Design of Stormwater Filtering Systems
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Design of Stormwater Filtering Systems Prepared by Richard A. Claytor and Thomas R. Schueler The Center for Watershed Protection 8391 Main Street Ellicott City, MD 21043 (410) 461-8323 Prepared for Chesapeake Research Consortium, Inc. P.O. Box 1280 Solomons, MD 20688 (410) 326-6700 with supplemental funding by U.S. Environmental Protection Agency, Region 5 The contents do not necessarily reflect the views and policies of the Chesapeake Research Consortium, Inc. or the Environmental Protection Agency, nor does the mention of trade names or commercial products constitute endorsement or recommendation for use. December 1996 Printed on Recycled Paper Abstract: Title: Design of Stormwater Filtering Systems Date: December, 1996 Authors: Richard A. Claytor and Thomas R. Schueler Center for Watershed Protection Publisher: Published under a cooperative agreement with the Chesapeake Research Consortium with supplemental funding by Region 5 of the Environmental Protection Agency by the Center for Watershed Protection. The Center for Watershed Protection is a non-profit organization dedicated to the protection, restoration, and stewardship of our nation’s water resources and watersheds through sensitive management of the land. The Center promotes the advancement of innovative and effective land and water management techniques, and serves as a forum for planners, engineers, landscape architects, and municipal officials engaged in watershed protection. Abstract: The project is oriented to create a unified design manual for stormwater filtering systems to remove pollutants from urban runoff generated at smaller sites within the Chesapeake Bay watershed. The primary audience for the manual are engineers, planners and landscape architects at the local or state level that need to comply with stormwater regulations in urban or suburban areas. The manual presents detailed engineering guidance on eleven different filtering systems. The term stormwater filter refers to a diverse spectrum of stormwater treatment methods utilizing various media, such as sand, peat, grass, soil or compost to filter out pollutants entrained in urban stormwater. These filters are typically designed solely for pollutant removal, and serve small development sites. The three broad groups include: sand filters (surface, underground, perimeter, organic, and pocket designs), bioretention, and vegetated channels (grass channels, dry swales and wet swales, filter strips, and gravel wetlands). The seven chapter design manual promotes a volume-based sizing criteria for all filtering systems utilizing principles of small storm hydrology, provides detailed guidance on the selection of appropriate filter types for various applications, reviews pollutant performance data and pathways for stormwater filtering systems, and provides detailed engineering design principles and guidance. The manual provides several design examples and contains over one-hundred tables and figures. Price: $30.00 Available from: The Center for Watershed Protection 8737 Colesville Road Silver Spring, MD 20910 (301) 589-1890 PREFACE This project is oriented to create a unified design manual for stormwater filtering systems to remove pollutants from urban runoff generated at smaller sites within the Chesapeake Bay watershed. The primary audience for the manual are engineers, planners and landscape architects at the local or state level that need to comply with stormwater regulations in urban or suburban areas. This manual continues the Center's efforts to produce urban stormwater practice design manuals targeted at specific categories of systems. Stormwater filtering is just one of these targeted areas. Existing and future manuals will cover areas such as wetland systems and pond systems. Primary funding support for the preparation of this manual has been provided by a grant from The Chesapeake Resource Consortium with supplemental funding by Region 5 of the U.S. Environmental Protection Agency to complete Chapter 6, and the appendices. ACKNOWLEDGMENTS Preparation of this manual would not have been possible without the contributions from many experts in the stormwater management filtering field. The authors would like to extend our appreciation and acknowledge the following individuals for their contributions and constructive comments: Robert Pitt, Warren Bell, Richard Horner, Earl Shaver, Hung Van Truong, Larry Coffman, and John Galli. We would expressly like to thank Earl, Warren and several of their co-workers for taking time out of their busy schedules to show us several examples of stormwater filters and their adaption to real world problems. This provided a much welcomed “reality check” for some of our design parameters contained herein. Special gratitude to Don Koch of Engineering Technologies Associate for his contributions on the bioretention water balance analysis, Tim Schueler and Rick Scaffidi of Environmental Quality Resources for their insightful comments and suggestions, Ricardo Gonzalez and Jeffery Everhart for their diligent work on the grass species selection guide, Keith Bowers for his contributions to the bioretention species selection guide, Dean Geiser and Donna deMars for their superb preparation of graphic figures and drawings, and Arlene Allegretto for her tireless efforts in making this manual a reality. I INTRODUCTION The manual presents detailed engineering guidance on ten different filtering systems. The term stormwater filter refers to a diverse spectrum of stormwater treatment methods which utilize an artificial media , such as sand, peat, grass, soil or compost to filter out pollutants entrained in urban stormwater. These filters are typically designed solely for pollutant removal (quantity bypassed), and serve small development sites (usually less than five acres). The three broad groups include: sand filters (surface, underground, perimeter, organic, and pocket designs), bioretention and vegetated channels (grass channels, dry swales wet swales, and filter strips). The underlying concept of the manual is that a common and unified approach was needed to design each type of stormwater filter, so that this useful technology can gain wider engineering acceptance at the local level. Therefore, each stormwater filter incorporates four standard engineering features: a flow regulator, a pretreatment mechanism , filter media and bed specification, and overflow channels. In addition, the manual presents a single volumetric sizing requirement for each filter which is to capture and treat 90% of the runoff producing events that occur each year. Many prior design approaches had been rate-based, and resulted in limited and unreliable pollutant removal rates. A third feature of the manual is that it utilizes new techniques for calculating runoff rates and volumes that reflect small storm hydrology from small, heterogeneous urban sites. Field research has indicated these methods are superior to traditional applications of the National Resource Conservation Service (NRCS) runoff forecasting models (such as TR-55 and TR- 20). The manual also includes numerous step-by-step design examples that help an engineer apply the new design techniques. Lastly, the manual synthesizes recent research and field experience on the pollutant removal performance, longevity, cost, and maintenance burden of each type of stormwater filter, drawn from a national literature and phone survey. This information has been condensed in a series of tables that help designers and municipal officials select the most effective stormwater filter for their situation, and compare the performance of stormwater filters to that of other stormwater BMP options (e.g., ponds, wetlands, and infiltration systems). Although stormwater filters can be applied to a diverse range of development conditions as a group, individual designs are limited to a more narrow range of site conditions . The most economical and feasible options are identified for five broad categories of development: ultra-urban, parking lots, roads, residential subdivisions, II DESIGN OF STORMWATER FILTERING SYSTEMS INTRODUCTION and backyard/rooftop drainage. Key feasibility factors that influence the selection of stormwater filters include space consumption, minimum head, maintenance burden, cost/acre and soil conditions. During the study, over thirty published and unpublished studies on the pollutant removal performance of stormwater filtering systems were consulted (and are abstracted in Appendix A and cited in the References). Estimated removal rates for each of the stormwater filters are derived in Chapter 4, based on monitoring studies, infiltration rates, modeling and inference from similar technologies. Despite their many differences in design, stormwater filters have many similarities in performance. The performance, feasibility, and environmental restrictions of stormwater filters are compared to three other groups of stormwater BMPs that are currently in widespread use by engineers in the Chesapeake Bay region-ponds, wetlands and infiltration systems. In general, stormwater filters are the most feasible option for smaller development sites (less than 5 acres) but are not typically cost effective beyond that drainage area. Other BMPs, most notably ponds and wetlands, also have higher or more reliable removal rates for nutrients, bacteria and hydrocarbons. Ponds and wetlands, however, cannot usually be applied on small development sites and ultra urban conditions. Another key advantage of stormwater filters as a group is their lack of environmental drawbacks,