Guest column: Saving Mirror Pond and dam safety
Jun 13, 2019
By Mark Davis
This last April I contacted the Oregon State Dam Safety Engineer to request a change of the hazard rating for the Bend Hydro Dam (Mirror Pond). After a visit to the Oregon Water Resources Department website it was apparent that the present rating of “significant” could be in error due to hazard creep resulting from the increased recreation at river parks. State law requires a “high” rating when the area below a dam is crowded with people, and this is the only dam on the Deschutes River without a high hazard rating. All safety measures are implemented based on the hazard rating and because of the significant hazard rating there has not been an inspection by the state engineer since 2015.
My interest in Mirror Pond was spurred on by Mirror Pond Solution’s proposal to dredge the Deschutes River, a plan that I believe will degrade water quality and is founded on a claim to the land under the pond without judicial confirmation. My hope is to redirect the community’s attention away from these benign sediments in Mirror Pond, and back to addressing the 109-year-old dam. Contrary to the beliefs of many, dredging will not save the pond and sediments will not eliminate it. The pond was created by a dam that is now at the end of its life and will need to be replaced or decommissioned. The timber-crib section was poorly designed, built and maintained. Of the 21 cribs, 14 have needle bays which were intended to pass the buildup of sediment in the pond, but in 1919 these were made inoperative by the addition of a concrete cap on the dam crest. The needle bays were never given a chance to pass sediments, which would have negated the need for dredging.
Only the replacement of the deteriorating timber-crib section will save Mirror Pond. The replacement should be a modern concrete dam with low-level sluice gates that allow the pond to run as a river for short periods of time each year so that dredging would never be needed. A “drawdown flush” similar to the one used by the U.S. Army Corps of Engineers at Fall Creek Reservoir is the most cost-effective and beneficial method of dam sedimentation management. This replacement work would trigger the requirement for fish passage and screens, but would also remove the last man-made barrier to fish passage below Wickiup.
This summer the state engineer will perform a dam breach inundation analysis and a downstream survey to determine if a high hazard rating is appropriate. In addition, pursuant to the recommendations of the 2015 OWRD routine inspection, there may also be “additional analysis of dam stability” due to the dam failure in 2018 and the addition of sheet-pile across the entire face of the dam.
PacifiCorp’s chief dam safety engineer reported in 2013 the need for extensive monitoring of dam deformation and biannual inspections by its dam safety engineer. The report described the deterioration of internal components leading to the loss of rock-fill.
Gannett Fleming Inc., contracted by the Bend Park & Recreation District in 2014, concluded, “The types of temporary repairs currently being implemented address only local seepage outbreaks and do not appear to address the overall long-term structural integrity of the structure.” Additionally, “for continued reliable and safe operation of the facility, it is recommended that the timber crib portion of the dam be completely rehabilitated or replaced.”
So why are we still talking about dredging the pond behind a failing dam? Saving Mirror Pond will require a holistic plan that includes all the components of the pond and all the stakeholders. The rush to dredging is a distraction from the collaboration required to save Mirror Pond. A vote to obtain a legitimate community consensus is needed to justify the expense of a realistic solution.
— Mark Davis is retired from the U.S. Army Corps of Engineers.
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