Corps Wants to Dredge Valley Channelby Eric Barker
Lewiston Tribune, December 14, 2012
The U.S. Army Corps of Engineers says it won't raise Lewiston's levees in the near term to guard against long-term flood risks caused by the accumulation of sediment in the slackwater of the Snake River.
But the agency is calling for dredging at the ports of Lewiston and Clarkston and in the shipping channel of the lower Snake and Clearwater rivers, according to a draft of the agency's Programmatic Sediment Management Plan and environmental impact statement.
The document that exceeds 1,000 pages is slated to be released today and be followed by a 55-day public comment period that begins next Friday. The agency has spent more than $16 million to prepare the plan that outlines a long-term strategy to deal with sediment accumulating in the slackwater behind the four lower Snake River dams.
When normal flows are slowed by the dams, suspended sand and silt drops out of the Snake and Clearwater rivers and clogs shipping channels and recreation sites, and robs the rivers of their ability to contain floodwater. The corps has been working on the plan to deal with the problem since 2005 and at one time it looked as though the agency would call for raising Lewiston's levees by about 3 feet. Such a move would guard against floodwaters topping the levee but is strongly opposed by community leaders who say higher levees would further cut off Lewiston from its historic waterfront.
Richard Turner, project leader for the corps at Walla Walla, said studies and mapping that detail how sediment accumulates and moves through the upper end of Lower Granite Reservoir show there has been no net accumulation of material in recent years and the agency feels no flood control action needs to be taken for the foreseeable future.
"It is at an acceptable level right now," he said. "We do not have to raise levees based on the conditions now."
That does not mean levees won't be raised in the future. The plan retains levee raising as a potential tool. It also points to dredging to protect Lewiston and
Clarkston if future sediment accumulation continues to rob the river of its ability to hold high flows. Corps officials will monitor sediment levels and determine the best action to take if and when flood risk becomes a concern.
"That is what this plan does, provide tools for taking action," said Sandy Sheline, environmental coordinator for the corps' Walla Walla district.
Lewiston Mayor Kevin Poole said dredging should be the first tool used to alleviate flood worries, and other actions should be favored over levee raising.
"Lewiston and Clarkston both always had a connection to the river," he said. "To put that barrier there to me just doesn't make sense when there are some other things they can do engineering-wise to handle the flood hydraulics."
Some salmon advocates have argued sediment reduction and actions like levee raising would not be needed if the dams were breached to recover threatened and endangered runs of wild salmon and steelhead. They saw a scenario where demand by the corps to raise levees might bring more residents of the port town to back dam breaching. That scenario now seems unlikely to materialize.
The plan calls for dredging the shipping channel at the confluence of the Snake and Clearwater rivers, the ports of Lewiston and Clarkston and the approach to Ice Harbor Dam as soon as next winter. Port of Lewiston Manager David Doeringsfeld welcomed that news and said shallow water in the port's berthing area is causing grain barge loading delays.
Congress has authorized the shipping channel to be maintained at a depth of 14 feet. But Doeringsfeld said the channel is as shallow as 12 feet in some places. Routine dredging has not occurred since the winter of 2005-2006.
"We will all be very pleased if we will be able to undertake dredging in the winter of 2013," he said.
The plan calls for removing about 3,000 cubic yards of sediment from the Port of Lewiston, more than 10,000 cubic yards from the Port of Clarkston and 407,000 cubic yards from the shipping channel. The spoils would be disposed downstream near Knoxway Canyon and used to create habitat for juvenile salmon and steelhead.
The plan's preferred alternative calls for a mix of tools to deal with long-term sediment problems. They include dredging, reservoir drawdowns, levee raising, relocation or reconfiguration of riverside facilities, the use of weirs and dikes to trap sediment, agitating the riverbed to stir up sediment so it can be transported downstream by river flows and erosion control. It was chosen over an alternative that called dredging the primary tool to deal with sediment accumulation.
The draft plan is available for viewing at 1.usa.gov/TRtk6p.
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