Attachment D.1 - MD SWM Volume 1.pdf

TOC-1 Supp. 1

Table of Contents Maryland Stormwater Design Manual

VOLUME ONE

Page List of Tables ................................................................................................................................... i List of Figures ................................................................................................................................. ii Preface.............................................................................................................................................iv

Chapter 1: Introduction to the Manual

1.0 Purpose of Manual ............................................................................................................1.1

1.1 Why Stormwater Matters: Impact of Runoff on Maryland Watersheds...........................1.3

1.1.1 Declining Water Quality .......................................................................................1.5

1.1.2 Diminishing Groundwater Recharge and Quality ................................................1.7

1.1.3 Degradation of Stream Channels ..........................................................................1.8

1.1.4 Increased Overbank Flooding.............................................................................1.10

1.1.5 Floodplain Expansion .........................................................................................1.11

1.2 General Performance Standards for Stormwater Management in Maryland..................1.13

1.3 How to Use the Manual ..................................................................................................1.16

1.3.1 Volume One ........................................................................................................1.16

1.3.2 Volume Two (Appendices) ................................................................................1.17

1.4 Revising the Manual .......................................................................................................1.19

1.5 What’s New ....................................................................................................................1.19

1.6 Symbols and Acronyms ..................................................................................................1.21

Chapter 2: Unified Stormwater Sizing Criteria

2.0 Unified Stormwater Sizing Criteria ..................................................................................2.1

2.1 Water Quality Volume (WQv) ..........................................................................................2.2

2.2 Recharge Volume Requirements (Rev).............................................................................2.5

2.3 Channel Protection Storage Volume Requirements (Cpv)................................................2.8

2.4 Overbank Flood Protection Volume Requirements (Qp2 or Qp10) .................................2.12

2.5 Extreme Flood Volume (Qf ) ..........................................................................................2.13

2.6 Design Examples: Computing Stormwater Storage Requirements ................................2.14

NOTE: The Maryland Stormwater Design Manual has been revised. Changes are identified as Supplements (e.g., Supp. 1) and occur throughout the design manual. When there are conflicts between supplemental and original requirements, the newest shall supersede.

Supp. 1 TOC-2

2.7 Acceptable Urban BMP Options ....................................................................................2.37

2.7.1 Urban BMP Groups ............................................................................................2.37

2.7.2 Structural BMPs that do not fully meet the WQv Requirement .........................2.39

2.8 Designation of Stormwater Hotspots .............................................................................2.41

Chapter 3: Performance Criteria for Urban BMP Design

3.0 Performance Criteria for Urban BMP Design ..................................................................3.1

3.1 Stormwater Ponds .............................................................................................................3.2

3.1.1 Pond Feasibility Criteria .......................................................................................3.8

3.1.2 Pond Conveyance Criteria ....................................................................................3.9

3.1.3 Pond Pretreatment Criteria..................................................................................3.10

3.1.4 Pond Treatment Criteria......................................................................................3.10

3.1.5 Pond Landscaping Criteria..................................................................................3.11

3.1.6 Pond Maintenance Criteria .................................................................................3.12

3.2 Stormwater Wetlands......................................................................................................3.16

3.2.1 Wetland Feasibility Criteria................................................................................3.21

3.2.2 Wetland Conveyance Criteria .............................................................................3.21

3.2.3 Wetland Pretreatment Criteria ............................................................................3.21

3.2.4 Wetland Treatment Criteria ................................................................................3.21

3.2.5 Wetland Landscaping Criteria ............................................................................3.22

3.2.6 Wetland Maintenance Criteria ............................................................................3.24

3.3 Stormwater Infiltration ...................................................................................................3.25

3.3.1 Infiltration Feasibility Criteria ............................................................................3.28

3.3.2 Infiltration Conveyance Criteria .........................................................................3.28

3.3.3 Infiltration Pretreatment Criteria ........................................................................3.29

3.3.4 Infiltration Treatment Criteria ............................................................................3.30

3.3.5 Infiltration Landscaping Criteria ........................................................................3.30

3.3.6 Infiltration Maintenance Criteria ........................................................................3.30

3.4 Stormwater Filtering Systems.........................................................................................3.31

