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Site Design GuidanceSITE DESIGN GUIDANCE TOOLS FOR INCORPORATING POST- CONSTRUCTION STORMWATER QUALITY PROTECTION INTO CONCEPT PLANS AND LAND DISTURBANCE PERMITTING Revision 1 - April 17, 2009 The Metropolitan St. Louis Sewer District, Developed in Conjunction with the St. Louis Municipalities Phase II Storm Water Steering Committee 2 Table of Contents 1. Introduction 4 2. Evaluation of Existing Conditions 6 3. Evaluation of Concept Plans 10 4. Integration with MSD Stormwater Quality Review 18 5. Conclusion 21 Table 1 Existing Natural Resource Considerations 7 Table 2 Site Development Goals, Questions, and Methods 11 Table 3 Post-Construction Structural BMPs and Allowed 14 Figures Figure 1 Generic Stormwater Site Design Approval Process 19 Appendices Appendix A Example Maps Appendix B Typical BMP Layouts and Sections Tables 3 The effect of intense urbanization on natural watercourses is well documented: as urbanization (and impervious area) increases, the diversity and quality of aquatic life that exists within urban streams decreases. The sources of impairment (for example, roadway runoff, fertilizer runoff, creek bank ero- sion, litter, etc.) to these streams are collectively referred to as non-point source pollution (NPS). The Clean Water Act Phase II Stormwater Regulations were promulgated to provide appropriate storm- water management of NPS pollution in urbanized areas, and these regulations apply to the water- sheds located within the Metropolitan St. Louis Sewer District (MSD). The Missouri Department of Natural Resources (DNR), through the Small MS4 Permit (Permit), sets requirements for stormwater management within the separate sewer portion of the MSD in St. Louis County. The St. Louis County Phase II Stormwater Management Plan (Plan) sets forth specific activities and schedules that the MSD, St. Louis County, and its fifty-nine municipal co-permittees must do to satisfy the Per- mit requirements. This document has been prepared to assist in im- plementing the Permit and Plan requirements re- lated to Post-Construction Storm Water Manage- ment in New Development and Redevelopment. These requirements apply to all new development and redevelopment projects that disturb greater than or equal to one acre, including (but not limited to) municipal roadway projects, residential infill and redevelopment projects, commercial and industrial development and redevelopment projects, and new residential subdivisions. While this topic’s descrip- tion implies measures that should be taken at pro- ject completion, its implementation begins in the project planning process, before project clearing and grading (i.e., land disturbance). The Permit states that water quality impacts must be prevented or minimized by mimicking pre-construction runoff conditions on new development projects to the maximum extent practicable, emphasizing practices that provide infiltration. Policies and ordinances must be implemented to protect sensitive areas, maintain green space, buffer water bodies, mini- mize impervious surfaces, and minimize disturbance of soils and vegetation. The Permit further re- quires that “The permittee shall assess site characteristics at the beginning of the construction de- sign phase to ensure adequate planning for storm water program compliance”. Thus, appropriate planning for mitigation of stormwater impacts must begin at the start of the project planning stage. A significant complicating factor in meeting this requirement is that MSD, who is responsible for im- plementing the stormwater facility design requirements, is not typically involved in the early planning stages of a development or redevelopment project. Municipal planners and other officials are more typically in contact with developers in the early stages of a land development project. Additionally, MSD does not issue land disturbance permits, which are issued by municipalities or St. Louis County. As a result, municipal/county plan review officials are the “first line” of watershed protection. However, post-construction stormwater quality and treatment considerations have not traditionally been part of the planning or land disturbance permit approval process. Thus, training and education 1 Introduction NPS Pollution, St. Louis County, MO 4 on stormwater management techniques that will meet the Permit and Plan are needed. This guid- ance is written for municipal/county plan review officials who will review development project concept plans and/or stormwater pollution prevention plans (SWPPPs) at sites located with the MSD. This guidance is consistent with Plan goals of Providing “…educational materials on Best Management Practices (BMPs) and promote the use of non-structural credits and the benefits of site storm water management planning prior to land dis- turbance” and “Provid(ing) storm water management BMP guidelines to public works and planning and zoning reviewers to assist in the concept review of plans.” While upfront stormwater planning is a requirement of the Permit, our Permit does not provide meth- odology for completing this requirement. This document presents “tools” and processes that plan re- view officials can use to evaluate whether development plans address Permit requirements and Plan goals for upfront stormwater planning. The tool develops a series of questions and actions that could be taken to meet Permit and Plan requirements. Not all of these questions and actions will be appli- cable to every project. Municipal/county plan reviewers even have the option of replacing this meth- odology with another one that accomplishes the same upfront stormwater planning objectives. The test is, when evaluated as a whole, 1. Does the project planning effort adequately document existing site conditions and identify im- portant natural resource considerations? 