ML19264D657
| ML19264D657 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/18/1983 |
| From: | Gonzales R Office of Nuclear Reactor Regulation |
| To: | Ballard R Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19264D658 | List:
|
| References | |
| FOIA-85-353 NUDOCS 8304260039 | |
| Download: ML19264D657 (7) | |
Text
boy APR ie 1s3 Docket ho. 50-344 MEMORANDut4 FOR: Ronald L. Balla' d, Chief Environmental & flydrologic Engineering Branch, DE THRU:
Myron Fliegel, Leader liydrologic Engineering Section Environmental & Flydrologic Engineering Branch, DE FROM:
Raymond Gonzales, Hydraulic Engineer itydrologic Engineering Section Environmental & Hydrologic Engineering Brancn, DE
SUBJECT:
SITE VISIT TO TROJAN PLANT, MOUNT ST. HELENS AND SPIRIT LAKE On March 17, 1983, I visited the Trojan Plant - Spirit Lake - Mount St. Helens areas along with Steve Drocoum and Dick McMullen of the Geosciences Branch.
Mr. Richard Janda of the USGS accoinpanied us to the Spirit Lake - Mount St.
Helens area.
Background
The debris avalanche that accompanied the May 18, 1980 emption of Mount St.
liciens buried the outlet channel of Spirit Lake to a depth ranging to 500 feet (seeattachedfigure). Since that time, the lake has had no natural outlet so its water surface and contents have increased significantly.
Studies of the debris stratigraphy, crosion of the surface of the blockage, and soil properties have led to a concern that the lake could breach-through or spill-over the debris blockage. A recent study by the USGS shows that such an event could create a mudflow that would be much more severe than the mudflow that followed the May 1980 cruption.
The Tmjan Plant is located on the west bank of the Columbia River about 6 miles upstream of the confluence of the Cowlitz River. Following the May 1980 cruption, the mudflow that came down the Cowlitz River formed a shoal in the Columbia River that blocked the navigation channel.
In the vicinity of the Trojan Plant, the depth of the channel was reduced by about 40 feet. The Trojan intake structure, however, was not affected by sediment.
Since the Spirit Lake debris failure postulated in the USGS Study would result in a mudflow of greater magnitude than the May 1980 event, there is a question as to what the effect would be on the Tmjan Plant.
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Ronald L. Ballard AP., 1 Fu Purpose The purpose of this site visit was to tour the Spirit Lake - Trojan Plant area to become familiar with the post-eruption condition of the streams and rivers that drain Mount St. Helens, to tour the Columbia River area from the confluence of the Cowlitz River to the Trojan Plant site and to determine what actions should be taken to assess the effect of failure of the Spirit Lake blockage on the Trojan Plant.
Site Visit On Thursday r:orning, March 17, we were met by Mr. Lichard Janda of the USGS at Castle Rock which is located just downstream of where the Toutle River flows into the Cowlitz River. Mr. Janda is an expert in ancient Mount St. Helens eruptions and pointed out that in the last 3500 years, there bnve been about 25 eruptions. Although the 1980 mudflows were devastating in their impacts on river channels, floodpleins and vegetation (not to mention all the people who died or are still missing), the effects were far less than those of some mudflows associated with earlier eruptions.
We proceeded along the Toutle River in an easterly direction toward Mount St. Helens and stopped where the. North and South forks of the Toutle River join. The river floodplain at this location is barren - devoid of any vegetation. The flow in the river is currently carrying a very large suspended sediment load as fine material deposited by the 1980 mudflow is croded by the flowing water. Mr. Janda pointed out that by measuring the difference in mudline elevations on each side of the river valley due to radial acceleration (banking) of mudflows around river bends, USGS is able to obtain a good estimate of mudflow velocities. This is important to us in that an estimate can be made, sith a high degree of confidence, as to the time that a mudflow due to a postulated failure of the Spirit Lake debris blockage, would arrive in the vicinity of the Columbia River - Trojan Plant.
We followed the North Fork Toutle River and stopped at a point about 25 kilometers from tbunt St. Helens. At this location there is a sediment control structure that was breached in three locations. Although this structure did not survive the mudflow, it did delay the arrival of the flood petk at populated areas downstream.
It is irunic that this structure which was built to retain sediment is now a sediment source. Mr. Janda mentioned that the Corps of Engineers is considering a new sediment-debris control structure which would be constructed in stages as sediment is deposited.
