Bioswales: A Guide to Low-Impact Development Design and Maintenance. Introduction and Compilation by: Leslie D. Bartsch And Jennifer S.

Transcription

1 Bioswales: A Guide to Low-Impact Development Design and Maintenance Introduction and Compilation by: Leslie D. Bartsch And Jennifer S. Raible Biological Engineering University of Arkansas Fayetteville, Arkansas

2 Introduction The low-impact development (LID) approach combines a hydrologically functional site with pollution remediation techniques to counter-act the effects of land development on water quality and hydrology on the site and surrounding areas. LID achieves these objectives by: Minimizing stormwater impacts through incorporating pervious pavements where possible, maintaining natural drainage courses, and reducing the use of pipes. Providing stormwater runoff storage through a variety of detention, retention, and runoff techniques. Maintaining predevelopment or even native conditions time of concentration through strategic routing of flow. Educating the public and property owners on the benefits of LID practices and encouraging them to employ pollution prevention measures. This reference manual contains multiple resources that address the issues of design, limitations, maintenance, and cost considerations of LID technology in great detail, and can be used to apply this technology to any development site within design limitations. Appendices 3 and 5 specifically focus on the methods of incorporating bioswale stormwater conveyance systems into LID communities. Low-Impact Development Design Strategies: An Integrated Design Approach (Appendix 1) is an inclusive guide to LID design specifically addressing site planning, hydrology, distributed integrated management practice (IMP) technology, erosion and sediment control, and public outreach. Green Technology: The Delaware Urban Runoff Management Approach (Appendix 2) contains design standards and specifications and detailed drawings of LID practices and landscaping standards and specifications. Analysis of Bioswale Efficiency for Treating Surface Runoff (Appendix 3) specific information on bioswales including the chemical processes, biological considerations, economic and regulatory considerations and design assessment. Maintenance Plan Stormwater Management System (Appendix 4) details guidelines for maintenance of LID systems. Basic Biofiltration Swale (Appendix 5) outlines the purpose, limitations, materials, and installation of bioswale systems. BIOSWALES General Description Bioretention is a low-impact development (LID) practice incorporated into commercial or residential areas in order to manage and treat stormwater runoff. Shallow depressions known as bioretention facilities, bioinfiltration cells, grassed swales, or bioswales,

3 contain conditioned planting soil beds and planting materials which are used to filter runoff stored within the depression. This method combines physical filtering and absorption with biological processes, thus enhancing ecological services on the site. The bioswale decreases speed of flows, acts as a stormwater detention structure, and allows suspended solids in runoff to settle out. As runoff passes slowly and evenly through the channel, aboveground vegetation filters particulates and their associated pollutants. The pollutants are then incorporated into the soil where they may be immobilized and/or decomposed by plants and microbes. The bioswale is considered a creative means of controlling runoff, and has the potential to provide ecological services such as improved water quality, mitigation of wetland loss, flood containment, and improved aesthetics of the project site. As such, the bioswale has hydrologic, chemical, and biological functions. Design Considerations and Limitations Many factors should be considered before designing and constructing a bioswale. Slope of the land, soil composition, available space, types of vegetation, climate, and dimensions of the bioswale and of the site itself will influence the success of the bioswale functionality. Permeable soils with infiltration rates greater than 0.27 in/hr are recommended, and soil limitations can be overcome with the use of underdrains. Minimum surface area range is sq. ft. with a minimum width of 5 10 ft, minimum length of ft., and a depth of 2 4 ft. depending on soil type. Hydrologic function such as infiltration, frequency, and volume of discharges and groundwater recharge are essential when considering bioswale construction. Bioswales will not thrive in constantly wet or shady conditions. Drying out between storms is necessary to allow air into the soil and root zone of the vegetation. Vegetation Selection Vegetation incorporated into the bioswale is critical in slowing runoff velocities, increasing infiltration, and filtering pollutants. Native herbaceous plant materials are the preferred choice for bioswales. Turf may be acceptable in formal landscaped settings. Woody vegetation should not be specified for the bottom of bioswales, since woody plants provide minimal filtering and will eventually shade out the herbaceous plants. Saturation tolerance, flooding tolerance, drought tolerance, and shade tolerance differ substantially among plants, and so the appropriate species must be selected for each planting zone, accordingly. Urban Pollutant Removal Common pollutants found in urban-setting runoff are phosphorous, hydrocarbons, nitrogen, heavy metals, herbicides and pesticides, bacteria, and floatable debris. Fertilizer is the primary source of phosphorous and nitrogen. Although these nutrients are essential to life, excess inputs can be harmful, resulting in algal blooms. Algal blooms reduce light penetration, causing submerged aquatic vegetation to die, which results in decreased dissolved oxygen levels. Hydrocarbons from motor oil, break fluid, and gasoline further pollute runoff. Floatable debris such as glass or paper can clog or slow

4 the movement of runoff through the bioswale. Bioswale vegetation function in reducing the conveyance of such pollutants to larger bodies of water. Maintenance and Costs Maintenance of bioswales is more demanding on the property owner but less demanding on the local government than traditional stormwater conveyance systems. Maintenance costs for traditional stormwater conveyance systems are significant and have become a burden for local governments. In comparison, bioswale maintenance requires little more than normal landscaping, and so is less expensive. To ensure proper functioning, the bioswale must be monitored frequently for debris and obstructions, especially after a rain event. Vegetation health is critical and should be maintained accordingly. Residents and/or property owners should be informed of proper maintenance of the site. Public Outreach A critical component to the successful operation of bioswales is the proper maintenance of the installed system by the property owners. Information should be provided to commercial and resident property owners regarding maintenance practices and effective prevention pollution practices. It is important that the developer and the local public agency effectively communicate the benefits of low-impact development to the property owners through distributed outreach materials. Property owners should also be educated about the necessity of not disturbing, compacting, or eliminating bioswale systems.

5 INDEX This handbook includes a compilation of the following previously developed LID manuals: Appendix 1: Low-Impact Developments Design Strategies: An Integrated Design Approach as prepared by Prince George s County, Maryland, Department of Environmental Resources, Programs and Planning Division. June Appendix 2: Green Technology: The Delaware Urban Runoff Management Approach. Standards, Specifications, and Details for Green Technology BMPs to Minimize Stormwater Impacts from Land Development. William C. Lucas. June Appendix 3: Analysis of Bioswale Efficiency for Treating Surface Runoff. Donald Bren School of Environmental Science and Management. University of California, Santa Barbara. By William Groves, Phillip Hammer, Karinne Krutsen, Sheila Ryan, and Robert Schlipf. Undated. Appendix 4: Maintenance Plan: Stormwater Management System. Ryerson Woods Conservation Area - Visitors Center. Lake County Forest Preserves. July Appendix 5: Basic Biofiltration Swale. Surface Water Design Manual. King County, Washington. Undated. Additional information concerning low-impact strategies and bioswales can be found at the following web pages: