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CITY OF SALEM, MASSACHUSETTS i Urban Stormwater Management Guidebook Issued December 31, 2005

PREFACE

This Best Development Practices Urban Design Manual accompanies the Salem Stormwater and Low Impact Development (LID) Ordinance, adopted by the Salem City Council on [MonthDay, Year]. This Guidebook will assist residents, designers and contractors with meeting the requirements of the Ordinance. The goal of the Guidebook is to clearly communicate the Citys expectations and, in turn, to expedite the review process.

The Urban Stormwater & Low Impact Development (LID) Ordinance and the Best Development Practices Guidebook are the result of innovative and environmentally proactive work by Mayor Stanley J. Usovicz, Bruce Thibodeau, City Engineer and Director of Public Services, and Joseph Nerden, Assistant City Engineer and Assistant Director of Public Services. Funding and technical support for the preparation of the Ordinance and this Guidebook was provided by the Coastal Nonpoint Source Grant Program made possible by NOAA and administered by the Massachusetts Office of Coastal Zone Management (CZM). Views expressed herein are those of the author(s) and do not necessarily reflect the views of EOEA or CZM.

Images on the cover (left to right): Harbor View from Fort Pickering, Peabody Essex Museum, and Forest River. Photos courtesy of the Welcome to Salem, Massachusetts The City Guide website at http://www.salemweb.com.

CITY OF SALEM, MASSACHUSETTS ii Urban Stormwater Management Guidebook Issued December 31, 2005

ACKNOWLEDGEMENTS

Creation of the Urban Stormwater & Low Impact Development (LID) Ordinance and the Best Development Practices Guidebook would not have been possible without the cooperation of City Staff and local stakeholders. Members of the Working Group should be recognized for their dedication to the project and their valuable insight throughout the project. Likewise, members of the Advisory Group should be acknowledged for their guidance during critical steps in the process.

Working Group

Julie Conroy and Jason Baker, Massachusetts Office of Coastal Zone Management (CZM) Joe Nerden, Asst. City Engineer and Asst. Director of Public Works Valerie Gingrich, Former Clerk to the Planning Board, Department of Planning and

Community Development (DPCD) Dan Merhalski, Clerk to the Planning Board, Department of Planning and

Community Development (DPCD) Frank Taormina, Planner/Conservation Agent (DPCD) Rebecca Longley, Environmental Planner (DPCD) Bob Rafferty, Project Manager - Woodard & Curran Emily Ferrazza, Engineer - Woodard & Curran

Advisory Group

Bruce Thibodeau, City Engineer and Director of Public Works Jean Pelletier, Ward 3 City Councilor Scott Patrowicz, Patrowicz Land Engineering Barbara Warren, Executive Director, Salem Sound Coastwatch Jean Levesque, Assistant A.D.A. Coordinator, Commission on Disabilities Kate Sullivan, Chief Administrative Aide, Office of the Mayor Joanne Scott, Health Agent, Salem Board of Health Jim Gilbert, Esq., City Solicitor Lynn Duncan, Director of Planning and Community Development and Executive Director of

the Salem Redevelopment Authority (SRA) Thomas St. Pierre, Building Inspector David Pabich, Conservation Commission

CITY OF SALEM, MASSACHUSETTS iii Urban Stormwater Management Guidebook Issued December 31, 2005

TABLE OF CONTENTS

SECTION PAGE NO.

PREFACE ................................................................................................................................................. I

ACKNOWLEDGEMENTS ............................................................................................................... II

INTRODUCTION ................................................................................................................................ 1

1. CHECKLISTS FOR DEVELOPERS.................................................................................... 3Stormwater Permit Eligibility Worksheet ...............................................................................................5City of Salem - Routing Slip for Stormwater Review ...........................................................................7Stormwater and Low Impact Development (SLID) Permit Requirements ..................................8Minor Impact Permit (MIP) Requirements ............................................................................................9Low Impact Development (LID) Credits Worksheet .........................................................................10

2. STORMWATER MANAGEMENT..................................................................................... 11Overview and Policies ..................................................................................................................................11Summary of Practices Recommended in Salem ..................................................................................12Discussion of Practices ................................................................................................................................131. Vegetated Swales .....................................................................................................................................132. Vegetated Filter Strips ..........................................................................................................................173. Constructed Wetlands ..........................................................................................................................214. Bioretention Areas (Rain Gardens) ..................................................................................................245. Cisterns & Rain Barrels ........................................................................................................................286. Infiltration Trenches and Dry Wells..................................................................................................307. Infiltration Drainfields ..........................................................................................................................338. Pervious Paving Surfaces .....................................................................................................................369. Roof Gardens ............................................................................................................................................409. Retention Basins......................................................................................................................................4311. Detention Basins ....................................................................................................................................4612. Underground Detention .....................................................................................................................4913. Catch Basins and Drain Pipes ............................................................................................................51

3. EROSION AND SEDIMENTATION CONTROL ........................................................ 53Overview and Policies ..................................................................................................................................53Summary of Practices Recommended in Salem ..................................................................................53Site Planning ................................................................................................................................................53Construction Site BMPs ..............................................................................................................................54

4. LOW IMPACT DEVELOPMENT CREDITS .............................................................. 55Overview and Policies ..................................................................................................................................55Summary of Practices Recommended in Salem ..................................................................................55Discussion of Practices ................................................................................................................................55Credit No. 1: Environmentally Sensitive Development Credit ........................................................58Credit No. 2: Disconnection of Rooftop Runoff Credit......................................................................60Credit No. 3: Disconnection of Non-Rooftop Runoff Credit............................................................63Credit No. 4: Stream Buffer Credit ..........................................................................................................64

CITY OF SALEM, MASSACHUSETTS iv Urban Stormwater Management Guidebook Issued December 31, 2005

Credit No. 5: Grass Channel Credit .........................................................................................................66Dealing with Multiple Credits ...................................................................................................................67Better Site Design .........................................................................................................................................67

5. LANDSCAPE DESIGN............................................................................................................ 71Overview and Policies ..................................................................................................................................71Water Sensitive Landscaping ..................................................................................................................71Recommended Plant Species ....................................................................................................................73

LIST OF TABLES

TABLE PAGE NO.

Table 2-1: Use of Stormwater Management Practices in Salem......................................................13Table 3-1: Guidelines for Identifying Sensitive Site Features ..........................................................54

APPENDICES

Appendix A: Homeowners Guide to Stormwater Permitting in Salem

Appendix B: Massachusetts Low Impact Site Design Fact Sheet

Appendix C: Stormwater Structures & Mosquitoes Fact Sheet by EPA

CITY OF SALEM, MASSACHUSETTS 1 Urban Stormwater Management Guidebook Issued December 31, 2005

INTRODUCTION

Stormwater management in Salem is controlled through a variety of local, state, and federal permits. The Conservation Commission is the official local agency specifically charged with the protection of Salems natural resources. For new development and redevelopment, the Conservation Commission (or Massachusetts Department of Environmental Protection (MA DEP) on appeal) implements the Massachusetts Stormwater Management Standards through an Order of Conditions whenever jurisdiction is established under the Wetlands Protection Act. The Environmental Protection Agency (EPA) and MA DEP jointly administer the National Pollutant Discharge Elimination System (NPDES) stormwater permitting program. NPDES permits are required for certain industrial activities and construction activities that disturb more than one acre of land.

Although the Wetlands Protection Act and the NPDES stormwater permitting programs have made tremendous improvements in stormwater management throughout Massachusetts, the City of Salem has created a Stormwater & Low Impact Development Ordinance to extend stormwater management beyond the current reach of these programs. This Ordinance will give the Salem Engineering Department authority to require and enforce adequate water quality and water quantity control measures on sites that would not otherwise be regulated. The City believes this Ordinance is a necessary step in protecting Salems coastal and inland water resources.

This Urban Stormwater Management Guidebook (Guidebook) is a set of guidelines for residents, developers, designers, and project reviewers intended to improve the quality of development in the City of Salem. The Guidebook describes the required and preferred design and construction practices in Salem related to stormwater management, erosion and sedimentation control, landscape design, and site planning. Any project proposed in Salem that meets the Urban Stormwater & Low Impact Development (LID) Ordinance thresholds shall comply with the requirements of this Guidebook.