2. Does the project plan provide features that minimize runoff and pollutant loading and mimic pre-construction runoff conditions (for new development projects) to the maximum extent practicable? Although emphasis has been placed in this introduction on the regulatory requirements, the goal of this document is to better serve our communities through better site development design. The public is served when new development and redevelopment projects implement practices that help avoid public hazards like flooding and NPS pollution and that protect important natural resources not fully appreciated until they are gone. 5 2 Evaluation of Existing Conditions Purpose: Documenting a site’s existing conditions is the first step in protecting natural resources. The primary purpose of evaluating existing conditions is to generate information that should be used as a basis for laying out the development and implementing strategies for protecting environmentally sensitive areas. Some of these areas may be protected by federal, state, and/or local regulations as well. Tool: Table 1, Existing Natural Resource Considerations, presents a list of natural resources, key ques- tions, and actions that should be taken by the developer to document a site’s existing resources. The plan reviewer should use the questions as a “springboard” of initial questions to be discussed. Additional probing will likely result from answers to the questions listed in Table 1, and documenta- tion of these additional questions is also important since a legal review of these issues could ensue. In the evaluation of existing conditions, the “actions” should be focused on delineating the location and extent of areas that warrant protection. The next step, concept planning, addresses specific “actions” that may be taken to protect natural resources and to mitigate the impacts. Key Terms: The following key terms define the natural resources and other technical terms evaluated in Table 1. Existing topography: mapping of the existing (pre-development) land surface elevations, water bodies, geology, and other features that describe a piece of property. Steep areas are generally un- stable and can result in extensive runoff caused by stormwater runoff. Flood plain: the area adjacent to a stream of body of water that is susceptible to flooding. The edge of a flood plain is defined by a water level for a given recurrence interval (i.e., probability of flooding), such as “100-year flood plain”. Development in the flood plain is at higher risk of flood damage, and reducing the floodwater storage volume in the flood plain will impact flood levels in other areas. Floodway: the channel of a river or other watercourse and the adjacent land areas that must be re- served in order to discharge the 100-year flood without cumulatively increasing the water surface elevation. [See 44 CFR 59.1] Karst: areas where the dissolution of limestone or other soluble rocks has produced cliffs, sinkholes, caves, underground streams, and other similar features. Karst areas may be direct “conduits” of stormwater “injection” to groundwater or streams. Increasing the rate and amount of urban stormwa- ter to karst features also increases the rate of limestone dissolution, which can exacerbate erosion problems. NPS pollution in urban runoff to karst areas can quickly contaminate groundwater wells and impact public health. Ponds: a small, still body of water. Lakes and reservoirs, including natural lakes and any impound- ments created by the construction of a dam across any waterway or watershed, are considered Wa- ters of the State. [See 10 CSR 20-7] Ponds (wet or dry) that were engineered and constructed for the purpose of flood detention or stormwater treatment are generally not considered Waters of the State. 6 Natural Re- source Questions Actions Wetlands Are wetlands on site? Are Army COE and/or MDNR permits needed (e.g., 404/401 permits)? Show all wetlands on map. Obtain COE/MDNR permits &/or docu- mentation before plan approval. Streams and Floodplains Are major waterways on the site? Are Army COE and/or MDNR permits needed? Is the site located within the 100 or 500- year flood plain? Is the municipal or county stream buffer (setback) shown? Is the site in a flooding or erosion prone area? Show major waterways. Obtain COE/MDNR permits &/or docu- mentation before plan approval. Show 100 and 500-year flood plains on map. Show stream buffer. Show areas prone to flooding. Show stream bank erosion areas. Karst Are sinkholes, springs, or seeps located on the site? What is the depth to bedrock? Show sinkholes, springs, seeps, and other karst features. Show areas with shallow depth to bed- rock. Existing To- pography What is the existing topography? Are there areas with slopes steeper than 20 percent? What are the site’s soil types? What is the existing stormwater drainage area and flow path? Show existing topography, identify ar- eas with slopes greater than 20 per- cent. Show site soil type. Show areas with erodible soils. Show gullies, swales, ditches, etc. Ponds Are there existing ponds on or adjacent to the property? Does the pond provide recreational bene- fits? Does the pond provide flood detention benefits? What is the condition of existing ponds (i.e., depth of sediment in pond, bank erosion, invasive plants)? Show all ponds on map, including any existing detention basins. Vegetated Cover Is the site forested? Are grassy/prairie areas on the site? Show forest and prairie areas. Show large trees (>12” dia). Existing Prop- erty Use What is the site’s current use? What buildings, structures, and other im- pervious surfaces are present? Are there utilities through the site? Show existing impervious areas and utilities. Surrounding Property Use What is the surrounding property use? Show property boundary and sur- rounding property uses. Table 1: Existing Natural Resource Considerations 7 Property use: evaluating the existing property use is important in understanding the impact from the proposed development or the project site. Evaluation of surrounding property use is important in evaluating its impact on the development site. Soil type (or hydrologic soil group): a term used to estimate the stormwater runoff potential. The USDA (NRCS) classifies soils as “A”, “B”, “C”, or “D” soils based on the soil’s potential for runoff. “A” soils have the lowest runoff potential (i.e., highest infiltration rate), while “D” soils have the highest runoff potential (i.e., lowest infiltration rate). Understanding what soil types exist before grading be- gins and leaving “A” and “B” soils undisturbed can reduce urban stormwater impacts by infiltrating stormwater runoff into the soil. Soil type information is easily retrieved through the web at http:// websoilsurvey.nrcs.usda.gov. Stream: a body of moving water in a natural channel, such as a creek or river. Regulated streams are those that are considered Waters of the State. Streams are considered regulated wetlands and are subject to State and Federal law, as well as local stream buffer ordinances that require areas next to the stream to remain undisturbed in natural condition. Stream Buffer: a natural area boundary between a development and stream that helps protect wa- ter