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Safe Clean Boating

Stormwater Runoff Best Management Practices For Marinas: A Guide for Operators
Jay Tanski, New York Sea Grant Extension Program, Cornell Cooperative Extension

Partial funding for this publication was provided by New York State Department of Environmental Conservation Section 319 Program and U.S. Environmental Protection Agency Peconic National stuary Program.

This information is provided for educational purposes only. Mention of trade names, products, companies or services does not imply approval, endorsement or recommendation by the New York Sea Grant Program, Cornell Cooperative Extension, N.Y. State Department of Environmental Conservation or U.S. Environmental Protection Agency. Although this work has been funded in part by the U.S. Environmental Protection Agency under Assistance Agreement No. 01-440-498-18-00019 to the Suffolk County Department of Health Services, it may not necessarily reflect the views of the Agency and no official endorsement is inferred.

Acknowledgements: The author wishes to thank Peter Needham, Coecles Harbor Marina; Allan Connell, U.S.D.A. Natural Resources Conservation Service; Tom McMahon, Suffolk County Soil and Water Conservation District; Gil Kelley, No-Chem Paint Stripping, Inc.; and Barry Dubin, Dumond Chemicals for their practical and technical expertise.

Covered BoatStormwater runoff and Best Management Practices, or BMPs, are buzzwords being used more and more when people are talking about environmental protection and water quality. They are also terms marina owners and operators are hearing more frequently in relation to their facilities. But what do these terms mean and how do they relate to marina operations from a practical perspective?

This guide provides a brief discussion of why stormwater runoff management is important at marinas and the types of things facility owners can do to address this potential problem. The emphasis is on providing information that will familiarize readers with the options available and help them begin to screen these options in terms of potential application at a specific site. Although intended primarily for marina owners and operators, this information should also be of interest and use to others involved with boating facilities and environmental management including government and agency officials, planners, consultants and the public.

Stormwater Runoff, Hull Maintenance and BMPs - They All Go Together
Stormwater runoff is simply the water from rain and snow melt that flows across the land. This water picks up pollutants left on the land and carries them into wetlands, creeks and bays where they can adversely impact water quality and threaten aquatic habitats. Any development can increase the amount of stormwater runoff, alter natural drainage patterns, and increase the concentration and types of pollutants carried by the runoff.

Boat - Just RelaxEven relatively simple practices can help reduce potential pollution from hull maintenance areas.
For marinas, runoff can be a concern particularly in areas used for boat hull maintenance. Due to the materials and compounds used on boats to control fouling and corrosion and for repairs, the wastes generated by sanding, scraping, painting, varnishing and fiberglassing can contain contaminants like metals, solvents and hydrocarbons.

Preparing a vessel for painting can generate paint chips, dust and particles that may contain metals such as copper, zinc, and lead. While some of these metals are relatively harmless on land, if they are not handled properly and allowed to get into the water, they can be toxic to marine organisms even at very low levels. (These levels are so low their harmful concentrations are often measured in terms of parts per billion). Since many contaminants tend to attach themselves to solid particles, even soil and dirt in hull maintenance areas can pick up potentially harmful materials. In addition to adversely affecting marine life, material washed into the water from hull maintenance areas can also contaminate sediments in the marina basin, posing problems for dredging and the disposal of dredged material. Finally, allowing pollutants to seep into the ground can eventually contaminate the site itself, posing problems if the marina is ever to be sold.

So, it is not hard to see why it is important to keep a close eye on hull maintenance areas to ensure that the associated wastes do not get into the water. Under the Federal Coastal Nonpoint Source Pollution Control Program, the States are required to develop plans to reduce pollution from stormwater runoff from a variety of sources, including marinas. While marinas are usually not considered a major contributor to water quality problems, existing marinas are being asked to reduce the total amount of solids in runoff from boat maintenance areas by 80 percent as part of this program. The question is how best to do this.

This is where Best Management Practices come in. Put simply, BMPs are really anything a marina operator can do to help prevent or reduce the amount of pollutants coming from his facility. BMPs can involve a wide range of activities including:

  • Building new structures,
  • Using new or different equipment or products,
  • Changing operating procedures and improving housekeeping practices.