None of the practices in the Guidebook are new, and many have already been used extensively in Massachusetts and other parts of the United States. The Guidebook codifies these practices as official City policy, thus taking some of the guesswork out of project design and review. The Guidebook also provides a single-source reference book for designers and reviewers working in Salem. Recognizing that many best development practices are site-dependent, the Guidebook identifies a range of practices that are relevant to development and redevelopment projects on a variety of sites.

Whenever possible, the City of Salem has chosen to encourage the use of Low Impact Development (LID) and Better Site Design. LID is an innovative approach to stormwater management in which an attempt is made to duplicate the hydrologic regime of an undeveloped watershed. This approach is implemented by engineering a site so that the post-development hydrologic functions remain close to pre-development conditions by using design techniques that infiltrate, filter, store, evaporate, and detain stormwater runoff close to its source. LID techniques can be applied to almost all components of the urban environment, including not only open space, but also rooftops, streetscapes, parking lots, sidewalks, and medians. LID is a versatile approach that can be applied equally well to new development, urban retrofits, and redevelopment / revitalization projects. Better Site Design is a fundamentally different approach to residential and commercial development. It seeks to accomplish three goals at every development site: to reduce the amount of impervious cover, to increase natural lands set aside for conservation, and to use pervious areas for more effective stormwater treatment. To meet


these goals, designers must scrutinize every aspect of a site plan its streets, parking spaces, setbacks, lot sizes, driveways, and sidewalks to see if any of these elements can be reduced in scale. At the same time, creative grading and drainage techniques reduce stormwater runoff and encourage more infiltration.

The Guidebook addresses the following topics:

x An overview of the Citys Urban Stormwater & Low Impact Development Ordinance;

x Checklists for Designers in Salem;

x Recommended stormwater best management practices for Salem;

x Low impact development credit system;

x Guidelines and criteria for site planning;

x Required erosion and sediment control practices; and

x Recommended sustainable landscape design.

References are provided for those who seek more information and design information for the various practices.


1. CHECKLISTS FOR DEVELOPERS

The following checklists are included in Chapter 1 and were created to assist the developer or designer in first determining applicability under the Stormwater and LID Ordinance and then guiding the applicant through the necessary steps for compliance under the Ordinance:

x Stormwater Permit Eligibility Worksheet

x City of Salem - Routing Slip for Stormwater Review

x Stormwater & LID Permit Requirements Checklist

x Minor Impact Permit Requirements Checklist

x Low Impact Development Credits Worksheet

Checklists that are applicable to each project seeking coverage under either a Stormwater & LID Permit or a Minor Impact Permit will be required to submit a copy of these checklists with the permit application.


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STORMWATER PERMIT ELIGIBILITY WORKSHEET

Project Name:________________________________________________ Date:____________________

Applicant Name:_______________________________________________________________________

Street:_____________________________ Town, State:_________________________ Zip:___________

Phone:____________________ Fax: ____________________ Email:_____________________________

1. Check all that might apply to your proposed project: Yes / No / Maybe

a) This is a new development or redevelopment project. b) There will be increased stormwater runoff or pollutants flowing

from a parcel of land. c) Activities on site will alter drainage characteristics of the parcel of land. d) There will be an increase in impervious cover where the total

impervious area is or will be greater than 15% of the property. e) This project requires a Curb Cut or Curb Work Permit. f) This project includes paving. g) The project includes the alteration, redevelopment or conversion of land

use to a hotspot as defined in Section 4.0 of the Ordinance.

If you checked No for all of the above, STOP. The Salem Stormwater & LID Ordinance does not apply to your project.

If you checked Yes or Maybe for any of the above, you may be required to meet the requirements of Salems Stormwater and LID Ordinance. Proceed to Question 2.

2. If you meet one of the following descriptions, you are exempt from Salems stormwater requirements:

a) Normal maintenance and improvement of land in agricultural use; b) Maintenance of existing landscaping, gardens, or lawn areas associated with

single-family dwellings that meet the Lawn Fertilizer Use restrictions described in Section 5.B of the Ordinance;

c) Repair or replacement of an existing roof of a single-family dwelling; d) The construction or repair of any fence or wall that will not alter the existing

terrain or drainage patterns;e) Construction of utilities (gas, water, electric, telephone, etc.) other than drainage,

which will not permanently alter terrain, ground cover, or drainage patterns; f) Emergency repairs to any Stormwater Management BMP that poses a threat to

public health or safety, or as deemed necessary by the City Engineer; or g) Redevelopment projects that meet the requirements under Section 5.B.9 of

the Ordinance.

If you checked any of the boxes in Question 2, STOP. You are exempt from Salems Stormwater & LID Ordinance.

If you do not meet any of these exemptions, you will need to apply for a Minor Impact Permit (MIP) or a Stormwater & Low Impact Development (SLID) Permit. Please proceed to Question 3 on the next page.


3. Minor Impact Permit: Does your project meet the following description?

a) The construction of any structure that will not alter existing terrain or drainage patterns, as described on Stormwater and LID Regulations, Section 5.C: Yes No

a. Patio b. Deck c. Retaining Wall d. Wall e. Shed f. Impervious Surfaces g. Swimming Pool h. Grade Change i. Other:____________________

If you said Yes to any of the project types listed in Question 3, STOP. You may qualify for a Minor Impact Permit (MIP). Proceed to the Minor Impact Permit Checklist on Page 8.

If you said No to all of the above, proceed to Question 4.

4. All other projects described in Question 1, but not in Questions 2 or 3, require a Stormwater & Low Impact Development (SLID) Permit. Please note that if you are building a structure listed in Question 3, but you have not met the guidelines established in Section 5.C of the Stormwater and LID Regulations, you will also need a Stormwater & LID Permit instead of a MIP. Proceed to the Stormwater & LID Permit Checklist on Page 7.


CITY OF SALEM - ROUTING SLIP FOR STORMWATER REVIEW

Per the Stormwater and LID Ordinance, the City Engineer is designated as the Stormwater Authority. The Stormwater Authority shall administer, implement and enforce the Ordinance. The Stormwater Authority may also designate another City Board to review all stormwater submittals and to issue stormwater permits for any project within that particular Boards (the Reviewing Board) jurisdiction. For additional information, please refer to Section 3 of the Ordinance. The City Engineer will use this Routing Slip to select additional Reviewing Boards when necessary and to collect the appropriate signatures for stormwater permitting only. Please attach a copy of this Routing Slip with your SLID Permit or MIP application.

To be filled out by the Applicant:


Applicant Name:_______________________________________________________________________


Phone:____________________ Fax: ____________________ Email:_____________________________

Application for (circle one): Minor Impact Permit Stormwater & LID Permit

Please do not write below this line. To be filled out by the Stormwater Authority and Reviewing Board(s) only.

Review Required for (circle one):

Department Contact SLID Permit MIP Signature & Date

Engineering Department Bruce Thibodeau (120 Washington St, 4th Flr)

Y Y

Building Thomas St. Pierre (120 Washington St, 3rd Flr)

Y Y

Conservation Commission Frank Taormina (120 Washington St, 3rd Flr)

Optional: Y / N N

Planning Board Dan Merhalski (120 Washington St, 3rd Flr)

Optional: Y / N N

HealthJoanne Scott (120 Washington St, 4th Flr)

Optional: Y / N N

Other:__________________ Optional: Y / N N


STORMWATER AND LOW IMPACT DEVELOPMENT (SLID)PERMIT REQUIREMENTS

Please submit a copy of this checklist with all documents required for the Stormwater & LID Permit application to the City Engineer.


Applicant Name:_______________________________________________________________________


Phone:____________________ Fax: ____________________ Email:_____________________________

PERMITTING PROCESSThe following steps must be taken to meet the Citys stormwater requirements. Completed?

Completed the Permit Eligibility Checklist Reviewed the current version of the Citys Urban Stormwater Management GuidebookReviewed the City Engineers Stormwater & LID Regulations Attached the required submittals per the Stormwater & LID Regulations, including:

Routing Slip for Stormwater Review (Guidebook p.6) Stormwater Management Plan (Section 6.L) Operation and Maintenance Plan (if applicable see Section 6.M) Erosion & Sediment Control Plan (Section 6.N) Applicable Fees (see Section 6.E)

Completed any additional permitting requirements through other City Boards. To the best of your knowledge, have you met the Performance Standards described in Section 7 of the Stormwater & LID Regulations, including Site Planning and Landscape Design Criteria?

Yes No

Have you used non-structural BMPs recommended in the LID Credit System to reduce reliance on structural BMPs? If so, please complete and attach the LID Credits Worksheet (Guidebook p.9).

Yes No

BEST DEVELOPMENT PRACTICES One or more of the following must be used to meet the Salems stormwater policies.

Incorporatedinto projects?

Vegetated Swales Vegetated Filter Strip Constructed Wetlands Bioretention Areas (Rain Gardens) Cisterns & Rain Barrels Infiltration Trenches & Dry Wells Pervious Paving Surfaces Roof Gardens Retention Basins Detention Basins Other:Catch Basins and Drain Pipes (discouraged, unless other stormwater collection and conveyance systems are not feasible due to site conditions)

Have you demonstrated that other systems are infeasible?

Applicant Signature: Date


MINOR IMPACT PERMIT (MIP) REQUIREMENTS

Please submit a copy of this checklist with all documents required for the Minor Impact Permit application to the City Engineer.


Applicant Name:_______________________________________________________________________


Phone:____________________ Fax: ____________________ Email:_____________________________

PERMITTING PROCESSThe following steps must be taken to meet the Citys stormwater requirements. Completed?

Completed the Permit Eligibility Checklist Reviewed the current version of the Citys Urban Stormwater Management Guidebook, specifically Appendix A: Homeowners Guide to Stormwater Permitting in Salem. Reviewed the City Engineers Stormwater and LID Regulations

Reviewed Projects Eligible for a MIP (Section 5.C and Table 1) Attached the required submittals per the Stormwater and LID Regulations, including:

Routing Slip for Stormwater Review Project Description Plans, drawings, and/or specifications Brief description of plans to prevent erosion & control sediments during

constructionCompleted any additional permitting requirements through other City Boards. BEST DEVELOPMENT PRACTICES Please check any stormwater management practices that you currently use or plan to use at your home or business? If so, please show location on site plan.Vegetated Filter Strip Bioretention Areas (Rain Gardens) Cisterns & Rain Barrels Dry Wells Pervious Paving Surfaces Catch Basins and Drain Pipes Disconnection of Rooftop Runoff (see Chapter 4) Disconnection of Non-Rooftop Runoff (see Chapter 4) Other:

Applicant Signature: Date


LOW IMPACT DEVELOPMENT (LID) CREDITS WORKSHEET

Chapter 4 of this document describes the LID Credits System and non-structural best management practices recommended in Salem. If you wish to use LID Credits to meet recharge, water quality, and, in some cases, water quantity controls, please fill out this worksheet and submit with your permit application.


Applicant Name:_______________________________________________________________________


Phone:____________________ Fax: ____________________ Email:_____________________________

PROPOSED PROJECTPlease fill in the following information regarding the proposed project:Is the proposed project in a resource area protected by the Wetlands Protection Act [Massachusetts General Laws (MGL) Chapter 131, Section 40]? If so, you may not be able to use LID Credits without MA DEP and Salem Conservation Commission approval.

Yes No

Circle your recharge calculation method: Percent Volume Percent Area Both What is the total recharge required on your site per Section 7.C.5 of the Stormwater & LID Regulations? Recharge Volume, Rev (acre-feet) = Recharge Area, Rea (acres) = Which LID Credit(s) will you use? (check all that apply) Credit 1. Environmentally Sensitive Development To the best of your knowledge, are there applicable restrictions to this credit, including impervious cover and natural conservation area requirements? Please provide appropriate documentation.

Yes No

Has applicable stormwater detention for all roadway and connected impervious surfaces been addressed?

Yes No

Credit 2. Disconnection of Rooftop Runoff and/orCredit 3. Disconnection of Non-Rooftop Runoff What is the Hydrologic Soil Group (A,B,C, or D) for the runoff recharge area? To the best of your knowledge, are there applicable restrictions to this credit, including hotspot land use and potential basement seepage limitations? Please provide appropriate documentation.

Yes No

Credit 4. Stream Buffers To the best of your knowledge, are there applicable restrictions to this credit, including overland slope and sheet flow requirements? Please provide appropriate documentation.

Yes No

Credit 5. Grass Channels To the best of your knowledge, have the design criteria been met, development density and channel slope requirements? Please provide appropriate documentation.

Yes No

Have multiple credits been claimed for an identical area of the site? Yes No

Applicant Signature: (Print name below) Date


2. STORMWATER MANAGEMENT

OVERVIEW AND POLICIES

Nonpoint source (NPS) pollution in Salem is a problem due largely to the quantity of densely populated urbanized land area and ever increasing impervious surface area. The negative impacts of urbanization on coastal waters have been well documented, and Salems many water resources, including rivers, streams, beaches, harbors, wetlands, and estuaries, are affected by polluted stormwater runoff.

Salem has become the educational, medical, legal, cultural, and banking hub of the North Shore. Therefore, there is a steady demand for housing, business, and industry in Salem. Impervious surfaces in the area have greatly increased over the last 50 years while the population has remained fairly stable.1 This increase in impervious surfaces can be attributed to development and redevelopment of small lots that are not regulated by state or federal stormwater regulations. Highly urbanized areas, such as downtown Salem, are susceptible to frequent flooding, which is both a public health and environmental concern.

Currently, the Environmental Protection Agencys National Pollution Discharge Elimination System (NPDES) Permitting Program only regulates stormwater runoff from construction sites one acre and larger. The minimum lot size required by the Salem Zoning Ordinance is 15,000 square feet (


SUMMARY OF PRACTICES RECOMMENDED IN SALEM

Conventional development strategies treat stormwater as a secondary component of site design, usually managed with pipe-and-pond systems that collect rainwater and discharge it off site.2 More recent studies have shown that conventional stormwater design and management practices are not sufficient to improve the water quality of surface water bodies. Therefore, the City of Salem has embraced Low Impact Development, which uses hydrology as an integrating framework for site design, not a secondary consideration. The Citys general preference is that stormwater be conveyed and treated in natural and vegetated systems such as vegetated swales, filter strips, constructed wetlands, and bioretention cells. While some of these practices may be new to Salem, they have been used successfully in other communities and states and have gained support from EPA because of their generally superior performance in attenuating peak runoff rates, filtering pollutants, recharging groundwater, and allowing retention of the natural landscape.3Chapter 4 discusses a Low Impact Development Credit System, which provides non-structural methods to potentially reduce the size and number of structural BMPs required for a project site.

The City recognizes that many of the aforementioned BMPs may not be feasible in the more urbanized, densely developed areas in Salem. For example, infiltration may not always be possible due to the dense development, low permeability soils, and/or high groundwater. Likewise, the use of infiltration practices without pretreatment is prohibited at hotspots in the Stormwater & LID Ordinance. Hotspots are uses or activities with higher potential pollutant loadings, as defined in the most recent version of the Massachusetts Stormwater Management Handbooks. Site conditions and potential impact on abutters should be carefully considered when selecting stormwater best management techniques. Thus, the City has also recommended a number of BMPs that utilize onsite storage in lieu of or prior to connection to the Citys municipal separate storm sewer system (MS4).

2 Metropolitan Area Planning Council (MAPC). Fact Sheet #8: Low Impact Site Design. Part of the Massachusetts Low Impact Development Toolkit. See Appendix B. 3 Town of Franklin, MA. Best Development Practices Guidebook. Version 1, November 2001.

WHAT IS REQUIRED?

Performance Standards:Stormwater and LID Criteria

(Refer to Section 7 of the Stormwater & LID Regulations)

To ensure a minimum level of water quality and water quantity control for development and redevelopment projects, Salem has adopted stormwater management performance standards. These standards allow the design engineer to select one or more stormwater management systems that are most appropriate and cost-effective for the particular site.