3.4.1 Filtering Feasibility Criteria ...............................................................................3.38

3.4.2 Filtering Conveyance Criteria.............................................................................3.38

3.4.3 Filtering Pretreatment Criteria ............................................................................3.38

3.4.4 Filtering Treatment Criteria ................................................................................3.39

3.4.5 Filtering Landscaping Criteria ............................................................................3.40

3.4.6 Filtering Maintenance Criteria............................................................................3.41

3.5 Open Channel Systems ...................................................................................................3.42

3.5.1 Open Channel Feasibility Criteria ......................................................................3.45

3.5.2 Open Channel Conveyance Criteria ...................................................................3.45

3.5.3 Open Channel Pretreatment Criteria...................................................................3.45

3.5.4 Open Channel Treatment Criteria.......................................................................3.46

3.5.5 Open Channel Landscaping Criteria...................................................................3.46

Table of Contents Maryland Stormwater Design Manual

TOC-3 Supp. 1

3.5.6 Open Channel Maintenance Criteria...................................................................3.46

Chapter 4: A Guide to BMP Selection and Location in the State of Maryland

4.0 Selecting the Best BMP at a Site ......................................................................................4.1

4.1 Watershed Factors.............................................................................................................4.3

4.2 Terrain Factors ..................................................................................................................4.6

4.3 Stormwater Treatment Suitability.....................................................................................4.8

4.4 Physical Feasibility Factors ............................................................................................4.10

4.5 Community and Environmental Factors .........................................................................4.12

4.6 Checklist: Location/Permitting Factors ..........................................................................4.14

Chapter 5: Environmental Site Design

5.0 Introduction.......................................................................................................................5.1

5.1 Design Process and Planning Techniques ........................................................................5.4

5.2 Addressing the Unified Sizing Criteria...........................................................................5.17

5.3 Alternative Surfaces........................................................................................................5.41

5.4 Treatment Using Nonstructural and Micro-Scale Practices ...........................................5.55

5.5 Redevelopment .............................................................................................................5.117

5.6 Special Criteria for Sensitive Waters ...........................................................................5.121

Glossary ..................................................................................................................................... G.1

References...................................................................................................................................R.1

Supp. 1 TOC-4

VOLUME TWO: APPENDICES

Appendix A: Landscaping Guidance for Stormwater BMPs

A.1 General Landscaping Guidance for All Stormwater BMPs.............................................A.2 A.2 Specific Landscaping Criteria for BMP Groups..............................................................A.4

A.2.1 Ponds and Wetlands.............................................................................................A.4 A.2.2 Infiltration and Filter Systems ...........................................................................A.12 A.2.3 Bioretention .......................................................................................................A.12 A.2.4 Open Channels ...................................................................................................A.18 A.2.5 Filter Strips and Stream Buffer ..........................................................................A.18

A.3 Plant Selection for Stormwater Facilities ......................................................................A.19 A.3.1 Hardiness Zones.................................................................................................A.19 A.3.2 Physiographic Provinces....................................................................................A.21 A.3.3 Hydrologic Zones ..............................................................................................A.25 A.3.3 Other Considerations in Stormwater BMP Landscaping...................................A.26

A.4 Stormwater Plant List ....................................................................................................A.27 A.5 References......................................................................................................................A.49

Appendix B: BMP Construction Specifications

B.1 NRCS-MD Code No 378 Pond Standards/ Specifications ..............................................B.1 B.1.1 Supplemental Pond and Wetland Stormwater Specifications........................B.1.1.1 B.1.2 MDE Dam Safety Small Pond Review Criteria.............................................B.1.2.1 B.2 Construction Specifications for Infiltration Practices...................................................B.2.1 B.3 Construction Specifications for Bioretention, Sand Filters, and Open Channels.........B.3.1 B.4 Construction Specifications for Environmental Site Design Practices.........................B.4.1

