[5/3/18] PUD Is Monitoring Increased Movement In The Chelan River Gorge

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Chelan County PUD staff reported to PUD Commissioners at their meeting this week, that a known area of ground movement in the Chelan River Gorge, is being closely monitored after a small section was recently discovered to be more active.

Bill Christman is the PUD Chief Dam Safety Engineer. He says the PUD has been watching that area for more than 25 years with little activity, but it recently caught their eye after the Chelan Trails Association asked to look into developing a trail in the area….

050318 PUD Gorge Slope 1 1:06 “…on a broader scale.”

What’s happening, Christman says, is a natural phenomena and is not related in any way to hydro operations.

As a precaution, he says, the area will be closed off to public access…

050318 PUD Gorge Slope 2 :22 “…probably for a long time.”

Until more information is known, the public is urged to stay out of the signed area.

Christman said there are no structures in the path of unstable ground- adding that a large ground movement could release materials that could block the gorge…

050318 PUD Gorge Slope 3 1:25 “..non event as possible.”

The PUD is contracting with a geotechnical firm to help with the assessment and what might occur if an earthen dam resulted from a large rockfall.

Christman says the consulting engineers will also study what impact an earthen dam would have on hydro operations as well as conditions further downstream in Reach 4…

050318 PUD Gorge Slope 4 :21 “…can also be aware of it.”

The PUD has reached out to state and federal agencies, stakeholders and tribes to advise them of the situation. All of those contacted agree with the approach being taken by the PUD.

If you would like to learn more about the Chelan River Gorge ground movement- check out their website at chelanpud.org.




Fact Sheet
About three miles down the Chelan River from the Chelan Dam, Site A is
approximately 750’ from the Chelan River Gorge road and Site B is
approximately 450’ from the road. The sites are about 150’ above the river
primarily on Chelan PUD property, with a portion of the northern edge of the
area of interest on private property. A total length of 1200’ is under active
monitoring by the PUD
Natural Phenomena
There is a mixture of loose dirt, sand and small rocks along with large
boulders found on this bluff. The area is prone to settling and sloughing due
to periods of drought and heavy rains, along with occasional earthquake
movement. The slopes drop off somewhat steeply about 150’ to the river.
Sites A and B
The area between Site A and Site B, approximately 1200’, is the area of
Site A has been monitored for more than 25 years. It is a known area of
slight ground movement, with no known releases into the river gorge.
Site B is more recent and more active within the past year. There has been a
rockfall release just below the Site B location.
Typography of the area
There are no structures either on or below the identified unstable area.
Likewise, this is an area without human access to the Chelan River, so it is
not a risk to human health and safety if precaution is used.
Reach 2 of the Chelan River Gorge is not an area where protected species
are found. There are resident fish in the general area.
Likelihood of a slide
Chelan PUD engineers and biologists have actively monitored the Site A area
for years. Site B is new. A geotechnical consulting company will join PUD
staff to do a full assessment of the entire area to determine the possibility of
a rockslide/rockslides into the Gorge. It is expected that there will continue
to be moderate rockfall similar to what has occurred during the past year.
What if there is a large rock release into the Gorge
Depending on the amount of the release, the rock composition (i.e. large or
small rocks) and the location, the likely response would be to increase the
river flow from the Chelan Dam to begin a natural “flushing” to move
material downstream toward the Columbia River, similar to what would
occur naturally.
If an earthen dam were to occur and it collapsed, the greatest risk would be
the release of a large volume of water and sediment further down into the
Gorge and into Reach 4, similar to what occurred in the 1990s.
The PUD will watch the situation carefully in order to protect fish habitat in
Reach 4 as well as minimizing water quality impacts.
Next steps
All interested and affected stakeholders, state and federal agencies and
tribes have been notified and concur with the PUD’s approach.
The PUD will bring on a geotechnical consultant, as well as continue
monitoring the sties.
The PUD is preparing an incident specific Emergency Action Plan (EAP) that
identifies all agencies and stakeholders who will need to be notified in the
event of significant slope action. The EAP describes the planned response
depending on the ground activity.
Immediately, The PUD will put signage in the area to warn the public to stay
out due to ground instability concerns.
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