At a minimum all projects shall comply with the performance standards of the most recent version of MA DEP Stormwater Management Policy and the accompanying Stormwater Management Handbooks. This Guidebook contains a list of BMPs that are recommended by the City. The applicant may propose alternative BMPs not listed in the Guidebook, subject to a full technical review and approval by the Stormwater Authority.


Table 2-1: Use of Stormwater Management Practices in Salem

Practice Salems Policy Recommended Uses Vegetated Swales Strongly encouraged Roadsides, parking lots Vegetated Filter Strips Strongly encouraged Roadsides, residential frontage areas,

parking lots, perimeter protection Constructed Wetlands Strongly encouraged Commercial and industrial sites, office

developments, subdivisions Bioretention Areas (Rain Gardens) Strongly encouraged Residential lots, parking lot islands Cisterns & Rain Barrels Strongly encouraged All areas of development Infiltration Trenches and Dry Wells Encouraged All areas of development Infiltration Drainfields Encouraged High density areas with limited space Pervious Paving Surfaces Encouraged Parking overflow areas Roof Gardens Encouraged Office/industrial buildings Retention Basins Neutral Subdivisions, office developments Detention Basins Allowed in combination with

other practices All areas of development, if necessary

Underground Detention Allowed in combination with water quality controls

High density areas with limited space

Drain Pipe/Catch Basin System Allowed only when other systems are not practical due to site constraints

All areas of development, if necessary

Note: This table does not include recommended non-structural techniques, such as site planning, water sensitive landscaping, disconnection of impervious areas, etc. discussed in subsequent chapters in this manual.

This Chapter of the Manual discusses thirteen stormwater management practices that can be used alone or in combination to meet Salems stormwater performance standards. The mention of trade names or commercial products does not constitute endorsement or recommendation for the use by the City of Salem. The applicant may propose alternative BMPs not listed in this Manual, subject to a full technical review and approval by the Stormwater Authority. At a minimum all projects shall comply with the performance standards of the most recent version of Massachusetts Department of Environmental Protection (DEP) Stormwater Management Policy, as well as the stormwater management performance standards specified in Section 7 of the Regulations.

DISCUSSION OF PRACTICES

Each of the thirteen stormwater management practices is discussed in more detail in this Chapter. References are provided at the end of each section for additional information on the use, design, and construction of these practices.

1. Vegetated Swales 4

Vegetated swales are an important Low Impact Development technique used to convey stormwater runoff. These open, shallow channels slow runoff, filter it, and promote infiltration

4 Information provided by the Massachusetts Low Impact Development Toolkit, produced by the Metropolitan Area Planning Council, in coordination with the I-495 MetroWest Corridor Partnership, with financial support from the US Environmental Protection Agency. See http://www.mapc.org/LID.html for more information.


into the ground. As a result, runoff volumes are smaller, peak discharge rates are lower, and runoff is cleaner. This approach contrasts with conventional stormwater strategies that rely on gutters and pipes that increase the velocity of runoff and provide no water quality improvement.

Swales are not just ditches under another namethey must be carefully designed and maintained to function properly. The vegetation in swales, usually thick grass, helps to trap pollutants (suspended solids and trace metals), and reduce the velocity of stormwater runoff; stormwater also percolates through the natural substrate.

Vegetated swales can replace curb and gutter systems as well as storm sewers that convey runoff. Swales require more room than curb and gutter systems but they require less expensive hardscaping; furthermore, the reduction in discharge rate and volume means that downstream treatment facilities can be smaller. Swales also double as landscaping features, increasing the value and attractiveness of the site, as well as its appeal to neighbors and regulatory boards.

Management Objectives:

x Provide water quality treatment; remove suspended solids, heavy metals, trash.

x Reduce peak discharge rate.

x Reduce total runoff volume.

x Infiltrate water into the ground.

x Provide a location for snow storage.

x Improve site landscaping.

Applications and Design Principles

Water quality swales are widely applicable on residential, commercial, industrial, and institutional sites. The amount of impervious cover in the contributing area to each swale should be no more than a few acres, and swales should not be used in areas where pollutant spills are likely. Grassed swales can be used in parking lots to break up areas of impervious cover. Roadside swales can be used in place of curb and gutter systems, except where there are numerous driveways requiring culverts. Where the sidewalk and road are parallel, the swale should be between the sidewalk and the road.

Vegetated swales may be parabolic or trapezoidal in cross section. Longitudinal slopes should be as low (i.e., flat) as possible, and never more than 4%; swales should follow natural topography and drainage patterns to the extent possible. Swales work best in sandy loams that facilitate infiltration; very sandy soils may be prone to erosion under high runoff velocities. Check dams placed along the length of the swale can help to slow the runoff and promote greater infiltration and pollutant removal. Careful hydrologic design is necessary to ensure adequate pretreatment of the water quality volume and nonerosive conveyance of large storms.

Figure 2: Swales can be used on commercial sites to convey runoff around the site and to help slow peak discharge rates. Photo: Lower Columbia River Estuary Partnership (http://www.lcrep.org).

Figure 1:This swale in a residential neighborhoodfits in with the landscaping.Photo:University of Connecticut,Jordan Cove UrbanMonitoringProject


In some applications, swales are designed with a 2- to 3-foot deep soil bed of loamy sand to promote greater infiltration; on denser sites, this bed may include a perforated underdrain to ensure rapid drainage of the swale if groundwater infiltration is slow. In such applications, the runoff would end up (via the underdrain or swale termination) in the conventional stormwater system, but the swale would still provide considerable quality, quantity, and rate benefits.

Figure 3: Vegetated swale design example from the Stormwater Managers Resource Center at http://www.stormwatercenter.net/.


Benefits and Effectiveness:

x Swales help to control peak discharges by reducing runoff velocity, lengthening flow paths, and increasing time of concentration.

x Infiltration through the natural substrate helps to reduce total stormwater runoff volume.

x Swales provide effective pretreatment for downstream BMPs by trapping, filtering and infiltrating particulates and associated pollutants. The design rate for TSS removal is 70%.

x Swales accent the landscape and may help to satisfy landscaping and greenspace requirements.

x Swales can provide a location for snow storage during winter months.

x Roadside swales effectively keep stormwater flows away from street surfaces.

x Construction costs are generally less than conventional curb and gutter systems.

Limitations:

x Each grassed swale can treat a relatively small drainage area of a few acres, depending on land use and soil type. Large areas should be divided and treated using multiple swales.

x Swales are impractical in areas with steep topography. If proper slope is not achieved, grassed channels will have very little pollutant removal.

x A thick vegetative cover is needed for these practices to function properly. Grass must not be mowed too short.

x Swales can be subject to channelization, if erosive velocity is exceeded. During construction, it is important to stabilize the channel before the turf has been established, either with a temporary grass cover, or the use of natural or synthetic erosion control products.

x Swales should be used carefully on industrial sites or areas of higher pollutant concentrations. If used, they should be part of a treatment train that includes other treatment BMPs.

x Soil compaction can reduce infiltration capacity.

x Grass swales do not appear to be effective at reducing bacteria levels in stormwater runoff.

Maintenance

Permits for water quality swales should specify schedules and responsibility for inspection and maintenance. Since swales may be located on private residential property, it is important to clearly outline the maintenance requirements to property purchasers.

x Inspect on a semi-annual basis; additional inspections should be scheduled during the first few months to make sure that the vegetation in the swales becomes adequately established. Inspections should assess slope integrity, soil moisture, vegetative health, soil stability, compaction, erosion, ponding, and sedimentation.


x Mow at least once per year, but do not cut grass shorter than the design flow depth because the effectiveness of the vegetation in reducing flow velocity and pollutant removal may be reduced. Grass cuttings should be removed from the swale and composted.

x Remove accumulated sediment when it is 3 deep or higher than the turf to minimize potential concentrated flows and sediment resuspension.

x Irrigate only as necessary to prevent vegetation from dying.

x The application of fertilizers and pesticides should be minimal.

x Reseed periodically to maintain dense turf.

x Remove trash or obstructions that cause standing water.

x Prevent off-street parking or other activities that can cause rutting or soil compaction.