Appendix C: Step-by-Step Design Examples

C.1 Shallow Wetland Design Example ...............................................................................C.1.1 C.2 Design Example 2 – Water Quality BMPs ...................................................................C.2.1 C.2.1 Design Criteria ..................................................................................................C.2.2 C.2.2 Preliminary Design ...........................................................................................C.2.2 C.2.3 BMP Design Option 1.......................................................................................C.2.8 C-2.3.1 Perimeter Sand Filter (F-3) .............................................................C.2.8 C-2.3.2 Pocket Sand Filter (F-5)................................................................C.2.12 C.2.4 BMP Design Option 2.....................................................................................C.2.16 C.2.4.1 Bioretention System (F-6) ..................................................................C.2.17 C.2.4.2 Infiltration Trench (I-1) ......................................................................C.2.18 C.2.5 BMP Design Option 3.....................................................................................C.2.21 C.2.5.1 Dry Swale (O-1)..................................................................................C.2.21

Table of Contents Maryland Stormwater Design Manual

TOC-5 Supp. 1

Appendix D: Assorted Design Tools

D.1 Testing Requirements for Infiltration, Bioretention, and Sand Filter Subsoils ............D.1.1 D.2 Geotechnical Methods for Karst Feasibility Testing ....................................................D.2.1 D.3 Short Cut Method for a Wetland Drawdown Assessment ............................................D.3.1 D.4 Stormwater Criteria for the Maryland Critical Area IDA Zone ...................................D.4.1 D.5 Documentation of BMP Ability to Meet the 80% TSS Removal Requirement ...........D.5.1 D.6 Industrial Stormwater NPDES Permit Requirements ...................................................D.6.1 D.7 MDE/WMA Overview of the NPDES Stormwater Program .......................................D.7.1 D.8 Miscellaneous Details for Compliance with Performance Criteria ..............................D.8.1 D.9 MD Stream Use Designations.......................................................................................D.9.1 D.10 Method for Computing Peak Discharge for Water Quality Storm .............................D.10.1 D.11 Method for Computing the Channel Protection Storage Volume (Cpv) .....................D.11.1 D.12 Critical Erosive Velocity for Grass and Soil ..............................................................D.12.1 D.13 Method for Designing Infiltration Structures .............................................................D.13.1 D.14 Eastern Shore (Delmarva) Dimensionless Hydrograph..............................................D.14.1 D.15 Miscellaneous MD SHA Design Charts for Determining Pipe Inlet Control.............D.15.1

Appendix E: Archived Material

E.1 Stormwater Credits for Innovative Site Planning ......................................................... E.1.1 i Supp. 1

LIST OF TABLES

No. Title Page

1.1 Typical Pollutant Concentrations Found in Urban Stormwater........................................1.6

1.2 NRCS Estimates of Annual Recharge Rates, Based on Soil Type ...................................1.7

1.3 Symbols and Acronyms ..................................................................................................1.21

2.1 Summary of the Statewide Stormwater Criteria ...............................................................2.1

2.2 Rainfall Depths Associated with the 1,2,10, and 100 Year 24-hour Storm Events........2.11

2.3 Summary of General Storage Requirements for Reker Meadows..................................2.19

2.4 Summary of General Design Information for Claytor Community Center ....................2.25

2.5 Summary of General Storage Requirements for Pensyl Pointe ......................................2.34

2.6 Classification of Stormwater Hotspots ...........................................................................2.42

