Town of Greenwich Drainage Manual Town of Greenwich

WASTE MANAGEMENT PLANTown of Greenwich Drainage Manual Low Impact Development and Stormwater Management
February 2012 Revised Feb 2014
Town of Greenwich Drainage Manual
Low Impact Development and Stormwater Management
Effective Date: January 1, 2012
46 Hartford Road Manchester, CT 06040
800.286.2469
8097 Windward Key Drive Chesapeake Beach, MD 20732
301.580.6631
Town of Greenwich Drainage Manual Low Impact Development and Stormwater Management
February 2012
The Town of Greenwich Drainage Manual is available on-line:
http://www.greenwichct.org/Government/Departments/Public_Works/Engineering_Division/Stormwater_Information/
Acknowledgements
Lead Staff
Scott Marucci, Senior Civil Engineer – DPW Engineering James Michel, P.E., Chief Engineer - DPW Engineering
David P. Thompson, Deputy Commissioner of Public Works – DPW Amy Siebert, Commissioner of Public Works – DPW
Fuss & O’Neill
Jennifer Cavanaugh, EIT, CPESC, Environmental Engineer Daniel DeLany, PE, Civil Engineer
Philip Forzley, PE, LEED AP, Civil Engineer M. James Riordan, AICP, LEED AP, Environmental Planner
Kenneth Sullenger, EIT, Civil Engineer
Larry Coffman, President, LNSB, LLLP
Stormwater Committee
Katie A. Blankley, Deputy Director P&Z/Assistant Town Planner – P&Z Michael N. Chambers, Director – IWWA
Frank Petise, Senior Civil Engineer – DPW Engineering Denise M. Savageau, Director – Conservation
Table of Contents
Town of Greenwich Drainage Manual i February 2014
1 Introduction .................................................................................................. 1 1.1 Purpose of the Manual................................................................................. 1 1.2 Organization of the Manual ........................................................................ 1 1.3 Relationship of the Manual to Local Land Use Review Process ................ 2 1.4 How to Use this Manual .............................................................................. 3
2 The Importance of Stormwater Management ............................................ 5 2.1 Impacts of Development ............................................................................. 5 2.2 The Three Components of Stormwater Management ................................ 8
3 Stormwater Management Standards ........................................................ 11 3.1 Introduction ............................................................................................... 11 3.2 The Stormwater Management Standards .................................................. 11 3.3 Applicability and Exemptions ................................................................... 20
4 Low Impact Development .......................................................................... 23 4.1 Introduction ............................................................................................... 23
4.1.1 Advantages of LID ......................................................................................... 24 4.2 Fundamental Concepts ............................................................................. 25 4.3 Incorporating LID Into the Site Planning and Design Process ............... 26
4.3.1 Process Overview ........................................................................................... 26 4.4 Non-structural LID Techniques ............................................................... 31
4.4.1 Minimizing Soil Compaction ........................................................................ 31 4.4.2 Minimizing Site Disturbance ......................................................................... 33 4.4.3 Protecting Sensitive Natural Areas ............................................................... 34 4.4.4 Protecting Riparian Buffers ........................................................................... 37 4.4.5 Avoiding Disturbance of Steep Slopes ........................................................ 39 4.4.6 Siting Relative to Permeable and Erodible Soils ........................................ 40 4.4.7 Protecting Natural Flow Pathways ............................................................... 40 4.4.8 Reducing Impervious Surfaces ..................................................................... 41 4.4.9 Stormwater Disconnection ............................................................................ 43
4.5 Structural LID Techniques ....................................................................... 46 4.6 LID Hydrologic Analysis .......................................................................... 46 4.7 LID Applications ....................................................................................... 47
4.7.1 Residential Development ............................................................................... 47 4.7.2 High Density and Commercial Development ............................................ 52 4.7.3 LID Roadway Design ..................................................................................... 54 4.7.4 Alternative Paving Surfaces ........................................................................... 59
5 Structural Stormwater Management Practices ....................................... 60 5.1 Introduction ............................................................................................... 60 5.2 Categories of Structural Practices ............................................................. 60
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5.2.1 Pretreatment BMPs ........................................................................................ 60 5.2.2 Treatment BMPs ............................................................................................ 61 5.2.3 Conveyance BMPs ......................................................................................... 61 5.2.4 Infiltration BMPs ........................................................................................... 61 5.2.5 Other BMPs and Accessories ....................................................................... 62
5.3 Proprietary Stormwater BMPs ................................................................... 62 5.3.1 Evaluating the Use of Proprietary Systems ................................................ 64
5.4 Treatment Train ........................................................................................ 65 5.5 Operation and Maintenance ...................................................................... 65 5.6 Design Criteria .......................................................................................... 66
5.6.1 Runoff Volume Reduction and Groundwater Recharge (Standard 4) ... 66 5.6.2 Peak Flow Control (Standard 5) .................................................................. 71 5.6.3 Pollutant Reduction (Standard 6) ................................................................ 74
5.7 Selection Criteria ....................................................................................... 79 5.7.1 Land Use Factors ........................................................................................... 79 5.7.2 Physical/Site Feasibility Factors .................................................................. 82 5.7.3 Downstream Resources ................................................................................ 86
5.8 LID Retrofits and Redevelopment ............................................................ 90 5.9 Design Guidance for Stormwater BMPs ................................................... 92
6 Drainage Facilities ...................................................................................... 93 6.1 General Design Requirements .................................................................. 93 6.2 Runoff Determination ............................................................................... 96
6.2.1 Design Storm Frequency .............................................................................. 96 6.2.2 Acceptable Methods ...................................................................................... 97 6.2.2.1 Rational Method…………………………………………………….. 97 6.2.2.2 Other Methods……………………………………………………...101 6.2.3 Hydrologic Analysis Submission Requirements ...................................... 103
6.3 Storm Drainage Systems ......................................................................... 104 6.3.1 General Requirements ................................................................................. 104 6.3.2 Pavement Drainage ...................................................................................... 104 6.3.3 Inlets/Catch Basins...................................................................................... 106 6.3.4 Manholes ....................................................................................................... 108 6.3.5 Storm Drains................................................................................................. 110 6.3.6 Headwalls and Trash Racks ........................................................................ 115 6.3.7 Outlet Protection ......................................................................................... 116
6.4 Culverts ..................................................................................................... 116 6.5 Channels ................................................................................................... 117 6.6 Bridges ...................................................................................................... 119 6.7 Storage Facilities ...................................................................................... 119 6.8 Erosion and Sedimentation Control ......................................................... 121 6.9 Structural Design ...................................................................................... 121
6.9.1 Reinforced Concrete Pipe ........................................................................... 121
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6.9.2 Plastic Pipe .................................................................................................... 121
7 Submittal Requirements .......................................................................... 124 7.1 Stormwater Management Report ............................................................ 124 7.2 Construction Plans .................................................................................. 126
7.2.1 Plan Set Standards ....................................................................................... 126 7.3 Supporting Documents and Studies ........................................................ 130 7.4 Operation and Maintenance Plan ............................................................ 131 7.5 Erosion and Sediment Control Plan ......................................................... 131 7.6 Plan and Report Revisions ...................................................................... 132
7.6.1 Plans ............................................................................................................... 132 7.6.2 Reports .......................................................................................................... 132
7.7 Certifications ............................................................................................ 132
8 References ................................................................................................ 133
Tables Page 1-1 Town of Greenwich Land Use Jurisdictions, Regulations, and Agencies 2 2-1 Summary of Development Impacts on Water Resources 6 5-1 Percent Annual Runoff Reduction for Various Stormwater BMPs 67 5-2 Recharge Target Depth by Hydrologic Soil Group 68 5-3 Stormwater Pretreatment BMPs 70 5-4 Stormwater BMPs for Peak Flow Control 73 5-5 Site Cover Runoff Coefficients 75 5-6 TSS Removal Efficiencies 77 5-7 Zoning and Land Use for Stormwater BMP Selection 80 5-8 Stormwater BMPs for High Load Areas 83 5-9 Stormwater BMPs for Shellfish Growing Areas and Public Swimming Beaches 88 5-10 Stormwater BMPs for Recharge Areas for Public Water Supplies 89 6-1 Design Elevations of Tidal Waters 95 6-2 Design Storm Frequencies 96 6-3 Runoff Coefficients for Various Surfaces (Rational Method) 98 6-4 Runoff Coefficients for Various Land Uses (Rational Method) 98 6-5 Runoff Coefficients for Greenwich, CT (Rational Method) 99 6-6 Frequency Factors (Rational Method) 100 6-7 Greenwich Catch Basin Grate Parameters 108 6-8 Manhole Sizing 110
Tables Page 6-9 Minimum Radii (Feet) for Curved RCP Installation 111 6-10 Manning’s Pipe Roughness Coefficients for Storm Drainage 112 6-11 Minimum Allowable Pipe Slopes to Ensure 2.5 ft/s in Storm Drains Flowing Full 115
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6-12 Storm Drainage Pipe Design and Installation Guidelines 122
Figures Page 2-1 Changes in Stream Hydrology as a Result of Urbanization 7 4-1 Minimally Disturbed Residential Development 33 4-2 Reduced Limits of Disturbance 34 4-3 Site Map with Natural Areas Delineated 35 4-4 Three-Zone Riparian Buffer 37 4-5 Relationship Between Slopes and Development Impacts 39 4-6 Conventional and LID Residential Design Concepts 49 4-7 Medium- to High-Density Lot Using LID Practices 50 4-8 Zero Lot Line Configuration 50 4-9 Conventional and LID Designs for Large Lots 51 4-10 LID Design Concept for Single Family Residential Lot 52 4-11 Example LID Site Plan for Commercial Office Building 53 4-12 Example LID Site Plan for Commercial Shopping Plaza 54 4-13 Example of a Dry Swale for Open Section Roadway Design 55 4-14 Example of Open Section Roadway Design 56 4-15 Narrow Road Section with Sidewalks, Shallow Swale and Porous Pavement Shoulders 56 4-16 Examples of Urban Roadway Bioretention Design 57 4-17 Bioretention Curb Extensions Used for Stormwater Treatment and Traffic Calming 57 4-18 Example of a Bioretention Cell Within the Road Right-of-Way 58 4-19 Example of a Curb Cut Leading to a Bioretention Area 58 4-20 Example of the Use of Alternative Paving Surfaces in Urban Roadway Design 59 5-1 NRCS Hydrologic Soil Groups 85 5-2 Critical Areas 87 6-1 Nomograph for Manning’s Equation for Flow in Storm Drains (Flowing Full) 113 6-2 Nomograph for Flow in Storm Drains (Partial Depth Flow) 114
Appendices A Water Quality – Town of Greenwich B Stormwater Infiltration/Recharge Requirements C Credits for Low Impact Development Best Management Practices D Suggested Sources of Information on the Effectiveness of Proprietary BMPs
Appendices E Recommended Process for Evaluating the Proposed Use of Proprietary BMPs F TSS Removal Efficiency Calculations G Design Guidance for Structural Stormwater BMPs H Stormwater Maintenance Declaration and Easement
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I Checklists J Standard Notes K Certification Forms L 24-Hour Design Storm Rainfall Amounts M Glossary
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information system (GIS) interface BMPs Best Management Practices C runoff coefficient CDM Camp, Dresser, & McKee C.O. Certificate of Occupancy cfs cubic feet per second CGS Connecticut General Statutes CN curve number CTDOT Connecticut Department of Transportation CTDEEP Connecticut Department of Energy and Environmental Protection DBH diameter at breast height DPW Department of Public Works DS downstream EIA effective impervious area ETV Environmental Technology Verification FEMA Federal Emergency Management Agency GIS geographic information system GRV groundwater recharge volume HDPE high density polyethylene HGL hydraulic grade line HSG hydrologic soil groups IDF intensity-duration-frequency LEED Leadership in Energy and Environmental Design LEED-ND LEED for Neighborhood Development LID Low Impact Development LIS Long Island Sound NAVD North American Vertical Datum NGVD National Geodetic Vertical Datum NPDES National Pollutant Discharge Elimination System NRCS U.S. Natural Resources Conservation Service O&M Operation and Maintenance PE Professional Engineer PVC polyvinyl chloride RCP reinforced concrete pipe RCV runoff capture volume RRV runoff reduction volume SARA Superfund Amendments and Reauthorization Act SCS Soil Conservation Service STEPL Spreadsheet Tool for Estimating Pollutant Loads SWPPP Stormwater Pollution Prevention Plan TAPE Technology Assessment Protocol – Ecology TARP Technology Acceptance Reciprocity Partnership TC time of concentration
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Acronyms and Abbreviations TMDL total maximum daily load TSS total suspended solids U.S. EPA United States Environmental Protection Agency US upstream USGBC United States Green Building Council USGS U.S. Geological Survey WB western basin WQF water quality flow WQV water quality volume
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Town of Greenwich Drainage Manual 1
February 2014
1 Introduction
1.1 Purpose of the Manual
The Town of Greenwich Drainage Manual provides guidelines for land development activities and stormwater management in the Town of Greenwich. The manual is applicable to new development and redevelopment activities undertaken by private or municipal entities, including public works projects and projects over which the Town Planning and Zoning Commission, Building Division, Inland Wetlands & Watercourses Agency, Health Department, and/or other commissions have responsibility for review and approval. The manual provides guidance for developers, engineers, and local regulatory authorities to design and review projects in a technically sound and consistent manner. This manual is intended to augment other existing design guidance, including the Connecticut Department of Energy and Environmental Protection (CTDEEP) Stormwater Quality Manual (as amended) and the Connecticut Department of Transportation (CTDOT) Drainage Manual (as amended). The Town of Greenwich Drainage Manual is generally consistent with these state-wide manuals to ensure consistency with state stormwater management policies and to eliminate potential redundancy with other existing guidance. This manual references applicable sections of the Connecticut Stormwater Quality Manual and Department of Transportation Drainage Manual, but also includes more detailed design guidance; greater emphasis on Low Impact Development (LID), sustainable site design, and green infrastructure; and specific Stormwater Management Standards tailored to the unique characteristics and issues facing the Town of Greenwich. The design practices described in this manual shall be implemented by professional engineers licensed to practice in the State of Connecticut. The design engineer is responsible for field investigations, data collection and analysis, and design of stormwater management and drainage facilities based upon the guidance contained in this manual. Stormwater management is an evolving field. Existing stormwater management practices are being refined and new practices are being developed on a regular basis. The Town may periodically amend this manual to reflect new or modified technologies, practices, and regulatory requirements.
1.2 Organization of the Manual
This manual emphasizes an integrated approach to stormwater management and drainage design, including stormwater quantity and quality issues. The manual also promotes LID, green infrastructure, and sustainable site planning techniques to maintain the integrity of natural site features during the development process, which can reduce or eliminate structural components of a stormwater management system. The organization of the manual reflects this integrated approach.
Section 1 – describes the purpose and organization of the manual, as well as how to use the manual.
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Section 2 – provides a summary of the stormwater problem and issues unique to the Town of Greenwich, and an overview of the stormwater management approaches contained in the manual.
Section 3 – describes stormwater management standards that address both water quality and quantity and their applicability to various types of projects and settings.
Section 4 – describes environmentally sensitive site design and Low Impact Development approaches for stormwater management.
Section 5 – contains a description of structural stormwater management practices that can be used to satisfy stormwater quality and quantity requirements, including their proper selection and design.
Section 6 – addresses the design of storm drainage facilities, including hydrologic and hydraulic analysis methods and specific design guidance and criteria for various types of drainage systems.
Section 7 – describes submittal requirements, including the Stormwater Management Report, construction plans, operation and maintenance plan, erosion and sedimentation control plan, and certifications.
1.3 Relationship of the Manual to
Local Land Use Review Process
The design guidance contained in this manual is applicable to new development and redevelopment activities on all properties within the Town of Greenwich, regardless of whether the manual is referenced by land use regulations or ordinances administered by the local agencies and jurisdictions listed in Table 1-1. This list will be updated as future land use regulatory programs are developed or modified.
