Water: Coastal Zone Act Reauthorization Amendments
Marina Flushing Management Measure - II. Siting and Design
II. Siting and Design
Siting and design are among the most significant factors affecting a marina's potential for water quality impacts. The location of a marina whether it is open (located directly on a river, bay, or barrier island) or semi-enclosed (located on an embayment or other protected area) affects its circulation and flushing characteristics. Circulation and flushing can also be influenced by the basin configuration and orientation to prevailing winds. Circulation and flushing play important roles in the distribution and dilution of potential contaminants. The final design is usually a compromise that will provide the most desirable combination of marina capacity, services, and access, while minimizing environmental impacts, dredging requirements, protective structures, and other site development costs. The objective of the marina siting and design management measures is to ensure that marinas and ancillary structures do not cause direct or indirect adverse water quality impacts or endanger fish, shellfish, and wildlife habitat both during and following marina construction.
Many factors influence the long-term impact a marina will have on water quality within the immediate vicinity of the marina and the adjacent waterway. Initial marina site selection is the most important factor. Selection of a site that has favorable hydrographic characteristics and requires the least amount of modification can reduce potential impacts. Because marina development can result in reduced levels of dissolved oxygen, many waters with average dissolved oxygen concentrations barely at or below State standards may be unsuitable for marina development.
A. Marina Flushing Management Measure
Site and design marinas such that tides and/or currents will aid in flushing of the site or renew its water regularly.
This management measure is intended to be applied by States to new and expanding marinas. Under the Coastal Zone Act Reauthorization Amendments of 1990, States are subject to a number of requirements as they develop coastal nonpoint source programs in conformity with this measure and will have some flexibility in doing so. The application of management measures by States is described more fully in Coastal Nonpoint Pollution Control Program: Program Development and Approval Guidance, published jointly by the U.S. Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce.
The term flushing or residence time is often misused in that a single number (e.g., 10 days) is sometimes given to describe the flushing time of an estuary or harbor. In actuality, the flushing time ranges from zero days at the boundary to possibly several weeks, depending on location within the marina waterbody.
Maintaining water quality within a marina basin depends primarily on flushing as determined by water circulation within the basin (Tsinker, 1992). If a marina is not properly flushed, pollutants will concentrate to unacceptable levels in the water and/or sediments, resulting in impacts to biological resources (McMahon, 1989; NCDEM, 1990, 1991). In tidal waters, flushing is primarily due to tidal advective mixing and is controlled by the movement of the tidal prism into and out of the marina waterbody. A large tidal prism relative to the mean total volume of the waterbody indicates a large potential for flushing because more of the "old" water has a chance to become mixed with the "new" water outside the boundary or opening to the waterbody.
In nontidal coastal waters, such as the Great Lakes, wind drives circulation in the adjacent waterbody, causing a velocity shear between the marina basin and the adjacent waterbody and thereby producing one or more circulation cells (vortices). Such cells can have a flushing effect on water within a marina. The current created by local wind conditions is influenced by its persistence in terms of velocity and direction. The depth of the affected water layer is controlled by temperature and how the salinity changes with depth. Several hours of consistent wind are required for full development of wind-driven currents. These currents can be 2 percent of the wind's velocity and are generally downwind in most shallow areas (Tobiasson and Kollmeyer, 1991). In many situations wind-driven currents will provide adequate flushing of marina basins.
The degree of flushing necessary to maintain water quality in a marina should be balanced with safety, vessel protection, and sedimentation. Wave energy should be dissipated adequately to ensure that boater safety and protection of vessels are not at risk. The protected nature of marina basins can result in high sedimentation rates in waters containing high concentrations of suspended solids. Methods for assessing and mitigating sedimentation rates are available (NRC, 1987).
3. Management Measure Selection
The measure was selected because it has been shown that adequate flushing will greatly reduce or eliminate the potential for stagnation of water in a marina and will help maintain biological productivity and aesthetics (Tsinker, 1992; SCCC, 1984). Presented below are some illustrative examples of flushing guidelines in different coastal regions and different conditions. In areas where tidal ranges do not exceed 1 meter, as in the southeastern United States, a flushing reduction (the amount of a conservative substance that is flushed from the basin) of 90 percent over a 24-hour period has been recommended. For example, a flushing analysis for a proposed marina/canal on the St. Johns River, Florida, was conducted to predict how an effluent would disperse and to determine the configuration that would provide for maximum flushing of a hypothetical conservative pollutant (Tetra Tech, 1988). The selected design provided the recommended flushing reduction of 90 percent over a 24-hour period. This study showed that employing modeling to demonstrate how to achieve the recommended flushing rate is effective at avoiding adverse water quality and other environmental impacts. In the Northwest, a minimum flushing reduction of 70 percent per day was judged to be adequate (Cardwell and Koons, 1981). The 70 percent value, which represents the overall mean flushing rate for the marina basin, was based on the prevailing 1.82-meter tidal range for a 24-hour period. However, if the marina was in a protected area, such as an estuary or embayment, where tidal ranges never attain 1.82 meters, then a minimum flushing reduction of approximately 85 percent per day was recommended.
