Conservation Design for Stormwater Management A Design Approach To Reduce Stormwater Impacts from Land Development and Achieve Multiple Objectives Related to Land Use A Joint Effort Between the Delaware Department of Natural Resources and Environmental Control and The Environmental Management Center of the Brandywine Conservancy September, 1997 Conservation Design for Stormwater Management A Design Approach To Reduce Stormwater Impacts From Land Development and Achieve Multiple Objectives Related to Land Use By the Sediment and Stormwater Program Delaware Department of Natural Resources and Environmental Control and The Environmental Management Center, Brandywine Conservancy Advisory Committee ! Greg Scott Consulting Engineers Council ! James McCulley Delaware Association of Homebuilders ! Rob McCleary Del. DOT - stormwater program ! Eileen Butler Delaware Nature Society ! Deborah Mills Water Resources Agency for N.C. Co. ! Mike Bennett Department of Planning (N.C. Co.) ! John Casey Delaware Contractors Association ! Roger Brickley Delaware Association of Surveyors ! Stever Abbott Natural Resources Conservation Service ! Bruce Jones N.C. Co. Public Works Hap Ryan N.C. Co. Public Works ! Joel Leidy Del. DOT - Subdivision Design ! Connie Holland Kent Co. Planning Department ! Alex Settles State Planning Coordination ! Mike Brady Sussex Co. Planning and Zoning ! Rich Baccino N.C. Conservation District ! John Garcia Kent Conservation District ! John Barwick Sussex Conservation District ! Connie Zipperer Delaware Department of Agriculture ! Vic Schuler Environmental Consulting Services, Inc. ! Cliff Short Burns & Ellis Realtors ! Harvey Rubenstein Delaware Section of the ASLA ! John Schneider DNREC - Water Resources ! John Maxted DNREC - Water Resources ! Albert Todd Chesapeake Bay Program Liaison September, 1997 Index Page Introduction Purpose i Audience ii Background ii Limitations of Structural Stormwater Management vi Conservation Design Approaches xii Watershed Wide Approaches xvi Manual Organization xvii Chapter 1 Importance of the Water Cycle in Stormwater Management The Water Cycle 1-1 Precipitation 1-4 Groundwater Recharge and Stream Baseflow 1-8 Stormwater (Surface) Runoff 1-12 Ecological Response of Urbanization and Stormwater 1-16 Other Impacts to the Water Cycle: Water Supply 1-20 and Wastewater Management Stormwater and Water Quality 1-22 Physical Types of Pollutants: Soluble vs. Particulate 1-24 Natural Mechanisms for Stormwater Pollutant 1-24 Reduction/ Mitigation Chapter 2 Site Resources and Limitations Introduction 2-1 Ecology and Landscape Position 2-1 Wetlands 2-10 Floodplains 2-13 Riparian Buffers 2-16 Forests 2-19 Meadows 2-24 Soils 2-26 Other Natural Features 2-30 Linkage with Site Development 2-33 Literature Cited 2-34 Page Chapter 3 The Conservation Design Approach: Nonstructural Conservation Techniques Defined Conservation Design Principles 3-2 Importance of Natural Processes in Conservation Design 3-3 Soil Linked Processes for Water Quality/Quantity 3-4 Management Vegetation - Based Processes for Water Quality and 3-6 Quantity Management Conservation Design: Approaches and Techniques 3-7 Conservation Design: Preventive and Mitigative 3-8 versus Structural and Nonstructural Conservation Design: Best Management Approaches (BMA’s) 3-9 and Practices (BMP’s) Conservation Design Approach: Comprehensive 3-11 Planning/Zoning (Macro/Areawide) Conservation Design Approach: Clustering/Lot Configuration 3-15 Conservation Design Approach: Reduction in Setbacks 3-24 Conservation Design Approach: Reduction in Imperviousness 3-24 Conservation Design Approach: Minimum Disturbance/ 3-34 Maintenance (Site Fingerprinting/Site Footprinting Conservation Design Practice: Vegetative 3-39 Filter Strips and Buffers Conservation Design Practice: Use of Natural Areas 3-48 Including Reforestation and Revegetation Conservation Design Practice: Terracing, Berming, 3-52 Special Grading (Terraforming) Conservation Design Practice: Grassed/Vegetative Swales 3-57 Chapter 4 The Conservation Design Procedure Introduction 4-1 The Conservation Design Procedure in Overview 4-2 The Conservation Design Procedure in Detail 4-4 Site Analysis Site Factors Inventory: What Site Physical 4-6 Factors Affect Conservation Design Site Factors Analysis: What Site Factors are Constraints 4-8 and Opportunities in terms of conservation design Use of Preventive Conservation Design Approaches 4-9 Lot Configuration and Design: How Can Lot Configuration 4-12 and Overall Site Design Prevent Stormwater Generation Impervious Coverage: Have Impervious Surfaces Been 4-14 Reduced as Much as Possible Page Minimum Disturbance/Maintenance: Has Disturbance of 4-15 Site Vegetation and Soils Been Minimized Use of Mitigative Conservation Design Practices: Which 