Which BMPs are really "best" for a particular facility depends on the marina.

Different Marinas, Different BMPs
When evaluating BMPs for use at a site, it is important to remember all marinas are different and, in some respects, unique. Marinas in different parts of the country can vary tremendously in terms of their average size, services offered and operating characteristics because of differences in boat use, number and size. Even in relatively small geographic areas there can be a great deal of variation in the facilities. In the New York/Long Island metropolitan region, marinas range in size from less than 10 to more than 500 slips. Gross annual revenues range from $14,000 to $15,000,000 with most of the facilities falling towards the lower end of the range.

Obviously, the diversity in size and types of marinas makes it difficult to generalize about BMPs for these facilities. A BMP that works well at one marina might be totally inappropriate for another because of location, site, economic or operational considerations. Each marina must be examined on a site-specific basis to ensure that the most effective and suitable BMPs are selected.

While there is no "one size fits all" set of BMPs for all marinas, there are a number of BMPs often suggested for reducing potential stormwater pollution from hull maintenance areas. Chances are one or more of the BMPs discussed here would be suitable for most facilities.

BMPs are often categorized into two types commonly known as Source-control BMPs and Stormwater-treatment BMPs. Source-control BMPs focus on keeping stormwater from coming into contact with pollutants. Stormwater-treatment BMPs usually involve building structures or installing devices to treat or manage runoff. Source-control BMPs are generally preferred because they usually cost less and can keep most, if not all, of the pollutants out of the water.

Indoor Maintenance Areas
Moving maintenance and repair work indoors or under roofs where it is not exposed to rainfall is one of the most effective ways to reduce contaminated stormwater runoff. However, it may also be one of the most impractical alternatives for many marinas due to cost, size and space limitations, and zoning restrictions, especially when traditional structures are used.

For marinas with enough room, temporary work enclosures, such as the one shown here, can be a relatively inexpensive way to protect maintenance areas from rain while extending the work season. The enclosures are pre-fabricated structures made of heavy-gauge polypropylene plastic stretched over a tubular metal frame. Although the plastic has a life expectancy of three years, this structure is seven years old and has survived a hurricane and a number of severe "northeaster" storms without major damage.

Planning Considerations
Indoor work areas should have hard floors to facilitate clean up. Floor drains should be avoided or covered while work is being done.

Temporary enclosures come in various sizes up to 100 feet long, 30 feet wide and 23 feet high.
Usually, enclosures come as a kit with materials and assembly instructions. The marina must supply the labor to actually build the structure. Construction does not usually require special tools or skills, but it can be time consuming.

Because they do not have permanent foundations and are considered temporary or portable, these structures may not require permits or zoning approvals in some locations and may also be exempt from capital improvement taxes. Check with your local building department regarding the laws and regulations in your area.

Moving certain types of work, like painting, indoors or into enclosed areas may require the use of special ventilation equipment, protective clothing and respirators, and safety equipment to meet Occupational Safety and Health Administration, Clean Air Act, and local fire safety requirements and regulations.

Buildings capable of handling boats can cost over $60 per square foot to construct in the New York area, not including land costs. (All cost estimates in this document are given in 1997 dollars unless otherwise noted.) Unless a building is already available, it is not feasible for most marinas to build a structure solely for maintenance activities. Typical costs for temporary work enclosures run between $3 to $5 per square foot for materials. This does not include labor.

Outdoor Maintenance Areas
Usually, it is not practical for marinas to do all maintenance work under a roof. If work has to be done outdoors, it should be done over dry land in specially designated areas designed for that purpose. These areas should be clearly marked with signs. Customers (and staff) should be discouraged from performing maintenance work outside these areas.

Planning Considerations
Locate maintenance areas as far away from the basin as possible to prevent contaminants from getting into the water.

Maintenance areas should have a hard, impermeable surface that can be easily vacuumed or swept to remove contaminants. Concrete surfaces are particularly easy to clean and resistant to solvents and petroleum products.

Maintenance areas should be swept or vacuumed regularly and the collected wastes disposed of properly. Special attention should be paid to weather conditions, with cleanups being done more frequently if there is a chance of material being dispersed by wind or rain. Avoid hosing down the work area.