Additional References:

x The Massachusetts Stormwater Technical Handbook (Volume 2) provides design details for Water Quality Swales. The handbook can be found on the MA DEP stormwater publications page (http://www.mass.gov/dep/brp/stormwtr/stormpub.htm), along with other useful stormwater publications.

x Grassed Swales; from The Wisconsin Stormwater Manual, University of Wisconsin-Extension Service; 2000. See http://cecommerce.uwex.edu/pdfs/G3691_7.PDF.

x Stormwater Management Fact Sheet: Grass Channel. Stormwater Managers Resource Center. See http://www.stormwatercenter.net.

2. Vegetated Filter Strips 5

Vegetated filter strips are low-angle vegetated slopes designed to treat sheet flow runoff from adjacent impervious areas. Filter strips (also known as grassed filters) function by slowing runoff velocities, filtering out sediment and other pollutants, and providing some infiltration into underlying soils. Because they use sheet flow and not channelized flow, filter strips are often more effective than swales at removing suspended solids and trash from runoff. They provide good pretreatment of stormwater that will then be routed to another technique such as a bioretention area.

5 Information provided by the MAPC Massachusetts Low Impact Development Toolkit.

Figure 4: A filter strip adjacent to this office development is vegetated with deep rooted native vegetation, which can remove sediment and organics from runoff. Photo: U.S. Army Corps of Engineers Chicago District (http://www.lrc.usace.army.mil/co-r/best_management_practices.htm).


Filter strips were originally used as an agricultural treatment practice, but have recently been used in more urban and suburban locations. They differ slightly from buffer strips, which are natural vegetated areas alongside streams and lakes; buffer strips are left undisturbed for habitat protection and visual screening, while filter strips are altered areas designed primarily for stormwater management. Like many other LID techniques, vegetated filter strips can add aesthetic value to development. They cost significantly less than hardscaped stormwater infrastructure and also provide a convenient and effective area for snow storage and treatment.

Figure 5: Vegetated filter strip design example from the Center for Watershed Protection publication entitled, Design of Stormwater Filtering Systems.


Management Objectives

x Remove suspended solids, heavy metals, trash, oil and grease.

x Reduce peak discharge rate and total runoff volume.

x Provide modest infiltration and recharge.

x Provide snow storage areas.



Filter strips are appropriate for roadside applications and along the edge of small- to medium-sized parking lots, so long as the tributary area extends no more than 60 feet uphill from a stream buffer strip. They can also be used to treat roof runoff that is discharged over a level spreader. Filter strips are ideal components of the outer zone of a stream buffer, or as pretreatment to another stormwater treatment practice. They are generally require too much land area for applications in urban areas. The contributing drainage area should generally be less than five acres.

Filter strips work best when they are at least 20 feet long (downhill axis), though shorter strips will still provide some treatment. They should have slopes between 1% and 15%, preferably in the lower end of that range. It is critical for filter strips to be planar or convex, since any undulation in the surface or obstructions can cause concentrated flow that leads to erosion, channelization, and loss of water quality benefits.

The design should seek to keep runoff velocity in the low to moderate range (less than 2 feet per second) to maximize water quality benefits. This can be done by limiting the size of the contributing impervious surface. Both the top and toe of the slope should be as flat as possible to encourage sheet flow. A pea gravel or cement level spreader (with a lip) at the top of the filter strip will improve sheet flow and will capture some sediment.

Some filter strips are designed with a pervious berm at the downhill end of the filter strip, to detain water temporarily, increasing infiltration and reducing peak discharge rates. This berm can significantly enhance water quality benefits if it is designed to impound the water quality volume.


x Filter strips provide runoff pretreatment by trapping, filtering and infiltrating particulates and associated pollutants. TSS removal rates range from 40%-90%. Effectiveness depends largely on the quantity of water treated, the slope and length of the filter strip, the type of vegetation, and the soil infiltration rate.

x Vegetated filter strips also reduce runoff velocities and increase the time of concentration as compared to channelized flow, resulting in a reduction of peak discharge rates.

x Filter strips may provide groundwater recharge as runoff infiltrates into soil; recharge may be considerable if design incorporates a ponding area at the toe of the slope.

x Filter strips can serve as a location for snow storage during winter months and will also help to trap and treat the salt and sand in snow when it melts.


x Filter strips are inexpensive to construct, especially when compared to conventional curb-and-gutter systems.

x Vegetated filter strips help to accent the natural landscape by providing green space adjacent to parking lots and roadways.

Limitations:

x Because filter strips infiltrate runoff to groundwater, they could be inappropriate at stormwater hotspots (such as gas stations) with higher potential pollutant loads. They should be combined with other BMPs to ensure adequate treatment of polluted runoff prior to discharge.

x Channelization and premature failure may result from poor design, imprecise construction, or lack of maintenance. Proper design requires a great deal of finesse, and slight problems in the construction, such as improper grading, can render the practice less effective in terms of pollutant removal.

x Filter strips have low removal rates for nutrients.

x Filter strips often require lots of space, often making them infeasible in urban environments where land prices are high.

Maintenance:

x Inspect level spreader monthly and remove built-up sediment.

x Inspect vegetation monthly for rills and gullies and correct. Fill any depressions or channels. Seed or sod bare areas.

x In the year following construction, inspect the filter strip regularly to ensure that grass has established. If not, replace with an alternative species. Allow natural succession by native grasses and shrubs if it occurs.

x Mow grass, as rarely as 2-3 times per year, to maintain 4" to 6 of dense grass cover. Grass clippings should be collected and composted elsewhere. Provide a minimum of fertilizer only when necessary. Mow when the soil is dry and firm to prevent rutting.

x Semi-annually, remove sediment that has accumulated to prevent berms or channels.

Additional Resources

x The Mass Highway Department Stormwater Handbook includes design details for filter strips. See http://166.90.180.162/mhd/downloads/projDev/swbook.pdf.

x Stormwater Management Fact Sheet: Grassed Filter Strip. Stormwater Managers Resource Center. See http://www.stormwatercenter.net.


3. Constructed Wetlands 6,7

Constructed wetlands (or stormwater wetlands) are shallow pools that create growing conditions suitable for marsh plants. These systems are designed to maximize pollutant removal through retention, settling, and uptake by wetland plants.8 Stormwater wetlands serve several benefits simultaneously. The primary purpose of constructed wetlands is to improve water quality by removing sediment and pollutants. However, these wetlands can also provide habitat for wildlife and waterfowl. A constructed wetland would be a suitable stormwater management practice for residential subdivisions and commercial developments.


Constructed wetlands must be designed considering the size of the contributing watershed area, amount of baseflow, soil type, and available space. The contributing watershed may be as small as 5 acres; however, the smaller the watershed area, the more difficult it is to create sufficient drainage and runoff to keep the wetland perpetually wet. Since wetlands need to maintain soil moisture throughout the year, it is important to have a dry-weather baseflow or a groundwater supply. In some cases, this water supply may need to be pumped from a well or surface water source. The preferred soil types for constructed wetlands are less-permeable soils that have relatively small pores and are less prone to evaporation.

The surface area of constructed wetlands should be at least 1% of the contributing drainage area, and the wetlands should have a length to width ratio of at least 1.5:1.9 To increase the efficiency of the retention pond, a sediment forebay must be incorporated as a pretreatment device. As with all other stormwater management practices, stormwater wetlands also require ongoing maintenance to retain their maximum effectiveness. However, several design features can decrease the amount of maintenance that a wetland needs. For example, a reverse-slope pipe or a weir outlet with a trash rack should be used to prevent clogging of the outlet; orifices should have diameters no less than 3; and direct maintenance access should be provided to the forebay to allow for sediment removal. Selection of plant species is one of the most important parts of creating a stormwater wetland, as the plants are largely responsible for the pollutant and sediment retention and uptake. Please refer to Chapter 5 for a list of plant species suitable for planting in constructed wetlands.

