Stormwater Best Management Practices: Guidance Document

Stormwater Best Management Practices: Guidance Document

January 2013

Stormwater Best Management Practices: Guidance Document

Revised by: Boston Water and Sewer Commission 980 Harrison Avenue Boston, MA 02119 and Prepared by: Geosyntec Consultants 289 Great Road, Suite 105 Acton, MA 01775

Stormwater Best Management Practices (BMP) Proposal and Guidance Document [THIS PAGE INTENTIONALLY LEFT BLANK]

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Stormwater Best Management Practices (BMP) Proposal and Guidance Document

TABLE OF CONTENTS

1.0 Selection of BMPs for an Urban Environment............................................................................................... 4

1.1 Purpose..................................................................................................................................................................... 4

1.2 General Considerations ..................................................................................................................................... 5

1.3 BMP Selection Matrix.......................................................................................................................................... 8

2.0 BMP Fact Sheets......................................................................................................................................................... 9

3.0 Technical guidance ................................................................................................................................................. 10

3.1 Green Infrastructure and Low Impact Development Techniques .................................................10

3.2 Massachusetts Stormwater Standards......................................................................................................11

3.3 Priority Pollutants..............................................................................................................................................19

3.4 Evaluation Methods for BMPs.......................................................................................................................22

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References............................................................................................................................................................. 23

APPENDICES

APPENDIX A: BMP Selection Matrix ............................................................................................................................ A-1

APPENDIX B: BMP Fact Sheets

1. Vegetated filter strip ....................................................................................................................................... B-1

2. Hydrodynamic separators............................................................................................................................ B-3

3. Baffle box ............................................................................................................................................................. B-5

4. Bioretention........................................................................................................................................................ B-7

5. Planter box .......................................................................................................................................................... B-9

APPENDIX C: Massachusetts Stormwater Handbook - 2008 ............................................................................ C-1

APPENDIX D: Stormwater Best Management Practices (BMP) Performance Analysis..................... D-1

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Stormwater Best Management Practices (BMP) Proposal and Guidance Document

1.0 SELECTION OF BMPS FOR AN URBAN ENVIRONMENT

1.1 PURPOSE

The purpose of this Stormwater Best Management Practices ("BMPs") Proposal and Guidance Document propose a suite of generic stormwater BMPs for potential application by the Boston Water and Sewer Commission ("BWSC") pursuant to Section VII, Part D, Paragraph 25 of the Consent Decree lodged in Conservation Law Foundation et al, v. Boston Water and Sewer Commission, U.S. District Court Civil Action No. 1:10-cv-10250-RGS. This document will identify a suite of proposed stormwater BMPS to address the range of known pollutant discharges, including general information, sizing requirements, critical design parameters and BMPs that are scalable as appropriate to match the discharge volumes, pollutant loads, sub-catchment areas served and anticipated site conditions within each sub-catchment area found within the BWSC's Municipal Separate Storm Water System (BWSC MS4). This proposal emphasizes structural BMPs incorporating both Green-Infrastructure ("GI") and Low-Impact Development ("LID") techniques. This BMP Proposal and Guidance Document will be used by BWSC during site plan review of development projects and when designing capital improvements to institute available BMPs (whether GI or LID) in both public and private development. This document contains discussion on selection of BMPs, fact sheets for a variety of BMPs, a BMP selection matrix, and a technical guidance section that focuses on Low Impact Development Techniques (LID), urban stormwater pollutants, current Massachusetts stormwater standards, and BMP design and sizing techniques. This document will also incorporate the standards found in Volume 2, Chapter 2 of the "Massachusetts Stormwater Handbook" (2008); as well as, the Stormwater Best Management Practices (BMP) Performance Analysis" (December 2008, revised March 2010) prepared by Tetra Tech for U.S. EPA Region 1. This document was originally prepared by Geosyntec in October 2011, but was revised and updated by the BWSC in January 2013. BWSC will continue to update this BMP Proposal and Guidance Document as appropriate as new technologies and applications are more widely employed and tested in the City of Boston and other urban areas.

