Section 2: Stormwater Treatment

Stormwater treatment best management practices (BMPs) are structural devices constructed to manage and treat runoff contaminated with pollutants. In some cases, these BMPs are also used to divert runoff away from areas where pollutants may occur. These devices normally work in one of three ways: 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.

There are many stormwater treatment BMPs, but not all of them are applicable to marinas because of space, cost and site conditions. However, the BMPs listed here are generally considered most suitable for conditions commonly found in marinas and for retrofitting existing boating facilities.

Learning Objectives

By the end of this section, you should be able to:

  • Consider using a prior site analysis (elevations, soil type, sources of runoff, etc.) as a guide for BMP selection.
  • Identify a suite of BMPs that will capture, infiltrate and treat stormwater runoff at your facility, thus minimizing reliance on traditional stormwater controls.

Best Management Practices

Use Site Analysis to Inform BMP Selection

This section proposes a variety of best management practices that address stormwater treatment; however, every site is different and the practices adopted should be specific to your situation. In order to select which BMPs are right for your marina, you should use an existing or conduct a new site analysis. This analysis should include assessment of elevations and drainage patterns (e.g., soil types, water table, and impervious surfaces), sources of runoff (e.g., impervious surfaces), and location and status of potential pollutant exposure (e.g., material storage, activities). In many cases, the services of a professional engineer or landscape architect may be required to provide this analysis.

Selection of BMPs will be further informed by cost and maintenance requirements. The information provided here is intended to help operators and owners evaluate and begin screening alternatives for potential applicability at their sites.

In many marinas, much of the runoff can come from offsite, so it may not be practical to capture and treat all of it. However, structural BMPs should be sized to collect the “first-flush” because it usually contains most of the pollutants. The term first flush refers to the first 1 to1.5 inches of rainfall runoff, which usually contains higher levels of pollutants.

Use Structural Controls as Necessary

Stormwater treatment BMPs are structural devices used to manage and treat runoff contaminated with pollutants. Types of stormwater treatment BMPs that may work at marinas include:

  • Have and maintain stormwater management structures that are appropriate for your property (e.g., retention ponds, extended detention ponds, constructed stormwater wetlands, sand filters, grassed swales, vegetated filter strips).


Stormwater pond systems capture and slowly release stormwater runoff. Ponds may hold water permanently (retention ponds) or temporarily (detention ponds).

Extended detention ponds (also known as dry ponds, extended detention basins, or simply detention ponds) are designed to detain the stormwater runoff for up to 24 hours after a storm. This time period allows particles and associated pollutants to settle.

Although these facilities do not have a large permanent pool, they are often designed with small pools at the inlet and outlet of the basin. They can also be used to provide flood control by including additional flood detention storage. This type of structure is effective for sites that are 10 acres or greater in size.

Diagram of a dry, extended detention pond. Source: OH CMP Guide – Appendix H, complements of T.R. Schueler.

Bioretention, Constructed Wetlands and Swales

Bioretention areas (rain gardens) are shallow surface depressions planted with specially selected native vegetation to capture and treat the first-flush of stormwater runoff from rooftops, streets, and parking lots. Bioretention areas reduce runoff volume, filter pollutants, provide habitat, and enhance site aesthetics. A rain garden is a type of bioretention system that is constructed in place. When additional treatment is required (e.g., runoff from parking lots where vehicle fluids are a concern), a biofiltration system can be designed with an underdrain and expanded subsurface infiltration bed (see image below). Biofiltration is a more specific type of bioretention that includes sub-root zone storage and an underdrain.

Rain gardens (left) are constructed in place. The use of an underdrain and storage zone with engineered soils differentiates the biofiltration system (right).

Rain gardens (left) are constructed in place. The use of an underdrain and storage zone with engineered soils differentiates the biofiltration system (right). Image Credit: Lind Severson.

In either case, water flows through the surface of the garden and either infiltrates to shallow groundwater or is collected by underdrains at the bottom that slowly release the water into the basin or an adjoining waterway.

Constructed wetlands are designed to mimic the ability of natural wetlands to cleanse and absorb stormwater runoff. One type of created wetland, the pocket wetland, is created by excavating to the high water table elevation. Pocket wetlands can serve drainage areas of 5 to 10 acres.

Diagram of a pocket wetland.

Diagram of a pocket wetland. Source: OH CMP Guide – Appendix H, complements of T.R. Schueler.

A vegetated swale (or grassed swale or bioswale) is a shallow stormwater channel that is densely planted with erosion-resistant vegetation. These conveyance channels are used as a substitute for curb and gutter systems and serve to redirect, slow, filter, and infiltrate stormwater runoff. Water generally moves more slowly over grassed swales. They improve water quality by filtering out particulates, taking up nutrients, and promoting infiltration. Grassed swales are not practical on very flat land, on steep slopes, or in wet or poorly drained soils.

Image and diagram of a grassed swale

Image and diagram of a grassed swale courtesy of Wisconsin Stormwater Manual. Photo credit: Cheryl Bauer.


A grassed swale helps direct stormwater to filter out contaminants.

A grassed swale helps direct stormwater to filter out contaminants.

Infiltration and Filter Systems

Infiltration systems, such as rain gardens and porous pavement, are designed to take advantage of soil’s natural infiltration and pollutant removal capacities. Infiltration systems should not be used where pollutants or chemical spills can mix with runoff. A dry well is another infiltration system, specifically designed to treat roof runoff. Water collected by downspouts is directed into a filter composed of crushed stone and fabric, then infiltrates through the subsoil rather than running over land.

Retention/Infiltration chambers are 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.

Triple stack stormwater chamber.

Triple stack stormwater chamber. Photo credit:

An infiltration trench is a variation on this idea, but is dug deeper and filled with stone (no constructed chamber). Water flows into the trench, percolates through three to eight feet of stone and into surrounding soil. These approaches divert water from running directly to surface waters.

Constructed filters “strain” runoff to remove pollutants. Constructed filters are structures or excavated areas containing a layer of sand, compost, organic material, peat, or other media that reduce pollutant levels in stormwater runoff by filtering sediments, metals, hydrocarbons, and other pollutants. Constructed filters are suitable for sites without sufficient surface area available for bioretention.

Proprietary water quality devices are a category of commercially available systems designed to remove non-point source pollutants from the conveyance system for stormwater runoff. These vary in size and function, but all use some form of filtration, settling, or hydrodynamic separation to remove particulate pollutants from overland or piped flow. Regular maintenance is critical for the continued proper functioning of water quality devices.

Oil grit separators are underground, multi-chamber structures typically used in parking lots to provide additional attention to filtration of petroleum-based products.

A swirl concentrator is a small, compact solids separation device with no moving parts. During wet weather water flows into the unit, creating a vortex that separates out the heavy grit in the first flush.

Side view diagram of an oil-grit separator.

Side view diagram of an oil-grit separator. Source: OH CMP Guide – Appendix H, complements of T.R. Schueler.

Some basic principles apply to all stormwater runoff management structures to ensure their effectiveness.

  • Select a stormwater management structure that is appropriate for your property.
  • Maintain all stormwater management structures properly.

Next: Unit Review