Dam Inspection for Mirror Pond Dam 2012

Oregon-Header-KitzOctober 16, 2012

Roger L. Raeburn, Manager Dam Safety
PacificCorp Energy
P.O. Box 3040
Portland, OR 97208

Re: Bend Hydro (Mirror Pond) Dam (B-99)- Inspection Summary

This dam was inspected on July 12, 2012. I performed the inspection with District 11 Watermaster Jeremy Giffin. You were there, as were Tom Becker and Nathan Higa from Pacific Corp. and provided very helpful dam history and safety information. The Water Resources Department conducts these routine inspections to identify safety, maintenance or operational issues that may affect dam integrity. Dams are assigned a hazard rating based on downstream hazard to people and property, not on the condition of the dam. Bend Hydro (Mirror Pond) dam is classified as a significant hazard dam. Significant hazard dams are inspected every 2-3 years.

The results of this inspection are illustrated and described in the following photos and text. This inspection includes recommendations to keep the dam safe

Results of Inspection:

The spillway is often the most important safety feature of a dam. The spillway is needle type structure, with multiple bays to wood stop and end timbers, and a more recent concrete cap.

Reservoir , log boom, and old needle style spillway section
Reservoir , log boom, and old needle style spillway section

A walkway constructed on top of the cap that allowed detailed inspection of top of the spillway section. The walkway was sound.

Old section, with old timbers on downstream face
Old section, with old timbers on downstream face

The rest of the spillway received visual inspection only. Some of the timbers show signs of significant decay. The concrete sections that support the bays, and their foundations near original, and a more thorough inspection at very low water would be prudent.

Leak controlled by sheet piling
Leak controlled by sheet piling

A leak through the spillway section was discovered by Watermaster Jeremy Giffin a couple of years ago. The leak was controlled by installation of sheet pi ling as shown above. The leak is an indicator that this part of the dam is showing its age, and in need of a thorough inspection to evaluate the base and the condition of the large timbers, and the overall needle structure.

New emergency gate control valve
New emergency gate control valve

The Emergency gate for this dam was just replaced with a new motor and controls. It was operated during the inspection (for a small part of its cycle, as the gate is not in the same condition as its control). The gate and control functioned well for this limited operation.

Release of water from gate during inspection
Release of water from gate during inspection

The gate structure is also old, but appears to be operational, and was opened for limited flow as described above. When closed, there is moderate leakage, mostly through gaps between the old timbers.

Concrete buttressed wall section
Concrete buttressed wall section

The concrete buttress wall forms the middle section of the dam. It is mostly the original section, so is also 100 years old. There are areas of minor to moderate spall, and some fairly minor cracking. Overall, the section appears sound. The area below the dam is well maintained grass, with no wet areas, and was maintained for easy inspections.

Minor seepage and old concrete deterioration
Minor seepage and old concrete deterioration

The location above shows the maximum deterioration seen in the buttressed wall section. Seepage loss was low, around one gallon a minute. This is not a concern at this time.

bend-hydro-power-house-2012
Powerhouse Section

The powerhouse wall is also one of the dam sections. This was inspected from the inside, and is in the best condition of any of the dam sections, with no leaks or significant cracks.

Access to and security at the dam was very good. It is, fenced with appropriate signage. This is a run of the river reservoir, and there are no signs of erosion around the dam site.

Recommendation(s):

  1. Continue with good maintenance and operations, including security, vegetation control, and security.
  2. Evaluate Deschutes River flow, and accompany me on an inspection of the base of the spillway structure at very low water. I will coordinate with you and Watermaster Jeremy Giffin on the timing of such an inspection.

We use a standard inspection form for all dams, and a copy of the field inspection sheet for this dam is attached. The next regular inspection is planned for 2015. Thanks for sending me the drawings of the dam, and please let me know if you have any questions about this inspection.

Sincerely,

Keith Mills, P.E., Dam Safety Engineer

Document: Bend Hydro Inspection 2012-1

Related: Dam Inspection Summary 2009Mirror Pond dam passes inspection

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