Stormwater Management Fact Sheet: On-Lot Treatment
The term on-lot treatment refers to a series of practices that are designed to treat runoff from individual residential lots. The primary purpose of most on-lot practices is to manage rooftop runoff and to a lesser extent driveway and sidewalk runoff. Rooftop runoff, and particularly residential rooftop runoff, generally has low pollutant concentrations compared with other urban sources (for more information see Sources of Urban Stormwater Pollutants Defined in Wisconsin, Article 7 in the Practice of Watershed Protection). The primary advantage of managing runoff from rooftops is to disconnect these impervious surfaces, reducing the effective impervious cover in a watershed. Many of the impacts of urbanization on the habitat and water quality of streams are related to the fundamental change in hydrologic cycle caused by the increase of impervious cover the landscape (for more information see The Importance of Imperviousness, Article 1 in The Practice of Watershed Protection).
Although there are a wide variety of on-lot treatment options, they can all be classified into one of three categories: 1) practices that infiltrate rooftop runoff; 2) practices that divert runoff or soil moisture 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 dry well. 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 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 with a perforated pipe within the gravel bed to distribute flow throughout the length of the trench.
Runoff can be diverted to a pervious area or to 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 Fact Sheet). Alternatively, rooftop runoff can simply be diverted to pervious lawn areas, as opposed to flowing directly to the street, and thus the storm drain system.
Practices that store rooftop runoff, such as cisterns and rain barrels, are the simplest in design of all 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 are particularly valuable in the arid southwest, where water is at a premium, rainfall is infrequent, and reuse for irrigation can save homeowners money.
Some form of on-lot treatment can be applied to almost all sites, with very few exceptions (e.g., very small lots or lots with no landscaping). Traditionally, on-site treatment of residential stormwater runoff has been encouraged, but has not been a widely used option to meet stormwater requirements. There are currently at least two jursidictions, however, that offer "credits" in exchange for the application of on-lot stormwater management practices. In Denver, Colorado, sites designed with methods to reduce "directly connected impervious cover," including disconnection downspout runoff from the storm system, are permitted to use a lower site impervious area when computing the required storage of stormwater facilities (DUDFC, 1992). Similarly, new regulations for Maryland allow designers to subtract each rooftop that is disconnected from the total site impervious cover when calculating required storage for stormwater management practices (CWP, 1998).
Siting and Design Considerations
Although most residential lots can incorporate on-lot treatment, the best option for a site depends on site constraints and the preferences of the homeowner. On-lot infiltration practices have the same restrictions regarding soils as other infiltration practices (see Fact Sheets on Infiltration Basins and Infiltration Trenches). If other design practices are used, such as bioretention or grassed swales, they need to meet the design requirements of those practices (see Fact Sheets on Bioretention and Grassed Swales). 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 stored water, and the practice must handle overflow and winter freezing conditions. Often, these practices are ideally suited for a homeowner that has an active interest in gardening or landscaping.
Although these practices are simple compared with many other stormwater practices, the design needs to incorporate the same basic elements of any stormwater practice. Pretreatment is important to ensure that they do not become clogged with leaf debris. Infiltration practices may be preceded with a settling tank or, at a minimum, a grate or filter in the downspout to trap leaves and other debris. Rain barrels and cisterns also often incorporate some sort of pretreatment, such as a mesh filter at the top of the barrel or cistern.
Both infiltration practices and storage practices typically incorporate some type of bypass so that larger storms flow away from the house. In rain barrels or cisterns, this bypass may be an overflow hose that directs runoff away from the practice and building foundation. A second hose is often set at the elevation of the bottom of the practice, which can be used to irrigate landscaping or for other uses by attaching this hose to a standard garden hose, and controlling flow with an adjustable valve. In infiltration practices, the bypass may be an above-ground opening of the downspout. As on-lot practices, grassed swales and bioretention can be designed on-line. That is, all flows are directed to the practice, but larger flows generally flow over the practice, and are not treated.
One important design feature for on-lot practices is to locate infiltration areas sufficiently far from house foundations to prevent undermining of the foundation or seepage into basements. These practices should be separated at least ten feet from the house to prevent these problems.
There are some limitations to the use of on-lot practices, including:
Bioretention, filters strips, and grassed swales require regular maintenance to ensure that the vegetation remains in good condition. Infiltration practices require regular removal of sediment and debris settled in the pretreatment area, and need to be replaced when the practice becomes clogged. 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 approximately once per year.
Although the practices used for on-lot applications can have relatively high pollutant removal, (see Fact Sheets on Infiltration Trench, Bioretention, Filter Strips, and Grassed Swales), it is not clear that these pollutant removal rates can be realized with the relatively low pollutant concentrations entering the practices. 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, 2000) (for more information see Irreducible Pollutant Concentrations Discharged from Stormwater Practices, Article 65 in the Practice of Watershed Protection). The primary role of these practices is to shave peak flows associated with small storms, thus reducing the required storage in downstream stormwater practices. This reduced storage can help alleviate the cost of these larger practices, and also reduce the possible impacts to wetlands and natural areas commonly associated with larger practices (for more information see Wet Ponds and Wetlands Fact Sheets). Another benefit of many on-lot practices is that they generally promote groundwater recharge, either directly through infiltration or indirectly by applying or direction runoff to pervious areas.
On a cost per unit area treated, on-lot practices are relatively expensive compared with other stormwater treatment options. Typical costs are: $100 for a rain barrel, $200 for a dry well or French Drain. On-lot application of bioretention is approximately 20% less than commercial applications, or roughly $5.50 per cubic foot of stormwater treated. 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.
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Schueler, T. 2000. The Importance of Imperviousness, Article 1 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.
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Schueler, T. 2000. Irreducible Pollutant Concentrations Discharged from Urban Stormwater Treatment Practices, Article 65 in The Practice of Watershed Protection. Ellicott City, MD.