SECTION 4: Management Measures

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4.1. MARINA FLUSHING

Management Measure Description

Water quality in a marina basin depends largely onhow well the basin is flushed, which depends inturn on how well water circulates within themarina. Studies have shown that adequateflushing improves water quality in marina basins,reduces or eliminates water stagnation, and helpsmaintain biological productivity and aestheticappeal. Flushing can reduce pollutant concentra-tions in a marina basin by anywhere from 70percent to almost 90 percent over a 24-hourperiod.1

When a single number (e.g., 10 days) is given asthe flushing time or residence time of a body ofwater (e.g., marina basin, harbor, or estuary), thisnumber represents an average and doesn’taccurately reflect what is happening inside themarina basin. Actually, flushing time in a marinabasin can range from zero days at the boundarywith the adjacent waterbody (at points of entryinto the marina basin) to as much as severalweeks within the marina basin at secludedlocations or where in-water structures preventwater from circulating.

In a poorly flushed marina, pollutants tend toconcentrate in the water and/or sediments.Pollutants and debris can collect in poorly flushedcorners or secluded or protected spots in thesame way that leaves collect in depressions inthe ground where they are protected fromwind. Stagnant, polluted water—with littlebiological activity, lifeless shorelines, andoffensive odors—can be the consequence.

In tidal waters, flushing is driven primarily bythe ebb and flow of the tide. A large tidalvolume relative to the total volume of a marinabasin provides excellent flushing because eachtidal exchange replaces a large amount of themarina basin water with “new” water fromoutside the marina basin. This condition is com-mon on coastal waters in northern New England,the Pacific Northwest, and Alaska, where tidalcirculation should adequately flush marinas.

In nontidal coastal waters, such as the GreatLakes, wind drives circulation in the wateradjacent to a marina. The circulating wateroutside a marina basin can have a flushing effecton water within the marina if the speed, persis-tence, and direction of the wind create a strongenough current. In many situations wind-drivencurrents can provide adequate flushing of marinabasins.

In river waters, with current flow, water usuallymoves into and out of the marina basin continu-ously unless the basin is built into the land or hasonly one small entrance channel.

The BMPs mentioned below are particularlyapplicable for incorporation into a marina’s designat new and expanding marinas. Marinas with poorwater quality that could be attributed to poorflushing might also benefit from using one or moreof the following BMPS, as appropriate. Entrancechannel design and wave protection structuresmust be designed with other factors in mind aswell. Adequate protection from wave energies,episodic storm currents, and ice floes andshoreline erosion protection must be consideredin the overall design strategy.

1 Cardwell and Koons, 1981; Tetra Tech, 1988.

Management Measure for Marina Flushing:

Site and design marinas such that tides and/or currents will aid in flushing ofthe site or renew its water regularly.

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National Management Measures Guidance

Applicability

This management measures primarily applies tonew and expanding marinas.

Best Management Practices

♦ Ensure that the bottom of the marina andthe entrance channels are not deeper thanadjacent navigable channels

Flushing rates in marinas can be improved andmaximized by proper design of entrance channelsand the basin. Areas with minimal or no tides orpoor circulation should have basin and channeldepths designed to gradually increase towardopen water to promote flushing.

Even where good flushing does occur, this alonedoes not guarantee that a marina’s deepestwaters will be renewed on a regular basis. Asmentioned previously, deep canals and depres-sions much deeper than adjacent waters might notbe adequately flushed by tidal action or wind-generated forces. Fine sediment and organicdebris will collect in them, and low dissolvedoxygen concentrations can result. In the warmermonths when dissolved oxygen concentrations arenormally low because of higher water tempera-tures, the even lower dissolved oxygen concentra-tions in these depressions can deteriorate waterquality and hinder biological activity in the water.

♦ Consider design alternatives in poorlyflushed waterbodies to enhance flushing.For example, consider

• An open design where a semienclosed designis not functional.

• Floating wave attenuators where fixedbreakwaters are not functional.

When selecting a marina site and developing adesign or when reconfiguring an existing marina,the need for efficient flushing of marina watersshould be a prime consideration.

Where a poorly flushed location is the only oneavailable or where a marina is already operationalin such a location, special arrangements may benecessary to ensure adequate flushing. Selectionof an open marina design may be considered.Open marina designs have no natural barriers torestrict the exchange of water between the largerwaterbody and the marina basin. To accommo-date both improved flushing and protection fromwave energy, floating wave attenuators can beuseful. Floating wave attenuators do not impedeflushing because water exchange is not restrictedby an underwater structure, yet the marina isprotected from limited wave action. Floating waveattenuators can provide effective protectionwhere waves do not usually exceed 3 feet, andopen area designs can be a viable alternativewhere they do not leave a marina exposed toexcessive wave action that could damage prop-erty and cause shoreline erosion.