4.1 BMP Selection Matrix No. 1: Watershed Factors ............................................................4.5

4.2 BMP Selection Matrix No. 2: Terrain Factors..................................................................4.7

4.3 BMP Selection Matrix No. 3: Stormwater Treatment Suitability ....................................4.9

4.4 BMP Selection Matrix No. 4: Physical Feasibility.........................................................4.11

4.5 BMP Selection Matrix No. 5: Community and Environmental Factors.........................4.13

4.6 Location and Permitting Factors Checklist.....................................................................4.15

5.1 Natural Resources & Corresponding Regulatory Authorities ..........................................5.7

5.2 Summary of Site Development Strategies ......................................................................5.10

5.3 Rainfall Targets/Runoff Curve Number Reductions Used for ESD...............................5.21

5.4 Effective RCNs for Extensive Green Roofs ...................................................................5.42

5.5 Effective RCNs for Permeable Pavements .....................................................................5.48

5.6 ESD Sizing Factors for Rooftop Disconnection .............................................................5.59

5.7 ESD Sizing Factors for Non-Rooftop Disconnection.....................................................5.62

5.8 Sheetflow to Conservation Area Sizing Factors .............................................................5.67

5.9 Solar Reflectance Indices for Typical Paving & Roofing Materials ............................5.122

LIST OF FIGURES

1.1 Water Balance at a Developed and Undeveloped Site .....................................................1.3

1.2 Relationship between Impervious Cover and the Volumetric Runoff Coefficient...........1.4

1.3 Decline in Stream Flow Due to Diminished Groundwater Recharge...............................1.8

1.4 Increased Frequency of Flows Greater than the Critical Discharge Rate in a Stream

Channel After Development… .........................................................................................1.9

1.5 Change in Hydrograph Following Development............................................................1.11

1.6 Change in Floodplain Elevations....................................................................................1.12

2.1 Location of the Eastern and Western Rainfall Zones in Maryland...................................2.3

2.2 Relationship between Impervious Cover and the Water Quality Volume........................2.3

2.3 Relationship between Rev and Site Impervious Cover .....................................................2.6

2.4 Regions of Maryland Not Subject to the Channel Protection Requirement (Cpv)............2.8

2.5 Example of Conventional Stormwater Detention Pond....................................................2.9

2.6 Reker Meadows ..............................................................................................................2.15

2.7 Reker Meadows: Pre Developed Conditions ..................................................................2.20

2.8 Reker Meadows: Developed Conditions ........................................................................2.21

2.9 Claytor Community Center.............................................................................................2.22

2.10 Claytor Community Center: Pre Developed Conditions ................................................2.26

2.11 Claytor Community Center: Developed Conditions.......................................................2.27

2.12 Pensyl Pointe...................................................................................................................2.29

2.13 Pensyl Pointe: Drainage Area 1-Post Developed Conditions.........................................2.35

2.14 Pensyl Pointe: Drainage Area 2–Post Developed Conditions ........................................2.36

3.1 Example of "Micropool" Extended Detention Pond.........................................................3.3

3.2 Example of Wet Pond .......................................................................................................3.4

3.3 Example of Wet Extended Detention Pond ......................................................................3.5

3.4 Example of Multiple Pond System ...................................................................................3.6

3.5 Example of Pocket Pond...................................................................................................3.7

3.6 Example of Shallow Wetland .........................................................................................3.17

3.7 Example of Extended Detention Shallow Wetland ........................................................3.18

3.8 Example of Pond/Wetland System .................................................................................3.19

3.9 Example of Pocket Wetland............................................................................................3.20

3.10 Example of Infiltration Trench .......................................................................................3.26

3.11 Example of Infiltration Basin .........................................................................................3.27

3.12 Example of Surface Sand Filter ......................................................................................3.32

Supp. 1 ii iii Supp. 1

3.13 Example of Underground Sand Filter .............................................................................3.33

3.14 Example of Perimeter Sand Filter...................................................................................3.34

3.15 Example of Organic Filter ..............................................................................................3.35

3.16 Example of Pocket Sand Filter .......................................................................................3.36

3.17 Example of Bioretention .................................................................................................3.37

3.18 Example of Dry Swale ....................................................................................................3.43

3.19 Example of Wet Swale....................................................................................................3.44

4.1 Map of Maryland Showing Key Terrain Factors..............................................................4.6

5.1 Design Process for New Development .............................................................................5.6

5.2 Cutaway of a Typical Green Roof ..................................................................................5.44