Care should be taken to ensure that stormwater runoff from other parts of the marina does not flow over the maintenance area. This can be done by proper siting or by using berms, curbs, or one or more of the stormwater treatment BMPs discussed below to divert water from the area.

If the maintenance area cannot be properly cleaned, rainwater falling on it should be directed to a stormwater treatment device before being allowed to run into the marina basin.

Maintenance areas should be located away from storm drains. If storm drains are nearby, they should be covered when work is being done to prevent material from being carried into the marina water.
If blasting or painting is done outdoors, vertically hung tarps or enclosures should be used to contain dust, abrasive grit and/or paint. Work should be monitored, especially on windy days, to ensure paint, dust or blasting grit is contained and stopped if conditions prevent containment.

Costs for hull maintenance areas vary widely depending on the specific marina. In some cases, existing infrastructure, such as parking lots or other paved surfaces may be converted to maintenance areas with minimal expenditures. Curbs or berms may be installed for approximately $10 to $12 per foot. Constructing a new concrete pad with appropriate runoff controls capable of handling large boats can cost $20,000 or more in the New York metropolitan area.

Work Outside of Designated Hull Maintenance Areas
In some marinas, it may not be possible to have a designated work area for all hull maintenance activities due to space limitations, time constraints, or cost. If work has to be done outside a designated area, precautions should be taken to contain wastes and debris and prevent them from entering the water.
Tarps used to contain paint chips and dust from hull maintenance activities.

Planning Considerations
In areas that do not have sealed or impervious surfaces, perform all work over tarps or drop clothes. Ideally, tarps should be placed beneath the cradle or boat stand.

If customers are allowed to do maintenance work on their boats, they should be required to clean up the area when done working. Separate, covered, and labeled containers should be provided for waste materials.

When maintenance work has to be done near water, consider the use of additional BMPs such as the innovative paint removal techniques and dustless vacuum sanders described below.

Innovative Paint Removal Techniques
Different paint removal technologies can be used as BMPs to eliminate or, at least, contain paint chips and dust associated with hull preparation activities. Special equipment, products or procedures can also help reduce the amount of waste material generated and ensure it does not get into the environment.

Plastic Media Blasting: New abrasive hull blasting technologies utilize a process to reclaim and reuse media made of plastic. In this procedure, the boat must be completely enclosed to trap the media and paint waste. The collected mixture is fed into specialized equipment (located in the truck in the photograph) which through a sifting process separates most of the paint dust and chips from the media, which can then be reused. In addition to containing paint dust and chips, this process can significantly reduce the amount of material that has to be disposed of when stripping a hull. For example, the plastic media stripping of a 19-foot boat produced a total of 200 pounds of waste and media. The equipment recovered 185 pounds reusable media (containing a small amount of paint), leaving only 15 pounds of paint for disposal.

Planning Considerations
Specialized equipment and training is required for plastic media blasting. In some areas companies have mobile equipment and will perform this service on site for a set fee.

Containment of dust and other debris and recovery of the media can considerably reduce clean up and disposal costs.

Since the media can not remove anything harder than itself, these techniques may not be effective for preparing all surfaces. For instance, plastic media may not remove corrosion or barnacles from props, shafts, or rudders. However, it will remove paint without damaging sound gel coat, rubber, chrome or glass surfaces.

Some softer media may not work well on very durable, pliable paints (like coal tar epoxies). Paint around edges may have to be removed by hand. Sanding is usually required before painting a blasted hull.

Care should be taken when blasting boats that have damaged or blistered gel coats since blasting may open blisters or voids that have to be filled before painting. Even sound gel coat may contain small voids that may have to be filled after blasting.

Equipment costs for a blasting system that incorporates media recovery and reclamation start at $25,000, not including training or the media. In New York, contractors with their own mobile equipment will blast hulls with plastic media on site for approximately $17 to $18 per foot, where length is calculated as the length of the boat at the waterline plus one-half the beam. (Approximate cost for blasting a "typical" 30-foot power boat is about $630.) Discounts may be available for volume work.