♦ Design new marinas with as few enclosedwater sections or separated basins aspossible to promote circulation within theentire basin.

Overall flushing in a marina is a function of thenumber of separate basins in the marina. Amarina in open water generally flushes better thana one-basin marina; a one-section marina, insteadof square corners, can eliminate stagnant cornerwater and can help produce strong circulation in amarina basin. A marina in open water flushesbetter than a one-segment marina, a one-segmentmarina generally flushes better than a two-sectionmarina, and so forth (Figure 4-1). Curved corners,instead of square corners, can eliminate stagnantcorner water and can help produce strong circula-tion within a marina basin.

♦ Consider the value of entrance channels inpromoting flushing when designing orreconfiguring a marina.

There are situations where it may benecessary to have areas deeper than therest of the marina basin. For example,Cove Haven Marina (Rhode Island) ser-vices large 12-meter America’s Cupsailboats with deep keels and needssufficiently deep water in and adjacent tothe boat haul-out facility to do so. In thiscase, the state allows the marina tomaintain this site dredged deeper than therest of the marina (USEPA, 1996: CleanMarinas—Clear Value).

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The alignment and number of entrance channelsmay affect flushing, along with many other site-specific factors. The following points generallyhold true and should be considered when design-ing or reconfiguring a marina:

• Entrance channels that follow the naturalchannel alignment and have only gradualbends promote flushing.

• Where the tidal range is small, a widerentrance may promote flushing.

• Where the tidal range is large, a single narrowentrance channel may improve flushing.

• In tidal and nontidal waters, entrance chan-nels aligned parallel to the direction of prevail-ing winds or water flow might enhanceflushing.

The orientation and location of a solitary entrancemight affect marina flushing rates and should beconsidered along with other factors that affectflushing. Consider the following points:

• In a square or rectangular marina basin, asingle entrance at the center of a marina maypromote flushing better than a single corner-located asymmetric entrance.

• In a circular marina basin, an off-centerentrance channel might promote bettercirculation.

♦ Establish two openings at the most appro-priate locations within the marina to pro-mote flow-through currents.

Where water-level fluctuations are small (e.g.,nontidal waters), alternatives in addition to theones previously discussed can be considered toensure adequate water exchange and to increaseflushing rates. An elongated marina situatedparallel to a tidal river may be adequately flushedby using two entrances to promote a flow-throughcurrent. A small outlet onto an adjacentwaterbody can be opened solely to enhanceflushing (Figure 4-2). Buried pipelines have beensimilarly used to promote flushing.

♦ Consider mechanical aerators to improveflushing and water quality where basin andentrance channel configuration cannotprovide adequate flushing.

Where poor water quality throughout a marinabasin or in secluded spots is a problem because ofpoor flushing, limited circulation, or other circum-stances, mechanical aerators (such as those usedfor ice protection) might be helpful.

These devices can raise the level of dissolvedoxygen in the water and circulate floating debrisout of corners into the rest of the basin, where itcan be flushed out naturally. Underwater airbubblers or submerged impeller-type motors canbe effective during short-term episodes that mightoccur during the summer. In certain circum-stances, such as in shallow and enclosed waters,water clarity improvement is often noted ifartificial aeration is used.

Figure 4-1. Example marina designs.

Both compressed air and agitatorsystems work in fresh water, salt water,and brackish water. They do not workwell in ice-covered rivers because rivercurrents destroy bubble or flow patternsand because of the lack of heat.Thermal mixing of river water is anatural process, and a river that hasformed an ice cover has alreadydissipated nearly all available heat.

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National Management Measures Guidance

Ice suppression systems available for marinashinder ice formation by using compressed airbubblers or in-water agitators. Bubbler systemsforce air to entrain warmer bottom water into arising plume, which reacts with and melts theunderside of the ice sheet. Water agitators workon the basis of thermal reserves of basin watersand surface currents to prevent freezing.

BMP Summary Table 1 summarizes the BMPsfor Marina Flushing mentioned in this guidance.

Figure 4-2. Puerto Del Ray Marina (Puerto Rico) has anoffshore rubble mound breakwater that protects thesoutheastern and eastern exposures of the marina. Twohundred feet of the southern breakwater was removed,creating a new south side breachway exit/enterance thatis still well protected but now allows for greatercircualtion in the basin. Water clarity improved after thealteration, and as a result new customers (a 3 percentincrease for the marina) relocated to Puerto Del ReyMarina (USEPA, 1996: Clean Marinas—Clear Value).

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alternatives in poorly recommended HIGH; helpful practice promotes

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