STORMWATER TREATMENT BMPs

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

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.

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.

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.

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.

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

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.

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.

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.s are removed by adsorption, straining, and decomposition by bacteria in the soil.

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

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

For 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

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.

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.

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.

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.

For More Information

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.