Table 1-1. Town of Greenwich Land Use Jurisdictions, Regulations, and
Agencies
Inland Wetlands Inland Wetlands & Watercourses Regulations
Greenwich Inland Wetlands & Watercourses Agency
Coastal Area Coastal Area Management Regulations and Policies
Greenwich Planning and Zoning Commission
Site Plan Approval Building Zone Regulations Greenwich Planning and Zoning Commission
Special Permit Building Zone Regulations Greenwich Planning and Zoning Commission
Building Permit CT State Building Code Building Zone Regulations
Division of Building Inspection, Greenwich Department of Public Works
Land Subdivision Subdivision Regulations Greenwich Planning and Zoning Commission
Septic Systems, Health Department Regulations Greenwich Health Department
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Table 1-1. Town of Greenwich Land Use Jurisdictions, Regulations, and
Agencies
Town Highways, Sidewalks, and Drainage Facilities
Subdivision Regulations Building Zone Regulations
Highway Division, Greenwich Department of Public Works
State Maintained Highways (Putnam Avenue and Drainage Facilities)
CTDOT Design Standards Connecticut Department of Transportation
1.4 How to Use this Manual
This manual can be used by engineers and designers, local land use boards and commissions, municipal department staff, municipal officials, and property owners. Engineers and designers who are responsible for site design and the design of stormwater management systems for new development and redevelopment projects are the most likely users of this manual. Before beginning a project, engineers and designers should familiarize themselves with the Stormwater Management Standards (Section 3) that their project will have to meet. Next, engineers and designers should review the LID site planning and design process described in Section 4 as well as the structural stormwater management practices and drainage design requirements described in Sections 5 and 6, respectively, to determine approaches that would work best at their site. Finally, engineers and designers should refer to the submittal requirements in Section 7 and any other applicable technical guidance in the manual appendices. The recommended process for using the manual is summarized below:
Step 1 – Review applicable zoning, subdivision, and other local land use planning and regulatory requirements and contact local officials to clarify uncertainties. At a minimum, designers are required to adhere to the Stormwater Management Standards and performance criteria in this manual and the requirements referenced herein.
Step 2 – Collect the necessary information and data to inventory and evaluate the site in order to begin developing site design concepts and a stormwater management approach. The information and data required to properly inventory and evaluate a site are discussed in Section 4 (Low Impact Development site planning and design process) and in Section 7 (submittal requirements). This information will allow engineers and designers to make the decisions necessary to develop effective LID site designs and stormwater management plans.
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Step 3 – Confirm the applicable Stormwater Management Standards (Section 3) and required design criteria (Section 5) depending on the type and location of the proposed development project. The basic criteria for both stormwater quality and quantity controls should be established at this stage.
Step 4 – Establish the basic site design and stormwater management approach utilizing the LID site planning and design techniques described in Section 4. Before project details are finalized, such as building and street layouts, engineers and designers must determine how to avoid, reduce, and manage impacts.
The initial objective of the LID site planning and design process is to avoid disturbance of natural features. This includes identification and preservation of natural areas; minimizing the hydrologic alteration of a site is just as important as stormwater treatment for resource protection. Once sensitive resource areas and site constraints have been avoided, the next objective is to reduce the impact of land alteration by minimizing impervious areas to reduce the volume of stormwater runoff, increase groundwater recharge, and reduce pollutant loadings. Runoff is generated primarily from impervious surfaces, such as rooftops, roadways or any hard surface that prevents water from absorbing into the ground. Impervious surfaces can often be reduced with thoughtful site planning.
After making all reasonable efforts to avoid and reduce potential development impacts, the final step is to determine the basic approach for effectively managing the remaining stormwater runoff to meet the stormwater standards using the approaches described in Sections 5 and 6.
Step 5 – Use the approach determined in Step 4 to develop a conceptual design plan at approximately the 25% design stage that utilizes LID site planning and design techniques to the maximum extent practicable as required by Standard 1; identifies the location and types of BMPs to be utilized, the approximate footprint needed, and construction and maintenance access requirements; and establishes the basic profile to verify physical constraints and the overall feasibility of each BMP (see Stormwater Management Report Part One in Section 7 submittal requirements). At this stage, coordination with the local approval agencies is recommended to address potential issues prior to final design. Additional data may need to be collected at this stage (including field testing of soils if necessary) to revise the concept before moving forward with final design.
Step 6 – Move forward with site design, ensuring that the proposed stormwater management system meets the standards described in Section 3, including preparation of a construction erosion and sediment control plan, a Stormwater Pollution Prevention Plan for high load areas, an Operation and Maintenance Plan, and a Stormwater Management Report (Part Two), construction plans, and completed checklists and certifications (see Section 7 for submittal requirements).
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2 The Importance of Stormwater Management
The Town of Greenwich’s exceptional quality of life depends on the development of successful programs to protect, manage and restore its water resources, aquatic living resources and natural environment. Program goals and objectives include protecting the quality and volume of existing and future water supplies; restoration of sensitive riparian and marine fisheries; protection of sensitive wetlands and critical areas; restoration of impaired waters; ensuring safe recreational water uses; addressing flooding impacts; and meeting regulatory requirements, such as Phase II of the National Pollutant Discharge Elimination System (NPDES) municipal stormwater permit program. Achieving these goals is extremely difficult and complicated by an array of political, social, economic and technological issues.
An effective urban stormwater management program is central to the successful protection and restoration of water resources and related water dependent uses. To guarantee protection and restoration goals are achieved, an urban stormwater program must maintain the natural hydrologic regime and maintain a watershed’s capacity to filter and purify runoff and capture/sequester pollutants. Unfortunately, experience over the last 30 years has clearly demonstrated that conventional strategies of natural resource conservation combined with end- of-pipe runoff treatment technology simply has not and cannot restore or prevent continued degradation of receiving waters from the cumulative impacts of urbanization.
The limitations of conventional urban stormwater management practices to meet receiving water goals was a major driver for development of more effective decentralized source control technologies generally referred to as Low Impact Development or LID. LID provides the necessary tools to plan and engineer sites in a manner that mimics predevelopment hydrology, protects water quality by treating runoff and reducing pollutant loads, and provides a wide array of strategies and tools for urban retrofits to restore impaired waters. Greenwich, like many local communities, historically has placed a strong emphasis on the stormwater basics of providing flood control and adequate drainage. The Town recognizes the limitations of these conventional strategies and the multiple benefits of a holistic approach to stormwater management through the use of more natural systems and LID techniques. The stormwater management design criteria and guidance adopted by this manual reflect the trend in stormwater management toward an integrated approach that combines effective site planning and structural stormwater controls to address the full range of hydrologic and water quality impacts resulting from development.
2.1 Impacts of Development
The hydrologic and water quality impacts from urban runoff can be significant. Streams, lakes, and estuaries in urban areas are often impaired by urban runoff. Impervious surfaces (e.g., rooftops, sidewalks, roads, and parking lots), disturbed soils, and managed turf associated with urban development can have multiple impacts on water quality and aquatic life. These impacts are summarized in Table 2-1.
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Increases in: Decreases in:
Impervious cover, compacted soils, managed turf, and other land covers and uses that contribute pollutants
Health and safety of receiving waters
Stormwater runoff volume Groundwater recharge
Stormwater runoff velocity Stream channel stability
Pollutant loads Health, safety, and integrity of water supplies, reservoirs, streams, biological communities, recreational opportunities
Stream channel erosion Stream habitat
Adapted from CWP, 2008.
Water bodies in the Town of Greenwich that are impaired (i.e., do not meet Connecticut Water Quality Standards) due to urban runoff or related nonpoint sources are summarized in Appendix A. Coastal waters are the primary impaired water bodies in the Town of Greenwich. The impairments are for shellfish harvesting, recreation, and habitats for fish and other aquatic life and wildlife. Non-point sources, including urban stormwater runoff and waterfowl, as well as marina/boating and sanitary on-vessel discharges are the predominant suspected sources of the impairments, which are caused by elevated concentrations of bacteria and nutrient enrichment/low dissolved oxygen. Urban development can also impact the timing and quantity of post-development runoff discharging to urban streams, as evidenced by a comparison between pre-development and post-development runoff hydrographs (Figure 2-1). Compared to the pre-development conditions, post-development stormwater discharges can increase the runoff volume, increase the peak discharge, and decrease the infiltration of stormwater, which thereby decreases baseflow in headwater streams and in wetlands. The changes to stream hydrology can have negative impacts on channel stability and the health of aquatic biological communities. Common problems include bank scour and erosion, increased downstream flooding, and loss of in-stream habitat for macroinvertebrates, fish, and other organisms (CWP, 2008). In addition, these impacts not only affect the aquatic environment, but also affect the ability of people to use these areas for recreation, both active and passive. For example, the discharge of stormwater commonly results in beach closures due to high bacteria and pathogen counts in the water. Increasingly, communities are looking for ways to maximize the opportunities and benefits associated with growth while minimizing and managing the environmental impacts of development. Where and how development occurs can dramatically affect a community’s watersheds, infrastructure, and water supplies.
