the alternatives as expeditiously as possible.
Preference should be given to the underground conduit through the debris embankment.
Although the construc-i-
tion cost of the conduit is greater than that of 6
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another alternative, it can be constructed in one i
season and eliminate the need to continue the interim
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pumping arrangement as soon as possible.
Like an open channel through the debris embankment, it would ap-5 proximate natural conditions, but it would have less of a visual impact on the National Volcanic Monument.
Sediment Control Alternatives, i
The long-term solution to the control of sediment deposition along the Toutle and Cowlitz Rivers is a most difficult one.
Compounding the difficulties is I
the uncertainty concerning the amount of sed iment which must be provided for and the timing of any move-ment that will occur.
The Corps has made its best estimate of both the amount and timing of this sed i-i ment mov emen t_.
- However, notwithstanding these estimates, any programmed solution should condi t ions.prov ide flexibility to adjust to actual The greatest uncertainty influencing the movement of sediment in the sho r t run is the intensity of future precipitation and the characteristics of resultant runoff.
Over time, stabilization will occur as a result of the cumulative effects of many natural processes.
Excessive precipitation with attendant flood run-off before stabilization takes place causes movement of large amounts of material which must be controlled in order to avoid downstream flooding and navigational hazards.
Temporary solutions involving the dredging of reaches of the Toutle and Cowlitz Rivers, and the construction of levees and small retention basins have prevented flooding and interference with navigation during the last three years.
These temporary so-
- lutions, while providing protection on an interim basis, are expensive to maintain and do not provide the long-term security necessary to the 50,000 to 60,000 residents of the Longview, Kelso and adjacent areau.
The Army Engineers have evaluated five alter-natives for sediment control.
The two alternatives which appear most feasible involve construction of t
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either one large sediment retention structure at the Green Valley site on the Toutle River or the construc-tion of three or more smaller structures at sites at and below the Green valley site.
i The Corps' preliminary estimates show the' single i
large retention structure to be of a lesser total cost
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under a range of erosion assumptions.
- However, it does not necessarily follow that a single large struc-ture would turn out to be the least costly in the long run or provide the desired flexibility.
More flexi-bility would be obtained by the construction of several smaller retention dams which would be con-structed as the need arises.
In addition, construc-tion of a
retention structure downstream from the Green Valley site would have the added advantage of intercepting those sediments which have been deposited to date below that site.
With respect to a comparison between the single large retention structure and the multiple retention structures, the multiple ' structures ultimately may be preferable for several reasons.
First, if the amount of material to be controlled is overestimated for the single retention structure, the result would be an 2
overdesigned foundation structure with its attendant y
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higher costs compared to that of a smaller structure which would be the first element of a
multiple.
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l structure plan.
Also, if the movement of the sediment is spread out over a
longer period of
- time, the present value of funds required for a
series of 4
smaller structures would be considerably less than proj ected by the' Corps.
This is an important consid-
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eration given today's high real interest rates.
In addition, estimates of the available capacity at LT-3 have varied widely from 64 million cubic yards to 150 million cubic yards during the course of the Corps' study.
The available storage at LT-3 has a direct bearing on which alternative might be the most desirable and the Corps should give this matter additional careful review and study.
An enlarged LT-3 storage site might also lend itself to staging.
On the other hand, if actual deposition turns out to be in the one-billion-cubic-yard range and essentially at h
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i rates projected by the Corps, it might be advantageous to construct a single retention structure at the Green Valley site.
Other factors involve fishery and safety con-cerns.
Further consultation is needed with 5, tate and Federal fishery agencies in order to assess the j
fishery impact of the two alternatives.
While the multiple retention structures might interfere with i
more miles of stream channel, it might be that the heights of the structures in the multiple structure plan could allow fish ladders or other bypass struc-tures.
With regard to safety provisions, the Corps could undoubtedly design and construct a safe struc-ture under either alternative.
However, people fear large dams because of the actual or perceived conse-quences of failure, and considerable opposition was expressed by local interests when a
single large structure was proposed in December of 1982.
In summary, there are compelling reasons favoring construction of retention structures to.contain the eroded material from the debris avalanche.' Multiple structures have an inherent flexibility which is not afforded by a single structure.
Follow-on studies should fully develop the multiple structure alter-native along with the single retention structure approach.
j Recommendations The Corps will begin to hold hearings on this report within.the next 45 days.
The Department's recommendations presented below are based on the investigations to date.
They will be presented to the public together with the analysis of alternatives.
Further planning of solutions to both long-term problems will be based upon public input, the views f rom State, Fed eral and other public agencies and the Corps' further investigations and coordination necessary to comply with all applicable laws and Executive Branch procedures.
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i With regard to Spirit Lake, the corps should I
proceed to construct a permanent outlet.
In the t
absence of compelling reasons why some other alter-native should be used, the Corps should complete the design for a buried conduit through the present debris embankment in order to insure the maintenance of Spirit Lake at a safe level.
A permanent outlet can be constructed without further congressional authori-zation.
Working with the. Office of Management and Budget and the Congress, we believe the necessary funds can be made available.
With respect to" the control of sediment, the Corps should complete the planning necessary to j
recommend congressional authorization and funding of a i
permanent solution.
Planning and design should con-centrate on the retention structure alternatives with the goal of early construction of a retention struc-ture (staged or otherwise) at the lowest feasible j
site.
Other ~ stages or upstream structures should be planned for subsequent construction if and when j
needed.
These feasibility plans will be reviewed at the Division and Office of the Chief of Engineers l
level, as well as the Board of Engineers for Rivers 4
and Harbors, before being sulxni t ted to the Secretary of the Army for approval and transmittal to the Congress.
i Pending the completion and implementation of i
structures to provide for long-term control of the l
level of Spirit Lake and movement of sed iment down-
- stream, the
- Corps, in full coordination with the i
Secretary's
- office, the Office of Management and j
Budget and local interests should provide such interim measures as are necessary to continue the protection afforded during the last three years.
$bw BY. '
4 William R.
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Assistant Secretary of the Army (Civil Works) 4 i
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!'E APPENDIX A Ig TABLE OF CONTENTS y
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?.a Prior Studies, Corps of Engineers
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Advance Measures Reports (June 1980) g Mount St. Helens Eruption long Term Program dl for Cowlitz and Toutle River Basins (July 1981)
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! bunt - St. Helens Eruptions:
Cowlitz County, Washington /
Cowlitz and Toutle Rivers Fiscal Year 1982 Action Plan (November 1981) lh _ Status Report of the Mount St. Helens Restoration ( April 1982)
I unt St. Helens Eruption:
Toutle,and Cowlitz Rivers /
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>T dowlitz and Skamania Countie's,' Tashington/ Spirit Lake "I
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- yf Interim Action Plan- ( Auguar 1982g upunt St. Helens Cow 11tzt.andiTsutig Rivers Sedimentation Study, f g/ ' 1980-82' (December.1982) $,['}"
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. Prid,. Studies, Other Agen'cies
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tr,it.Iake Flood Hazard. Task Eorc :.' Situation Assessment i;
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and RecommendationiP, U.S.'For'es Sd vice (July 1982)
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ytle-Cowlitz Waters l}ed Mariageme Pla n, wl[tz Coun' y, WA (Mar 1983.)
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t TF." STAD.S OF EXISTING PROJECTS AND IMPRO MENTS.
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