quality by filtering pollution and infiltrating stormwater runoff. Stream buffers also help alleviate stream bank erosion and provide room for the normal lateral movement of the stream channel. Vegetated Cover: vegetation can heavily influence the runoff potential from a site. Wooded sites, sites with heavy brush, and sites planted in warm-season native grasses have the lowest runoff po- tential. Large trees help stabilize sites, reduce runoff, and reduce thermal warming of waters. Pre- serving areas with quality vegetated cover, and replanting buffer areas with deep-rooted trees and native plants are techniques that minimize site development runoff. Turf grass (i.e., bluegrass, fes- cue, etc.) has a shallow root structure and, accordingly, a higher runoff potential. Wetland: wetlands occur at the transition zone between land and water. For regulatory purposes under the Clean Water Act, the term wetlands means "those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil con- ditions. Wetlands generally include swamps, marshes, bogs and similar areas.” [See 40 CFR 230.3 (t).] Wetlands are protected from disturbance under the Clean Water Act. The Army Corps of Engi- neers and DNR issue permits for any work impacting a wetland. Waters of the State: All rivers, streams, lakes, and other bodies of surface and subsurface water lying within or forming a part of the boundaries of the state which are not entirely confined and located com- pletely upon lands owned, leased, or otherwise con- trolled by a single person or by two or more persons jointly or as tenants in common and includes waters of the United States lying within the state. [See 10 CSR 20-7] This term is important because the small MS4 Permit authorizes discharges to waters of the state (i.e., the Permit applies to and protects waters of the State). Waters of the state should not be used as BMPs, but rather be protected by BMPs. Ramona Lake, Berkeley, MO 8 Deliverables: At a minimum, the developer should provide the reviewer an Existing Site Resources Map of the site that documents the location of any of the features listed in Table 1. A table or legend stating what features are (and are not) of concern on the site would be helpful. Presenting this information on a separate large size sheet (preferably on 24” x 36” paper) will assist the reviewer in evaluating the site’s information. An example Existing Site Resources Map is provided in Appendix A. The presence of some features will require additional assessment beyond the Existing Site Re- sources Map. A site visit and digital photographs of the features are also helpful. Additionally, a pro- ject which impacts wetlands or waters of the U.S. or State (jurisdictional waters) will likely be accom- panied by an additional assessment of the feature as required for Army Corps of Engineers and/or Missouri Department of Natural Resources under the Clean Water Act section 404/401 permitting requirements. The plan reviewer should request and review this information, as it may be helpful to them as well in determining the location, extent, and quality of these features. 9 3 Evaluation of Concept Plans Purpose: Once the Existing Site Resources Map is presented, the next step is locating the proposed buildings and other impervious features (e.g., parking lots, roadways, sidewalks, etc.) and managing the stormwater runoff from these areas. To the maximum extent practicable, the development plan should preserve and/or protect existing natural resource areas that facilitate pollutant removal and reduce runoff. With today’s construction technology, the tendency is to completely disturb and grade a site based on the future use of that property, but without regard to a site’s existing natural resources or potential for minimizing stormwater runoff. While this approach facilitates the speed of development, fre- quently it is at odds with conservation of natural features and soil characteristics (e.g., soil perme- ability) that mitigate development impacts and have longer-term community and water quality bene- fits. Additionally, many have approached compliance with “MSD’s water quality requirements” in a similar way to traditional flood detention, in that an engineered structural BMP is installed near the low point of the site. This mindset needs to be replaced with one that also includes practices that minimize the volume of runoff, because treatment alone does not meet the Permit intent’s for mim- icking pre-construction runoff or implementing water quality strategies to the maximum extent practi- cable. Tool: Table 2, Site Development Goals, Questions, and Methods, presents an approach to site design that plan reviewers can look for, and designers and developers can take, to minimize impacts to the envi- ronment from stormwater. While specific solutions will be tailored on a site-by-site basis, the major concepts that these solutions embody (in order of preference) include: Limiting disturbance and preserving existing pervious areas (i.e., green space) and sensitive ar- eas such as flood plains and stream buffers. Reducing the amount of impervious area (rooftops, parking lots, sidewalks, roads, etc.). Disconnecting impervious areas stormwater from the storm sewer system via overland flow to vegetated buffers and other “green” infrastructure techniques that promote infiltration. Using pervious surface materials, such as permeable paver blocks, porous asphalt, porous con- crete, and green roofs. Installing engineered systems that treat stormwater runoff and/or reduce peak stormwater runoff rates using techniques that employ vegetation and infiltration. Underground structural techniques should be used as a last resort. A significant portion of MSD is already developed. Strategies for managing stormwater quality at re- development sites are sometimes more limited because of existing utilities, space restrictions, and other factors. MSD provides greater flexibility for meeting water quality requirements to smaller (<5 acre) redevelopment projects. Additionally, MSD wants to encourage redevelopment of existing im- pervious areas and infill development as a component of regional stormwater management. (Redeveloping a parking lot, commercial district, or already degraded site allows a community to en- joy the benefits of growth and improved water quality without further increasing net runoff.) However, MSD anticipate that larger redevelopment projects will have an opportunity to employ techniques 10 Table 2: Site Development Goals, Questions, and Methods Goal Questions Methods (To the Maximum Extent Practicable) Minimize Stormwater Generation Can land disturbance be mini- mized? Can additional green space be preserved? Can proposed development be located in already devel- oped areas? Limit clearing, grading, and earth disturbance. Use clustered development with open space de- signs. Use narrower, shorter streets, right-of-way, and sidewalks. Allow smaller radii for cul-de-sacs. Reduce parking space requirements. Preserve and protect forested areas, especially areas with large trees. Show tree preservation areas on plans. Allow for shared driveways and parking areas. Provide incentives for site redevelopment. Can stormwater safely flow overland to buffer areas (i.e., avoid piping)? Grade to allow stormwater to sheet flow into buffer or conservation easement areas. Limit use of curb and gutter streets. Use grass channels for street drainage and stormwater conveyance. Allow roof downspouts to flow overland into vegetated cover. Can stormwater be captured and infiltrated into the ground? Rainwater infiltration systems. Examples include rain gardens, dry wells, and other landscape infil- tration methods. Emphasize managing stormwater at the point of generation. Can stormwater be captured and reused for irrigation or décor? Rainwater harvesting systems. Examples include rain barrels, cisterns, shallow ponds, and under- ground chambers. Emphasize managing stormwater at the point of generation. Could permeable surface ma- terials be used to promote in- filtration and limit runoff? Use permeable pavements in low traffic areas. Use green roofs. Direct rooftop runoff to pervious surfaces, such as amended soils. Minimize Ero- sion of Site Soils Can land disturbance be re- stricted to less sensitive ar- eas? Is the development located outside the 100-year flood plain? Land disturbance SWPPP requirements apply. Avoid grading areas with steep slopes and erod- ible soils. Limit disturbance areas within the 100-year flood- plain. Minimize Stream Bank Erosion Is the development located outside the stream bank set- back buffer? Development should not encroach municipality’s stream bank buffer. Show stream buffer on pre- liminary plan. Does the development war- rant engineering channel pro- tection controls (because of development size or stream bank erosion problems)? MSD rules and regulations require channel pro- tection detention for the 1-year 24-hour rainfall event. Show detention basin on preliminary plan. Locate outside limits of 100-year floodplain. If feasible, stabilize the stream bank using other engineered methods. 11 (Bold items reflect project requirements.) Minimize Im- pact to Envi- ronmentally Sensitive Ar- eas Does the development plan avoid sensitive areas? Untreated stormwater should not discharge into sinkholes, wetlands, fishing ponds, and other sensitive areas. Provide a buffer around sensitive areas. Preserve the existing stormwater flow path. Adequately Treat Storm- water Before Discharge Does the site development plan utilize stormwater cred- its? Does the development plan show structural BMPs? What is the acreage of drain- age to the BMP? Will the BMP be above or below ground? Show locations of any (non-structural) “credit” areas and show locations of any structural stormwater BMPs on preliminary plan. Locate structural BMPs outside the 100-year flood plain. Provide a BMP drainage area map. Only certain wet ponds and wetlands may be used for drain- age areas larger than 10 acres. Encourage stormwater credits, managing stormwater at the point of generation, and aboveground stormwater BMPs. “Regional BMPs” and underground BMPs should be avoided when possible. As a rule of thumb, the development should provide 35% minimum green space for a structural BMP(s). Stormwater Controls Shall Be Maintain- able and En- forceable Who will be responsible for maintaining stormwater con- trols? Are the structural BMP shown on the plan appropriate for the entity or person responsible for maintenance? The property owner or subdivision association will maintain BMPs. Underground BMPs, large surface filters, and other maintenance intensive BMPs should be avoided on residential developments. Minimize Downstream Flooding Is over 1 acre of impervious area being added? Is the development tributary to any existing basins that need to be upgraded? A stormwater detention basin will be needed. Show location on plan. Developments feeding basins that do not cur- rently meet MSD flood detention requirements will need to be upgraded. Frequently this re- quires enlargement of the existing basin. 12 that both minimize stormwater generation and treat stormwater pollution. For example, larger rede- velopment and new development projects may employ alternative surfaces (e.g., porous pavement) that reduce stormwater runoff, as well as decentralized technologies that infiltrate and treat stormwa- ter runoff at the source. At smaller redevelopment projects, these alternative surfaces and technolo- gies may not be practical due to space constraints, utility conflicts, traffic flow, cost, or other factors. A common misconception is that a traditional “dry” flood detention basin meets MSD water quality requirements. Dry basins may be used in conjunction with water quality BMPs for flood control, but by themselves they do not provide effective stormwater quality management. Additionally, while con- structed wet ponds and wetlands provide effective water quality management, developments may not use existing features that are considered “waters of the State” as a BMP. This includes wetlands, lakes, and ponds located on public property (e.g., park land), as well as the streams that feed them (including intermittent streams). MSD recognizes that some of the methods that can be used to mitigate stormwater impact may be in conflict with other development requirements (e.g., reducing cul-de-sac radii, reducing parking spaces, use of narrower streets and sidewalks, alternative pervious materials, etc.). Municipal/ county/state officials and MSD must identify impediments to these practices, determine what flexibil- ity is available, and modify rules as appropriate. St. Louis area local governments are increasingly supportive of green infrastructure and have an incentive to be supportive as a Phase II co-permittee. Until all barriers and conflicts are removed, co-permittees are encouraged to allow greener stormwa- ter management solutions and allow exemptions to conflicting requirements where it makes sense. The property owner must maintain stormwater treatment devices, and