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Source: CWP, 2008.
Figure 2-1. Changes in Stream Hydrology as a Result of Urbanization Impervious cover, which is one measure of the amount of development within a community and watershed, has been widely cited as an indicator of stream health in urbanizing watersheds. However, the original body of work upon which the impervious threshold relationships to stream health has been disputed on many levels. Although there is a correlation between impervious cover and health of streams in urbanizing watersheds, it is an indirect and complex one. Since this relationship is a general correlation and not a direct cause-and-effect relationship, establishing absolute impervious thresholds for stream health cannot be made reliably. In fact, the cause and effect of stream health degradation is not imperviousness itself but, instead, the way that impervious surfaces have been used to quickly collect and dispose of stormwater runoff. This good drainage paradigm results in a change from the natural hydrologic regime to one that increases runoff volume, velocity and flows; loss of recharge; reduced capacity of the landscape to capture and assimilate pollutants; loss of habitat structure; increased water temperatures, etc. The cumulative changes cause physical, chemical and biological alterations of vital ecological functions to the terrestrial and aquatic ecosystems that result in degraded stream health. If stormwater management/drainage systems were engineered to mimic nature by using LID techniques, there would be no cause and effect of urbanization and no correlation between imperviousness and stream health. LID’s philosophy, principles and practices provide the tools to mimic vital natural watershed functions, allowing development and urbanization to occur without stream degradation and independent of the amount of imperviousness. This concept is also known as effective impervious cover. Impervious cover that is hydrologically disconnected from the drainage system through the use of LID techniques effectively functions similar to pervious surfaces.
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Stormwater Management
Consistent with the Connecticut Stormwater Quality Manual, the Greenwich manual also promotes the following hierarchy of stormwater management methods to initially reduce stormwater impacts through site design and source control methods, followed by structural stormwater management controls to collect, detain, and treat stormwater:
First, reduce runoff and site disturbance through design: Use LID site planning and design techniques to reduce effective impervious cover, disturbed soils, and stormwater runoff volume.
Second, reduce pollutants carried by runoff: Use source control and pollution prevention practices to reduce exposure of pollutants to rainfall and runoff.
Third, capture, detain and treat runoff: Design stormwater BMPs to collect, detain and treat the stormwater that is generated after applying the LID site planning and design and source control methods described above.
Site Planning and Design Effective site planning and design consists of preventive measures that address the root cause of stormwater problems by attempting to maintain pre-development site hydrology. Stormwater programs that rely heavily on conventional end-of-pipe stormwater controls can miss opportunities to reduce stormwater impacts because they collect and treat runoff after it has already been generated. This manual emphasizes the use of site planning and design techniques early in the site development process to achieve greater stormwater quantity and quality benefits. Section 4 of the manual describes LID site planning and design techniques (i.e., non- structural LID BMPs). The following site planning and design techniques are recommended for use in Greenwich:
Preservation of undisturbed natural areas
Preservation or restoration of riparian buffers, floodplains, and shorelines
Minimize grading and clearing
Preservation of natural topography
Avoidance of sensitive areas
Conservation development
Shorter or shared driveways
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Use of open drainage systems
Lengthen flow paths and maximize sheet flow
Disconnection of roof runoff
Saving and replacing topsoil or the use of compost-amended soils
Source Control Practices and Pollution Prevention Source control practices and pollution prevention are operational practices that reduce or eliminate the exposure of pollutants to rainfall and runoff. Similar to the Connecticut Stormwater Quality Manual, the Greenwich manual emphasizes the use of source control practices and pollution prevention, together with effective site planning and design. The following source control and pollution prevention practices are recommended for use at residential and nonresidential sites in Greenwich: Residential Nonresidential
Natural landscaping
Tree planting
Downspout disconnection
Natural landscaping
Structural Stormwater Management Practices Structural stormwater management practices (also referred to as stormwater Best Management Practices or BMPs) are designed to collect, detain and treat the stormwater that is generated after applying the LID site planning and design and source control methods described above. Structural practices are typically used to meet multiple objectives such as reducing runoff volume, attenuating peak flows, capturing and treating runoff, and providing groundwater recharge.
1Yard waste composting reduces or eliminates the need for outside inputs of chemical fertilizers, thus resulting in
the reduction of pollutants exposed to rainfall and runoff.
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Stormwater management can be accomplished through small-scale, distributed practices close to the source of runoff (also referred to as structural LID BMPs), such as the use of rain gardens, filter strips, and permeable pavement, in combination with the LID site planning and design techniques that are described in Section 4 of this manual. Traditional end-of-pipe controls such as stormwater basins should only be used, if necessary, after exhausting LID approaches. The following structural stormwater BMPs are recommended for use in Greenwich. Structural LID BMPs Other Structural BMPs
Rainwater harvesting (e.g., rain barrels, cisterns) for property irrigation
Bioretention systems including rain gardens, tree filters, stormwater planters, and curb extensions
Dry wells and subsurface infiltration systems (decentralized, small-scale practices distributed throughout the site)
Green roofs
Permeable pavement
Stormwater basins
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Stormwater management standards establish minimum stormwater management criteria for new development and redevelopment activities. The Town of Greenwich stormwater management standards promote the use of Low Impact Development techniques to protect water quality, reduce runoff volume, maintain groundwater recharge, and address peak flows and flooding during larger storms. The standards are generally consistent with the stormwater management approaches and design guidance contained in the Connecticut Department of Energy and Environmental Protection Connecticut Stormwater Quality Manual and the Connecticut Department of Transportation Drainage Manual, but also reflect the town’s unique natural resources and development characteristics.
3.2 The Stormwater Management
Standards
Standard 1: Low Impact Development Low Impact Development (LID) site planning and design techniques (see Section 4 of this manual) shall be used to the maximum extent practicable2 to reduce the generation of stormwater runoff and pollutant loads. LID practices, both non-structural and structural, are to be given preference over conventional structural stormwater controls.
Standard 2: Protection of Natural Hydrology
A. Site disturbance shall be minimized. The area outside the project disturbance area shall be maintained at natural grade and retain existing, mature vegetated cover. The project disturbance area shall be depicted on the design, construction, and mitigation plans and shall be delineated in the field prior to commencing land disturbance activities. The project disturbance area shall include only the area necessary to reasonably accommodate construction activities. Low areas on a lot shall not be dewatered and filled in unnecessarily.
B. Soil compaction on site shall be minimized by using the smallest (lightest) equipment possible and minimizing travel over areas that will be revegetated (e.g., lawn areas) or used to infiltrate stormwater (e.g., bioretention areas). In no case shall excavation
2 Project proponents seeking to demonstrate compliance with this standard to the maximum extent practicable
shall demonstrate that: 1. They have made all reasonable efforts to meet the standard; 2. They have made a complete evaluation of possible site planning and design techniques (non-structural
LID BMPs), source control practices and pollution prevention, and structural LID BMPs as described in Section 4 of this manual; and
3. If full compliance with this standard cannot be achieved, they are implementing the highest practicable level of stormwater management, which must be documented by the proponent.
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equipment be placed in the bottom of an infiltration area during construction. All areas to be used to infiltrate stormwater shall be delineated with protective fencing prior to the start of construction.
C. The post-development time of concentration (TC) should approximate the pre-
development3 values when possible. Flow velocity in areas that are graded to natural drainage patterns should be kept as low as possible to avoid soil erosion.
D. Development shall follow the natural contours of the landscape. A grading plan shall be
submitted as part of the site plan review process showing both existing and finished grades for the proposed development. Retaining walls must comply with the requirements of the Building Zone Regulations. Basements that reach grade should be constructed as walk-outs.
E. Compost-amended soils shall be used for areas of fill on development sites prior to
vegetation establishment. Amending a soil with compost increases the soil’s permeability and water holding capacity, thereby delaying and often reducing the peak stormwater runoff flow rate, and decreasing irrigation water requirements. Soil amendments also enhance a lawn’s long-term aesthetics while reducing fertilizer and pesticide requirements.
F. No ground disturbed as a result of site construction and development shall be left as
exposed bare soil at project completion. All areas exposed by construction, with the exception of finished building, structure, and pavement footprints, shall be de- compacted (aerated) and covered with a minimum thickness of six inches of non- compacted topsoil, and shall be subsequently planted with living vegetation such as grass, groundcovers, trees, and shrubs, and other landscaping materials (mulch, loose rock, gravel, stone).
G. Priority shall be given to maintaining existing surface waters and systems, including, but
not limited to, perennial and intermittent streams, wetlands, vernal pools, natural swales, and low-lying areas.
H. Where roadway or driveway crossings of surface waters cannot be eliminated,
disturbance to the surface water shall be minimized, hydrologic flows shall be maintained, direct discharge of runoff from the roadway to the surface water is strongly discouraged, and the area shall be re-vegetated after construction.
I. Roadway and driveway crossings over streams shall comply with the Connecticut
Department of Energy and Environmental Protection Stream Crossing Guidelines (as amended) to accommodate high flows, minimize erosion, and support aquatic habitat and wildlife passage.
3 Refer to the Glossary at the end of this manual for a definition of pre-development conditions.
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Standard 3: Stormwater Best Management Practices Non-structural and structural stormwater Best Management Practices (BMPs) shall be used to meet the conditions below for control of runoff volumes and peak flows, pollutant reduction, and maintenance of groundwater recharge.
A. Stormwater management practices shall be selected to accommodate the unique hydrologic and geologic conditions of the site.
B. Proponents shall demonstrate how the proposed control(s) will comply with these
standards, including runoff reduction, groundwater recharge, peak flow control, and pollutant reduction. The proponent must provide design calculations and other back-up materials necessary.
C. At the discretion of the approving authority, conventional structural stormwater
management systems (non-LID BMPs) shall incorporate designs that allow for shutdown and containment (as feasible) in the event of an emergency spill or other unexpected contamination event.
D. Pumping of stormwater (excluding rainwater harvesting systems such as cisterns),
including, but not limited to, from yards, driveways, and roofs, is strongly discouraged and will be prohibited in most situations as part of a proposed stormwater management system design. This is because of the significant runoff volumes, maintenance requirements, standby power requirements, and overflows associated with large storms. All other feasible approaches must be investigated to avoid the use of pumps in stormwater management system designs. In the event the project proponent determines that pumps for stormwater are necessary the proponent must submit required backup information as described in this manual for review by the approving authority. For the use of a pump for stormwater to be approved by the approving authority, the proponent will be required to provide the following at a minimum:
Include on-site stormwater BMPs that are designed to accommodate and manage the pumped stormwater in accordance with the other standards contained in this manual. The stormwater BMPs shall include a system for reuse of the pumped stormwater for lawn or landscape irrigation.
Maintain a backup generator associated with the pump,
Design the system, at a minimum, for the 25-year, 24-hour design storm (the system may be required to be designed for the 50-year or 100-year, 24-hour design storm as determined by the approving authority),
Provide documentation that the pump and stormwater re-use system are designed and inspected by a Professional Engineer licensed in the State of Connecticut.
E. Pumping of uncontaminated groundwater, including, but not limited to, from
basements, and foundations, is discouraged for new development or in the case of redevelopment involving the upgrade of existing sump pump systems. The replacement of an existing sump pump system is acceptable when a direct replacement of the pump
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is needed. All other feasible approaches (footing drains to daylight, slab on grade, crawl space, etc.) must be investigated to avoid the use of pumps in groundwater management system designs for new development or redevelopment. In the event the project proponent determines that pumps are necessary to manage groundwater for new development or redevelopment applications, the proponent must submit required backup information as described in this manual for review by the approving authority. For the use of a pump to manage groundwater to be approved by the approving authority, the proponent will be required to provide the following at a minimum:
Include on-site BMPs that are designed to accommodate and manage the pumped uncontaminated groundwater in accordance with the other standards contained in this manual. A system for re-use of the pumped groundwater for lawn or landscape irrigation shall be considered. Overflows must be directed to an on-site level spreader or connected to the Town drainage system (Highway Permit Required) with review and approval.
A backup generator associated with the pump is recommended but not required,
Design the on-site BMPs to meet the maximum pumping rate of the proposed pumping system.
Provide documentation that the pump and on-site BMPs are designed and inspected by a Professional Engineer licensed in the State of Connecticut.
Standard 4: Runoff Volume Reduction and Groundwater Recharge
A. Runoff Reduction – Control post-development runoff volumes to the corresponding pre-development runoff volumes for up to the 1-year, 24-hour storm to the maximum extent practicable4 through the use of LID site planning and design techniques and structural stormwater BMPs.
B. Groundwater Recharge – Loss of annual recharge to groundwater shall be eliminated or
minimized to the maximum extent practicable through the use of infiltration measures including LID site planning and design techniques, structural stormwater BMPs, and good operation and maintenance. At a minimum the annual recharge from the post- development site shall approximate the annual recharge from the pre-development site conditions. Compliance with the runoff reduction standard in Item A. above using stormwater infiltration shall be considered adequate to demonstrate compliance with the groundwater recharge standard.
C. Runoff Capture – Runoff must be retained on-site for new stormwater discharges
located within 500 feet of and that ultimately discharge to tidal wetlands, which are not
4 For purposes of this standard, to the maximum extent practicable means that:
1. The project proponent has made all reasonable efforts to meet the standard, 2. The project proponent has made a complete evaluation of all possible management measures, including
Low Impact Development site planning and design techniques and structural stormwater BMPs, 3. If the post-development runoff volumes and/groundwater recharge do not at least approximate the
runoff volumes and/or annual recharge under pre-development conditions, the project proponent has demonstrated that the highest practicable methods for runoff reduction and/or infiltration have been implemented.
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fresh tidal wetlands, through the use of LID site planning and design techniques and/or structural stormwater BMPs. Compliance with the runoff reduction standard in Item A. above shall be considered adequate to demonstrate compliance with this standard.
Standard 5: Peak Flow Control
A. Stream Channel Protection – Control the post-development peak flow rate (typically the 2-year storm or smaller) that results in bankfull streamflow conditions and the shape and form of stream channels.
B. Conveyance Protection – Provide adequate passage for flows leading to, from, and
through stormwater management facilities.
C. Peak Runoff Attenuation – Control the post-development peak flow rates to the corresponding pre-development peak flow rates.
D. Emergency Outlet Sizing – Size the emergency outlet to safely pass the post-
development peak runoff from large storms in a controlled manner without eroding the outlet works, downstream drainage systems, and property more than would occur during a similar event under pre-development conditions.
Standard 6: Pollutant Reduction
A. Stormwater management systems shall be designed to remove 80% of the average annual post-construction load of Total Suspend Solids (TSS)5.
Standard 7: High Load Areas Stormwater discharges from land uses with higher potential pollutant loads (referred to as High Load Areas) require the use of specific source controls, pollution prevention measures, and stormwater BMPs, approved by the approving authority for such use.
A. The uses or activities identified in Section 5.7.1 are considered high-load areas, with the potential to contribute higher stormwater pollutant loads, and shall comply with the requirements in Section 5.7.1.
5 Since removal efficiency may vary with each storm, 80% TSS removal is not required for each storm. It is the
average removal over the year that is required to meet the standard.
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B. In addition to site-specific stormwater BMPs, high-load areas shall include a stormwater pollution prevention plan (SWPPP) describing methods for source reduction and pretreatment.
C. If a high-load area demonstrates, through a SWPPP, the use of BMPs that result in no exposure of regulated substances to precipitation or runoff or release of regulated substances, it shall no longer be considered a high-load area.
Infiltration of stormwater from high-load areas is prohibited within critical areas (see Stormwater Management Standard 8). Infiltration of stormwater from high-load areas outside of critical areas (see Stormwater Management Standard 8) is allowed if adequate treatment is provided.
Standard 8: Critical Areas
A. Stormwater discharges to or near6 critical areas (defined in Section 5 of this manual) require the use of source control and pollution prevention measures and structural stormwater BMPs that are suitable for managing discharges to such areas.
B. Infiltration of stormwater runoff from land uses with higher potential pollutant loads (high load areas) near or within a critical area is prohibited.
Standard 9: Redevelopment
A. Redevelopment is defined as construction, alteration, or improvement that disturbs the ground surface or increases the impervious area on previously developed sites. Redevelopment includes maintenance and improvement of existing roadways including widening less than a single lane, adding shoulders, correcting substandard intersections, and improving existing drainage systems; development, rehabilitation, expansion and phased projects on previously developed sites including residential teardowns (i.e., demolition and reconstruction or replacement of an existing residential dwelling with another residence of any size); and remedial projects specifically designed to provide improved stormwater management. A redevelopment project is any project site that undergoes redevelopment. The project site can be entirely under redevelopment or the project site can be a combination of redevelopment and new development.
B. Redevelopment of previously developed sites must meet the standards to the maximum
extent practicable7 for the portion of the site undergoing redevelopment. Projects involving redevelopment or reuse activities shall also improve existing conditions.
6 Near a critical area means there is a strong likelihood of a significant impact occurring to a critical area, taking into account site-specific factors. 7 For the purposes of this standard, To the maximum extent practicable means that: Proponents of redevelopment projects have made all reasonable efforts to meet the standard, considering the benefits of redevelopment as compared to development of raw land with respect to stormwater;
1. They have made a complete evaluation of possible stormwater management measures including LID site planning and design techniques and stormwater BMPs; and,
2. If not in full compliance with the applicable Standard, they are implementing the highest practicable level of stormwater management.
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C. The portion of a property that is currently undeveloped is not a redevelopment and thus does not fall under Standard 9. Any development on previously undeveloped portions of a site shall fully comply with all of the other Stormwater Management Standards.
D. For all redevelopment projects, stormwater controls (retrofits or new controls) shall be
incorporated into the design and result in a reduction in annual stormwater pollutant loads from the site. Proponents of redevelopment projects shall make full use of opportunities for controlling the sources of pollution and reducing runoff volumes by incorporating LID site planning and design techniques, including filtration (e.g., flow- through bioretention planters or rain gardens) and runoff capture and reuse for irrigation. This is particularly important for constrained redevelopment sites where it may not be possible to install BMPs that treat the entire water quality volume or meet the full runoff reduction standard. Redevelopment projects shall also incorporate measures that address water quantity issues by reducing the runoff volume and peak runoff from the site and by increasing groundwater recharge. Actions to improve existing conditions shall address known water quality and water quantity problems such as documented water quality impairments, low stream flow, or flooding.
E. Redevelopment activities shall not infiltrate stormwater through materials or soils containing regulated or hazardous substances or areas with soil or groundwater contamination. In such instances, the approving authority may waive the requirement to comply with other stormwater management standards that may require infiltration.
Standard 10: Construction Erosion and Sediment Control
A. A plan to control construction related impacts, including erosion, sedimentation, and other pollutant sources during construction and land disturbance activities (construction period erosion, sedimentation, and pollution prevention plan) must be developed and implemented in accordance with the Connecticut Guidelines for Soil Erosion and Sediment Control (as amended) and the requirements of the CTDEEP General Permit for the Discharge of Stormwater and Dewatering Wastewaters from Construction Activities for CTDEEP-regulated activities.
B. All development, regardless of the area of disturbance, must implement erosion and sedimentation controls prior to and during construction. Additionally, temporary controls shall be removed from a site and disposed of properly after the site has been stabilized.
Standard 11: Construction Inspections
A. The approving authority may require the proponent to post a bond, cash or other
acceptable surety. The form of the surety shall be approved by the Town, in an amount deemed sufficient to ensure that the work will be completed in accordance with the approved plans, but not less than the total estimated construction cost of the stormwater management facilities. A portion of the surety may be released as each phase of the project is completed in compliance with the approval, but shall not be fully released until the approving authority has received and approved the final inspection
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report. A portion of the surety may be held after issuance of a Certificate of Occupancy until all site work is completed.
B. The proponent shall notify the approving authority before starting land-disturbing
activity. The proponent shall also notify the approving authority before constructing the key components of the stormwater management system.
C. Periodic inspections of the stormwater management system construction shall be
conducted by the approved professional engineer of record (see Engineer of Record certification form in Appendix K). The Town reserves the right to conduct inspections at any time. Written inspection reports shall include: the inspection date and location; evaluation of compliance with the stormwater approval; and any deviations from the approved plans.
D. At a minimum, inspections shall include: an initial site inspection prior to approval of
any plan; inspection of site erosion controls; inspection of the stormwater management system prior to backfilling of any underground drainage or stormwater conveyance structures; and a final inspection before the surety is released. The stormwater system shall be inspected to verify its as-built features, and the inspector shall also evaluate the system during a storm event8. If the inspector finds the system adequate, this shall be reported to the approving authority.
E. If the system is found to be inadequate due to operational failure, even though built
according to the approved plans, the system shall be corrected by the proponent before final approval is granted by the approving authority. If the proponent fails to act, the approving authority may use the surety to complete the work. If the system does not comply with the approved plans, corrective action shall be required and a Stop Work order shall be issued until any violations are corrected and all work previously completed has received approval by the approving authority.
F. Upon completion, the proponent shall certify that the project is in accordance with
approved plans and specifications, and shall provide inspections to adequately document compliance. The approving authority will issue a letter certifying completion upon its receipt and approval of the final inspection and reports, and/or upon otherwise determining that all work was completed in conformance with the approved plans.
Standard 12: Operation and Maintenance
A. A long-term Operation and Maintenance (O&M) Plan shall be developed and implemented to ensure that stormwater management systems function as designed. This plan shall be reviewed and approved as part of the review of the proposed permanent (post-construction) stormwater management system. Execution of the O&M Plan shall be considered a condition of approval of a development plan. The approving authority shall require a project proponent to establish a homeowners association or similar entity
8 A storm event shall mean a storm forecasted for 0.5 inches or more of precipitation in a 24-hour period.
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to maintain the stormwater management system. For high-load areas or activities under Stormwater Management Standard 7, the O&M Plan shall include implementation of a SWPPP.
B. The O&M Plan shall identify all items described in Sections 5 and 7.
C. The proponent shall include with the development plan a mechanism for implementing and enforcing the O&M Plan. The proponent shall identify the lots or units that will be serviced by the proposed stormwater BMPs. The proponent shall also provide a copy of the legal instrument (deed, homeowner’s association, utility trust or other legal entity) that establishes the terms of and legal responsibility for the operation and maintenance of stormwater BMPs. In the event that the stormwater BMPs will be operated and maintained by an entity, municipality, state agency or person other than the sole owner of the lot upon which the stormwater management facilities are placed, the proponent shall provide a plan and easement deed that provides a right of access for the legal entity to be able to perform said operation and maintenance functions, including inspections. The owner shall keep the O&M Plan current, including making modifications to the O&M Plan as necessary to ensure that BMPs continue to operate as designed and approved. Proposed modifications of O&M Plans including, but not limited to, changes in inspection frequency, maintenance schedule, or maintenance activity along with appropriate documentation, shall be submitted to the approving authority for review and approval within thirty days of change.
D. Parties responsible for the operation and maintenance of a stormwater management system shall keep records of the installation, maintenance and repairs to the system, and shall retain records for at least five years.
E. Parties responsible for the operation and maintenance of a stormwater management system shall provide records of all maintenance and repairs during inspections and/or upon request.
F. When the responsible party fails to implement the O&M Plan, including, where applicable, the SWPPP, the municipality is authorized to assume responsibility for their implementation and to secure reimbursement for associated expenses from the responsible party, including, if necessary, placing a lien on the subject property.
Standard 13: Stormwater Management Report
A Stormwater Management Report shall be prepared for all development and redevelopment activities that are subject to the Stormwater Management Standards. This report shall document how the proposed project complies with the Stormwater Management Standards and shall be submitted with the stamp and signature of a Professional Engineer (PE) licensed in the State of Connecticut.