Bottom of Boat
Chemical paint stripper and covering cloth applied to boat hull

"Environmentally-Sensitive" Chemical Paint Strippers
Chemical paint strippers can actually eliminate paint chips and dust associated with sanding, scraping and blasting. There are now less toxic and less hazardous alternatives to strippers that use methylene chloride and other organic solvents. New products are non-chlorinated, biodegradable, have low volatility and are not listed as hazardous. Some of the more environmentally-sensitive strippers may be water based and use less toxic materials (look for dibastic esters, semi-aqueous terpene-based products, detergents and C9 to C12-based hydrocarbon strippers). While the new strippers themselves may be considered non-hazardous, metals and chemicals from the paint they remove may be hazardous, so all residue and wash water must be collected and disposed of properly.

Planning Considerations
Environmentally-sensitive paint strippers are usually made without toxic or caustic chemicals, so they do not burn skin and will not release harmful fumes like some of the more aggressive chemical strippers. This can reduce or eliminate the need for special ventilation equipment.

The more environmentally-friendly strippers may require more experience and expertise to apply correctly, as well as more time to work effectively. Some may have to remain on the hull for 2 to 24 hours depending on the condition of the hull and air temperature. Lower temperatures require longer times, and some products do not work well below 32F.

When stripping, place plastic around and under the work area to catch any drips. Some products come with a special paper placed over the stripper after it has been applied that helps contain the chemicals and dissolved paint.

Strippers may not work on all paints, such as 2-part epoxies or chlorinated rubbers. Check with the manufacturer for specific applications.

Machine used to pressure wash and collect washwater in one step from chemically stripped hull.
Stripping residue and washdown water may be contaminated by paint and must be collected and disposed of properly, possibly as a hazardous waste. Special machines that pressure wash the hull and collect the washwater in a one-step process are available for this purpose. It is estimated that a 30-foot boat would generate approximately 30 gallons of waste, including washdown water.

Costs will vary depending on product used, conditions, and layers of paint to be removed. Non-toxic, water-based strippers can cost $40 to $60 per gallon. According to one manufacturer, a gallon of their product will cover an average of 50 square feet of hull so the estimated cost of materials is one dollar per square foot. A 30-foot boat may require five to six gallons. The manufacturer also estimated disposal cost for the residue to be about $30 per boat depending on size. Complete pressure washing/vacuum collection systems cost $5,000 but costs can be reduced if the marina has an existing pressure washer and/or vacuum system.

Dustless Vacuum Sanders: Dustless vacuum sanders are sanders (or grinders) attached to a vacuum system that starts automatically when the sander is turned on. These units can trap up to 98 percent of the dust generated by hull sanding, making them particularly suitable for situations where work must be done near the water.

Planning Considerations
By containing dust, vacuum sanders keep work areas and workers clean, saving time and money in clean up.

To recover costs, some marinas rent the sanders to customers for use on their own boats for fees ranging up to $15 per hour. Advertising and training is necessary to encourage use.

Studies indicate that sanders may collect an average of two ounces of dust per foot of boat sanded (3.75 pounds for a 30-foot boat). This material must be collected and disposed of properly.

The cost of vacuum sanders varies depending on size and features. Typical costs range between $1,100 and $1,400.

Stormwater treatment BMPs are structural devices used to manage and treat runoff contaminated with pollutants. In some cases, these BMPs can also be used to divert runoff away from areas where pollutants may occur. These devices normally work by capturing runoff and allowing it to filter into the ground (infiltration), holding the runoff long enough for pollutants to settle out (detention/retention) or some combination of these two processes.

When considering treatment BMPs, a good understanding of the site in terms of the drainage patterns and the rate, direction and volume of water coming from different areas is key to designing an effective system. In many cases, the services of a professional engineer may be required to provide this analysis. The information provided here is intended to help operators and owners evaluate and begin to screen alternatives for potential applicability at their sites. It should not be considered a design manual or a substitute for professional engineering guidance. See "For More Information" for a listing of more technical publications on this topic.

In general, marinas should try to reduce the total amount of runoff coming from the entire facility by using permeable material like gravel or shell whenever possible outside of maintenance areas. Permeable coverings slow runoff and allow water to filter into the ground rather than run directly into the basin. Redirecting slopes away from the shoreline can also help.

In many marinas, much of the runoff comes from offsite so it may not be practical to capture and treat all of the runoff. However, structural BMPs should be sized to collect and treat at least the first 0.5 to 1.0 inches of rainfall from impervious work areas. This is often called the "first-flush" because it usually contains most of the pollutants. To prevent premature failure, BMPs should also incorporate provisions for handling overflow from rainfalls greater than the design rainfall.

There are a large number of stormwater treatment BMPs, however, not all of them are applicable to marinas because of space, cost and site conditions. High groundwater tables, limited space, and aesthetic and safety concerns are just a few of the factors commonly found at existing marinas that may limit the type of BMPs that can be employed. The BMPs discussed here do not cover the full range of practices available, but they do represent those BMPs that are generally considered most suitable for conditions commonly encountered in marinas and for retrofitting existing boating facilities.

Vegetated Filter Strip
What It Is
Vegetation planted as a buffer along the water’s edge to filter stormwater runoff and remove contaminants and soil particles before they reach surface waters. Filter strips can be particularly effective at removing pollutants that are in the form of large particles, such as paint chips.

How It Works
Runoff carrying sediments, chemicals and nutrients is slowed by the vegetation, which allows particles carrying pollutants to settle out before reaching the surface water. Some rainwater may filter into the vegetation strip before it can run into the marina basin. In some cases, nutrients or chemicals in the runoff may be taken up by the vegetation, rather than going into the surface water.

Potential Benefits

  • Helps prevent pollutants from entering waterways, protecting water quality and keeping sediments in the marina basin free from contaminants that may impact future dredging operations.
  • Can help reduce sediment deposition in the marina basin, reducing the need for dredging.
  • Creatively landscaped strips can provide aesthetic and recreational amenities, such as a picnic area, at a marina if allowed activities do not disturb the vegetation.

Planning and Technical Considerations
Filter strips must be a minimum of 20 feet wide to be effective. Wider strips are better in terms of filtering sediment and pollutants.

Filter strips are most effective on slopes of 5 percent or less and will not function well on slopes greater than 15 percent. Steeper slopes require wider strips. As a rule of thumb, an additional 4 feet of width should be added for each additional one percent of slope.

Filter strips can only handle runoff from relatively small areas (1 to 5 acres). Care must be taken to ensure that all of the water from the upland area passes through the strip and cannot bypass it.
Since water has to flow evenly over the strip for it to be effective, the landward edge of the strip must be at a constant elevation (no dips, depressions or gullies). A shallow stone trench can be used to spread the flow evenly at the edge of the strip.

Plants suitable for the particular area and climate must be used. In marine areas, salt-tolerant species such as salt meadow cordgrass (Spartina patens) or "salty alkaligrass" (Puccinellia distans) should be considered. Your local U.S.D.A. Natural Resources Conservation Service office can provide information on the best species for your location.

Strips require regular maintenance. Reseeding, watering, fertilization and some mowing may be required to maintain the necessary dense growth of vegetation. Annual inspections should be conducted and rills, gullies and channels repaired as soon as possible.

Filter strips are one of the least expensive stormwater runoff control measures you can implement. Seeding costs can range from $20 to $100 per 1,000 square feet with sod costing $125 per 1,000 square feet ($0.40 to $6.25 per linear foot for a 20- to 50-foot wide strip) depending on site conditions.

Infiltration Trench
Infiltration TrenchWhat It Is
A shallow trench, usually 3 to 8 feet deep, filled with stone to create an underground reservoir that holds runoff, allowing it to slowly percolate through the bottom into the surrounding soil.

How It Works
Runoff carrying pollutants is diverted to the trench before it reaches surface waters. The trench retains all or some of the runoff, depending on the design. The stormwater slowly filters through the soil below, where pollutants are removed by adsorption, straining, and decomposition by bacteria in the soil.

Potential Benefits

  • Helps prevent pollutants from entering waterways, protecting water quality and keeping sediments in the marina basin free from contaminants that may impact future dredging operations.
  • Properly designed and maintained, trenches can provide effective treatment for dissolved pollutants as well as particulate matter.
  • Relatively easy to fit into margins and around perimeters of developed areas with limited space like marinas.