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Stormwater Best Management Practices (BMP) Proposal and Guidance Document

1.2 GENERAL CONSIDERATIONS

An integral step in selecting suitable BMPs for a site within the BWSC MS4 is to assess the existing site conditions. This section focuses on a number of factors that should be considered in regards to the implementation of BMPs in an ultra-urban environment as the BWSC MS4.

Site Suitability

There are a wide range of BMPs available, from rain barrels that require little space to constructed stormwater wetlands that require a much larger footprint. The scale of the BMP is directly related to the size of the construction project. For example, it may be impractical for private landowners of a brownstone in Back Bay to be required to design and install a stormwater wetland for a redevelopment project. However, private landowners could install rain barrels or planter boxes, to capture roof runoff and use for on-site watering demands. Parks or grassed areas adjacent to roadways may be ideal location for the implementation of larger scale BMPs such as a water quality swales or bio-retention areas.

Physical constraints at a site may include soil conditions, watershed size, depth to water table, depth to bedrock and slope. For redevelopment projects, physical constraints may include already compacted soils or the location of underground utilities. Even with physical site constraints, BMPs can be modified and adapted to fit a site needs with site planning and design to meet the applicable stormwater standards.

The BMP selection matrix, found in Section 1.3, contains a column labeled "General Suitability" that designates whether specific BMPs are potentially suitable for private landowners, public property (i.e. parks, sidewalks, roadways, etc.) or both land uses.

Soils

Soils regulate the process of surface runoff, infiltration and percolation, and are a major controlling factor in evapotranspiration through the capacity of the soil to store and release water. Furthermore, soils play an important role in removing pollutants from the water column via sorption to soil particles. The characteristics of soils at any particular site should be carefully considered during site planning.

Soil types and subsequent infiltration rates in the BWSC MS4 Area vary widely and can include glacial till, a wide variety of fill types, possible contamination, organic material such as blue clay, and bedrock such as Roxbury Conglomerate. As a result, soils should be evaluated carefully at each site and BMPs that provide groundwater recharge (i.e., dry wells) should be selected only if the site is comprised of soils that meet the infiltration requirements stated in the Massachusetts (MA) Stormwater Handbook, attached and incorporated herein as Appendix C-1.

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Stormwater Best Management Practices (BMP) Proposal and Guidance Document

Groundwater

The depth of seasonal high groundwater is an important factor to consider when selecting BMPs, suitable for groundwater recharge requirements. Information such as distance between the ground surface and the groundwater table, depth and direction of groundwater flow, seasonal groundwater variation, regional geology, and the slope of the water table are important factors to consider. The groundwater table acts as an effective barrier to exfiltration through the BMP soil media and soils below and can prevent an infiltration BMP from draining properly.

The typical depth to groundwater in BWSC MS4 Area ranges from 8 to 20 feet; however, it can vary widely based on topography, soil types, and underlying bedrock.

Available Pervious Area

Pervious areas in the urban environment are critical to providing effective stormwater treatment. Several stormwater BMPs require pretreatment, which can consist of a vegetated buffer strip and all infiltration BMPs largely depend on pervious areas. For sites with high areas amounts of development, new pervious areas may be created by removing impervious surfaces. Similarly, for sites with poor permeability, a suitable BMP may be to over-excavate and backfill the area with a more permeable substrate to increase the capacity of the drainage system by increasing subsurface stormwater storage availability and the overall permeability.

Existing Infrastructure

One primary step is for the BWSC to perform a review of its existing infrastructure during the review of private site plan submissions and capital infrastructure design planning. BWSC should also evaluate other existing infrastructure (roadways, bridges, building foundations, retaining walls, etc) within close proximity to proposed or planned infiltration BMPs. The use of conveyance BMPs or structural proprietary devices in areas where existing infrastructure may not allow for other practices will be considered. BWSC should also consider other impacts caused by BMP placement; for example, trees or vegetation placed in filter boxes could obscure traffic signs or obstruct road visibility at corners.