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Standard 14: Illicit Discharges All illicit discharges to the stormwater management system are prohibited. The stormwater management system is the system for conveying, treating, and infiltrating stormwater on site including stormwater best management practices and any pipes intended to transport stormwater to the groundwater, surface water, or municipal separate storm sewer system. Illicit discharges to the stormwater management system are discharges that are not entirely comprised of stormwater. Notwithstanding the foregoing, an illicit discharge does not include discharges from the following activities or facilities:
Landscape irrigation,
Irrigation water,
Discharges of flows from fire fighting activities (except training),
Discharges containing no chemical additives (including chlorine) from the flushing of fire protection systems, and
Naturally occurring discharges such as rising groundwater, uncontaminated groundwater infiltration, springs, and flows from riparian habitats and wetlands.
Redevelopment projects shall demonstrate that no illicit discharges exist on the redevelopment site by use of a dry-weather illicit discharge survey.
3.3 Applicability and Drainage Report
Exemptions
The Greenwich Stormwater Management Standards apply to new development, redevelopment, and other activities that will result in an increased amount of stormwater runoff and/or water pollutants flowing from a parcel of land or any activity that will alter the drainage characteristics of a parcel of land (prior to the application of stormwater Best Management Practices), unless exempt. Two types of exemptions may apply.
Categorical Exemptions The Greenwich Stormwater Management Standards shall not apply to the following categorically exempt activities, although application of the standards is still strongly encouraged:
Normal maintenance and improvement of land in agricultural use (as defined by Connecticut General Statutes), provided such activity conforms to acceptable management practices for pollution control approved by the Connecticut Department of Energy and Environmental Protection and the Greenwich Inland Wetlands and Watercourses Commission. This exemption does not apply to construction activities that are not directly related to the farming or agricultural operation.
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Routine maintenance of existing landscaping, gardens (excluding structural modifications to stormwater BMPs including rain gardens) or lawn areas including those maintained by the Town of Greenwich Parks and Recreation Department and Board of Education.
Resurfacing of an existing impervious area on a non-residential lot such as repaving an existing parking lot or drive with no increase in impervious cover.
Routine maintenance to existing town roads that is performed to maintain the original width, line, grade, hydraulic capacity, or original purpose of the roadway.
Customary cemetery management.
Emergency repairs to any stormwater management facility or practice that poses a threat to public health or safety, or as deemed necessary by the approving authority.
Any emergency activity that is immediately necessary for the protection of life, property, or the environment, as determined by the approving authority.
Repair of an existing septic system.
Construction of utilities (gas, water, electric, telephone, etc.), other than drainage, which will not permanently alter terrain, ground cover, or drainage patterns.
Repair or replacement of an existing roof of a single-family dwelling.
Construction of a second (or higher) floor addition on an existing building.
Construction of a maximum 12 foot x 12 foot shed. The construction must include the installation of a 1 foot wide x 1 foot deep crushed stone trench along the sides of the shed that discharge the roof runoff.
The repair of an existing wood, composite, or plastic deck with no proposed enlargement of the deck surface.
The reconstruction or construction of a wood, composite, or plastic deck with the decking boards spaced at least 3/16 of an inch and a pervious surface below the deck. The pervious area below the deck must have the soil tilled 12 to 16 inches and finished with grass seed, sod, or crushed stone. The minimum depth for the crushed stone is 4 inches. A site plan showing the proposed location of the deck and construction details for the deck must be submitted.
The construction of any fence that will not alter existing terrain or drainage patterns.
Conditional Exemptions Requiring PE Certification
Projects Adding Up to 500 Square Feet of Impervious Surfaces Projects adding up to 500 square feet of impervious surfaces9 are exempt from the Greenwich Stormwater Management Standards, provided that all of the following conditions are met:
The project design, including the proposed drainage design, if any, will not have an adverse effect on offsite properties or offsite drainage infrastructure, as certified by a professional engineer.
At least one of the following measures shall be implemented on the project site to help mitigate the effects of site disturbance and new impervious surfaces within its on site watershed and point of concern:
9 Refer to the glossary in the Town of Greenwich Drainage Manual for a definition of impervious surface.
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o Disconnection of roof down spouts that meet the Simple Disconnection standards in the Town of Greenwich Drainage Manual February 2012 as amended
o A zero increase in peak flow to all points of concern for the 1, 2, 5, 10, and 25- year design storms
o The runoff volume from the new impervious surfaces shall be infiltrated for the 10-year design storm
o Constructing a bioretention area for the Water Quality Volume of the contributing watershed of the project area. The design standards in the Town of Greenwich Drainage Manual February 2012 as amended must be met
o Creating a buffer with a length greater than or equal to the length of the project area and a minimum width of 10 feet planted as a meadow
o Restoring a riparian buffer (may require IWWA permit)
The project proponent submits an exemption request, including professional engineer certification, in lieu of a Stormwater Management Report (Form SE-100).
This exemption can only be used until the cumulative addition of impervious surfaces on a site exceeds 500 square feet, regardless of ownership changes. For projects adding up to 500 square feet of impervious surfaces, application of the Greenwich Stormwater Management Standards is still strongly encouraged. Residential teardowns are not exempt unless the project meets the Conditional Residential Teardown Exemption Requirements. Commercial teardowns are not exempt.
Projects Adding Between 500 and 1,000 Square Feet of Impervious Surfaces Projects adding between 500 and 1,000 square feet of impervious surfaces are exempt from the Greenwich Stormwater Management Standards, provided that all of the following conditions are met:
The project design, including the proposed drainage design, if any, will not have an adverse effect on offsite properties or offsite drainage infrastructure, as certified by a professional engineer,
At least one of the following measures shall be implemented on the project site to help mitigate the effects of site disturbance and new impervious surfaces within its on site watershed and point of concern:
o Disconnection of roof down spouts that meet the Simple Disconnection standards in the Town of Greenwich Drainage Manual February 2012 as amended
o A zero increase in peak flow to all points of concern for the 1, 2, 5, 10, and 25- year design storms
o The runoff volume from the new impervious surfaces shall be infiltrated for the 10-year design storm
o Constructing a bioretention area for the Water Quality Volume of the contributing watershed of the project area. The design standards in the Town of Greenwich Drainage Manual February 2012 as amended must be met
o Creating a buffer with a length greater than or equal to the length of the project area and a minimum width of 10 feet planted as a meadow
o Restoring a riparian buffer (may require IWWA permit)
Town of Greenwich Drainage Manual 22-A February 2014
At least one of the following measures shall be implemented on the project site using LID or conventional stormwater BMPs to help mitigate the effects of site disturbance and new impervious surfaces:
o A zero increase in peak flow to all points of concern for the 1, 2, 5, 10, and 25-year design storms
o The runoff volume from the new impervious surfaces shall be infiltrated for the 10- year design storm
The project proponent submits an exemption request, including professional engineer certification, in lieu of a Stormwater Management Report (Form SE-100).
This exemption can only be used until the cumulative addition of impervious surfaces on a site exceeds 1,000 square feet, regardless of ownership changes. For projects adding between 500 and 1,000 square feet of impervious surfaces, application of the Greenwich Stormwater Management Standards is still strongly encouraged. Residential teardowns are not exempt unless the project meets the Conditional Residential Teardown Exemption Requirements. Commercial teardowns are not exempt.