In ultra urban areas (i.e., densely developed urban areas in which little pervious surface exists), it is difficult to use wet ponds because of the land area each wetland consumes. They can,

6 Description and design considerations provided by the Town of Franklin Best Development Practices Guidebook, Version 1, November 2001. 7 The description for applicability, benefits, effectiveness, limitations, and cost were provided by The Stormwater Managers Resource Center; Stormwater Management Fact Sheet: Stormwater Wetland. See http://www.stormwatercenter.net/.8 NVPDC Nonstructural Urban BMP Guidebook. 9 The Stormwater Managers Resource Center. Stormwater Management Fact Sheet: Stormwater Wetlands. See http://www.stormwatercenter.net.

Figure 6: Constructed wetland at Spragues Cove in Sippican Harbor in the Town of Marion, MA. Photo: Buzzards Bay Project National Estuary Program, Spragues Cove Stormwater Remediation Project.See http://www.buzzardsbay.org/sprafact.htm.


however, be used in these environments if a relatively large area is available downstream of the site.

Wetlands can accept runoff from stormwater hotspots but need significant separation from groundwater if they will be used for this purpose. A stormwater hotspot is an area where land use or activities generate highly contaminated runoff, with concentrations of pollutants in excess of those typically found in stormwater (e.g., a gas station). Caution also needs to be exercised for stormwater wetlands to ensure that pollutants in stormwater runoff do not work their way up the food chain of aquatic organisms living in or near the wetland.

A stormwater retrofit can be put into place after development has occurred to improve water quality, protect downstream channels, reduce flooding, or meet other watershed restoration objectives. When retrofitting an entire watershed, stormwater wetlands have the advantage of providing both educational and habitat value. One disadvantage to wetlands, however, is the difficulty storing large amounts of runoff without consuming a large amount of land. It is also possible to incorporate wetland elements into existing practices, such as wetland plantings (see Retention/Detention Basins).

Management Objectives:

x To allow for the settlement of particulate pollutants.

x To allow for the biological uptake of pollutants by wetlands plants.

x To reduce peak discharges and reduce occurrence of downstream flooding.


x One objective of stormwater treatment practices can be to reduce the flood hazardassociated with large storm events by reducing the peak flow associated with these storms. Wetlands can easily be designed for flood control, by providing flood storage above the level of the wetland surface.

x One result of urbanization is the channel erosion caused by increased stormwater runoff. When used for channel protection, wetlands have traditionally been designed to control the two-year storm. It appears that this design storm has not been effective in preventing channel erosion, and recent research suggests that control of a smaller storm may be more appropriate.10 Choosing a smaller design storm (one-year) and providing longer detention time (12 to 24 hours) are thought to be the best methods to reduce channel erosion.

x While no studies are available on wetlands in particular, there is some evidence to suggest that wet ponds may provide an economic benefit by increasing property values. The results of one study suggests that "pond frontage" property can increase the selling price of new properties by about 10%.11 Another study reported that the perceived value (i.e., the value estimated by residents of a community) of homes was increased by

10 MacRae, C. 1996. Experience from Morphological Research on Canadian Streams: Is Control of the Two-Year Frequency Runoff Event the Best Basis for Stream Channel Protection? In: Effects of Watershed Development and Management on Aquatic Ecosystems. American Society of Civil Engineers. Edited by L. Roesner. Snowbird, UT. pp. 144-162. 11 US EPA. 1995. Economic Benefits of Runoff Controls. Office of Wetlands, Oceans, and Watersheds. Washington, DC Publ. 8410S-95-0022.


about 15 to 25% when located near a wet pond.12 It is anticipated that well-designed wetlands, which incorporate additional aesthetic features, would have the same benefit.

x Wetlands are among the most effective practices for removing stormwater pollutants. Over thirty-five research studies have estimated the effectiveness of wetlands. Wetlands have high pollutant removal rates, and are more effective than any other practice at removing nitrate and bacteria.

Limitations:

Some limitations of stormwater wetlands include:

x Wetlands usually cannot provide groundwater recharge. The build-up of sediment and organic matter debris at the bottom of the wetland prevents the downward movement of water into the subsoil.

x Wetlands consume a relatively large amount of space, making them an impractical option on many sites where surface land area is constrained or land prices are high.

x Although design features can minimize the potential of wetlands to become a breeding area for mosquitoes13, there can be public perception that wetlands are a mosquito source.

x Wetlands require careful design and planning to ensure that wetland plants survive and flourish after construction.

x Some evidence exists that stormwater wetlands can release some nutrients during the non-growing season if plants are not harvested.

x Designers should ensure that wetlands are not built in natural wetlands or high quality forest.

x Cold climates present many challenges to designers of wetlands. During the spring snowmelt, a large volume of runoff occurs in a short time, which carries a relatively high pollutant load. In addition, cold winter temperatures cause freezing of the shallow pool as well as freezing up inlet and outlet structures. Finally, high salt concentrations are spread by road salting which can impact wetland vegetation. Also sediment loads from road sanding can be high, and cause premature loss of treatment capacity.

Maintenance:

Several regular maintenance and inspection practices are needed for stormwater wetlands as outlined below:

x After construction, replace wetland vegetation to maintain at least 50% surface area coverage in wetland plants after the second growing season.

x Inspect for invasive vegetation and remove where possible twice per year.

x Annually inspect for damage to the embankment and inlet/outlet structures. Repair as necessary.

12 Emmerling-Dinovo, C. 1995. Stormwater Detention Basins and Residential Locational Decisions. Water Resources Bulletin, 31(3): 515-52113 McLean, J. 2000. Mosquitos in Constructed Wetlands - A Management Bugaboo?. Article 100 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.


x Annually note signs of hydrocarbon build-up, and deal with appropriately.

x Annually supplement wetland plants if a significant portion have not established (at least 50% of the surface area). Harvest wetland plants that have been "choked out" by sediment build-up if necessary.

x Frequently (3-4 times per year) clean and remove debris from inlet and outlet structures and mow side slopes.

x Removal of sediment form the forebay every 5 to 7 years.

x Annually monitor for sediment accumulation in the facility and forebay. Remove sediment when the pool volume has become reduced significantly, plants are "choked" with sediment, or the wetland becomes eutrophic. If the constructed wetland is designed properly, this should not be required for 20 to 50 years.

Additional Information:

x The Massachusetts Stormwater and Technical Handbook (Volume 2) provides design details for constructed wetlands. The handbook can be found on the MA DEP stormwater publications page (see http://www.mass.gov/dep/brp/stormwtr/stormpub.htm), along with other useful stormwater publications.

x Case study: Spragues Cove Project. This constructed wetland was designed to manage pollution from a stormwater discharge in the Town of Marion, MA causing shellfish bed closures. Please visit the Buzzards Bay Project National Estuary Program Wetlands Protection page at http://www.buzzardsbay.org/wetlands.htm.

4. Bioretention Areas (Rain Gardens) 14

Bioretention is an important technique that uses soil, plants and microbes to treat stormwater before it is infiltrated or discharged. Bioretention cells are shallow depressions filled with sandy soil, topped with a thick layer of mulch, and planted with dense vegetation. Stormwater runoff flows into the cell and slowly percolates through the soil (which acts as a filter) and into the groundwater; some of the water is also taken up by the plants. Bioretention areas are usually designed to allow ponded water 6-8 inches deep, with an overflow outlet to prevent flooding during heavy storms. Where soils are tight or fast drainage is desired, designers may use a perforated underdrain, connected to the storm drain system.

Bioretention areas can provide excellent pollutant removal and recharge for the first flush of stormwater runoff. Properly designed cells remove suspended solids, metals, and nutrients, and can infiltrate an inch or more of rainfall.

14 Information provided by the MAPC Massachusetts Low Impact Development Toolkit.

Figure 7: The plants in this garden are all hardy and native to the area. This garden is for sunny areasplaces receiving more than six hours of direct sunlight per day. Photo: City of Maplewood, Minnesota. Public Works, Engineering Division Rainwater Gardens website (http://www.ci.maplewood.mn.us).


