It's safe to say that marina tenants ap-preciate placid moorages, but many

of them have little or no idea how much

science and engineering goes into plan-ning and designing them. As a struc-tural engineer specializing in marinadesign for the last 20 years, I can tellyou that designing a tranquil marina isno easy task.

A smooth ride in a marina deliversmore thanjust a higher level of comfortto tenants and a less stressful berth fortheir boats. It can also turn a parking lotfor boats into a revenue-producing ma-chine. As the saying goes, "The moretranquil the marina basin, the morelikely the tenants will come to the ma-rina and use its services."

A good example of this saying can befound at Elliat Bay Marina, nestled inthe harbar in Seattle, Wash., with itspanaramic views of the cityscape, Mt.Rainier, and the Olympic Mountainsrange. The marina is so pleasant thatbusinessmen leave their office build-ings to lunch with clients and have adrink on their boats. What most .ofthesebusinessmen don't know is that it takesa combination of wave attenuator,wavewall, and rubble mound to protect themarina and provide this tranquil setting.

In addition to sheltering the marinabasin, wave attenuators provide otherrevenue benefits. Many marinas usethem for fuel docks and visitormoorage, as well as floating platfarmsfor bait shaps, graceries, and harbor-master on-dock servicebooths. Marinas

can even use the offshore side of many

22 MARINADOCKAGE, JANUARYIFEBRUARY2006

Floatingwave

attenuatorsprovidepeace,

protection,and

tranquilityonan

affordablebudget

By Craig Funston, P.E.

attenuatars for revenue generation. As adesigner, however, I don't recommendthem as permanent moorage. Whenthings get rough, they do move. Per-manent, rentable moorage should be in-side, protected by the wave attenuator.

Construction planningWhen selecting a wave attenuator in

terms of costs and benefits, marinastypically depend on the knowledge andexpertise .ofcansultants and manufac-turers. Everett Babbitt, president ofBellingham Marine, Bellingham,Wash.offers a manufacturer's perspective."Wave attenuatorsare generallyless ex-pensive than fixed structures such asrubble mound breakwaters," said Bab-bitt. "Like all components .ofa marina,we design them to be site specific.Fetch distances, wind speeds, waterdepths and an array of other factors willaffect the cast."

When determining the most cast-ef-fective attenuator, Babbitt stressed theimportance .ofhaving a skilled prafes-sianal analyze the marina site. "If aflaating attenuator looks like a possi-bility, then budgets can be farmulatedwith the preliminary engineering," hesaid. "This is well worth the effort, At-tenuatars have praven durable and suc-cessful far decades, .often with verylittle maintenance."

This marina in Oak Harbor, Wash. usesa castellated concretefloating waveattenuator developed especially for thesite by Bellingham Marine.

..

"A great example of this formulationis the city marina in Oak Harbor,Wash.," Babbitt continued. "Their at-tenuator creates wonderful public ac-cess and picnic areas and hosts one ofthe region's most popular sailboatracing venues."

Reflection VS. attenuationTaking the rock and roll out of a wavehas only two solutions. First, one canreflect the wave off of an impervious,immovable object. A rubble-moundbreakwater or a solid wall are the mostobvious examples of this method.

A floating concrete wave attenuatorperforms the same function for that partof the structure that fronts the wave.Today's concrete wave attenuators typ-ically have so much mass and inertiathat they appear solid to an incomingwave. In practice, the floating concretewave attenuator will reflect most of thewave energy in the same manner as afixed breakwater or wall.

A unique option far keeping the ma-rina placid is to install a recreational is-land. The city of Ft. Pierce, Fla. is

considering such a solution after itsbattle with Hurricane Frances. It's a

sure solution to the problem of longfetches in the Intracoastal Waterway.(See "Ft. Pierce Marina Tells a Storyof Hurricane Fury and Destruction,"MarinaDock Age, May/June 2005).

