Report fingers groundwater as Kenai bluff erosion culprit

Corps study finds wind, waves don’t help, but seepage big issue

Posted: Monday, September 18, 2006

A recently completed Army Corps of Engineers technical report on the Kenai River bluff indicates it is stable, but its slope face is susceptible to sloughing.

“Although the bluff is receding, geotechnical analyses indicate that the slope is stable and that massive slope failures are not contributing factors to the erosion,” the report states.

“Both the sand and clay slope faces, however, are susceptible to surface raveling, sloughing and wind and water erosion,” it adds.

The technical report, which focuses on environmental impacts of possible bluff stabilization efforts, is only one part of the study process for the bluff project, according to Kenai City Manager Rick Koch.

“We’re moving forward with the feasibility and design phase,” he said.

The corps said a number of things contribute to bank erosion at the mouth of the Kenai River, including wind, waves, foot traffic, overland drainage and river currents, but the primary contributor is groundwater seepage out of the bank face.

“Further studies and bank stabilization project designs should first address ground-water seepage,” the report states.

Koch said $500,000 has already been expended on the study and a second half million dollars is funding the study now.

“It’s my understanding Sen. (Lisa) Murkowski has put in a $1.5 million appropriations bill (for the project) this fall,” Koch said.

In his opinion, the most technically feasible method for keeping groundwater seepage from eroding the bluff would be using a “horizontal collection gallery.”

“Essentially it’s a culvert with holes in it ... that water can run into,” he said.

The groundwater would then be channeled into pipes leading down through the bluff for eventual discharge into the river.

At the river’s edge, the bank would be stabilized using armor rock — essentially large rocks piled on the shore — and higher up the slope of the bluff would be cut to a shallower grade and be revegetated in the prevailing preferred design.

That design was conceptualized by Peratrovich, Nottingham and Drage Inc. in February 2002.

“If the Peratrovich plan is feasible in terms of cost-effectiveness, I prefer armor rock to sheet pile,” said Koch, explaining a second alternative could be to drive steel pilings down along the bluff face to stop the erosion.

The area under study stretches from Erik Hansen Scout Park to the property line of Pacific Star Seafoods Inc., the first seafood processing plant on the right side of Bridge Access Road heading out of Kenai.

Environmentally, the stabilization project is expected to cause direct wildlife habitat loss from construction in the intertidal area, according to the corps report, and a potential loss of nesting habitat for swallows if the bank grade is altered.

Change in the grade also would result in the removal of spruce trees on top of the bluff, commonly used by eagles as perches.

When subsequently revegetated, the slope is likely to provide some bird habitat, the report said.

Impacts to the sand dunes from an erosion control project are expected to be minor. Armor rock placed along the bank would decrease the amount of sediment entering the riparian system, but the amount is small compared to the overall amount of sediment contributed from other sources in the river, according to the report.

Koch said if the funding appropriation makes it through Congress this fall, project design work could continue and be completed along with permitting in 18 months.

“The earliest construction could begin would be the 2009 or 2010 construction season,” Koch said.

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