they will be periodically in- spected by MSD to ensure proper maintenance is occurring. (A maintenance plan is recorded with the property and is transferable with property ownership.) Failure of the property owner to maintain their BMPs is a violation of MSD ordinance and the site maintenance agreement. Therefore, what- ever BMPs are selected, they need to be maintainable by the end user of the property. It is im- portant to recognize that all designed systems will require maintenance, and as a rule, the more “engineered” the solution, the more frequent and expensive maintenance will be. There are eco- nomic tradeoffs: while establishing buffer areas and managing stormwater using non-structural BMPs may reduce the amount of land available for development, maintenance costs and head- aches on future property owners are reduced. However, only using non-structural BMPs may make some developments cost prohibitive due to land value, and some commercial users will have the re- sources needed to maintain the engineered structural BMPs. The best solution is the one that bal- ances future maintenance costs, given the resources of the user, with the cost of development. Un- derground vaults, filters, and manufactured separation devices (which are maintenance intensive) are not appropriate for residential development. Except for approved hydrodynamic separation de- vices installed by municipalities in roadway right-of-way, MSD will not maintain stormwater treatment devices. Post-Construction Non-Structural BMPs: Non-structural BMPs are development strategies that minimize the impact of land development on natural resources. Many of the “methods” in Table 2 are non-structural techniques that can be used to treat, as well as reduce, site runoff. In the Maryland Stormwater Manual, some non-structural techniques are grouped together as a “credit” that satisfies MSD water quality requirements (as a stand-alone practice) or reduces the volume or rate of water that must be managed as part of the stormwater management plan. These credits include Natural area conservation Disconnection of rooftop runoff 13 14 Disconnection of non-rooftop runoff Sheet flow to buffers Open (grass) channel use (for roadways), and Environmental sensitive development (i.e., low-impact development). In most cases, non-structural practices must be combined with structural practices to meet stormwa- ter requirements. Nationally and at MSD, non-structural practices are increasingly recognized as a critical and economical feature of stormwater management. Post-Construction Structural BMPs: In all new development and redevelopment, MSD rules and regulations require the use of BMPs to treat a water quality volume of runoff from 1.14 inches of rainfall. MSD regulations specify and allow certain structural BMP’s for use. The structural BMPs allowed for stormwater quality control are di- vided into six general categories as shown in Table 3. Typical BMP layouts and sections are pro- vided in Appendix B. The full scope of BMP selection, design, and construction is beyond this docu- ment and the goal of conceptual plan review. However, concept plan reviewers may find this table of BMP categories helpful in evaluating whether a proposed BMP, at least by name, is allowable and whether adequate space has been allocated in the design. For more specific guidance on structural BMPs, MSD relies upon Chapters 3 and 4 of the Maryland Department of the Environment’s (MDE), April 2000, “Stormwater Design Manual, Vol. I and Vol. II.” The “Maryland Manual” outlines the se- lection, design and construction of various structural BMPs. (At present, there is not a national or Missouri state design manual for these devices. The Maryland Manual is a comprehensive BMP de- sign manual, and its principles can be adapted to the St. Louis region.) Table 3. Post-Construction Structural BMPs Allowed within MSD 1Hydrodynamic Separator Devices are manufactured by a variety of vendors. A list of approved devices and vendors is available through the MSD website, www.stlmsd.com. 2 Alternative surfaces reduce the impervious area, reduce the volume of runoff requiring treatment, and provides some pre-treatment benefit; but are not recognized as “stand-alone” treatment BMPs. They may be a component of a BMP that does provide treatment (i.e., infiltration or filtration). Stormwater Ponds Micropool Extended-Detention (ED) Ponds Wet Ponds Wet ED Ponds Multiple Pond System “Pocket Ponds” Stormwater Wetlands Shallow Wetland ED Shallow Wetland Pond/Wetland System “Pocket Wetland” Stormwater Infiltration Infiltration Trench Infiltration Basin Stormwater Filtering System Surface Sand Filters Underground Sand Filters Perimeter Sand Filters Organic Filters Pocket Sand Filters Bioretention Proprietary Cartridge Devices Open Channel Systems Dry Swale Wet Swale Hydrodynamic Separator Devices MSD Approved Devices1 Alternative Surface Materials2 Green Roof Permeable Pavement Many of the post-construction structural BMPs incor- porate plants that add functional and landscape value to the BMP. MSD references the “Landscape Guide for Stormwater Best Management Practice Design, St. Louis, Missouri” for recommendations on selecting plant species and on planting methods used in BMPs. (This guide can be accessed through the MSD web- site, www.stlmsd.com.) Key Terms: Amended soils: soil that has been improved by practices that preserve site topsoil, reduced soil com- paction, and/or blended with organic and/or inorganic materials that reduce soil runoff capacity. BMP drainage area map: a topographical map that shows the drainage areas that feed individual BMPs or credit areas. The BMP drainage area map also provides limits of disturbance, improve- ments, and a table showing the water quality volumes required and provided in BMPs. Disconnection of runoff: Credit focused on reducing the water quality volume by disconnecting im- pervious areas (rooftops, parking lots, etc.) from the sewer and directing them to pervious areas where stormwater can either infiltrate into the soil or filter over it. The credit is usually obtained by grading the site to promote overland flow or by providing bioretention/rain garden areas on single- family residential lots. Engineered channel protection: to protect natural channels from erosion and consequent pollu- tion, MSD rules and regulations require some developments to provide extended detention of the one-year, 24-hour storm event. A detention pond or underground chamber is generally needed to meet channel protection requirements. Environmentally