The second method is to let the wavepass-but dampen or suppress its en-ergy as much as possible as it passes.Some fixed wave walls are open atthe bottom for ecological purposes, andessentially act as wave attenuators.These walls have openings designedto prevent "dead" areas that interruptcurrent and tide flow, and where thewater can become anoxic (lacking inoxygen), which is obviously unhealthyto life.

Marinas considering rubble-moundbreakwaters and fixed walls (whethervented or not) will find that they areoften much more expensive than atten-uators; can't be easily moved in futureremodeling projects; are potentiallyharmful to the harbor's ecology; and re-quire the harbor to be reasonablyshallow at the installation point. For

many marinas, anyone of these issueswould point them toward a floatingwave attenuator.

Concrete attenuatorIt is important to note that the modemconcrete floating wave attenuator is nota wave eliminator.

The modem floating wave attenuatordesign works as both a reflector and anenergyattenuator(Fig. 1). For the partofthe wave energy from the top of thewave to the bottom of the wave attenu-

atar, the wave is reflected. The energybelow that point is dampened by themass and breadthof the wave attenuator.

But there are limits. In the final

StandardFloat

WaveAttenuator

--In"",,,,, depth \

Wave ,tteo..riO",;ml'" to, do"d box

Figure 1. Dock and WaveAttenuatorComparison.

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MARINADOCKAGE,JANUARYIFEBRUARY2006 23

analysis, wave attenuators can only re-duce the energy in a wave and act as ashock absorber to dampen its move-ment. In extreme conditions, which arerare except for open water situations,the wave length can be so long the waveattenuator moves in synch with thewave and there is no dampening at all.Given the semi-protectedwaters that aremost common for U.S. marinas, this ex-treme-case phenomenon is very un-likely.

Attenuator designSo how does one design wave attenua-tors for marinas? The first step is todetermine the worst-case storm eventthat can be expected in 15, 25, or 50years. Marinas can and should hireconsultants to collect and analyze me-teorological and topological data todetermine a worst-case event. This willbe an educated guess and will never beperfect, because we're dealing withnature.

But don't lose sight of the value ofsite-specific engineering. For 99.9% ofthe time, a well-engineered solution

will help most marinas survive stormsand protect the marina investment andits tenants.

To complete the engineering and de-sign, the marina must address severalquestions.What are the expected winds,and from where will they come? Howlong is the fetch? Do topographical orstructuralobstructionscreate reflections

and multiply the effects of the waves?How shall the wave attenuator be situ-ated relative to the threat? What arethe height and period of the waves?

With this wave information, engi-neers and scientists can apply a statis-tical approach to provide a usefulprediction of the conditions the newwave attenuator will perform against.From this analysis one can build a waveattenuator best suited to the site.

Energy below the seaMost marina managers tend to think ofa wave as extending from the trough tothe crest, but much of its energy ex-tends down into the sea-especiallywhen the bottom is closer than half thelength of the wave. For the purposes of

this article, the wave begins to interactwith the bottom at about 30 to 40 feet of

depth.Once the wave reaches shallow water,

the motion within the wave is elongatedfrom circular to elliptical. The waterbelow the surface becomes more agi-tated and the wave energy is more con-tinuous from the surface to the bottom.

Figs. 2 and 3 show this effect.

How they workWave attenuatorswork by reflecting thepart of the wave that fronts the wave at-tenuator. Therefore, the deeper thewave attenuator, the more effective itwill be. But there are limitations.

Installing very deep wave attenuatorscreates tremendous forces on the mari-na's mooring systems. Thus, engineerscompromise between depth and func-tion so that wave energy is allowed toflow beneath the wave attenuator.

So what happens to the energy be-neath the wave attenuator? Wave en-

ergy is dampened by the mass andinertia of the attenuator as the wavepasses under it. The wider the attenu-

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'J

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Figure 2. Wave energy diminishes withdepth in a deep water wave.

ator, the more energy it can reduce. Forwaves with a period greater than fourseconds, the width of wave attenuatorthat would be needed becomes imprac-tical for most applications.