Planning and Technical Considerations
Trenches are only feasible where soils are well-drained (sandy). The bottom of the trench should be at least 3 feet above the seasonal high groundwater table and 4 feet above bedrock or other impervious surfaces (clay).

This approach should only be used when the contributing drainage area is less than 5 acres and/or the slopes are less than 5 percent.

Trenches can be designed to collect all or some of the expected stormwater runoff. "Water quality" trenches that are designed to catch only the "first flush" of stormwater, which contains most of the pollutants, may be the only trenches suitable for many marinas because of space considerations. (To get a rough estimate of first flush volumes, multiply the square footage of the drainage area by 0.3 gallons/square foot. About 40 percent of the total volume of the trench will be available to hold water.)
Because they are susceptible to clogging, infiltration trenches should only be used in conjunction with vegetated filter strips or some other method for trapping coarse sediments before it reaches the trench.
Clean, washed 1.5- to 2.5-inch stone should be used to fill the trench to prevent clogging. Blue stone aggregate should be avoided. A layer of filter fabric placed 6 to 12 inches below the surface can help trap sediment before it clogs the entire trench, reducing maintenance costs.

Shallow, wide trenches (as opposed to narrow, deep trenches) enhance pollutant removal, but care should be taken to ensure that the stone fill extends below the frost-line so the trench functions in cold weather.

Trenches should be sited away from building foundations. If the trench is down slope, it should be a minimum of 10 feet from the building, and 100 feet away if the trench is up slope.

Trenches should be designed to hold water for at least 6 hours after a rain and to drain completely within 3 days after a storm. A perforated PVC pipe should be installed as a monitoring well.

Infiltration trenches are one of the most economical stormwater BMPs for small sites. Costs vary depending on the site and the specific design. Estimates from the Washington D.C. area for a 150-long trench, 6 feet wide and 6 feet deep ranged between $56 and $122 per linear foot of trench and averaged $83 per foot.

Dry Well
Dry WellWhat It Is
An excavated pit filled with clean stone typically 3 to 12 feet deep that is usually designed to collect and store stormwater from rooftops or other relatively "clean" runoff.

How It Works
Runoff enters the dry well through an inflow pipe (such as a roof gutter downspout) and from surface infiltration. The water then infiltrates down through the subsoil rather than running over land.

Potential Benefits
Dry wells can be used to manage peak discharges from storms and reduce the overall volume of stormwater runoff from a marina site. This, in turn, may help eliminate the need for other stormwater management measures or reduce the size needed.

Because they normally collect relatively clean water, they can provide good quality groundwater recharge.

Planning and Technical Considerations
Because dry wells have limited pollutant removal capabilities, they may not be effective in areas with high pollutant loadings unless the runoff is pretreated before entering the well.

Dry wells are only suitable for sites where soils are well drained (sandy) and the well can be designed so the bottom is a minimum of 3 feet above the seasonal high groundwater table, bedrock or other impervious surface (clay).

The total contributing surface area for a dry well system should not be more than one acre.

Wells are susceptible to clogging and possible failure from sediment. They should not be used where they will receive runoff that carries high sediment loads.

To prevent clogging and promote infiltration, the well should be filled with 1 to 3 inch diameter clean (washed) stone and lined with filter fabric.

Locate wells a minimum of 10 feet away from building foundations.

Dry wells should be designed to capture, at minimum, roof runoff from a two-inch rainfall (roughly equivalent to a two year storm on Long Island). Two inches of rain would generate approximately 1,250 gallons of water per 1,000 square feet of impervious roof (1.25 gallons per square foot). As a first estimate of the size of the well needed for a particular application, assume about 40 percent of the total volume of the well will be available to hold water.

An observation well should be installed in each dry well to make sure it is draining properly. A perforated PVC pipe installed vertically in the well can be used for this purpose. The pipe should have a removable cap on top and be anchored with rebar at the bottom.

Costs for dry wells vary depending on the site and design specifications. Because of similarities in construction, costs for dry wells should be close to, or perhaps slightly higher, than those for infiltration trenches which can range between $56 and $122 per linear foot for a 6-foot wide, 6-foot deep trench.

