Making waves

Johnson Lake wake study aims to prevent erosion of riverbanks

Posted: Thursday, July 27, 2000

The quiet waters of Johnson Lake, near Kasilof, are being shaken and stirred by boats this week.

A 20-foot Willie Predator with a 50-horsepower motor roars past, sending a wake rolling toward shore. But this isn't just a boatload of weekenders out to have fun, and the wake it produces is being carefully monitored and measured by a team of specialists on shore.

It is all part of a comprehensive study on boat wakes, which ultimately will be used to help prevent riverbank erosion on the Kenai River. The study is being conducted by staff from the United States Army Corps of Engineers' Waterways Experiment Station in Vicksburg, Miss., in cooperation with Alaska State Parks, the Alaska Department of Fish and Game and the U.S. Geological Survey.

It is led by Dr. Steve Maynord, a noted expert in the field. Maynord has studied boat wakes for the Corps of Engineers for 28 years in many types of situations, but this is the first such study he has done in Alaska.

The boat passes three gauges set up in a line on poles, which measure the height and magnitude of the boat's wake at various distances from shore. A fourth gauge measures the angle at which it hits the bank, and a radar gun measures wake speed.

Wires connect the gauges to computers set up in a wall tent on shore, where Wallace Guy, another staff member from Vicksburg, watches the results come in. These take the form of graphs, with a smooth ripple representing the natural movement of the lake's surface, interrupted by a sudden crash of sharp peaks and valleys which represent the wake.

A different graph comes up for each gauge, and Guy compares them.

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

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 conduct tests on Johnson Lake until Tuesday, when further tests will be done on the Kenai River. Dates for the river tests were chosen to be between the end of the open season for king salmon on Monday, and the start of the season for coho on Aug. 4.

It is hoped that there will be fewer boats on the river on those days so there is less interference with the tests. Results from both lake and river will be compared for the final analysis.

Four different boats are being used in the tests -- the 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 on these craft will be a 50 horsepower and 50 horsepower de-tuned to 35 horsepower on the 20-foot boats, and 40 horsepower and 40 horsepower de-tuned to 35 horsepower on the 16-foot boats.

These sizes are representative of typical boats used on the Kenai, said Degernes.

A de-tuned motor is one that has been altered with a throttle restricter to keep the motor from running at full volume, thus lowering the actual horsepower. Motors larger than 35 horsepower were banned on the Kenai in 1987. A lot of boats on the river have de-tuned motors because of this restriction, Degernes said.

"People just keep pushing the limit. We have bigger boats on the river now than in 1987, too. "

To measure passenger loads, sandbags are used to simulate the weight of real people, 165 pounds of sandbags for each "passenger."

"I don't know how many 165-pound anglers we have on the Kenai," Degernes said. "But that is the national standard used by the Coast Guard."

Each boat will be 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 all possible combinations of boats, motors and passenger loads.

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

The study was first recommended by the Kenai River Special Management Area advisory board in 1998, after several public meetings were held on the Kenai River Comprehensive Management Plan in 1996-97. The advisory board consists of representatives from the Department of Natural Resources, the U.S. Forest Service, U.S. Fish and Wildlife Service, the Department of Environmental Conservation, Alaska Department of Fish and Game, the cities of Kenai and Soldotna and the Kenai Peninsula Borough, as well as nine citizen members.

The board secured funding for the project, from Fish and Game and DNR, with matching funds coming from the Army Corps of Engineers.

A previous study done on the Kenai River in 1996, by the USGS, laid the groundwork for the current effort. In the 1996 study, erosion was measured by stakes in the riverbank, with the highest levels of erosion 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.

The study proved that boat wakes cause bank erosion. The new study will attempt to define just what sizes of boat, motor and passenger weight cause the largest wakes, and how they are 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, it appears to be true. A boat with six passengers consistently makes a larger wake than one with three, according to the tests that already have been run, Maynord said. "But there's no definite conclusions yet."

More research will be done in the future after this study is completed. The next study will investigate soil, vegetation and other components of Kenai riverbanks and their susceptibility to erosion.

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