Case Study: Protecting a Dock System from Boat Wakes

Case Study:
Protecting a Dock System from Boat Wakes

Marina Dock Age January/February 2005
By Anna Ossanna

Bay Points Village new wave attenuator system meets
stringent environmental guidelines set by local, state, and federal requirements.

Missouri's Lake of the Ozarks is a snaking, 92-mmile long manmade lake that has undergone extensive development. It offers residents some breathtaking views of wooded foothills and open water. But amidst this idyllic setting rests some wave problems that are similar to those experienced by marinas across the country.

Sometimes the wave action at Bay Point Village Condominiums on Lake of the Ozarks was so rough that mooring cables broke loose and docks would swing into each other. Boaters struggled through choppy wakes to steer their boats into the slips of the dock system at Bay Point Village Condominiums. A combination of spring storms and three-foot wakes stirred by passing offshore-style cruisers were the culprits behind these difficulties, with the real problem being the type of boats on the lake and their size, according to Ron Nelson, a director on the condominium association's board. "These wakes caused the most damage to the waterfront property," Nelson said.

Though Bay Point Village, which has 155 slips, is not so much a marina as a collectively-owned dock system, this condo association experiences the boating industry's trend toward larger crafts much like any commercial marina. In fact, this trend toward bigger boats prompted many marinas and dock owners to increase their slip sizes. While 60' slips may be an alluring addition to a dock system, it is not without problems. This fact was painfully driven home at Bay Point Village after a holiday weekend of heavy boat traffic.

The Problem
On this national holiday weekend, Bay Point Village's wave break sank under the barrage of boat wakes. This was unexpected, according to Ron Nelson, because this wave protection device wasn't that old and, a the time of its installation, was properly engineered for Bay Point Village's site conditions. But, as Nelson explained, "The lake is an evolving thing."

Years ago, the largest wake on the Lake of the Ozarks came from a 25-foot powerboat towing a water skier. "Today, the ones that really roll the breakers are the big cruisers," Nelson said. "A 30-, 40-,50-foot Carver draws about two feet of water. Within the time that wake rolls off, two or three more go by." Three-foot wakes have become commonplace on Lake of the Ozarks, and when these waves combined, they spelt trouble for the dock owners.

Concerned for boaters' safety and dock repair cost that were rising an average of 15% per year, Nelson called the agency overseeing the lake for advice. The 58,000-acre Lake of the Ozarks was created in 1931 when Union Electric constructed Bagnell Dam as part of the Great Osage River Project.

Today, AmerenUE, the local energy provider, regulates the lake's use. The company told Nelson that if Bay Point Village wanted to install a new wave attenuator, it would need a system designed to meet both AmerenUE's guidelines, as well as the stringent environmental and safety requirements set by local, state, and federal regulators.

Criteria
Given these parameters, Bay Point Village looked at wave attenuators from a new perspective. On the local scene, for example, the Lake of the Ozark's water level rises and falls an average of seven feet annually, fluctuating with changes in rainfall and demand for hydroelectric power. Thus, any wave attenuator installed by Bay Point Village had to withstand emergency drawndown levels. For Bay Point Village, this meant finding a wave attenuator that could be adjusted vertically as much as 10 feet.

The new wave attenuator had to be both strong and sturdy, but also aesthetically pleasing to the residents. Nelson and the other members of the Bay Point Village's Board of Directors wanting to see "proven results" sought out other docks owners on the lake who had installed wave attenuator systems.

Being ont he Lake of the Ozarks, the group had easy access to a wide variety of technologies. Because of its severe wave action, Lake of the Ozarks has been described as the New York of waterfront equipment manufacturing: if you can make it there, you'll make it anywhere.

After many phone calls and site visits, the Board decided to replace its sunken wave break with a floating concrete wave attenuator they'd witnessed in action, a ShoreMaster 850.

The wave attenuator on Missouri's Lake of the Ozarks stretches more than 1,300 feet
and is connected to concrete blocks on the bottom of the reservoir.