Sensitive Development (or Low-Impact Development (LID)): mimicking a site's predevelopment hydrology by using decentralized “micro-scale” techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. These micro-scale techniques include rainwater harvesting (e.g., rain barrels), dry wells, small rain gardens, bioswales, and other small-scale techniques that reduce runoff. Grass channel credit: Credit that is given when open grass chan- nels are used in lieu of curb and gutter and when designed to reduce the volume of runoff during smaller storms. Green roof: also referred to as vegetated roofs, roof gardens, or eco-roofs, green roofs replace conventional roof materials with a pro- tective (water barrier), planting media, and vegetation. The planting media and vegetation assists in attenuation of stormwater impacts. Hydrodynamic Separator (HDS) Devices: A pre-fabricated stormwater treatment structure utilizing settling and vortex separation to remove coarse sediment and trash from storm runoff. HDS devices are only allowed as “stand-alone” treatment devices for highway runoff and small redevelopment Surface Sand Filter, St. Louis County, MO Typical section of a green roof 15 sites. They may also be used for pre-treatment to filters, ponds, or other types of structural BMPs. Infiltration basin (or trench): BMPs that capture and temporarily store the water quality volume while allowing infiltration into the soil over a defined period. Infiltration basins should be limited to tributary areas smaller than 10 acres, and infiltration trenches should be limited to areas smaller than 5 acres. Infiltration techniques should be located in areas with A or B soil types. These tech- niques should not be located where seepage could damage adjoining or downstream property. Maintenance agreement: a recorded agreement between MSD and the owner of the property where the BMP is located. The agreement requires the property owner to maintain the stormwater facilities and provides a means for MSD to rehabilitate these facilities and place a lien on the prop- erty, if necessary. Natural Area Conservation: Credit focused on conserving natural areas, thereby retaining the pre- development and water quality characteristics. This strategy reduces the water quality volume that must be managed by placing conservation areas in permanent protection through a conservation easement, reserve area, or other means that preserves the pervious area in its natural state. Open channel systems: BMPs that are de- signed to capture and treat the water quality vol- ume within swales formed by check dams or other means of shallow ponding. They are typi- cally limited to roadways and low-density residen- tial projects with drainages areas 5 acres or less. Permeable pavement: pavement that is porous, allowing stormwater to infiltrate into the subbase (gravel) and soil below it. Permeable pavements are effective for reducing imperviousness in park- ing lots, driveways, sidewalks, and areas with low traffic load. Permeable pavement areas are also exempt from MSD’s stormwater service fee. Rain Garden: a rain garden, or bioretention area, is a type of filtering system where stormwater is allowed to temporarily pond in a planted shallow depression. Rain gardens are planted with native vegetation that can tolerate periods of inundation and remove pollution in stormwater. (Rain garden is a “buzzword” that is sometimes inaccurately used to describe many filtering systems. They may or may not meet MSD Rules and Regulations, based on whether they meet the applicable filter system design.) Redevelopment site: within MSD, a site (i.e., total property area) is considered a redevelopment site if at least 20% of the existing site was impervious coverage as of January 15, 2000. Regional stormwater treatment: post-construction structural BMPs that serve multiple properties, property owners, and sub-watersheds. While regional BMPs focus maintenance in large, centralized, treatment cells, by themselves they do not encourage methods that reduce runoff at the source and can complicate maintenance responsibilities. Sheet flow: flow over plane surfaces (e.g., a rooftop or roadway) that typically occur in the headwa- ters of the watershed. As the volume of water in sheetflow increases, it concentrates and begins to Permeable Pavement During Rainfall 16 (quickly) flow in channels. By maintaining sheet flow conditions in buffers, the length of time storm- water is allowed to infiltrate increases. Sheet Flow to Buffer: Credit that reduces the water quality volume when a natural buffer or a for- ested area effectively treats stormwater runoff. Effective treatment is achieved when pervious and impervious area runoff is discharged to a grass or forested buffer through overland sheet flow. Stormwater filtering systems: BMPs that capture and temporarily store the water quality volume and then pass it through a bed of sand, organic matter, soil, or other media. Filtered water is then collected and returned to the conveyance system (or allowed to infiltrate). Examples of stormwater filters include sand filters, bioretention (rain gardens), and manufactured filter devices. Due to clog- ging, experience suggests the drainage area to underground filters should be limited to a couple acres; aboveground surface sand filters may serve up to 10 acres. The filtering medium may need to be removed and replaced in the future, as sediments build up over the filter and ponding times in- crease. Stormwater ponds: BMPs that have a permanent pool of water, or combination of extended detention or shallow wetland with a permanent pool, equivalent to the water quality volume. Stormwater ponds re- move solids by settling. The shallow bench around stormwater ponds is planted with aquatic plants that further remove pollution. To ensure the pond remains “charged” with water, stormwater ponds should be limited to sites with a drainage area of 10 acres or greater. Stormwater wetlands: BMPs that create shallow wetland areas to treat urban stormwater and often incorporate small permanent pools of water and or extended detention storage to contain the full water quality volume. Stormwater wetlands should be lim- ited to sites with a drainage area of 25 acres or greater. Water Quality Volume (WQv): the storage needed to capture and treat the runoff from 90 percent of the recorded daily rainfall events. [See 4.080.02 of MSD’s Rules and Regulations]. Deliverables: The Concept Plan submittal should include a Site Development Plan (preferably on 24” x 36” paper) and