Water: Best Management Practices
The term "on-lot treatment" refers to a range of practices designed to treat runoff from individual residential lots. The primary purpose of most on-lot practices is to manage runoff from rooftops and, to a lesser extent, driveways and sidewalks. Rooftop runoff, particularly from residential roofs, generally has low pollutant concentrations compared to other urban sources (Schueler, 1994b). Managing runoff from rooftops effectively disconnects these impervious surfaces, reducing a watershed's overall imperviousness. This is important because many of the deleterious effects of urbanization on water quality can be traced to fundamental changes in the hydrologic cycle caused by increases in impervious materials, like roofs, covering the landscape (Schueler, 1994a).
Although a variety of on-lot treatment options exist, all can be placed in one of three categories: 1) practices that infiltrate rooftop runoff; 2) practices that divert runoff to a pervious area; and 3) practices that store runoff for later use. The best option depends on the goals of a community, the feasibility at a specific site, and the preferences of the homeowner.
The practice most often used to infiltrate rooftop runoff is the drywell. In this design, the storm drain is directed to an underground rock-filled trench that is similar in design to an infiltration trench (see Infiltration Trench fact sheet). French drains or Dutch drains can also be used for this purpose. In these designs, the relatively deep dry well is replaced with a long trench equipped with a perforated pipe buried within the gravel bed to distribute flow throughout the length of the trench.
Runoff can be diverted to a pervious area or a treatment area using site grading, or channels and berms. Treatment options can include grassed swales, bioretention, or filter strips. The bioretention design can be simplified for an on-lot application by limiting the pre-treatment filter and, in some cases, eliminating the underdrain (see Bioretention (Rain Gardens) fact sheet). Alternatively, rooftop runoff can simply be diverted to pervious lawns, as opposed to flowing directly onto the street and then to the storm drain system.
Practices that store rooftop runoff, such as cisterns and rain barrels, are the simplest of all of the on-lot treatment systems. Some of these practices are available commercially and can be applied in a wide variety of site conditions. Cisterns and rain barrels can be particularly valuable in the arid southwest, where water is at a premium, rainfall is infrequent, and reuse for irrigation can save homeowners money.
With very few exceptions (e.g., very small lots or lots with no landscaping), some sort of on-lot treatment can be applied to most sites. Traditionally, on-site treatment of residential stormwater runoff has been encouraged, but it has not generally been an option to meet stormwater requirements. Currently, at least two jurisdictions offer "credits" in exchange for the application of on-site stormwater management practices. In Denver, Colorado, sites designed with methods that reduce "directly connected impervious cover," methods like disconnecting downspout runoff from the storm sewer system, are permitted to use a lower site impervious area when computing the required storage of stormwater facilities (DUDFCD, 1992). Similarly, Maryland regulations allow designers to subtract each disconnected rooftop from the site's total impervious cover when calculating required storage in stormwater management practices (MDE, 2000).
Siting and Design Considerations
Although most residential lots can incorporate on-lot treatment, options for a site design depend on design constraints and homeowner preferences. On-lot infiltration practices have the same restrictions regarding soils as other infiltration practices (see Infiltration Basin and Infiltration Trench fact sheets). If bioretention, grassed swales or other design practices are used, they need to meet the siting requirements of those practices (see Bioretention (Rain Gardens) and Grassed Swales fact sheets). Of all of the practices, cisterns and rain barrels have the fewest site constraints. In order for the practice to be effective, however, homeowners need to have a use for the water collected. The design must also accommodate overflow and winter freezing conditions. These practices are best suited to an individual who has some active interest in gardening or landscaping, since the stored water may be used for irrigation.
Although simple compared to other post-construction stormwater practices, on-lot infilitration needs to include the same basic elements of any stormwater practice. Pretreatment is important to ensure they do not clog with leaf debris. A settling tank or, at a minimum, a debris-trapping grate or filter in the downspout, may precede infiltration practices. Some sort of pretreatment, such as a mesh filter, is often also found at the top of rain barrels and cisterns.
Both infiltration and storage practices typically incorporate some type of bypass to direct heavy runoff flows away from homes. For example, a hose mounted at the bottom of the barrel or cistern, is typically used for irrigating gardens or for landscaping. This hose can be attached to a standard garden hose or to drip tape, which is a hose with small holes that is laid on the ground surface to allow the water to soak into the ground. Then the flow can be controlled with an adjustable valve. In infiltration practices, an above ground opening in the downspout can serve as the bypass. In on-lot practices, grassed swales and bioretention cells can be designed to absorb all but the largest of stormwater flows. In extreme cases, flows generally flow untreated over the practices.
When designing infiltration practices, it is important to locate the infiltration area far enough away from the house's foundation to prevent the undermining of the foundation or basement seepage. The infiltration area should be at least 10 feet away from the house.
There are limitations to the use of on-lot practices, including the following:
- These practices require homeowners to perform some basic maintenance.
- For homeowners who do not enjoy landscaping, it may be difficult to find uses for water stored in rain barrels or cisterns, since the water is not potable.
- Some of these practices may be impractical on small lots.
- Even if applied to every home in a watershed, these practices would only treat a relatively small portion of the watershed imperviousness, which is largely composed of roads and parking areas (see Narrower Residential Streets and Green Parking fact sheets).
Bioretention areas, filter strips, and grassed swales require regular maintenance to ensure that the vegetation remains in good condition (see Bioretention (Rain Gardens) ; Vegetated Filter Strip; and Grassed Swales fact sheets). Infiltration practices require regular removal of sediment and debris settled in the pretreatment area, and the media might need to be replaced if it becomes clogged (see Infiltration Trench fact sheet).
Rain barrels and cisterns require minimal maintenance, but the homeowner needs to ensure that the hose remains elevated during the winter to prevent freezing and cracking. In addition, the tank needs to be cleaned out about once per year. Furthermore, rain barrels and cisterns should be checked periodically to ensure that they are properly sealed to prevent mosquito breeding.
Although the practices used for on-lot applications can have relatively high pollutant removals (see Infiltration Trench; Bioretention (Rain Gardens) ; Vegetated Filter Strip; and Grassed Swales fact sheets), it is unclear that these pollutant removal rates can be realized, since the pollutant concentrations entering the systems are generally low. Some data suggest that, at least for stormwater ponds, there may be an "irreducible concentration" below which no further pollutant removal can be achieved (Schueler, 1996). Another benefit of many on-lot practices is that they generally promote ground water recharge, either directly through infiltration or indirectly by applying or directing runoff to pervious areas.
On a cost per unit-area treated, on-lot practices are relatively expensive compared with other stormwater treatment options. It is difficult to make this comparison, however, because the cost burden of on-lot practices is born directly by homeowners. Typical costs are $100 for a rain barrel and $200 for a dry well or a French drain. For many of these practices, homeowners can reduce costs by making their own on-lot practice rather than purchasing a commercial product.
Some treatment practices, such as rain barrels and on-lot bioretention, offer additional benefits to the homeowner that may offset the cost of applying the practice. Similarly, maintenance costs are essentially free, with the exception of replacement of a dry well system, which may require outside help.
Denver Urban Drainage and Flood Control District (DUDFCD). 1992. Urban Storm Drainage Criteria Manual: Volume 3¿Best Management Practices. Denver Urban Drainage and Flood Control District, Denver, CO.
Maryland Department of the Environment (MDE). 2000. Maryland Stormwater Design Manual. [www.mde.state.md.us/environment/wma/stormwatermanual ]. Accessed May 22, 2001.
Schueler, T. 1994a. The importance of imperviousness. Watershed Protection Techniques 1(3):100¿111.
Schueler, T. 1994b. Sources of urban stormwater pollutants defined in Wisconsin. Watershed Protection Techniques 1(1):30-32.
Schueler, T. 1996. Irreducible pollutant concentrations discharged from urban BMPs. Watershed Protection Techniques 1(3):100-111.
City of Tucson, Arizona, Stormwater Quality Program. 1996. Water harvesting fact sheets. City of Tucson Stormwater Quality Program, Tucson, AZ.
Konrad, C., B. Jensen, S. Burges, and L. Reinelt. 1995. On-Site Residential Stormwater Management Alternatives. University of Washington, Seattle, WA.
Prince George's County, Maryland, Department of Environmental Resources. 1997. Low Impact Development. Prince George's County, Maryland, Department of Environmental Resources, Laurel, MD.
Schueler, T. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments, Washington, DC.
Center for Watershed Protection. 1998a. Better Site Design: A Handbook for Changing Development Rules in Your Community. Center for Watershed Protection, Ellicott City, MD.