SECTION 4: Management Measures

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4.1. MARINA FLUSHING

Management Measure Description

Water quality in a marina basin depends largely on

how well the basin is flushed, which depends in

turn on how well water circulates within the

marina. Studies have shown that adequate

flushing improves water quality in marina basins,

reduces or eliminates water stagnation, and helps

maintain biological productivity and aesthetic

appeal. Flushing can reduce pollutant concentrations in a marina basin by anywhere from 70

percent to almost 90 percent over a 24-hour

period.1

When a single number (e.g., 10 days) is given as

the flushing time or residence time of a body of

water (e.g., marina basin, harbor, or estuary), this

number represents an average and doesn’t

accurately reflect what is happening inside the

marina basin. Actually, flushing time in a marina

basin can range from zero days at the boundary

with the adjacent waterbody (at points of entry

into the marina basin) to as much as several

weeks within the marina basin at secluded

locations or where in-water structures prevent

water from circulating.

In a poorly flushed marina, pollutants tend to

concentrate in the water and/or sediments.

Pollutants and debris can collect in poorly flushed

corners or secluded or protected spots in the

same way that leaves collect in depressions in

the ground where they are protected from

wind. Stagnant, polluted water—with little

biological activity, lifeless shorelines, and

offensive odors—can be the consequence.

In tidal waters, flushing is driven primarily by

the ebb and flow of the tide. A large tidal

volume relative to the total volume of a marina

basin provides excellent flushing because each

tidal exchange replaces a large amount of the

marina basin water with “new” water from

outside the marina basin. This condition is common on coastal waters in northern New England,

the Pacific Northwest, and Alaska, where tidal

circulation should adequately flush marinas.

In nontidal coastal waters, such as the Great

Lakes, wind drives circulation in the water

adjacent to a marina. The circulating water

outside a marina basin can have a flushing effect

on water within the marina if the speed, persistence, and direction of the wind create a strong

enough current. In many situations wind-driven

currents can provide adequate flushing of marina

basins.

In river waters, with current flow, water usually

moves into and out of the marina basin continuously unless the basin is built into the land or has

only one small entrance channel.

The BMPs mentioned below are particularly

applicable for incorporation into a marina’s design

at new and expanding marinas. Marinas with poor

water quality that could be attributed to poor

flushing might also benefit from using one or more

of the following BMPS, as appropriate. Entrance

channel design and wave protection structures

must be designed with other factors in mind as

well. Adequate protection from wave energies,

episodic storm currents, and ice floes and

shoreline erosion protection must be considered

in 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 of

the site or renew its water regularly.
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National Management Measures Guidance

Applicability

This management measures primarily applies to

new and expanding marinas.

Best Management Practices

♦ Ensure that the bottom of the marina and

the entrance channels are not deeper than

adjacent navigable channels

Flushing rates in marinas can be improved and

maximized by proper design of entrance channels

and the basin. Areas with minimal or no tides or

poor circulation should have basin and channel

depths designed to gradually increase toward

open water to promote flushing.

Even where good flushing does occur, this alone

does not guarantee that a marina’s deepest

waters will be renewed on a regular basis. As

mentioned previously, deep canals and depressions much deeper than adjacent waters might not

be adequately flushed by tidal action or windgenerated forces. Fine sediment and organic

debris will collect in them, and low dissolved

oxygen concentrations can result. In the warmer

months when dissolved oxygen concentrations are

normally low because of higher water temperatures, the even lower dissolved oxygen concentrations in these depressions can deteriorate water

quality and hinder biological activity in the water.

♦ Consider design alternatives in poorly

flushed waterbodies to enhance flushing.

For example, consider

• An open design where a semienclosed design

is not functional.

• Floating wave attenuators where fixed

breakwaters are not functional.

When selecting a marina site and developing a

design or when reconfiguring an existing marina,

the need for efficient flushing of marina waters

should be a prime consideration.

Where a poorly flushed location is the only one

available or where a marina is already operational

in such a location, special arrangements may be

necessary to ensure adequate flushing. Selection

of an open marina design may be considered.

Open marina designs have no natural barriers to

restrict the exchange of water between the larger

waterbody and the marina basin. To accommodate both improved flushing and protection from

wave energy, floating wave attenuators can be

useful. Floating wave attenuators do not impede

flushing because water exchange is not restricted

by an underwater structure, yet the marina is

protected from limited wave action. Floating wave

attenuators can provide effective protection

where waves do not usually exceed 3 feet, and

open area designs can be a viable alternative

where they do not leave a marina exposed to

excessive wave action that could damage property and cause shoreline erosion.

