Four options for Mirror Pond
May 02, 2013
Officials consider dam removal, dredging plans, or taking no action
By Scott Hammers
The group responsible for finding a solution to silt buildup in Mirror Pond has settled on four options, and is expected to be ready to present the public with a look at the possibilities early next month.
Tuesday, the Mirror Pond Management Board picked its four options from a list of eight developed by GreenWorks, a Portland-based landscape architecture firm that has been studying the situation the last several months.
The options include doing nothing, allowing the pond to continue filling with silt but potentially damaging views, water quality and recreational opportunities. A dredging-heavy option similar to the dredging performed in 1984 is also on the list, as is a partial dredging, in which much of the sediment dredged up would be left on-site to create new areas of dry land. The final option calls for the removal of the Newport Avenue Dam — a choice which would require the cooperation of PacifiCorp, the dam's owner — and some in-stream work to manage existing sediment and possibly develop a fixed river channel.
Jim Figurski, a consultant working with the Bend Park & Recreation District to oversee the operations of the management board and the public outreach process, said he hopes to have detailed illustrations of what each of the alternatives might look like and rough cost estimates ready in time public open houses planned for June.
Though detailed cost estimates are still a few weeks out, GreenWorks offered management board some idea what it might cost to dredge and remove silt from the pond, placing the price at between $30 and $50 per cubic yard. The estimate, Figurski said, reflects the cost of vacuuming silt off the bottom, pumping it to a nearby location where the silt can be spread out and dried, and disposing of it.
At the estimated price, it would cost between $1.8 million and $3 million to duplicate the 1984 dredging of 60,000 cubic yards of silt, which was done for $312,000.
The pond is currently estimated to contain 380,000 cubic yards of silt, up from the 350,000 cubic yards estimated to be on the bottom prior to the 1984 dredging.
Figurski said dredging all of the sediment from Mirror Pond was never really on the table. As GreenWorks begins filling in the details of the dredging-oriented options, their goal will be to find the “sweet spot," Figurski said, how much sediment would need to be removed from where in the pond to put off additional dredging as long as possible.
Ryan Houston, director of the Upper Deschutes Watershed Council and a member of the management board, said figuring out how much to dredge — if dredging is to be the solution — is a tricky proposition.
The more sediment you dredge from a river, the slower the water will move, Houston said, and the slower the water moves, the more sediment falls out of suspension and begins piling up on the bottom.
“Taking out twice as much sediment will not necessarily get you twice as much time," Houston said.
The partial sediment removal option would use sediment dredged from the bottom to create shallows or dry land, Figurski said, most likely around the islands in the upper part of Mirror Pond or on the western Harmon Park side. Doing so should not alter the views enjoyed by any of the private property owners along the edge of the pond, he said.
However, the material — characterized as “goo" in the GreenWorks reports — would not stay in place without reinforcement, Figurski said. The option would likely involve bringing in large rocks to stabilize the artificial banks and hold the dredged material in place.
“It would be as natural looking as we could make it," he said. “There's no need for it to be a concrete-lined channel.
Houston said he finds the partial removal option intriguing, as the wetlands it could create could help neutralize contaminants that are currently being emptied into the bond through a series of storm drains.
Figurski said its difficult to know what might happen if the dam were removed, whether that's through the dam removal alternative on the management board's list, or as a result of future action by PacifiCorp.
Aerial photos show the river has established a channel that has remained reasonably consistent over the last several years, he said, but it's unclear if an un-dammed river would erode its way down to the underlying bedrock, or where that bedrock is.
Depending on the course chosen by an un-dammed river, it could be desirable to remove sediment currently on the bottom of the pond, or to create an artificial channel unlikely to change significantly over the longer term, Figurski said.
Each of the three options beyond the “do nothing" option are quite similar in terms of the regulatory hurdles that would need to be cleared to proceed, Figurski said.
Houston said none of the alternatives selected by him and his fellow board members were particularly surprising, and that the board could well have arrived at the same four choices months ago before the public outreach process began. However, board's consideration and rejection of other options — one of the final eight possibilities called for dam removal with no sediment management, another for the partial removal of the dam and the construction of stepped water terraces — should streamline the process from here forward, Houston said.
“The fact they were uncovered, they were brought to the surface, that at least means the probability of these coming up at the 11th hour and throwing a wrench in to the process, I think that probability is less," he said.
Sediment removal — Dredge and remove sediment from the pond, as was done in 1984. Estimates suggest there could be 380,000 cubic yards of sediment in Mirror Pond, above the 350,000 estimate developed prior to the removal of 60,000 cubic yards 29 years ago.
Do nothing — Allow the pond to continue filling with silt carried from upstream. No financial costs, but the pond's scenic attributes, water quality, and recreational opportunities could be diminished.
Partial sediment removal — Use sediment dredged from the bottom as fill, possibly expanding the island mid-pond or creating dry land adjacent to Harmon Park. Water could move faster, and artificial banks would have to be created to protect the newly-created land from erosion.
Dam removal — Requires PacifiCorp to agree to removal of Newport Avenue Dam, and the likely dredging of some sediment on the bottom of the pond. If the un-dammed river created significant erosion in undesirable locations, artificial banks could be constructed to create a channel.
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