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Ronald L. Ballard APii ; 61983 The entire geororphic and hydrologic conditions of the rivers have been altered drastically. The 1980 eraption completely obliterated the upper reaches of the Toutle River.
h' hat used to be stable incised river channels are now braided wide channels that extend across entire valleys. These braided channels are very unstable; they are constantly shifting as they attempt to adjust to altered conditions and reach some sort of equilibrium.
Just south of Elk Rock, we boarded a helicopter for an aerial reconn.aissance of the area. From the air you can see that there are hot spots that are still steaming two years after the eruption.
In particular, the done wMch is forming in the Fount St. Helens crater was emitting a vapor plume high above the crest of the volcano. Although more than.?O0 square miles of dense furest were devastated, most of the fallen trees have been harvested by the Weyerhaeuser Company thus reducing the possibility of future river blockage by tree debris.
In addition to Spirit Lake, there are two other bedies of water that were created by debris blockage. These are Coldwater Lake and South Castle !.ake.
These water bodies are also of some concern because groundwater is mounding underneath them. This rising groundwater makes the surrounding slopes more susceptible to sliding into the lakes and breaching the debris blocket;en The probability of these lakes releasing their contents due to overtopping or breaching through the debris blockages is probably greater than the probability of the Spirit Lake blockage failing. However, the content of these two lakes is much smaller so the potential hnzard to populated down-stream areas is also much less.
Spirit Lakes still has much debris, mostly fallen trees floating on the surface. A floating barge is anchored in a cove of the lake. Using pumps mounted on this barge, the Corps of Engineers is currently pumping water through a buried pipeline at a rate of 180 cubic feet per second (cfs) in an attempt to stabilize the lake level and reduce the likelihood of a breakout of the lake.
We deboarded from the helicopter on the surface of the debris blockage to examine the debris material. On the surface, the material consists aninly of a loose, fine, granular, easily-crodible soil. Steep-walled channels as deep as 50-60 feet have already eroded in this material and are working their way toward the crest of the debris blockage. After examining the debris material it became apparent why there is a concern that the Spirit Lake debris blockage could fail. The movement of water through the blockage could pick up and move the fine soil particles thus creating channels within the mass of the debris blockage. This phenomenon is known as piping. Also, because the material is very loose, the blockage could fail before the lake level actually reached an elevation that would overtop.
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DISTRIBUTION:
Dockets EHEB Rdg MFliegel Ronald L. Eallard RGonzales APE 1 5 1333 Pumping of water by the Corps is at best only an interim solution.
Cecause of this the Corps is doing a stability analysis to determine the elevation at which the lake would be stable. Once this elevation is determined, a method has to be developtd to lower and maintain the lake at a safe elevation.
Two opticas are being considered. One of these is constructing an open channel across the debris to allow water to flow out. This option is risky however, because of the highly erodible nature of the material. The second option is to drill a tunnel through bedrock to Smith Creek. This appears to be a better solution except that Spirit Lake is now highly polluted having a high concentration of heavy metals and various types of parasites and viruses.
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Diversion of Spirit Lake to Smith Creek could pollute several tributaries and three reservoirs before reaching the Columbia River.
In suunary, there is no easy solution as to the best and safest way of reducing the danger of a Spirit Lake Breakout.
After the aerial reconnaissance of Spirit Lake and the surrounding area we proceeded to the vicinity of the Columbia River - Trojan Power Plant. The river floodplain in the vicinity of the Trojan Plant is very wide and would be capable of dispersing a large amount of the mudflow debris that would be generated by a Spirit Lake debris failure. liowever, whether or not the Trojan Plant would be affected is not known at this time because the studies that have been done by the USGS and/or the Corps have not included the Columbia River. FEMA has requested that the USGS excend their previous mudflow inundation study to include the Columbia River. At this time, however, the USGS estimates that such a study would take about a year. The Corps, which is responsible for maintaining the navigation channel in the Columbia River, has requested the USGS do a quick analysis to try to get some idea of what the effect would be on the Columbia River. The USGS is reluctant to do thh so there is some uncertainty as to when the USGS wit 1 complete its study. As far as NRC is concerned, at the present time there are no bases for determining whether or not the safety of the Trojan Plant would be affected by a Spirit Lake debris blockage failure. The Corps will keep us informed as to any progress made concerning accelerating the Columbia River mudflow inundation stucy.
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49 Raymond Gonzales, PJdraulic Engineer Hydrologic Engineering Section Environmental 6 flydrologic Engineering Branch Division of Digineering
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