Conditional Residential Teardown Exemption Requiring Professional Engineering Certification Projects for residential teardowns that reconstruct where the impervious surfaces10 within each point of concern is less than or equal to pre-development conditions and the peak flow and runoff volume for the 1, 2, 5, 10, 25, 50, and 100-Year Storms has a zero increase to all points of concern are exempt from the Greenwich Stormwater Management Standards, provided that all of the following conditions are met:
The project design, including the proposed drainage design, if any, will not have an adverse effect on offsite properties or offsite drainage infrastructure, as certified by a professional engineer
A Stormwater Management Report must be submitted with the following included: 1. Project Narrative 2. Site Inventory & Evaluation
a. Topography b. Soil Evaluation (Soil Evaluation Test Results (Form SC-101) Shall Be Used)
i. Initial Feasibility Evaluation (NRCS Web Soil Survey and similar sources of information)
ii. Concept Design Testing (test pits/borings and saturated hydraulic conductivity testing, as per Appendix B)
3. Evaluate Pre-Development Site Hydrology to all points of concern (Runoff Volume and Peak Flow Rate – 1, 2, 5, 10, 25, 50 and 100-Year Storms)
a. Watershed Map Pre-Development b. NRCS Runoff Curve Numbers Pre-Development c. Time of Concentration Pre-Development
4. Evaluate Post-Development Site Hydrology to all points of concern (Runoff Volume and Peak Flow Rate – 1, 2, 5, 10, 25, 50 and 100-Year Storms)
a. Watershed Map Post-Development
10 Refer to the glossary in the Town of Greenwich Drainage Manual for a definition of “impervious surface.”
22-B Town of Greenwich Drainage Manual
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b. NRCS Runoff Curve Numbers Post-Development c. Time of Concentration Post-Development
5. Peak Runoff to all points of concern must have a zero increase for the 1, 2, 5, 10, 25, 50, and 100-Year Storms
6. Runoff volume to all points of concern must have a zero increase for the 1, 2, 5, 10, 25, 50, and 100-Year Storms
7. Compare & Summarize Pre-&-Post Development Site Hydrology for peak flow and runoff volume to all points of concern
8. Conveyance Protection: 10, 25, 50 & 100-Year Depending on Peak Flow Rate for Downstream Stormwater Facilities
9. Outlet Protection Calculations – Based on Conveyance Protection 10. Emergency Outlet Sizing: Safely Pass the 100-Year 11. Supporting Documents 12. Sealed and Signed By a Professional Engineer
The application of the Greenwich Stormwater Management Standards is still strongly encouraged. For projects that meet the above criteria, the project proponent needs to submit plans which include all items on the:
1. Checklist for Construction Plans – Form CL-102 2. Checklist for Driveway Profile and Sight Distance Plan – Form CL-103
For projects that meet the above criteria, the project proponent must submit an Operations and Maintenance Plan Report. The Operations and Maintenance Plan must be submitted following the Checklist for Operations & Maintenance Plan Report CL-104. For projects that meet the above criteria, the project proponent needs to submit the items on the Checklist for Certificate of Occupancy – Form CL-105 with the request for Certificate of Occupancy. The Improvement Location Survey must include the items on the Checklist for Improvement Locations Survey Depicting „As-Built” Conditions CL-106. The use of this exemption removes any future additional construction on the property from using the Conditional Exemption regardless of ownership changes.
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4.1 Introduction
Traditionally, stormwater has been managed using large, structural practices installed at the low end of development sites – essentially as an afterthought – on land segments left over after subdividing property. This approach, sometimes referred to as end-of-pipe stormwater management, yields the apparent advantages of centralizing control and limiting expenditure of land. Unfortunately, it is much less efficient than it appears. In the last decade, alternative approaches have been established that employ environmentally sensitive site design and Low Impact Development (LID) techniques with results that surpass the end-of-pipe approach. LID is the cornerstone of stormwater management. LID is an innovative stormwater management approach that uses the basic principle modeled after nature: manage rainfall where it lands. The goal of LID is to mimic a site’s pre-development hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. Techniques are based on the premise that stormwater management should not be seen as stormwater disposal. Instead of conveying and managing/treating stormwater in large, costly end-of-pipe facilities located at the bottom of drainage areas, LID addresses stormwater through small, cost-effective landscape features located at the lot level. LID is a versatile approach that can be applied equally well to new development, urban retrofits, and redevelopment projects. Effective LID includes the use of both non-structural and structural stormwater management measures that are a subset of a larger group of practices and facilities known as Best Management Practices or BMPs. The BMPs utilized in low impact development, known as LID BMPs, focus first on minimizing both the quantitative and qualitative changes to a site’s pre- developed hydrology through non-structural practices and then providing treatment as necessary through a network of structural facilities distributed throughout the site. In doing so, LID places an emphasis on non-structural stormwater management measures, seeking to maximize their use prior to utilizing structural BMPs. Non-structural BMPs used in LID seek to reduce stormwater runoff impacts through environmentally sensitive site planning and design. Non-structural LID BMPs include such practices as minimizing site disturbance, preserving important site features, reducing and disconnecting impervious cover, flattening slopes, utilizing native vegetation, minimizing turf grass lawns, and maintaining natural drainage features and characteristics. Structural BMPs used to control and treat runoff are also considered LID BMPs if they perform these functions close to the runoff’s source. As such, they are typically smaller in size than standard structural BMPs. Structural LID BMPs include various types of basins, filters, surfaces, and devices located on individual lots in a residential development or throughout a commercial, industrial, or institutional development site in areas not typically suited for larger, centralized structural facilities. Standard 1 of the Greenwich Stormwater Management Standards requires the use of LID site planning and design techniques to the maximum extent practicable to reduce the generation of
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stormwater runoff and pollutants. Project proponents shall demonstrate why the use of LID site planning and design techniques is not possible before proposing to use traditional, structural stormwater management measures alone.
4.1.1 Advantages of LID
LID can provide a number of advantages over traditional stormwater management approaches that rely solely on end-of-pipe controls, depending on site-specific factors. Some of these potential advantages include:
Reduced consumption of land for stormwater management. LID practices engage the natural capacity of undisturbed land to absorb precipitation thus reducing the need for structural controls. When structural controls are used, they are small, close to the source of runoff, often installed below grade and made to fit well into the general landscape. Little land is expended for stormwater management.
Reduced construction costs. Traditional stormwater management requires significant sewering and earthwork. LID methods apply controls as close to sources of runoff as possible. Wherever practical, conveyances incorporate natural flow paths and swales instead of pipes. Structures installed tend to be smaller, thus reducing the need for excavation and construction materials.
Ease of maintenance. LID practices require limited maintenance. Much of the maintenance that is required can be accomplished by the average landowner.
Takes advantage of site hydrology. LID management mimics natural site hydrology and exploits the tendency of undisturbed land to retain and absorb runoff from impervious surface. Runoff that is absorbed recharges groundwater and stream baseflow and does not need to be managed or controlled by an end-of-pipe practice. Reduced end-of-pipe discharge is also beneficial for streambank stability and habitat.
Better quality of discharge. Recent research indicates that most constructed technologies are unable reduce pollutant concentrations below certain thresholds, which may exceed water quality standards. Landscapes that utilize LID practices minimize discharge and often retain all runoff from events smaller than the 2-year, 24-hour design storm. Pollution is minimized because discharge is minimized.
More aesthetically pleasing development. Traditional stormwater management tends to incorporate the use of large, unnatural looking practices such as detention ponds. When neglected, these practices may present safety and mosquito concerns. LID practices utilize pre-development land features that are small and fit well into the natural landscape.
Improved marketability and property values. The advantages of LID management translate into the marketplace. The benefits to developers include reduced land clearing and earth disturbance costs, reduced stormwater management costs,
February 2014
4.2 Fundamental Concepts
Successful application of LID is maximized when it is viewed in the context of the larger design process. Using LID to its fullest potential involves adhering to the following fundamental concepts:
Prevent. Then mitigate. A primary goal of LID is preventing stormwater runoff by incorporating non-structural practices into the site development process. This can include preserving natural features, clustering development, and minimizing impervious surfaces. Once prevention as a design strategy is maximized, then the site design — using structural BMPs – can be prepared.
Minimize disturbance. Limiting the disturbance of a site reduces the amount of stormwater runoff control needed to maintain the natural hydrology.
Manage stormwater as a resource — not a waste. Approaching LID as part of a larger design process enables us to move away from the conventional concept of runoff as a disposal problem (and disposed of as rapidly as possible) to understanding that stormwater is a resource for groundwater recharge, stream base flow, lake and wetland health, water supply, and recreation.
Mimic the natural water cycle. Stormwater management using LID includes mimicking the water cycle through careful control of peak rates as well as the volume of runoff and groundwater recharge, while protecting water quality. LID reflects an appreciation for management of both the largest storms, as well as the much more frequent, smaller storms.
Disconnect. Decentralize. Distribute. An important element of LID is directing runoff to BMPs as close to the generation point as possible in patterns that are decentralized and broadly distributed across the site.
Integrate natural systems. LID includes careful inventorying and protecting of a site’s natural resources that can be integrated into the stormwater management design. The result is a natural or green infrastructure that not only provides water quality benefits, but greatly improves appearance by minimizing infrastructure.
Maximize

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