♦ Design new marinas with as few enclosed

water sections or separated basins as

possible to promote circulation within the

entire basin.

Overall flushing in a marina is a function of the

number of separate basins in the marina. A

marina in open water generally flushes better than

a one-basin marina; a one-section marina, instead

of square corners, can eliminate stagnant corner

water and can help produce strong circulation in a

marina basin. A marina in open water flushes

better than a one-segment marina, a one-segment

marina generally flushes better than a two-section

marina, and so forth (Figure 4-1). Curved corners,

instead of square corners, can eliminate stagnant

corner water and can help produce strong circulation within a marina basin.

♦ Consider the value of entrance channels in

promoting flushing when designing or

reconfiguring a marina.

There are situations where it may be

necessary to have areas deeper than the

rest of the marina basin. For example,

Cove Haven Marina (Rhode Island) services large 12-meter America’s Cup

sailboats with deep keels and needs

sufficiently deep water in and adjacent to

the boat haul-out facility to do so. In this

case, the state allows the marina to

maintain this site dredged deeper than the

rest of the marina (USEPA, 1996: Clean

Marinas—Clear Value).
SECTION 4: Management Measures

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The alignment and number of entrance channels

may affect flushing, along with many other sitespecific factors. The following points generally

hold true and should be considered when designing or reconfiguring a marina:

• Entrance channels that follow the natural

channel alignment and have only gradual

bends promote flushing.

• Where the tidal range is small, a wider

entrance may promote flushing.

• Where the tidal range is large, a single narrow

entrance channel may improve flushing.

• In tidal and nontidal waters, entrance channels aligned parallel to the direction of prevailing winds or water flow might enhance

flushing.

The orientation and location of a solitary entrance

might affect marina flushing rates and should be

considered along with other factors that affect

flushing. Consider the following points:

• In a square or rectangular marina basin, a

single entrance at the center of a marina may

promote flushing better than a single cornerlocated asymmetric entrance.

• In a circular marina basin, an off-center

entrance channel might promote better

circulation.

♦ Establish two openings at the most appropriate locations within the marina to promote flow-through currents.

Where water-level fluctuations are small (e.g.,

nontidal waters), alternatives in addition to the

ones previously discussed can be considered to

ensure adequate water exchange and to increase

flushing rates. An elongated marina situated

parallel to a tidal river may be adequately flushed

by using two entrances to promote a flow-through

current. A small outlet onto an adjacent

waterbody can be opened solely to enhance

flushing (Figure 4-2). Buried pipelines have been

similarly used to promote flushing.

♦ Consider mechanical aerators to improve

flushing and water quality where basin and

entrance channel configuration cannot

provide adequate flushing.

Where poor water quality throughout a marina

basin or in secluded spots is a problem because of

poor flushing, limited circulation, or other circumstances, mechanical aerators (such as those used

for ice protection) might be helpful.

These devices can raise the level of dissolved

oxygen in the water and circulate floating debris

out of corners into the rest of the basin, where it

can be flushed out naturally. Underwater air

bubblers or submerged impeller-type motors can

be effective during short-term episodes that might

occur during the summer. In certain circumstances, such as in shallow and enclosed waters,

water clarity improvement is often noted if

artificial aeration is used.

Figure 4-1. Example marina designs.

Both compressed air and agitator

systems work in fresh water, salt water,

and brackish water. They do not work

well in ice-covered rivers because river

currents destroy bubble or flow patterns

and because of the lack of heat.

Thermal mixing of river water is a

natural process, and a river that has

formed an ice cover has already

dissipated nearly all available heat.
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National Management Measures Guidance

Ice suppression systems available for marinas

hinder ice formation by using compressed air

bubblers or in-water agitators. Bubbler systems

force air to entrain warmer bottom water into a

rising plume, which reacts with and melts the

underside of the ice sheet. Water agitators work

on the basis of thermal reserves of basin waters

and surface currents to prevent freezing.

BMP Summary Table 1 summarizes the BMPs

for Marina Flushing mentioned in this guidance.

Figure 4-2. Puerto Del Ray Marina (Puerto Rico) has an

offshore rubble mound breakwater that protects the

southeastern and eastern exposures of the marina. Two

hundred feet of the southern breakwater was removed,

creating a new south side breachway exit/enterance that

is still well protected but now allows for greater

circualtion in the basin. Water clarity improved after the

alteration, and as a result new customers (a 3 percent

increase for the marina) relocated to Puerto Del Rey

Marina (USEPA, 1996: Clean Marinas—Clear Value).
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