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Researchers study wakes to battle Kenai erosion

Posted: Friday, July 28, 2000

KENAI (AP) -- Researchers are stirring the quiet waters of Johnson Lake near Kasilof with fishing boats this week as part of a wake study by the Army Corps of Engineers that's intended to help fight erosion on the Kenai River.

Test boats pass three gauges that measure the wake at various distances from shore. A fourth gauge measures the angle when it hits the bank, and a radar gun measures wake speed.

The gauges report to computers in a wall tent on shore, where Wallace Guy watches the results. The resulting graphs show a smooth ripple, the natural movement of the lake's surface, interrupted by sharp peaks and valleys when the wake rolls in.

''You can see that the wake is stronger close to the boat, and decreases as it moves away,'' he said.

Steve Maynord, who is directing the study, pulls up a set of figures on another monitor and calculates the height of the wake.

''That one was about 10 1/2 inches,'' he said. Maynord has been studying wakes for the corps for 28 years.

Chris Degernes, Kenai area superintendent for Alaska State Parks, also monitored the tests.

''This is intended to be a Kenai river study,'' she said. ''Johnson Lake was chosen to start with because it is a more controlled environment, with no river currents or other boat traffic to interfere with the tests.''

The team will be on Johnson Lake until Tuesday, then move to the Kenai River.

Four boats are being used -- a semi-V Willie Predator and another 20-foot boat with a flat bottom, and two 16-footers, also with semi-V and flat bottom. Motors for the 20-footers are a 50 horsepower and a 50 de-tuned to 35 horsepower. The 16-foot boats are powered by a 40 horsepower and a 40 de-tuned to 35 horses.

That's representative of boats used on the Kenai, said Degernes. Motors larger than 35 hp were banned on the Kenai in 1987. A lot of boats on the river have de-tuned motors because of this restriction, Degernes said.

Sandbags are used to simulate passenger loads, with 165 pounds of sandbags for each one.

The boats are tested for loads of two to six people to determine the effects of weight on their wakes. A total of 492 separate tests are planned, using the various combinations of boats, motors and loads.

''We have to repeat the same test enough times so that it's statistically sound,'' Maynord said.

The study was recommended by the Kenai River Special Management Area advisory board in 1998 after several hearings on the Kenai River Comprehensive Management Plan.

That board secured research money from the state Department of Fish and Game and the Department of Natural Resources. Matching funds came from the Corps of Engineers.

A 1996 study on the Kenai by the U.S. Geological Survey measured erosion using stakes in the riverbank. Heaviest erosion was found at Big Eddy and the Kenai Keys, areas which also had the highest boat traffic -- up to 1,100 boats per day at Big Eddy.

That study showed boat wakes cause bank erosion. The new study will attempt to define just what combinations of boat, motor and passengers cause the largest wakes, and how they're affected by the boat's speed and distance from shore.

''There's been a lot of statements made over the years that heavily loaded boats close to shore cause more erosion,'' said Degernes. ''This will help quantify those statements, and prove whether or not that's true.''

So far, results bear out the assumption. A boat with six passengers consistently makes a larger wake than one with three in the 45 tests already run, according to Maynord.

More research will be done. The next study will investigate soil, vegetation and other components of Kenai riverbanks and their susceptibility to erosion.



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