SECTION 4: Management Measures

4-7 

## 4.1. MARINA

## F LUSHING



### 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 aestheticappeal. 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 effecton 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.** 4-8 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. Asmentioned previously, deep canals and depressions much deeper than adjacent waters might notbe 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 arenormally 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 largerwaterbody 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 toexcessive 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 stagnantcorner 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

4-9 The alignment and number of entrance channelsmay affect flushing, along with many other sitespecific factors. The following points generallyhold 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 maypromote 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 ofpoor 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 mightoccur 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.4-10 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 ).SECTION 4: Management Measures

4-114-12 National Management Measures Guidance