As discussed more fully at the beginning of this chapter and in Chapter 1, the following practices are described for illustrative purposes only. State programs need not require implementation of these practices. However, as a practical matter, EPA anticipates that the management measure set forth above generally will be implemented by applying one or more management practices appropriate to the source, location, and climate. The practices set forth below have been found by EPA to be representative of the types of practices that can be applied successfully to achieve the management measure described above.
- a. Site and design new marinas such that the bottom of the marina and the entrance channel are not deeper than adjacent navigable water unless it can be demonstrated that the bottom will support a natural population of benthic organisms.
Existing water depths can affect the entire marina layout and design. Therefore, if depth information is not available, bathymetric surveys should be conducted in the proposed marina basin area as well as in those areas that will be used as channels, whether existing or proposed (Schluchter and Slotta, 1978). Flushing rates in marinas can be maximized by proper design of the entrance channel and basins. For example, in areas of minimal or no tides, marina basin and channel depths should be designed to gradually increase toward open water to promote flushing (USEPA, 1985a). Otherwise, isolated deep holes where water can stagnate may be created (SCCC, 1984).
Good flushing alone does not guarantee that a marina's deepest waters will be renewed on a regular basis. Several studies have concluded that deep canals and holes deeper than adjacent waters are not adequately flushed by tidal action or by wind-generated forces and thus cause stagnant or semi-stagnant conditions (Walton, 1983; Barada and Partington, 1972). Lower layers in canals and basins can act as traps for fine sediment and organic detritus and exhibit low dissolved oxygen concentrations. Lower-layer stagnation can occur in holes of depths less than 10 feet (Murawski, 1969). The low DO concentrations, resulting from an oxygen demand exerted by resuspended sediments and decaying organic matter, can impact aquatic life in the warmer months when the normal DO concentration is lower because of higher temperatures (Sherk, 1971). Fine sediments trapped in deep holes may form a thin surface ooze, which gives poor internal oxygen circulation and leads to oxygen reduction both within the sediments and in the overlying water (USEPA, 1976).
- b. Design new marinas with as few segments as possible to promote circulation within the basin.
Flushing efficiency for a marina is inversely proportional to the number of segments. For example, a one-segment marina will not flush as well as a marina in open water, a two-segment marina will not flush as well as a one-segment marina, and so forth. Figure 5-1 presents examples of marinas with one segment and more than one segment. The physical configuration of the proposed marina as determined by the orientation of the marina toward the natural water flow can have a significant effect on the flushing capacity of the waterway. The ideal situation is one in which the distance between the exchange boundary and the inner portion of the basin is minimized. As the shape of the basin becomes more elongated (i.e., more than one segment) with respect to total surface area, the tidal advective or other dispersive mixing processes become more confined along a single flow path, and it takes longer for a water particle originating in the inner part of the basin to travel the greater distance to the boundary.
The marina's aspect ratio (the ratio of its length to its breadth) should be used as a guideline for marina basin design with respect to flushing. This ratio should be greater than 0.33 and less than 3.0, preferably between 0.5 and 2.0 (Cardwell and Koons, 1981). For rectangular marinas with one entrance connected directly to the source waterbody, the length-to-breadth ratio should be between 0.5 and 3.0 to eliminate secondary circulation cells where mixing and tidal flushing are reduced (McMahon, 1989).
Marina configurations that promote flushing exhibit, in general, better dissolved oxygen conditions than those with restrictions or stagnant areas such as improper entrance channel design, bends, and square corners (NCDEM, 1990). These areas also tend to trap sediment and debris. If debris are allowed to collect and settle to the bottom, an oxygen demand will be imposed on the water and water quality will suffer. Therefore, square corners should be avoided in critical downwind or similar areas where this is most likely to be a problem. If square corners are unavoidable because of other considerations, then points of access should be provided in those corners to allow for easy cleanout of accumulated debris.
In tidal waters, marina design should replace conventional rectangular boat basin geometry with curvilinear geometry to eliminate the stagnation effects of sharp-edged corners and to exploit the natural hydraulic patterns of flow and prevent the occurrence of areas where flushing is negligible (Cardwell and Koons, 1981). By combining these elements in the design of a marina, analytical studies have suggested that a strong internal basin circulation system could develop, resulting in acceptable water quality levels (Layton, 1991).
