ML20132C836

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Forwards Revised SER Input Re Effects of Breakout of Spirit Lake on Plant.Conclusions in Previous SER Based on Unpublished USGS Rept & Did Not Mention Independent Study Performed by Applicant
ML20132C836
Person / Time
Site: Trojan File:Portland General Electric icon.png
Issue date: 03/28/1985
From: Johnston W
Office of Nuclear Reactor Regulation
To: Lainas G
Office of Nuclear Reactor Regulation
Shared Package
ML19264D658 List:
References
FOIA-85-353 NUDOCS 8504050310
Download: ML20132C836 (8)
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, . .. MEMORANDUM FOR: Gus G. Lainas, Assistant Director l

for Operating Reactors, DL FROM: William V. Johnston, Assistant Direcuer ,

Materials, Chemical & Environmental Technology, DE

SUBJECT:

SPIRIT LAKE / TROJAN PLANT FLOODING -SER On March 6, 1985, we provided you SER input addressing the effects of a breakout of Spirit Lake on the Trojan Plant. The conclusions in our SER were based on an unpublished USGS report and did not mention an independent study that had been performed by the applicant. The project manager, Charles Trammell, requested that we revise our SER to include the licensee's evaluation. In response to that request, attached is our revised SER input.

This input replaces our March 6,1985 input entirely. A copy of this revised

input was provided to C. Trammell on March 13, 1985.

William V. Johnston, Assistant Director.

Materials, Chemical & Environmental Technology .

Division of Engineering .

Attachment:

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HYDROLOGIC ENGINEERING SAFETY EVALUATION REPORT j CONCERNING A POSTULATED BREAK 0UT OF SPIRIT LA){E l AND ITS IMPACTS ON THE TROJAN PLANT 3

1. Purpose of Report l On May 18, 1980, Mount St. Helens erupted and caused a massive n'udflow l flood that deposited a large amount of sediment and debris in the 4l Columbia River near the mouth of the Cowlitz River. The Trojan Nuclear I Plant which is located on the west bank of the Columbia River about 4.5 miles upstream of the mouth of the Cowlitz River (See Figure 1) was not directly affected by the sediment deposited in the Columbia River,

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l althcugh large deposits' w'ere measured near the intake structure. In

, addition to the mudflow flood, the eruption also resulted in an avalanche of rock, mud and ice that swept down the slopes of Mount St. Helens into Spirit Lake and the Upper Toutle River Valley. As the debris avalanche slid into Spirit Lake, it displaced the lake level upward by more than -

200 feet. Th'e avalanche also deposited an estimated 3.9 billion cubic yards (bey) of sediment in the upper 17 miles of the North Fork Toutle _.

River Valley and blocked the outlet channel of Spirit Lake with~ debris

[ ranging in depth to 500 feet. This debris-blockage, which is a massive ard unstable deposit, caused a dramatic increase in the volume of water stored in Spirit Lake. Because of the unstable nature of the debris

, blockage, there was a concern that Spirit Lake could breach the blockage c and cause a mudflow flood that could possibly affect the safe operation of the Trojan Plant. This report addresses the potential effects of such a mudflow flood at the Trojan Plant.

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, , 2. Introduction i

Mudflows associated with the May 1980 eruption moved down the Toutle j

River and carried more than 50 million cubic yards (mcy) of material 1

into the Cowlitz River and its overbank areas. An additional 45 mcy y .

were deposited in the Columbia River, mostly in.a nine mile reach of the river extending from about 5 miles downstream of the mouth of the Ol Cowlitz River to 4 miles upstream.

q In the fall of 1981, a task force organized by the U.S. National Forest Service determined that the effective crest of the debris blocking the a

i Spirit Lake outlet would deteriorate from elevation of 3490 ft to 3475 ft i

during the winter of 1982 _83 because of subsidence and erosion.

It was estiEa~ted that Spirit Lake would fill to elevation 3475 ft (314,000 acre-feet) sometime in March 1983 assuming that no action was taken to reduce the level and volume of the lake and that the annual inflow was average. Were this to happen, there would be a very high potential for )

failure of the blockage and catastrophic flooding downstream. To reduce the potential for failure, the Corps of Engineers, as an interim measure, f

constructed a pumping facility at Spirit Lake and pumping began en -.

November 5, 1982 (see Figure 2). The goal of the pumping operation was to stabilize the lake level at an elevation of about 3,462 ft, which corre-

sponds to a lake volume of about 275,000 acre-feet. However, greater-than-normal rainfall, failure or disruption of the pumping system and/or
addition of debris into Spirit Lake from a subsequent eruption could cause i the lake level to rise excessively. In an extreme case the debris

, blockage could fail. Because of the potential public safety hazard associated with a breakout of Spirit Lake, the Corps of Engineers has developed a plan to permanently lower the level of Spirit Lake. This plan consists of drilling a tunnel from Spirit Lake to South Coldwater '

Creek (See Figure 2). This will allow Spirit Lake to drain so that the lake level can be maintained at an elevation low enough to remove the I

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T threat of a Spirit Lake debris dam failure and consequent mudflow flood.