Vegetated Swale
Vegetated SwaleWhat It Is
A vegetated channel that looks similar to but is wider than a ditch, with a gentle slope designed to transport and treat stormwater runoff. Vegetated swales are also sometimes called "biofilter" swales and are commonly used as a substitute for curb and gutter systems.

How It Works
Surface water is directed to a vegetated channel where gentle slopes and dense vegetation slow water flow. The reduced flow in combination with the vegetation provide moderate to high removal rates of particulate pollutants from runoff by trapping, filtering and infiltration into the soil.

Potential Benefits

  • Can help protect water quality by removing more pollutants from runoff than gutters, pipes and ditches or other conveyances.
  • Can be used to divert runoff from areas that may be contaminated with pollutants, like hull maintenance areas.
  • Generally less expensive than curb and gutter or other drainage systems they replace.
  • Can enhance the natural landscape and provide aesthetic amenities.

Planning and Technical Considerations
Swales are most suitable for relatively small sites (less than 10 acres) with low to moderate density development where the percentage of impervious cover is small and in parking lots where they can be used to break up the impervious cover.

Because they have limited capacity to accept runoff from large storms, other BMPs may have to be used in conjunction with swales depending on site conditions and the level of runoff treatment required.
The slope of the swale along its axis should be as close to zero as possible while still allowing drainage and should never exceed 4 percent. Side slopes should be no greater than 3:1 (horizontal to vertical).
The site should have well drained soil. Because soil compaction can inhibit performance, swales should not be used for boat storage or parking. The bottom of the swale should be at least 2 feet above the seasonal high groundwater level.

Swales slopes should be designed to prevent erosion during a two-year storm and sized to handle the flow from a Ten-year storm. A stabilized outlet should be provided at the down slope end to prevent scour and erosion.

Check dams (railroad ties sunk halfway into the swale with a weep hole and stone on the downstream side) can be used to flatten slopes and promote infiltration.

A dense cover of vegetation must be established and maintained. (Contact your local U.S.D.A. Natural Resources Conservation Service office for the best species for your location.) Vegetation should be kept at a height of at least 4 to 6 inches to promote infiltration.

Maintenance is minimal and primarily involves periodic mowing, spot reseeding, debris removal and watering, if necessary.

Swales are relatively inexpensive. Costs for a 15-foot wide swale with 3:1 side slopes are estimated to be approximately $7.00 to $13.00 per linear foot depending on the method of seeding.

Deep Sump Catch Basin and Water Quality Inlet
What It Is
Deep sump catch basins (also called oil and grease or hooded catch basins) and water quality inlets (known as oil/grit separators) are underground retention systems designed to remove trash, debris and a portion of the sediment and oil and grease from stormwater runoff.

Deep Sump Catch BasinHow It Works
Runoff is directed or channeled into the top of an underground chamber or series of chambers that contain a permanent pool of water. The discharge is located below the inlet pipe. Oil and grease float on the surface of the water and eventually attach to the sediment trapped in the chamber, which settles to the bottom.

Potential Benefits

  • Can help improve water quality by reducing the amount of trash, sediment and petroleum hydrocarbons reaching marina waters.
  • Underground installation minimizes space requirements. Usually suitable for retrofits where larger BMPs are not feasible.
  • Can provide pretreatment for other BMPs such as swales or infiltration trenches.

Planning and Technical Considerations
Inlets and sump catch basins are should only be used when the drainage area is less than one acre of impervious cover.

Because they provide limited pollutant removal, these devices are often only recommended as pretreatment devices for other runoff treatment practices.

Water Quality InletFor catch basins, the discharge pipe should be located at least four feet below the inlet pipe and the depth of the permanent water pool should be four times the diameter of the inlet pipe. In water quality inlet chambers, the permanent pool of water should have a minimum volume 400 cubic feet of water per acre of impervious drainage area and a minimum depth of four feet.

Oil-absorbent pads or material can be installed in the basin to further enhance hydrocarbon removal.
In areas with high sediment loads, inlets and basins should be inspected and cleaned after every storm. At minimum, they should be inspected monthly and cleaned four times per year.