Distributed around a property, vegetated bioretention areas can enhance site aesthetics. In residential developments they are often described as rain gardens and marketed as property amenities. Routine maintenance is simple and can be handled by homeowners or conventional landscaping companies, with proper direction.


x Provide water quality treatment; remove suspended solids, metals, nutrients

x Increase groundwater recharge through infiltration.

x Reduce peak discharge rates.




Bioretention systems can be applied to a wide range of development in many climatic and geologic situations; they work well on small sites and on large sites divided into multiple small drainages. Common applications for bioretention areas include parking lot islands, median strips, and traffic islands. Bioretention is a feasible retrofit that can be accomplished by replacing existing parking lot islands or by re-configuring a parking lot during resurfacing. On residential sites they are commonly used for rooftop and driveway runoff.

Bioretention cells are usually excavated to a depth of 4 feet, depending on local conditions. Generally, cells should be sized (based on void space and ponding area) to capture and treat the water quality volume (the first 0.5 or 1 of runoff, depending on local requirements.) Some manuals suggest a minimum width of 15, though much narrower bioretention cells have been installed in parking lot islands and are functioning well. Regardless of size, some type of filter should cover the bottom of the excavation. Filter fabric is commonly used but can be prone to clogging; consequently some engineers recommend a filter of coarse gravel, over pea gravel, over sand.

The cell should be filled with a soil mix of sandy loam or loamy sand. The area should be graded to allow a ponding depth of 6-8 inches; depending on site conditions, more or less ponding may be appropriate. The area should be planted with a mix of herbaceous perennials, shrubs, and (if conditions permit) understory trees that can tolerate intermittent ponding and occasionally saline conditions (due to road salt.) The soil should be covered with 2-3 of fine-shredded hardwood mulch.

In very permeable soils, some bioretention areas can be designed as off-line treatment structures (no overflow necessary), but in most situations they will be an on-line component of the stormwater management system, connected to downstream treatment structures through an overflow outlet or an overflow drop inlet installed at the ponding depth and routed to the sites

Figure 8: This rain garden in the City of Maplewood, Minnesota was designed to turn stormwater management into an accepted formal landscape element for residences. Photo: Rain Gardens of West Michigan (http://www.raingardens.org).


stormwater management system. Ideally, overflow outlets should be located as far as possible from runoff inlets to maximize residence time and treatment. In general, bioretention area should be designed to drain within 72 hours. In slowly permeable soils (less than 0.3 inches/hour) a perforated underdrain can be installed at the bottom of the excavation to prevent ponding.

Bioretention areas work best if designed with some pretreatment, either in the form of swales or a narrow filter strip. A stone or pea gravel diaphragm (or, better yet, a concrete level spreader) upstream of a filter strip will enhance sheet flow and better pre-treatment.

Figure 9: An example bioretention schematic. Image from the Stormwater Managers Resource Center.



x Bioretention areas remove pollutants through filtration, microbes, and uptake by plants; contact with soil and roots provides water quality treatment better than conventional infiltration structures. Studies indicate that bioretention areas can remove 75% of phosphorus and nitrogen; 95% of metals; and 90% of organics, bacteria, and total suspended solids. Bioretention areas qualify as an organic filter according to the Massachusetts Stormwater Policy.

x In most applications, bioretention areas increase groundwater recharge ascompared to a conventional pipe and pond approach. They can help to reduce stress in watersheds that experience severe low flows due to impervious coverage.

x Low-tech, decentralized bioretention areas are also less costly to design, install, and maintain than conventional stormwater technologies that treat runoff at the end of the pipe. The use of decentralized bioretention cells can also reduce the size of storm drain pipes, a major driver of stormwater treatment costs.

x Bioretention areas enhance the landscape in a variety of ways: they improve the appearance of developed sites, provide wind breaks, absorb noise, provide wildlife habitat, and reduce the urban heat island effect.

Limitations:

x Because bioretention areas infiltrate runoff to groundwater, they may be inappropriate for use at stormwater hotspots (such as gas stations) with higher potential pollutant loads. On these sites, the design should include adequate pretreatment so that runoff can be infiltrated, or else the filter bed should be built with an impermeable liner, so that all water is carried away by the underdrain to another location for additional treatment prior to discharge.

x Premature failure of bioretention areas is a significant issue that results from lack of regular maintenance. Ensuring long-term maintenance involves sustained public education and deed restrictions or covenants for privately-owned cells.

x Bioretention areas must be used carefully on slopes; terraces may be required for slopes >20%.

x The design should ensure vertical separation of at lease 2 from the seasonal high watertable.

Maintenance:

Bioretention requires careful attention while plants are being established and seasonal landscaping maintenance thereafter. In many cases, maintenance tasks can be completed by a landscaping contractor working elsewhere on the site.

x Inspect pretreatment devices and bioretention cells regularly for sediment build-up, structural damage, and standing water.

x Inspect the soil and repair eroded areas monthly. Re-mulch void areas as needed. Remove litter and debris monthly.

x Treat diseased vegetation as needed. Remove and replace dead vegetation twice per year (spring and fall.)


x Proper selection of plant species and support during establishment of vegetation should minimizeif not eliminatethe need for fertilizers and pesticides.

x Remove invasive species as needed to prevent these species from spreading into the bioretention area.

x Replace mulch every two years, in the early spring.

x Upon failure, excavate bioretention area, scarify bottom and sides, replace filter fabric and soil, replant, and mulch.

Additional Information:

x Design Manual for Use of Bioretention in Stormwater Management; Department of Environmental Resources, Prince Georges County, MD; 1993.

x The EPA has published a Storm Water Technology Fact Sheet about Bioretention(http://www.epa.gov/owm/mtb/biortn.pdf) as well as a study of Bioretention Applications: Inglewood Demonstration Project, Largo, MD and Florida Aquarium, Tampa, FL (http://www.epa.gov/owow/nps/bioretention.pdf).

x The Low Impact Development Center has prepared specifications and details for construction of bioretention areas. See http://www.lowimpactdevelopment.org/epa03/biospec.htm.

x The Federal Highway Administration has also prepared a Bioretention Fact Sheet for Ultra-Urban areas. See http://www.fhwa.dot.gov/environment/ultraurb/3fs3.htm.

5. Cisterns & Rain Barrels 15

Cisterns and rain barrels are simple techniques to store rooftop runoff for reuse for landscaping and other nonpotable uses. They are based on the LID approach that treats rooftop runoff as a resource that should be reused or infiltrated. In contrast, conventional stormwater management strategies take rooftop runoff, which is often relatively free of pollutants, and send it into the stormwater treatment system along with runoff from paved areas.

The most common approach to roof runoff storage involves directing each downspout to a 55-gallon rain barrel. A hose is attached to a faucet at the bottom of the barrel and water is distributed by gravity pressure.

A more sophisticated and effective technique is to route multiple downspouts to a partially or fully buried cistern with an electric pump for distribution. Where site designs permit, cisterns may be quite large, and shared by multiple households, achieving economies of scale. Stored rain water can be used for lawn irrigation, vegetable and flower gardens, houseplants, car 15 Information provided by the MAPC Massachusetts Low Impact Development Toolkit.

Figure 10: This rain barrel is used to collect rooftop runoff from a gutter/downspout system. Photo: EPA BMP Factsheets: On-Lot Treatment ( http://cfpub.epa.gov/npdes/stormwater/menuofbmps/post.cfm).


washing, and cleaning windows. When rain barrels or cisterns are full, rooftop runoff should be directed to drywells, stormwater planters, or bioretention areas where it will be infiltrated.


x Reduce water demand by providing an alternative source for irrigation needs.

x Reduce peak discharge rates and total runoff volume.

Applications and Design Principles:

Cisterns and rain barrels are applicable to most commercial and residential properties where there is a gutter and downspout system to direct roof runoff to the storage tank. They take up very little room and so can be used in very dense urban areas. Rain barrels and cisterns are excellent retrofit techniques for almost any circumstance.