- c. Consider other design alternatives in poorly flushed waterbodies (open marina basin over semi-enclosed design; wave attenuators over a fixed structure) to enhance flushing.
In selecting a marina site and developing a design, consideration of the need for efficient flushing of marina waters should be a prime factor along with safety and vessel protection. For example, sites located on open water or at the mouth of creeks and tributaries usually have higher flushing rates. These sites are generally preferable to sites located in coves or toward the heads of creeks and tributaries, locations that tend to have lower flushing rates.
In poorly flushed waterbodies, special arrangements may be necessary to ensure adequate overall flushing. In these areas, selection of an open marina design and/or the use of wave attenuators should be considered. Open marina designs have no fabricated or natural barriers, which tend to restrict the exchange of water between ambient water and water within the marina area. Wave attenuators improve flushing rates because water exchange is not restricted. They are also attractive because they do not interfere with the bottom ecology or aesthetic view. Other advantages include their easy removal and minimization of potential interference with fish migration and shoreline processes (Rogers et al., 1982).
The effectiveness of wave attenuators is usually dependent on their mass (Tobiasson and Kollmeyer, 1991). The greater the horizontal and draft dimensions, the greater their displacement and effectiveness. Floating wave attenuators have limitations on their use in extreme wave fields, and site-specific studies should be performed as to their suitability.
- d. Design and locate entrance channels to promote flushing.
Entrance channel alignment should follow the natural channel alignment as closely as possible to increase flushing. Any bends that are necessary should be gradual (Dunham and Finn, 1974). In areas where the tidal range is small, it is recommended that the marina's entrance be designed as wide as possible to promote flushing while still providing adequate protection from waves (USEPA, 1985a). In areas where the tidal range is large, however, a single narrow entrance channel, if properly designed, has proven to provide adequate flushing (Layton, 1991).
Entrance channel design and placement can alleviate potential water quality problems. In tidal and nontidal waters, marina flushing rates are enhanced by wind action when entrance channels are aligned parallel to the direction of prevailing winds because wind-generated currents can mix basin water and facilitate circulation between the basin and the adjacent waterway (Christensen, 1986).
Shoaling may be significant in areas of significant bed load transport if the entrance channel is located perpendicular to the waterway. Increased shoaling could require extensive maintenance dredging of the channel or create a sill at the entrance to the marina basin. Shoaling at the marina entrance can lead to water quality problems by reducing flushing and water circulation within the basin (Tetra Tech, 1988; USEPA, 1985a). In Panama City, Florida, a study of bathymetric surveys before and after the construction of an artificial inlet showed that the areas of deposition and erosion in the natural bay rapidly changed as a result of alterations of channel positions and depths (Johnston, 1981).
The orientation and location of a solitary entrance can impact marina flushing rates and should be given consideration along with other factors impacting flushing. When a marina basin is square or rectangular, a single entrance at the center of a marina produces better flushing than does a single corner-located asymmetric entrance (Nece, 1981). This results in part because the jet entering the marina on the flood tide is able to circumnavigate a greater length of the sub-basin perimeter associated with each of the two gyres than it could in a single-gyre basin with an asymmetric entrance. If the marina basin is circular, an off-center entrance channel will promote better circulation. Off-center entrance channels also promote better circulation in circular canals.
- e. Establish two openings, where appropriate, at opposite ends of the marina to promote flow-through currents.
Where water-level fluctuations are small, alternatives in addition to the ones previously discussed should be considered to ensure adequate water exchange and to increase flushing rates (Dunham and Finn, 1974). An elongated marina situated parallel to a tidal river can be adequately flushed using two entrances to establish a flow-through current so that wind-generated currents or tidal currents move continuously through the marina. In situations where both openings cannot be used for boat traffic, a smaller outlet onto an adjacent waterbody can be opened solely to enhance flushing. In other situations a buried pipeline has been used to promote flushing.
- f. Designate areas that are and are not suitable for marina development; i.e., provide advance identification of waterbodies that do and do not experience flushing adequate for marina development.
For example, the physical characteristics of some small tidal creeks result in poor flushing and increased susceptibility to water quality problems (Klein, 1992). These characteristics include:
- Bottom configuration Flushing is retarded when a depression exists that is lower than the entrance to the waterway.
- Entrance configuration A constricted entrance will decrease flushing.
- Tributary inflow Higher freshwater inflow will increase flushing.
- Tidal range Increased tidal range will increase flushing.
- Shape of the waterway As the configuration of a waterway becomes more convoluted and irregular, flushing tends to decrease.