It's a clear day at the lake. Waves lap at the shore as sailboats cast their colorful banners across the water. Serenity itself. Then the noise begins—the blast of motor boats, the roar of jet skis. They churn into view trailing choppy wakes.
The noise is one thing. But the turbulence is what owners of lakefront property are complaining about. They may have owned property for ten to 20 years, and over that period they've seen the degradation of the waterfront, says David Hill, an assistant professor of civil and environmental engineering at Penn State. It went from being a real nice, clear lake, to a green lake. They look at that, they look at the increase in boat use, and basically blame all the boats.
Hill, who grew up near a lake in Wisconsin, became interested in the problem after learning about the complaints from the Wisconsin Department of Natural Resources. Lake managers had responded with restrictions on boat type, size, and speed, but the problems hadn't gone away. So in July 2001, Hill and graduate student Michele Beachler traveled to Wisconsin to study the effects of boating in shallow water.
Stirring up lakebed sediment, Hill explains, can inhibit plant growth and kill fish and other organisms; it can also recirculate contaminants from pollution, such as heavy metals, that had been buried in the sediment. According to Hill, sandy-bottomed lakes are not much affected. Sand resettles very quickly. This is a bigger issue where you have bottoms that are characterized by really fine, soft organic mud, he says. If you disturb it, it gets swept up and takes a very long time to settle back. In shallow water the sediment travels directly to the surface, where it can affect swimmers, shoreline organisms, and the beauty of the water.
Hill and Beachler conducted a two-week experiment on two national forest lakes, Butternut Lake and Franklin Lake, each about a thousand acres in size with a maximum depth of 30 to 40 feet. They placed instruments to measure water velocity and turbidity on the bottom of each lake, then ran different types of boats over them at a wide range of speeds. Then we repeated the experiment at different depths, Hill says.
After testing boats with both outboard and inboard motors and personal watercraft (jet skis), they came up with a surprising result: At depths between two and seven feet, it was the boats cruising at medium speeds that caused the most damage.
Boats putting along at a very slow speed really have negligible impact at any depth, Hill explains. On the other hand, a boat traveling at high speed, 20 to 30 miles per hour, has already planed and is lifted up out of the water, significantly decreasing the waves it produces. But when the boat is traveling at medium speeds, just about to plane, it generates the most waves. There's this range of speeds in the middle where you have a very significant stirring of the bottom at surprising depths, Hill says. That range depends on a number of factors, he adds, including the size of the boat and the water's depth.
What this finding suggests is a need for more strategic regulations, Hill says. This idea of a blanket speed limit is probably not the best approach. A better approach is to impose a slow no wake' zone, where there's no speed limit. Regardless of the boat, your speed has to be sufficiently slow that it creates no waves behind the boat.
Hill and Beachler have created a computer model to help lake managers make such decisions. By plugging in a little bit of information about a boat, for a given water depth, Hill says, you'll be able to tell whether you are going to have disturbance at the bottom.
David F. Hill, Ph.D., is assistant professor of civil and environmental engineering in the College of Engineering, 231D Sackett Bldg., University Park, PA 16802; 814-863-7305; dfh4@psu.edu. Michele Beachler graduated in May 2002 with a master's degree in civil and environmental engineering. This study was funded in part by the U.S. Department of Interior. The results were presented in November 2001 at the North American Lake Management Society Symposium in Madison, WI. The paper, which Beachler co-authored, was part of her master's thesis.