Utilities such as gas lines, water and sewer lines, electricity, telephone, and optical cables are often located underground and can further complicate placement and selection of BMPs. Construction activities that involve excavation or the use of large construction equipment must be carefully planned and executed to avoid costly and potentially dangerous damage to overhead or underground utilities. Utility conflicts are particularly applicable to the Boston MS4 Area due to the vast number of utilities that exist in the City of Boston, its streets and sidewalks.

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Stormwater Best Management Practices (BMP) Proposal and Guidance Document Cost Constraints Cost is an important factor to consider for the implementation of stormwater BMPs. Cost estimation can be difficult due to the following:

? Project site variability; ? Unforeseen site conditions, particularly with retrofit situations, including subsurface

conflicts, space constraints, site accessibility, safety and security; ? Differences and quality of planners, designers, and contractors; ? High cost of engineering, permitting and construction management; and ? Construction related issues such as change orders, accelerated constructions schedules,

unsuitable designs, and the use of non-standard components. The BWSC's BMP Selection Matrix and BMP Fact Sheets provide general construction cost estimate ranges for each discussed BMP. The estimated costs are general guidelines and BWSC should also consider the additional factors listed above that can influence the final cost for implementation of the various BMPs listed below. BWSC will also consider Whole Life Cycle (WLC) costs as a selection criterion for evaluating BMP alternatives. WLC costs take into account both capital, operations and maintenance costs over the long term. A study published by the Water Environment Research Federation (WERF) provides whole life cycle cost models for an array of stormwater BMPs. The WLC models are a set of spreadsheet tools that have been developed to facilitate automation of a WLC approach for stormwater BMPs. The models allow users to systematically combine capital costs and on-going maintenance expenditures in order to estimate costs (WERF 2009).

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Stormwater Best Management Practices (BMP) Proposal and Guidance Document

1.3 BMP SELECTION MATRIX

The BWSC MS4 BMP Selection Matrix was developed to facilitate the site planning and capital design planning process whether by a developer, private landowner or BWSC. During the site planning process or capital design planning, a developer, private landowner or BWSC will review the proposed site and/or location conditions and suitability, including any site constraints and stormwater management goals. Upon completion of review, the developer, private landowner, public agency and/or BWSC will then apply the BMP Selection Matrix, to choose appropriate BMPs for the particular site of interest.

The BMP Selection Matrix, attached hereto and incorporated as Appendix A, includes the following categories:

? BMP Class - Massachusetts Department of Environmental Protection (Mass DEP) divides stormwater BMPs into several categories which include:

o Pretreatment ? designed to treat the entire water quality volume (on-line) or a specified discharge volume or rate (off-line), prior to a treatment, infiltration or other BMP.

o Treatment ? designed to provide peak rate attenuation, removal of pollutants through retention and settling or filtration through soil media.

o Infiltration ? designed to provide volume reduction through groundwater recharge.

o Conveyance ? designed to collect and transport stormwater to BMPs for treatment or infiltration and may provide temporary storage.

o Other ? BMPs that do not fit into the categories above.

? Construction Cost Range ? provides an estimated typical range of construction cost for each BMP. This cost may vary depending on site hydrology, soils and location.

? Applicability ? provides an estimate of the typical land use suitability, general space requirements, if BMP requires groundwater recharge and if the BMP requires pretreatment.

? Unit Processes ? provides a summary of the unit processes the BMP is capable of including volume reduction, peak reduction, filtration and sorption and biological process, as designated in the Mass. Stormwater Handbook. These unit processes are ranked by H (high), M (medium) and L (low).

? Target Pollutants ? provides a summary of the pollutant removal capabilities of a BMP for sediment, nutrients, bacteria and metals as designed in the MA Stormwater Handbook. The

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