Rain barrels are typically 50-100 gallon covered plastic tanks with a hole in the top for downspout discharge, an overflow outlet, and a valve and hose adapter at the bottom. They are used almost exclusively on residential properties. Since rain barrels rely on gravity flow, they should be placed near, and slightly higher than, the point of use (whether a garden, flower bed, or lawn.) The overflow outlet should be routed to a dry well, bioretention area, or rain garden. It is important for property owners to use the water in rain barrels on a regular basis, or else they fill up and no additional roof runoff can be stored. It is recommended that each house have at least two rain barrels; a one inch storm produces over 500 gallons of water on a 1,000 square foot roof.

Cisterns are partially or fully buried tanks with a secure cover and a discharge pump; they provide considerably more storage than barrels, as well as pressurized distribution. Cisterns can collect water from multiple downspouts or even multiple roofs, and then distribute this water through an electric pump. Property owners may use one large tank or multiple tanks in series. Either way, the overflow for the systems should be a drywell or other infiltration mechanism, so that if the cistern is full, excess roof runoff is infiltrated, and not discharged to the stormwater system. Some cisterns are designed to continuously discharge water at a very slow rate into the infiltration mechanism, so that the tank slowly empties after a storm event, providing more storage for the next event.


x Rain barrels and cisterns can reduce water demand for irrigation, car washing, or other nonpotable uses. Property owners save money on their water bills and public water systems experience lower peak water demand and less stress on local water supplies.

x Property owners who have cisterns and rain barrels can use stored water for landscape purposes, even during outdoor watering bans.

Figure 11: Construction of a large underground cistern at a commercial site; this vault consists of a weight-bearing skeleton wrapped in a waterproof membrane. Photo: Rainwater Recovery, Inc. (http://www.rainwaterrecovery.com).


x If installed and used properly, rain barrels and cisterns can reduce stormwater runoff volume through retention, and will also help to reduce the peak discharge rate through retention.

Limitations:

x The stormwater volume/peak discharge rate benefits of cisterns and rain barrels depend on the amount of storage available at the beginning of each storm. One rain barrel may provide a useful amount of water for garden irrigation, but it will have little effect on overall runoff volumes, especially if the entire tank is not drained in between storms. Greater effectiveness can be achieved by having more storage volume and by designing the system with a continuous discharge to an infiltration mechanism, so that there is always available volume for retention.

x Rain barrels and cisterns offer no primary pollutant removal benefits. However, rooftop runoff tends to have fewer sediments and dissolved minerals than municipal water and is ideal for lawns, vegetable gardens, car washing, etc.

x Rain barrels must be childproof and sealed against mosquitoes. Please refer to the Stormwater Structures and Mosquitoes fact sheet provided in Appendix C.

x The water collected is for nonpotable uses only.

Maintenance:

Rain barrels and cisterns require minimal maintenance, but the homeowner needs to ensure that the hose remains elevated during the winter to prevent freezing and cracking. In addition, the tank needs to be cleaned out approximately once per year.

Additional information:

x Rainwater Recovery, Inc. is a local contractor that builds rainwater harvesting and storage systems for commercial and residential sites in Eastern Massachusetts. See http://www.rainwaterrecovery.com/index.html.

6. Infiltration Trenches and Dry Wells

Infiltration trenches and dry wells are standard stormwater management structures that can play an important role in Low Impact Development site design. Dispersed around the site, these infiltration structures can recharge groundwater and help to maintain or restore the sites natural hydrology. This approach contrasts with conventional stormwater management strategies, which employ infiltration as a secondary strategy that occurs in large basins at the end of a pipe.

Dry wells and infiltration trenches store water in the void space between crushed stone or gravel; the water slowly percolates downward into the subsoil. An overflow outlet is needed for runoff from large storms that cannot be fully infiltrated by the trench or dry well. Bioretention, another important infiltration technique, is discussed in another fact sheet. Infiltration trenches do not have the aesthetic or water quality benefits of bioretention areas, but they may be useful techniques where bioretention cells are not feasible.







Infiltration structures are ideal for infiltrating runoff from small drainage areas (



x Dry wells and infiltration trenches reduce stormwater runoff volume, including most of the runoff from small frequent storms. Consequently, downstream pipes and basins are smaller, and the local hydrology benefits from increased base flow.

x Dry wells and infiltration trenches also reduce peak discharge rates by retaining the first flush of stormwater runoff and creating longer flow paths for runoff.

x These techniques have an unobtrusive presence; they do not enhance the landscape (like bioretention areas do), but they have a lower profile than large infiltration basins.

Figure 12: Infiltration trench design example from the Stormwater Managers Resource Center at http://www.stormwatercenter.net/. A dry well schematic is provided in Chapter 4.


Limitations:

x Infiltration trenches and dry wells cannot receive untreated stormwater runoff,except rooftop runoff. Pretreatment is necessary to prevent premature failure that results from clogging with fine sediment, and to prevent potential groundwater contamination due to nutrients, salts, and hydrocarbons.

x Infiltration structures cannot be used to treat runoff from portions of the site that are not stabilized.

x Infiltration structures are moderately expensive to construct.

x Rehabilitation of failed infiltration trenches and dry wells requires complete reconstruction.

x Infiltration structures are difficult to apply in slowly permeable soils or in fill areas. Do not use trenches or dry wells where soils are >30% clay or >40% silt clay.

x Where possible, the design should maintain a minimum separation from paved areas (generally 10, depending on site conditions) to prevent frost heave.

x Unlike bioretention areas, infiltration trenches and dry wells do not help meet site landscaping requirements.

Maintenance:

x After construction, inspect after every major storm for the first few months to ensure stabilization and proper function.

x On a monthly basis, remove sediment and oil/grease from pretreatment devices, overflow structures, and the surface of infiltration trenches.

x Semi-annually, check observation wells 3 days after a major storm. Failure to percolate within this time period indicates clogging

x Semi-annually, inspect pretreatment devices and diversion structures for sediment build-up and structural damage.

x If ponding occurs on the surface of an infiltration trench, remove and replace the topsoil or first layer of stone and the top layer of filter fabric.

x Upon failure, perform total rehabilitation of the trench or dry well to maintain storage capacity within 2/3 of the design treatment volume and 72-hour exfiltration rate.

Additional information:

x The Massachusetts Stormwater Technical Handbook (Volume 2), found on the MA DEP stormwater publications page, includes design details for infiltration trenches and dry wells. See http://www.mass.gov/dep/brp/stormwtr/stormpub.htm.

7. Infiltration Drainfields 16

Infiltration drainfields are innovative technologies that are specially designed to promote stormwater infiltration into subsoils. Infiltration systems are one of the few techniques that provide significant groundwater recharge in areas with a high percentage of impervious surface

16 Information regarding infiltration drainfields was paraphrased from the U.S. Environmental Protection Agency (EPA) Stormwater Technology Fact Sheet: Infiltration Drainfields. September 1999.


area. The system is usually composed of a pretreatment structure, a manifold system, and a drainfield. Runoff is first diverted into a storm sewer system that passes through a pretreatment structure such as an oil and grit separator to remove coarse sediment, oils, and grease from the runoff. The stormwater runoff then continues through a manifold system, consisting of a perforated pipe which distributes the runoff evenly throughout the infiltration drainfield. The runoff then percolates through an underlying aggregate sand filter and filter fabric into the subsoils. An example of this system is provided in Figure 13.

Common design modifications to the infiltration drainfield best management practice include the insertion of an emergency overflow pipe in the oil and grit pretreatment chamber. The overflow pipe allows runoff volumes exceeding design capacities to discharge directly to a trunk storm sewer system.





x Provide recharge in areas with a high percentage of impervious surface area.


Infiltration drainfields are most applicable on sites with a relatively small drainage area (less than 15 acres). They can be used to control runoff from parking lots, rooftops, impervious storage areas, or other land uses. Infiltration drainfields should not be used in locations that receive a large sediment load that could clog the pretreatment system, which in turn would plug the infiltration drainfield and reduce its effectiveness.

Soils in areas where the installation of an infiltration drainfield is being constructed should have field-verified permeability rates of greater than 0.5 inches per hour and should include a 4-foot minimum clearance between the bottom of the system and the bedrock or water table.

Infiltration drainfields can have a short life span

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