narrative to support the design. The narrative should describe how natural resources will be pre- served and protected, and explain how stormwater quality and flood protection requirements will be achieved. At a minimum, the concept should include the following: Location of site natural resources. Proposed limits of clearing and grading. Location of proposed impervious areas (buildings, roadways, parking, and sidewalks). Location of existing and proposed utilities. Locations of proposed buffer areas and BMPs. Rain Garden in St. Louis County, MO 17 As alluded to earlier, a challenge to meeting better site designs for stormwater management is coor- dination between municipal/county plan reviewers and MSD plan reviewers. Complicating this fur- ther, the process by which municipalities and/or St. Louis county review preliminary development plans and land disturbance plans varies. For an overview, Figure 1 presents a general process of how the preliminary and detailed plan development pieces “fit” together. While each municipality and St. Louis County can use a different process for assessing site plans, the elements of the process (establishing existing site conditions, identifying natural resources that warrant protection, establish- ing a preliminary layout that illustrates post-construction buffers and BMPs, etc.) should be common. The municipality and/or St. Louis County would review the initial steps in the development review process: establishing site conditions and presenting a preliminary development plan. For projects that need “rezoning”, the initial stormwater planning steps should occur before or during the rezoning process. In many cases, it will make sense to incorporate key components (e.g., protection buffers and “credit” areas) as conditions of rezoning. If rezoning is not required, the planning authority should coordinate with MSD to ensure stream buffers and non-structural BMPs are reserved in the recorded maintenance agreement. The more detailed site development phase begins after the planning authority approves the prelimi- nary plan. The resource protection component of the approved preliminary plan will need to be coor- dinated between the SWPPP authority (St. Louis County or municipality) and post-construction per- mit authority (MSD). The SWPPP authority will address the actions to be taken to protect resources from runoff during construction activities. MSD will permit the actions need to protect resources after construction activities are complete. SWPPP reviewers should note the relationship between construction and post-construction BMPs. Construction BMPs in a SWPPP are designed to minimize impacts during the active construction phase, and they do not always translate into BMPs applicable for post-construction. Post- construction BMPs must treat long-term runoff from the newly constructed or redeveloped site. In some cases, construction and post-construction BMPs can be located in the same area. (For exam- ple, a sediment control basin may be converted into a wet pond and flood detention basin.) How- ever, at the majority of sites, construction and post- construction sites will be located on different parts of the site. This is needed to preserve the soil structure necessary for BMPs that rely upon infiltration (such as infiltration basins). Also, post-construction BMPs (such as filters, bioretention/rain gardens, dry swales, and infiltration areas) must be installed after contribut- ing drainage areas are stabilized in order to prevent them from clogging with construction sediment. MSD plan review engineers will review the post- construction stormwater management plan. This will include review of plans showing how stormwater will be conveyed and treated (and detained), the BMP drainage area map, maintenance plans, calculations, 4 Integration with MSD Stormwater Quality Review 18 Surface Sand Filter Clogged with Construction Site Sediment, Berkeley, MO Figure 1. Generic Stormwater Site Design Approval Process yesEstablish Existing Site Conditions -Identify & locate natural resources -Establish which areas merit special protectionSubmittal: Existing Site Conditions MapEstablish Preliminary Development Layout -Establish strategies for preservation/protection of sensitive areas -Locate stormwater credits and post-construction BMPs -Draft BMP Drainage Area map.Submittal: Preliminary Site Development LayoutPreliminary Site Development Layout Approved by Planning Authority?noBegin Site Development Phase (Post-Construction Component) -Finalize site layout, topography, & drainage areas -Finalize Post-Construction Stormwater Management Plan (calculations, sizing, details, etc.) -Finalize post-construction strategies for preservation/protection of sensitive areasMSD Engineering Plan Approval?yesFinalize Design and Proceed to Permits -Complete Maintenance Agreements, Easements, and Other Land Use Restrictions Needed to Implement Post-Construction Stormwater Management Plan -Finalize Site Development Design -Determine Performance Deposits to be CollectedBegin Site Development Phase (Improvement Plan & SWPPP component) -Finalize Stormwater Pollution Prevention Plan (obtain municipal & state land disturbance permit) -Finalize strategies for preservation/protection of sensitive areas during construction -Provide overlay of sediment control plan & post-construction BMP planCounty/City Grading Plan Approval?yesFinalize Design and Proceed to Permits -Finalize Site Development (Improvement Plans) Design -Determine Performance Deposits to be CollectednonoCoordination BMP sizing, and other details. MSD and the owner of the property(s) where the BMP(s) is located will enter into a maintenance agreement, and the agreement will be recorded with the property(s). MSD will also collect a BMP construction deposit and field-inspect the post-construction BMPs. It is not the task of the planning and zoning or SWPPP permitting staff to review detailed post- construction stormwater management plans and calculations. These reviewers need only to be fo- cused on ensuring that post-construction stormwater management is a component of preliminary design and that the development approach considers sensitive areas, buffer areas, and methods that re- duce (as well as treat) stormwater runoff. It is very difficult to provide “rules of thumb” and general guidance on stormwater management de- sign, given the site-specific nature of design and the range of BMP options available. In some cases, specific questions about post-construction stormwater management should be addressed to MSD, and assistance is available on three levels. First, MSD plan review engineers are available through the MSD permit office (314-768-6272) to answer general questions. Second, where a site specific and/or more defined scope of stormwater management requirements is requested, MSD can provide a (paid) conceptual review. The conceptual review provides a quicker “desktop” review of the devel- opment plan and comments on the overall direction of stormwater management, but does not typi- cally address project details and calculations. Plan approvals are not provided in conceptual re- view. Where a detailed project review and plan approval is needed, the developer and engineer should apply for a formal plan review. It is important to note that the developer/engineer can submit plans to MSD for formal plan review at any point in the process (including preliminary site layout). 