4-19 Practices are most Effective and How Can Their Positive Effects be Maximized The Conceptual Stormwater Management Plan: How Can 4-22 All Preventive Approaches and Mitigative Techniques be Integrated Into and Optimal Conservation Design Plan Stormwater Calculations: How has Conservation Design 4-24 Affected Stormwater Calculations? What Conventional Stormwater Techniques are Necessary to Manage Any Residual Stormwater Need not Mitigated by Conservation Design Selection of Additional Stormwater Controls: If Conservation 4-28 Has Not Fully Met All Stormwater Requirements, What Additional Requirements Must Be Provided Chapter 5 Conservation Design Case Study Analysis Conservation Design Case Studies 1 and 2: Chapel Run 5-4 Conservation Design Case Study 3: Buckingham Greene 5-17 Conservation Design Case Studies 4 and 5: Tharpe Knoll 5-26 Conservation Design Case Study 6: Pleasant Hill Farm 5-38 Appendix A Biofiltration Design Procedure from the State of Washington (to be completed) Appendix B Natural Resource Conservation Service Maryland Conservation Practice “Standard Ripar- ian Forest Buffer”, July, 1996. Conservation Design for Stormwater Management Introduction “Conservation Design” is a design approach to site develop- ment that protects and incorporates natural site features into the stormwater management plan. Purpose The primary The primary purpose of this manual is to provide guidance for site de- purpose of this sign which incorporates conservation into land development. The intent is to provide an incentive for land developers to retain and incorporate manual is to natural site features into the site development process and thereby re- provide guidance duce or eliminate the need for structural stormwater management con- for site design trols. Other benefits are certainly realized through conservation design, which such as more closely approximating the predevelopment water budget, protection of habitat, and reduced overall impact to the receiving sys- incorporates tem. Site features which will be discussed in the manual include: conservation into land development. ! Wetlands ! Floodplains ! Forested areas The intent is to ! Meadows provide an ! Riparian buffers incentive for land ! Soils ! Other natural features developers to retain and Design procedures are provided which allow site designers to incorpo- incorporate rate practices, inherently known to be good, but which have not had a natural site sound rational basis to ensure plan approval. That rational basis will be provided in this manual for a variety of situations. The design approach features into the will be flexible enough to allow for various conservation practices to be site development combined on one site and quantify the benefits of that combination. process and It must be emphasized that structural controls will still be essential on thereby reduce or many sites. A heavily-wooded site having a significant portion of the eliminate the need tree canopy removed will still have a significant increase in stormwater for structural runoff, even with aggressive conservation planning. The practices de- stormwater tailed in this manual are provided as additional tools in the stormwater management toolbox. They may supplement structural control practices management and may, in some situations, replace or reduce the need for structural controls. practices while providing attractive site amenities. i Introduction Audience The manual is primarily directed toward residential land developers and users. Commercial and industrial land uses generally have a much higher level of site imperviousness, which limits the degree that conservation design can be utilized. Conservation practices are still recommended on commercial and industrial sites, but the ability of conservation prac- tices to eliminate or significantly reduce the need for structural practices on commercial or industrial sites may be limited. The Sediment and Stormwater Law July, 1989 flood in New Castle County was created in Christina Watershed response to increasing land development Background pressures which Stormwater management has been implemented at a statewide level in caused increased Delaware since July 1, 1991. The Sediment and Stormwater Law was levels and created in response to increasing land development pressures which frequency of caused increased levels and frequency of flooding, further degraded water quality, and impacted aquatic organisms and habitat. The major flooding, further catalyst for approval of the law was a flood in July, 1989 which caused degraded water significant property damage. This flood heightened public awareness to quality, and the cumulative nature of flooding problems caused by continued land impacts to aquatic development. organisms and In addition