[ Construction of the tunnel was initiated in July 1984. 'It is expected

[. that the tunnel will be completed by April 1985. ,

3. Analysis I

In a report titled " Analysis of Flood Level at Trojan Plant Associated with Hypothetical Failure of Spirit Lake Blockage," by Simons, Li and Associates, Inc.; the licensee addressed the potential impact of a Spirit Lake breakout on the Trojan Plant. The depth of flooding in the Columbia River would be dependent Jni the coincident discharge in the river at the time the breakout flood peak arrived at the mouth of the Cowlitz River.

Thus the licensee determined water surfage elevations corresponding to vari 5'u~

s coincident discharges in the Columbia River. Resu.lts of their

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study were as follows:

SIMMONS AND LI REPORT Resultant Water Surface Elevations in _

the Columbia River at the Trojan Plant due to a postulated breakout of Spirit Lake Coincident Columbia Riv.er Discharge in cfs 125,000 400,000 800,000 Colurbia River Water Surface Elevations in feet at the 31 35 39 Trojan Plant This table shows that a postulated breakout of Spirit Lake occurring coincident '.

with an 800,000 cfs flood flow in the Columbia River would not affect the safe operation of the Trojan Plant because the plant grade elevation, at 45 feet, is 6 feet higher than the flood level would be.

A potential breakout of Spirit Lake was also of concern to the U.S. Federal Emergency Management Agency (FEMA) because flooding from a breakout could .

have a severe impact on public safety and on the regional econcay. To enable -

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. , .. it to plan for such an event, FEMA requested the USGS to study the impacts of a Spirit Lake breakout on the entire lower Columbia River. The USGS has comp 10 tad its study and its report is now (February 1985) b,eing reviewed by i the USGS Federal Headquarters in Reston, Virginia. The study addresses the

impacts of a postulated breakout of Spirit Lake on the Columbia River from j Bonneville Dam to the mouth. The Trojan Plant is located within this stretch l of the river. -

7 The USGS, using a sediment transport computer model, concluded that following a postulated breakout of Spirit Lake, a large sediment j blockage would form in the Columbia River at the confluence of the Cowlitz, with subsequent irnpoundment of water upstream of the blockage.

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This blockage would result in a slowly rising water level at the Trojan Planf khich would continue for 16 day's after the blockage', formed.

As stated above, the depth of flooding in the Columbia River would be dependent on the coincident discharge in the rf ver at the time the breakout flood peak arrived at the mouth of the Cowlitz River. The USGS q

also determined surface water elevations corresponding to various coinci-dent discharges in the Columbia River. Results of their study were as m.

follows:

USGS REPORT Resultant Water Surface Elevations in the Columbia River at the Trojan Plant due to a postulated breakout of Spirit Lake Coincident Columbia River Discharge in cfs 233,000 410,000 610,000 750,000 820,000 f Recurrence Interval in years (average) (2 year) (10 year) (50 year) (100 year)\

Columbia River Water Surface 32 38 41 43 44 i Elevation in feet at the ~ ' '

T Trojan Plant The results of the USGS study are similar to the licensee's study in that a

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a postulated breakout of Spirit Lake occurring coincident with a flood in the -

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4. Summary and Conclusion By pumping water out of Spirit Lake the Corps of Engineers has been successful in maintaining the Lake level at an elevation that has minimized the chances for a breakout and the potential threat to the Trojan Plant. The staff however, remained concerned because it was uncertain about whether or not a lake breakout and the ensuing flood could affect the safe operation of the plant. The licensee and the USGS studies, however, provide ~'

information that indicates that even if Spirit

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Lake did break out, the resuitant flood level in the Colubbia River at the Trojan Plant would not rise as high as plant grade even if the breakout occurred coincident with a 100 year'ficed discharge in the Columbia River.

The tunnel being constructed between Spirit Lake and South Coldwater

, i Creek (See Figure 2) will provide a permanent outlet for Spirit Lake.

This tunnel will permanently lower the level of Spirit Lake and remove _ _ .

the threat of a breakout and the potential for high water in'th'e Columbia River at the Trojan Plant.

The staff concludes that a breakout of Spirit Lake will not affect the safe operation of the Trojan Plant. Furthermore, once the Spirit Lake outlet tunnel is completed, there will be no safety concern regarding a breakout as far as the Trojan Plant is concerned.

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