University Park, Pa. – A "No Wake Zone" may be better than a speed limit to prevent the pollution and water quality problems that can occur when pleasure boats stir up a lake bottom, a Penn State study has shown.
Dr. David Hill, assistant professor of civil and environmental engineering, says, "One might think that putting in a sign in shallow water that says, '8 mph’ would be a good way to prevent turbulence from prop wash that can stir up shallow lake bottoms. However, our study shows that imposing a uniform speed limit can lead to significantly different impacts for boats of different size.
"In addition, we found that at between 6 and 8 mph, in waters shallower than 6 to 8 feet, there is maximum potential for prop wash to stir up lake sediments. So, an 8 mph speed limit could aggravate rather than reduce turbulence problems."
Hill will present his results, Friday, Nov. 9, at the North American Lake Management Society Symposium in Madison, Wis., in a paper, "The Hydrodynamic Impacts of Recreational Watercraft on Shallow Lakes." Michele Beachler, a master of science candidate in civil and environmental engineering, is co-author. The study is part of her master’s thesis.
Hill and Beachler conducted their study at two lakes in northern Wisconsin used by recreational boaters including, water skiers, fishermen and personal watercraft fans. The two Penn State engineers instrumented the lake bottoms at depths from 3 to 7 feet with an acoustic Doppler velocimenter to measure the water velocity induced by passing boats and an optical backscatter sensor to measure the turbidity of the water. Then they passed different watercraft, including inboard and outboard boats, at different speeds and different depths over the instrumented lakebeds.
"We did not see much impact from personal watercraft in water depths greater than 3 to 4 feet," Hill says. "There was not a big difference between inboard and outboard boats, either."
With water skiing boats, including a 16-foot, 150-horsepower outboard and a 19-foot, 275-horsepower inboard, the Penn State engineers found that at very low speeds, as well as at very high speeds, there was little impact. However, at speeds near 6 to 8 mph, where the boat was "near plane" or close to skimming the water, there was maximum potential to stir up the lake bottom.
Using the data from the study, Hill and Beachler have developed a computer program that can predict the water velocity at the lake bottom at different boat speeds and water depths. They hope to produce guidelines that can be used by lake managers to decide what speeds can be allowed in shallow parts of a lake.
The Penn State engineer says previous studies by other researchers have shown that stirring up the sediments on a lake bottom can cause less light to get to aquatic plants growing there and adversely affect them. Water clarity also affects water temperature as well as quality and has impacts on human lake users as well as wildlife. In addition, stirring sediments can lead to increased levels of nutrients and contaminants in the water and allow them to be transported to other regions of a lake.
Hill points out that the study also holds important implications for commercial boats, for example, ferries. He notes that ferries often leave their propellers turning while docked which could cause turbulence that could stir up a lakebed.