5.3 Examples of Permeable Pavements ................................................................................5.49

5.4 Example of Disconnection of Rooftop Runoff ...............................................................5.58

5.5 Example of Non-Rooftop Disconnection........................................................................5.64

5.6 Example of Non-Rooftop Disconnection........................................................................5.65

5.7 Example of Sheetflow to Conservation Area..................................................................5.68

5.8 Example of Rain Barrels.................................................................................................5.74

5.0 Example of Cistern .........................................................................................................5.75

5.10 Example of Submerged Gravel Wetland ........................................................................5.79

5.11 Example of Landscape Infiltration .................................................................................5.85

5.12 Example of Infiltration Berms ........................................................................................5.89

5.13 Example of Dry Wells ....................................................................................................5.93

5.14 Example of Micro-Bioretention (Variation 1) ................................................................5.99

5.15 Example of Micro-Bioretention (Variation 2) ..............................................................5.100

5.16 Example of Micro-Bioretention (Variation 3) ..............................................................5.101

5.17 Example of Rain Garden...............................................................................................5.107

5.18 Example of a Bio-Swale ...............................................................................................5.111

5.19 Example of a Wet Swale...............................................................................................5.112

5.20 Example of Enhanced Filters ........................................................................................5.116

5.21 Design Process for Redevelopment ..............................................................................5.120

A publication of the Maryland Department of the Environment in cooperation with the Maryland Department of Natural Resources Coastal Zone Management Program pursuant to National Oceanic and Atmospheric Administration Award No. NA67OZ0302.

Preface

Stormwater management has evolved dramatically in Maryland since it became the first state to adopt stormwater quality regulations some fifteen years ago. Much has been learned about what works in the field and what does not. The goal for this project is to compile this hard-won knowledge and experience into a single comprehensive design manual that is truly useful to engineers and plan reviewers who design and construct stormwater practices.

We would like to acknowledge the many people who helped us get to this point. In particular, we are grateful to the staff at the Water Management Administration of the Maryland Department of Environment who played an active role in getting the manual to this point. In particular, the patience and support of Stewart Comstock (Project Manager), Ken Pensyl, Brian Clevenger and James Tracy is gratefully acknowledged. In addition, the assistance of other MDE staff was most helpful.

The manual could never have been produced without the talents, experience and hard work of our project team partners, Environmental Quality Resources, Inc. (EQR), and Loiederman Associates, Inc (LAI). In particular, we would like to acknowledge the great contributions of Timothy Schueler, Richard Scafidi, and Joanne Reker of EQR, and Karen Carpenter, Michael Wagner, Josie Greenberg, and Cliff Deward of LAI.

Thanks are also extended to the members of the Stormwater Management Regulations Committee (SMRC), whose insightful comments and local perspective in reviewing earlier drafts were most helpful in improving the manual. SMRC members who graciously gave their time and advice included: David Bourdon (Prince George’s Soil Conservation District), Richard Brush (Montgomery County), Michael Clar (Engineering Technology Associates), Andrew Daneker (Howard County Bureau of Highways), Neal Fitzpatrick (Audubon Naturalist Society), John Galli (Metropolitan Washington Council of Governments), James Gracie (Brightwater, Inc.), Terrence McGee (Washington County), John Mickley (Washington County Soil Conservation District), Joseph Necker (The Rouse Company), Steven Oder (Cavalier Development), Daniel O’Leary (Parsons Brinkerhoff, Inc), John Redden (Wicomico County), James Slater (Carroll County), Susan Straus (City of Rockville), William Street (Chesapeake Bay Foundation), Raja Veeramachaneni (Maryland State Highway Administration) and Tom Vidmar (Baltimore County).

The authors would also like to acknowledge Tom Devilbiss (Carroll County), John Redden (Wicomico County), and Terrence McGee (Washington County) for their help in providing their perspectives on stormwater management in unique terrain areas of the State.