The solution
In selecting the wave attenuator, the Board looked at strength and reliability as the two major considerations, especially in light of what happened to the previous wave break. The Board chose ShoreMaster's concrete floating wave attenuator, even though the attenuator differed from conventional models in several ways. Conventional systems rely on wood whalers and through-rods to connect the attenuator's concrete pontoons, placing considerable stress on both the wood and concrete. ShoreMaster uses hydraulically post-tensioned cables running through tubes molded inside the concrete bridge design, this system minimizes the amount of forced transferred to the concrete, which in turn minimizes the stress fractures that can undermine a concrete wave attenuator's structural integrity.

Before installing Bay Point Village's wave attenuation system, engineers from ShoreMaster performed a thorough site analysis. Wind and wave analyses are vital to an attenuator's success, because even within a single owner's water space, an engineer may find significant differences in wind exposure, underwater currents, or boat traffic patterns. Because Bay Point Villages's approximately 1,500-foot shoreline is so exposed, the site would require a wave attenuation system of considerable size.

In spite of this extensive shoreline, the Board of the Directors was initially limited to installing just two 50-foot units of the floating concrete wave attenuator. Association by-laws allowed them to replace broken equipment, but new expenditures would require approval from the 133-member condominium association.

"After purchasing those two sections, the homeowners saw the [attenuator's] results," Nelson said. Association members also visited installations around the lake, and in the end, voted in favor of installing the wave attenuator. "We had a harder time permitting than we did with the homeowners," Nelson noted. With the association's support, Bay Point Village planned an addition of 24 concrete pontoons, which would bring the wave attenuator system's total length to 1,300 feet.

Design factors
In designing Bay Point Village's wave attenuator system, ShoreMaster engineers faced a number of challenges. Doug Pluth, chief engineer of ShoreMaster's GalvaFoam Division, described the design process as a balancing act between shoreline protection requirements, permitting regulations, and the owners' desire for an aesthetically pleasing wave attenuation system. Because the local regulations disallow attenuators constructed as a continuous barricade, Pluth broke up the modular attenuator system's pontoons into smaller groups, providing an approvable yet protective configuration, which also improving the boater's access to the lake. Further, Pluth explained, "If Bay Point Villages makes changes to their dock system, the modular design can be reconfigured and reanchored according to their needs."

Lakeside condominium owners won't trade their view for anything, least of all a wave break. With its low profile, the ShoreMaster attenuator maintains Bay Point Village's unobstructed vistas. But because the system is so unobtrusive, the water patrol was concerned that the attenuators might become a nighttime navigation hazard. The manufacturer alleviated this concern by attaching four-foot solar-powered blinking lights on the structure, creating a compromise that satisfied both homeowners and regulators.

To accommodate the lake's potential 10-foot water level fluctuation, ShoreMaster engineers utilized a self-adjusting rope-and-chain anchoring system. Situated in approximately 50 feet of water, the wave attenuator is connected to the concrete blocks resting on the bottom of the reservoir. Project engineers used a four-to-one scope for the anchor line, which substantially reduced the amount of change in the anchor line's length as the water rises and fall. With this anchoring system in place, Bay Point Village's wave attenuator will require no adjustments in the emergency drawndown of the lake.

The results
In addition to this user-friendly anchoring system, the wave attenuator itself requires very little maintenance. Unlike conventional construction, which requires hundreds of nuts and bolts requiring regular attenuation, ShoreMaster's "Tested by the North Sea" post-tension connection system uses a streamlined design that eliminates the need for such time-intensive tasks. Since the attenuator system's completion in July 2004, Bay Point Village has seen a significant reduction in waterfront repairs and maintenance for their now protected docks. Joe George, president of SunDown Property Management, said that in 2004 Bay Point Village homeowners saved more than 60% in dock system maintenance expenditures compared with the previous year.

As for the system's performance, Ron Nelson said, "The proof's in the pudding." With a new wave attenuator system in place, Bay Point Village enjoys the sounds of the lake, not the dock system.

Anna Ossanna is a free-lance writer who lives in the Twin Cities of Minneapolis/St.Paul, Minnesota. She can be reached at 651-238-8443.