20 Stormwater quality in our community will best be managed when all of us coordinate our stormwater management strategies. Stormwater management should be focused on minimizing and treating stormwater runoff. One development or redevelopment project, alone, is unlikely to improve regional stormwater quality. Rather, meaningful results will only be achieved after many redevelopment and development projects that incorporate good stormwater management practices and years operation. MSD looks forward to working with our co-permittees and stakeholders on improving water quality. Questions about stormwater management plans and approaches may be directed to these MSD managers. Mr. Bruce Litzsinger, P.E. Environmental Compliance Manager 314-436-8757 BLITZSIN@stlmsd.com Mr. Francis Kaiser, P.E. Program Manager, Development Review 314-768-6315 FXKAIS@stlmsd.com Mr. John Grimm, P.E. Principal Engineer, Development Review (east team) 314-768-2743 JCGRIM@stlmsd.com Mr. -D\ +RVNLQV, P.E. Principal Engineer, Development Review (west team) 314--27 JSHOSK@stlmsd.com Internet Resources: http://www.stlmsd.com/MSD/PgmsProjs/planreview/Search. MSD plan review information, including links to rules and regulations, proprietary devices, and the Landscaping Guide. http://www.stlmsd.com/MSD/PgmsProjs/PhaseII. MSD Phase II information website, a clearinghouse on many stormwater related items. http://www.dnr.mo.gov/env/wpp/stormwater/sw-local-gov-programs.htm. Phase II stormwater re- quirements and resources Information for local governments from Missouri DNR. http://www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/ index.asp. The Maryland Stormwater Design Manual may be downloaded from this website. http://websoilsurvey.nrcs.usda.gov/app/. Web soil survey application from the USDA-NRCS. http://www.cwp.org/. The Center for Watershed Protection is a non-profit organization that develops watershed protection strategies, researches their effectiveness, and publishes guidance to water- shed managers across the U.S. 5 Conclusion 21 Appendix A Example Site Resources Map Example Concept Plan for Stormwater Management Example BMP Drainage Area Map ?" APPR f2' r �4 >CM 58 H".E.._ \i^ \ 410.14 ASDRES NO. Xry; 1,023,968 SO. 23.507 ACRES± / J OMB SPAT WIERSTA1E2I2 RoIMA1E LOCATION FIBER CPRC (LMil1CORE) Natural Resource Presence? Additional lnformOtion Wetlands Yes COE/MDNR 404/401 are required for any work in wetlands. Streams and Yes Watkins Creek tributaries now through site. Floodplains Silo is located outside of 100-year Soodplain. Stream buffer requirements apply. Karst No Not identified during site walk.. Ponds Yes 'Farm pone is not waters of the Stale. 1111111111111, Requlal L Waters of Stale or Nation Ny*de gie Group B Soh irydotogic Group d Sons Hydrologic Group 0 Soils >SWlde stream BUNsr EASEMENT 1074 10; 5%. TRIO EASEMENT Sttq P0. 124 151 NATURAL GAS EASEMENT De. srls. PO. 200 XltlJ Example Existing Site Resources Map /440P MO I Vacant Commercial Property 3d Scala 1" =1OOt Total Disturbance Acreage: 18.6 AcTotal Existing Imperviousness: 17.1 AcTotal Proposed Site Imperviousness: 13.5 AcOnly surface BMPs will be used and more pervious area is created. No flood or channel protection detention is required.Post-construction stormwater quality management is required. Stormwater BMP/Credit Summary TableBasin Description of Area BMP TypeDrainage Area (Acres) ImperviousnessWQv(Cu. Ft.)A Roof, roadway, and green space Surface Sand Filter9.0476%27,000BParking areasBioretention 9.5670%27,000Total18.673%54,000 PRIVATE LACIER A5O INSPECTION SAID FILTER 4" VAULT A o- I 2 I I 1 I I I I 1 ✓ 4 4 4 4 FINE ARTS DRIVE 4 Q Q INSPEcnaM I I I �. 1 -- J__I I I I ofrAkOr An 11122=1122 —;11;11-1.114t40.i> , Erawitamv.m...:=..ff, ir -7111 Li L I I,-�e I I 7 ? 7 -i -I _,r I I I I I 1 I ,_ I I 1 I {. r, I I I I: - I I I I I: 1 I I l: I I I 1_o- .._.._.._.._.._.. _.. _.. _..—.... _.._.. _.._.._..� l •.( IAUIT1R TER t 1 1 I 1 I' 1 I I I -I -I i -j- I I I I I I 1 I ItIIIIILIIIIII' 1 I I �{P tO1V I I�� I I 11'fnnni'nrfmmtr Tvii / /.Y Y- F I I 1 I I B IN6 I 1 i I. 14�; I 1 1 I 1 e I I 1 I l I I 1 I 1 1 I I I I 1 t- t I t t— I 1 I I <I . IT I; I L 1 I 1; J r 1 I L 1'f==?"1"r=.�!e�-% 711. ige (14 1: TT, Yf Y7 nlrYTT _ .... tk AN-AILTBRY7Q �IATpA1R MAL•BInCTIPADI-fit NM BOER IBA rosIa1110212 e N-sanYL-nt A11o11-NW: ro. LIMITS OF SITE Al DISTURBANCE Py i' BASIN 30 0 30 60 90: , • 30' Heel LEGEND BASIN 5 ASIN 6 • Fp ■ ■ BAP DRAINAGE BOUNDARY WATER QUALITY TREATMENT BOUNDARY AREA COLLECTED FOR TREATMENT UNCOLLECTED AREA ACCOUNTED FOR IN BMP SIZING WATER QUALITY TREATMENT SYSTEM NOTES: 1. ANY FUTURE LAID DISTURBANCE AND/OR INCREASE IN IIPERVIOUS AREA ON THIS SITE MAY REQUIRE ADDITIONAL STORM WATER MANAGEMENT PER RED REGULATIONS IN PLACE AT THAT TILE (INCLUDING TOTAL LOAD DISTURBANCE MID/OR IIFERVIOUSLESS ADDED CAI THIS PLAN). 2. DETENTION FOR ANY FURTHER DEVELOPMENT SHALL INCLUDE THESE IAPROVEAENTS AS YELL. 3. LAND AREA DISTURBED • 7.080 ACRES ANY FURTHER DEVELOPMENT SHALL INCLUDE THESE IAPROVEIENTS AS YELL. 4. PROJECT ADDRESS IS ONE FINE ARTS DRIVE, ST. LOUIS, MISSOURI, 63110 AND IS LOCATED IN LOT 33 OF *CATLIN TRACT.. BMP/STORMWATER CREDIT SUMMARY TABLE BASIN REQUIRED TREATMENT AREA (ACRES) COLLECTED AREA (ACRES] DESCRIPTION BA' STORINATER CREDIT WO, REDUCTION TYPE II OF 3 TYPES) 99. REQUIRED ICU FTI 75f of VAN MO, PROVIDED (CO FT) 1 020 021 NORTHWEST SAID FILTER -- -- 540 540 5 4.24 2.96 50UTH BIOREIENTION POND -- -- 2368 8026 6 229 222 NORTHEAST SAID FILTER -- -- 5524 5524 TOTAL 6.73 TOTAL 5.39 MSD BASEMAP 19H3 & 19144 WB,MIA &WIBre BMP DRAINAGE MAP MSD-2 Appendix B Typical BMP Layouts and Sections Appendix – Examples of BMP Designs