VOLUMES I AND II

MARYLAND STORMWATER DESIGN MANUAL

v

Special thanks are extended to Hye Yeong Kwon of the Center for Watershed Protection (CWP) for her heroic efforts to assemble and knit together the many pieces of the manual into a final product.

Chris Swann and Deborah Caraco also provided valuable input and review.

This manual was prepared for the Water Management Administration under a cooperative agreement between that agency and the Maryland Department of Natural Resources pursuant to National Oceanic and Atmospheric Administration Grant No. NA67OZ0302. Reference to any particular commercial products and trade names in this manual does not in any way constitute an endorsement by the State.

Thomas Schueler Richard Claytor

Center for Watershed Protection

Chapter

1.0

In tr od uc ti on

Chapter 1. Introduction to the Manual..................................................... Introduction and Purpose

1.0 Introduction and Purpose of Manual

Title 4, Subtitle 2 of the Environment Article of Annotated Code of Maryland states that “…the management of stormwater runoff is necessary to reduce stream channel erosion, pollution, siltation and sedimentation, and local flooding, all of which have adverse impacts on the water and land resources of Maryland.” The program designed in the early 1980s to address this finding of the General Assembly concentrated primarily on controlling runoff increases and mitigating water quality degradation associated with new development. The counties and municipalities in Maryland are responsible for administering effective stormwater management programs that “…maintain after development, as nearly as possible the predevelopment characteristics…” These localities have performed remarkably in establishing Maryland as a national leader in stormwater management technology. Over the last 14 years, tens of thousands of best management practices (BMPs) have been constructed in an attempt to meet program mandates. However, the experience gained since Maryland’s stormwater statute was enacted has identified necessary improvements and revealed a need to refocus the approach to fulfill the original intent of this essential water pollution control program.

Recently, increased emphasis on water quality, resource protection needs, increased BMP maintenance costs, and identified shortcomings in Maryland’s program have all contributed to basic philosophical changes regarding stormwater management. The “Maryland Stormwater Design Manual” is an effort to incorporate the significant experiences gained by the State’s stormwater community and accommodate much needed improvements for managing urban runoff. It is hoped that the design standards and environmental incentives provided below will produce better methods and advance the science of managing stormwater by relying less on single BMPs for all development projects and more on mimicking existing hydrology through total site design policies. Additionally, the inherent philosophical change should produce smaller less obtrusive facilities that are more aesthetic and less burdensome on those responsible for long-term maintenance and performance.

The purpose of this manual is threefold:

to protect the waters of the State from adverse impacts of urban stormwater runoff, to provide design guidance on the most effective planning techniques, and nonstructural and structural BMPs for development sites, and to improve the quality of BMPs that are constructed in the State, specifically with regard to performance, longevity, safety, ease of maintenance, community acceptance and environmental benefit.

The BMPs and the required design criteria below represent conventional stormwater management technology for controlling runoff from new development projects. Based upon current available research, the Maryland Department of the Environment, Water Management

1.1 Supp. 1

Chapter 1. Introduction to the Manual..................................................... Introduction and Purpose

Supp. 1 1.2

Administration (MDE/WMA) has evaluated each BMP group and the associated design variants and has developed standards for each so that all perform similarly. The “General Performance Standards” outlined in this manual (see Section 1.2, page 1.13) specify those criteria that were used to create runoff control options that would perform equally. The BMPs contained in this manual are by no means exclusive. MDE encourages the development of innovative practices that meet the intent of Maryland’s stormwater management law and can perform according to the standards in Section 1.2. In the future, should structural or non-structural practices be developed that meet the standards specified below, MDE will approve their use for controlling new development runoff.

MDE encourages wise, environmentally sensitive site designs to reduce the generation of runoff borne pollution. Additionally, Maryland has adopted “Smart Growth” policies that are geared toward concentrating development where it currently exists thereby reducing “suburban sprawl.”

Therefore, redevelopment is encouraged. A stormwater management policy for redevelopment is established in the Code of Maryland Regulations (COMAR 26.17.02). Additionally, redevelopment is defined in both COMAR and this manual.