Accumulated sediment and hydrocarbons may be considered a hazardous waste in some areas. Check with local officials regarding applicable guidelines and regulations for proper disposal.

Compared to other BMPs, catch basins and inlets are considered moderately expensive. Individual catch basins can cost $1,000 to $1,500 to install. Water quality inlets can cost $5,000 to $15,000 or more. Maintenance costs can also be high due to the need for periodic cleaning.

Retention/Infiltration Chamber
What It Is
High-density polyethylene chambers designed to store runoff underground. The chambers have an open bottom and permeable sides to promote infiltration of the runoff into the surrounding soil. The units can be linked together to increase capacity and are designed to be used in place of stone, pipe, surface ponds and dry wells.

How It Works
Runoff is directed to a catch basin or other suitable inlet connected to a chamber or system of chambers buried underground that retain some or all of the water, depending on the design. The open bottom and permeable sides allow the water to slowly filter through the soil where pollutants are removed by adsorption, straining or decomposition by bacteria in the soil.

Potential Benefits
Helps prevent pollutants from entering waterways, protecting water quality and keeping sediments in marina basin free from contaminants that may impact future dredging operations.

Used as infiltration devices, chambers provide effective treatment for dissolved as well as particulate pollutants.

Can be installed under parking lots and work areas, freeing up surface space in marinas.

The low profile (12 to 30 inches) of the units makes them particularly suitable for use in areas like marinas with high water tables.

Planning and Technical Considerations
In general, the chamber systems function in much the same way as the previously-described infiltration trenches and the same guidelines for site conditions, sizing, and siting apply.

The individual chambers come in various sizes but are generally 6 to 7 feet long, 3 to 4.3 feet wide and 1.3 to 2.5 feet high and have capacities between 75 and 416 gallons. Units are lightweight (22 to 78 pounds) and can be installed by one to two men without the need for cranes or heavy equipment.

Depending on size and design, systems may only require the excavation of a 3-foot wide, 3-foot deep ditch.

With 18 inches of properly compacted backfill cover, the chambers are designed to withstand loads up to 32,000 pounds per axle, making them suitable for use in areas used for heavy equipment traffic and boat storage.

A minimum of 3 inches of 0.75- to 1.5-inch diameter crushed, washed stone should be placed under and along the sides of the chambers.

Inlets should be equipped with a catch basin, sediment trap or similar device to intercept sediment and debris to minimize maintenance.

Installations should be inspected once a year for sediment buildup. Sediment can be removed by re-suspending with water and pumping the chamber using access ports built into the units.

Costs for individual chambers vary depending on size. A 122-gallon unit costs about $50 to $60 dollars. Manual installation of the units can also provide cost savings. In New York, system of chambers designed to handle 1,144 cubic feet of runoff was installed for $1,400 (not including a site analysis or design costs, or the catch basin) which is approximately one-third of the cost for a system of similar capacity using traditional infiltration devices. One manufacturer estimates an installed system costs between $3 and $3.25 per cubic foot of runoff capacity.

The information presented here was derived from a large number of different sources. Readers seeking more detailed information on BMP siting planning, selection and design should refer to the publications listed below. Your regional state environmental protection agency, Natural Resources Conservation Service, County Soil and Water Conservation District, or Sea Grant office can also provide additional information and assistance on storm water BMP planning and design.

Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs (1987) by Thomas R. Schueler, available from the Metropolitan Washington Council of Governments, 777 North Capital Street N.E., Suite 300, Washington D.C. 20002-4201. Phone: (202) 962-3256

Reducing the Impacts of Stormwater Runoff From New Development (1992) by William B. Morton available from the Empire State Chapter of the Soil and Water Conservation Service, P.O. Box 1686, Syracuse, NY, 13201-1686.

Storm Water Management. Volume 2: Stormwater Technical Handbook (1997) by Massachusetts Department of Environmental Protection and Office of Coastal Zone Management available from the Massachusetts Department of Environmental Protection, 1 Winter Street, Boston, MA, 02108. Phone: (617) 292-5500.

CT | ME/NH | MIT (MA) | WHOI (MA) | NY | RI Sea Grant This page was last updated 1 May 2000 and designed by Elisabeth Sylvan of MIT Sea Grant.