Note: To get the desired effect it isn'tnecessary to have a solid bottom.Marinas can get the same results byplacing flanges or skirts front and back

Shallow Water Wave

, .:-.~',~..::.. :;:,.,': ,:~~'." '.~~.',.::,:,:~';~;

Figure 3. Once the wave "feels" thebottom, the wave energy descends downand does not taper off.

that extend the depth facing the waveto create a "box" open at the bottom(see Fig. 1). Tests have shown that anopen-bottom box works about as wellas a closed one, but is less expensive tomanufacture.

Six things to rememberWhen it comes to installing wave at-tenuators,marinas should remember sixthings:.Performance of a wave attenuator is

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.1

determined first by depth, then width.Deeper wave attenuators do a better jobof reflecting waves, but they also createpowerful forces on the mooring system.Wider wave attenuators create friction

that takes energy from the wave, but re-quire less rugged pilings or moorings..For waves with a period longer than

about four seconds, a floating wave at-tenuator may be impractical..In designing and constructing waveattenuators, marinas should avoid"hinged" connections, and note thatcontinuously-joined longer structureswork best..Be very selective about putting per-

manent moorage slips on floating waveattenuators because they do move instorms. Using the outside of a wave at-tenuator for transient moorage is fine,but not recommended during storms..Wave attenuators typically cost less

than the cost of a rubble-mound break-water or sea wall, and can be used indeep water..Wave attenuators are ecologicallyfriendly and can actually stimulate ad-ditional sea life in the marina because

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they attract vegetation and bivalves,which attract fish.

Beneficial resultsThe floating wave attenuator has madean important contribution to the marinaindustry. Here are two examples of thebenefits attained from wave attenuatorsat marinas.

Barb Herzog of Sunset Marina, KeyWest, Fla. credits the marina's wave at-tenuator with saving the facility fromextensive damages during HurricaneWilma (October 24, 2005).

The marina is situated in a protectedbasin, with the western opening ex-posed to the Gulf of Mexico. Two waveattenuators are located on each side ofthe entrance channel to protect the westside. The section closest to the marinaalso serves as the home base for a 115-ft. charter fishing boat that providesboth mooring revenue and protectionand wave attenuation for the basin.

"Wilma could have caused extensivedamage to the marina and all the boatsinside, but it didn't," Herzog said. "Thetwo wave attenuatorsdid theirjobs. The

windsreached 135mph, the stormsurgelifted the marina-side wave attenuatorto within a foot of the top of the pilings.When the surge subsided, it came backdown and everything was fine."

The wave attenuator on the other sideof entrance was severely damaged, butit also did its job, according to Herzog."The wave attenuator was lifted abovethe piling and is a mess," she said, "Butit saved the marina. We only had oneboat with cosmetic damage and thedocks were fine."

In Sausalito, Calif. Ken Pedersen,president of Clipper Yacht Co., which

manages a marina here, said the oldestpart of the marina (Basin Two) wouldn'tbe standing today without a properly de-signed and installed wave attenuator.

Pedersen said that Basin Two at the

marina is a fixed wooden dock mooragedecked with one-by-six planks. In 1997,an El Nino caused extensive damages atthe marina. The insurance money al-lowed the marina to put in a 480-ft.wave attenuator. "Without it [the waveattenuator], we wouldn't be able to keepthe marina open in Basin Two," saidPedersen. "The wave attenuator has

protected those old wooden docks allthis time as we continue to press for ap-proval of our remodeling project."

So, wave attenuators continue to im-

prove the lives of boaters everywhereand is clearly winning the recognition itdeserves. .j;.

Craig Funston is a senior principal ofRedpoint Structures, and has designedwave attenuatorsfor nearly 20 years.He can be reached at:

craig@redpointstructures.com.

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MARINADOCKAGE,JANUARYIFEBRUARY2006 27