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Boat wake study released

No conclusions being drawn despite plethora of data

Posted: Tuesday, April 09, 2002

To what extent is boat traffic eroding the banks of the Kenai River?

So far, the only answers to that question have come in the form of either anecdotal evidence from property owners and other user groups or piles of data compiled through government studies.

Last December, the U.S. Army Corps of Engineers added to that pile of data when it released "Boat Waves on Johnson Lake and Kenai River, Alaska," a 149-page study detailing the effects of various boat types, loads and motors on the size and energy of waves produced.

The study was suggested by the Kenai River Special Management Area Advisory Board in response to public demand to further study the issue. The Corps study followed a 1996 study done by the U.S. Geological Survey, which indicated boat wakes are indeed responsible for increased erosion.

The 1996 study found that as much as 80 percent of the total energy dissipated against stream banks on the river during peak traffic periods was the result of boat wakes. The Corps study was done to find out exactly which types of boats were likely to cause the most damage.

The KRSMA Advisory Board has been instrumental in gathering and interpreting the data that has been coming in. Last month, the group heard from Gordon Nelson, district chief for the USGS in Anchorage. The USGS has been approached by the KRSMA board to complete the next phase of the study, which would determine the types of soils and bank characteristics of the river, and attempt to correlate that information with the data already gathered.

Nelson told the board that if it agrees to support the next phase, the USGS would focus more closely on the actual, rather than theoretical, impacts of boat wakes on the river bank.

"We're going to study the heck out of one section of river," Nelson said.

He said the USGS planned to bring in several pieces of high-tech equipment, including video cameras and side scanning sonar to map and study the riverbank in detail.

Plans for the next phase have yet to be finalized.

Nelson said Thursday from Anchorage that he had been in discussion with the KRSMA board but no agreement had been reached to start work on the next part of the study. He said he hopes to meet with KRSMA board members sometime in the next two weeks to determine whether the project will go forward.

The earlier Corps study was conducted by staff from the Corps of Engineers' Waterways Experiment Station in Vicksburg, Miss., in cooperation with Alaska State Parks, the Alaska Department of Fish and Game and the USGS.

The study focused primarily on determining what kinds of boats and loads produced the largest wakes, and, thus, more pressure on stream banks. For the study, the Corps compared the effects of two kinds of 20- and 16-foot boats, V-hulled and flat-bottomed. Although the study stopped short of drawing a direct correlation between larger boats and increased erosion, the evidence strongly points in that direction.

For example, the Corps found that boats with V-hulls produce larger wakes than flat-bottomed boats. In fact, the report concluded that hull type caused the most variation in wake size and energy, and that larger, V-hulled boats loaded with more passengers caused the most disturbance.

When coupled with the 1996 report by the USGS, the Corps report indicates that greater levels of erosion on the Kenai River are caused by V-hulled boats than by flat-bottomed boats. What is still not known is how different parts of the river are being affected and what areas are most sensitive to damage from boat wakes.

That's what Nelson plans to do if the next part of the study is done.

"We're also going to look at the river channel in excruciating detail," he said.

Nelson said the study also would look at various bank profiles along the river. This data will be used to further enhance the understanding of how wakes in various parts of the river affect erosion.

The reason the KRSMA board called upon Nelson to further study the problem is that the previous two studies have not yet shown a direct correlation between various wave strengths, boat sizes and vegetation and soil conditions.

Nelson said his study may not be able to achieve that goal, but it will certainly add to the knowledge people have about the situation.

"We're going to have a large amount of data. What do we do with it? We don't know. What we'll have is a good string of documentation, a good string of evidence," Nelson said.

Not everyone is pleased with the amount of time it's taken to come to some definitive conclusions about boat wakes and their effects on the river.

"I'm wondering where we're going," board vice president Brett Huber said.

Board member Paul Shadura agreed.

"It's disheartening that there's no conclusive data. I'd say we need something more quantitative," he said.

The KRSMA board is hoping once all the data is in, new measures can be put in place to protect the bank.

"The hope was to get information on to distinguish high- vs. low-wave energies to tie to a management scheme," Huber said.

Nelson said he hopes to begin the next phase of the study sometime this spring, but since no agreement has been reached between the USGS and KRSMA, the plans are still up in the air. If the study goes forward, the USGS will do experiments and gather data throughout the summer, then compile and analyze the data next fall.



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