To Calm Troubled Waters - How to choose the right floating dock wave attenuator

To Calm Troubled Waters -
How to choose the right floating dock wave attenuator

Marina Dock Age
January/February 2004
Written by: Anna Ossanna

A concrete wave attenuator such as the ShoreMaster Model 850 withstands winds of 65 mph.

The complaint that "all the good ones are taken" seems to hold true for marina sites, at least when it comes to waves. Beaches and waterfronts with natural protection were snatched up years ago, and with recreational boats growing in size, it's a rare sheltered cove that doesn't need some help in the struggle against damaging waves. Exacerbating the problem are today's strict regulations, which often frustrate marina owners who seek the right line of defense. In the search for protection that's tough on waves - but easy on the environment and community aesthetics - many have found a solution in floating wave attenuators. But how do we know which one works best with our particular situation?

Before selecting an attenuator, understand that no one should attempt to install a wave attenuation system without a thorough site analysis, including a wind and wave study. The success of any attenuator depends on how well the system is designed to meet the challenges of a specific environment. Second, floating wave attenuators will not be very effective in conditions where wave heights exceed four feet, or where wave periods - the time between wave crests - are greater than four seconds. Finally, be aware that no allowable wave protection system will completely block all waves. Thoroughly blocking wave movement can permanently alter the marine environment, so wave protection systems that promise 100 percent wave reduction are both expensive and strictly regulated.

How does a floating wave attenuator work?
The goal of a wave attenuator is not so much to block a wave, but to dissipate or refract the wave's surface with a fixed object attenuates, or lessens, the wave's horizontal motion. And because a wave travels in an elliptical motion that rotates to a depth of one-half its height, most attenuators also utilize a hanging vertical component to dissipate energy movement within the water column. As the horizontal and vertical, or draft, dimensions increase, so does the attenuator's ability to displace the wave's energy. Ultimately, the effectiveness of an attenuator is a product of its mass: The greater the mass, the greater the wave dissipation.

A case study
On Lake Lanier, north of Atlanta, Gainesville Marina sits on a small peninsula lined on two sides by covered and uncovered docks. While one side of the peninsula is relatively sheltered, the other side faces two miles of open fetch, where wind and considerable boat traffic combine to form three-foot waves. Because Lake Lanier is a reservoir where water levels are managed to prevent flooding downstream, Gainesville Marina needed a wave attenuation system that could adjust to the lake's fluctuation. And, like many marinas, Gainesville is planning for future expansion to better accommodate the growing population of larger boats. The owners hoped to find an attenuator that could perform multiple duties.

This heavy-duty breakwater, the ShoreMaster Model 850, stands six feet, six inches in height.

After a detailed site analysis, Gainesville Marina found that specialized concrete wave attenuator would appropriately tame the harbor's choppy waters. Though the heavy-duty attenuator is more commonly used in rough, saltwater applications, Gainesville's inland site experienced wave action requiring stronger protection than most freschwater attenuators could provide. At six feet, six inches in height, this concrete attenuator supplied the amount of wave damping that Gainesville Marina needed. Further, this system uses vertical "legs" to trap water inside the floating structure, thus increasing the attenuator's mass and, in turn, its effectiveness. To accommodate Lake Lanier's fluctuating water levels, a cambled anchoring system was designed. This included winches that allow the attenuator to be raised and lowered for optimum performance.

Lake Lanier's rough waves could easily undermine the integrity of some attenuators, requiring ample tools and fastners, and the need for regular attenuation. The staff at Gainesville, however, found that their investment, with its streamlined design, didn't need intensive maintenance. Modeled after suspension bridges, the concrete attenuator uses hydraulically post-tensioned cables to connect its 50-foot sections. This connection system creates the rigidity necessary to constrain the water's movement, while also allowing enough flexibility to prevent the stress fractures and increased maintenance common among attenuators subjected to taxing conditions.

Since the installation of this concrete attenuator, the marina has smoothed the waters and paved the way for an expanded dockage area. The Gainesville facility also has plans to connect the attenuator to existing docks, then build piers for additional moorage. This proves that the good marina sites aren't all taken: They can be created.

Author Anna Ossanna is a freelance writer with a degree from Iowa State University of Science and Technology.

Wave attenuator considerations

Site Conditions

What is the greatest wave height and frequency in this location?
From what direction(s) and source(s) do waves come?
What is the distance to opposite shore (or, how great is the open fetch)?
Will currents, sedimentation, or flushing be affected by the chosen installation?
How much boat traffic and what size boats will pass the attenuator? At what distance?
How much does this body of water fluctuate?
What are the worst annual conditions this site will experience? Will ice, snow, salinity, or high winds be a concern?

Function and versatility

Is it feasible to use this attenuator for multiple purpose, such as moorage?
Will pedestrians be allowed on this attenuator?
Could it double as a fishing pier?

Budget

Will this attenuator provide a financial return thanks to additional slip rental?
Will maintenance or insurance costs be reduced with the addition of an attenuator?

Finally...

What performance do you expect from the attenuation system?
How much are you willing to pay for?
What can you expect in return?

These questions are absolute musts in the selection process.