The policy required in COMAR for redevelopment basically specifies a 50% reduction in impervious surface area below existing conditions. Because this may be impractical due to site constraints, environmental site design (ESD) practices are to be used to the maximum extent practicable (MEP) to meet the equivalent in water quality control of a 50% decrease in impervious surface area. Various alternative BMPs that do not necessarily meet the performance criteria established in this manual may be implemented for redevelopment projects provided that it is demonstrated that impervious area reduction and ESD have been implemented to the MEP.

These alternative BMPs may also be implemented to satisfy the pretreatment volume requirements established in Chapter 3 below. Individual project designers should contact the appropriate approval authority for the specific practices allowed for redevelopment and pretreatment purposes.

The approval of new control technologies, modifications to the practices contained in this manual, and alternative policies regarding stormwater management for new development is the responsibility of MDE. Typically, information is submitted to the WMA that describes the policy or practice. For new BMPs, monitoring data need to be submitted that demonstrate that the performance criteria in this manual can be met. WMA then reviews this material to determine if the proposed practice is appropriate for use on new development projects. Because of local variations in ownership policies, maintenance abilities, cost, design standards, hydrology, etc., information on practices to be used for redevelopment and pretreatment should be submitted to the appropriate authority for approval.

NOTE: The Maryland Stormwater Design Manual has been revised. Changes are identified as Supplements (e.g., Supp. 1) and occur throughout the design manual. When there are conflicts between supplemental and original requirements, the newest shall supersede.

Chapter 1. Introduction to the Manual ................................................Impact of Runoff

1.3

Section 1.1 Why Stormwater Matters: Impact of Runoff on Maryland’s Watersheds

Urban development has a profound influence on the quality of Maryland’s waters. To start, development dramatically alters the local hydrologic cycle (see Figure 1.1). The hydrology of a site changes during the initial clearing and grading that occur during construction. Trees, meadow grasses, and agricultural crops that had intercepted and absorbed rainfall are removed and natural depressions that had temporarily ponded water are graded to a uniform slope. Cleared and graded sites erode, are often severely compacted, and can no longer prevent rainfall from being rapidly converted into stormwater runoff.

Figure 1.1 Water Balance at a Developed and Undeveloped Site (Source: Schueler, 1987)

Surface runoff is minimal in an undeveloped site, but dominates the water balance at a highly impervious site.

Chapter 1. Introduction to the Manual ................................................Impact of Runoff

1.4

The situation worsens after construction. Roof tops, roads, parking lots, driveways and other impervious surfaces no longer allow rainfall to soak into the ground. Consequently, most rainfall is converted directly to stormwater runoff. This phenomenon is illustrated in Figure 1.2, which shows the increase in the volumetric runoff coefficient (Rv) as a function of site imperviousness.

The runoff coefficient expresses the fraction of rainfall volume that is converted into stormwater runoff. As can be seen, the volume of stormwater runoff increases sharply with impervious cover.

For example, a one acre parking lot can produce 16 times more stormwater runoff than a one acre meadow each year (Schueler, 1994).

The increase in stormwater runoff can be too much for the existing natural drainage system to handle. As a result, the natural drainage system is often “improved” to rapidly collect runoff and quickly convey it away (using curb and gutter, enclosed storm sewers, and lined channels). The stormwater runoff is subsequently discharged to downstream waters such as streams, reservoirs, lakes or estuaries.

Figure 1.2 Relationship between Impervious Cover and the Volumetric Runoff Coefficient

The runoff coefficient (Rv) expresses the fraction of rainfall that is converted into runoff.

The data points reflect over 35 monitoring stations in the U.S.

Chapter 1. Introduction to the Manual ................................................Impact of Runoff

1.5

1.1.1 Declining Water Quality

Impervious surfaces accumulate pollutants deposited from the atmosphere, leaked from vehicles, or windblown from adjacent areas. During storm events, these pollutants quickly wash off and are rapidly delivered to downstream waters. Some common pollutants found in urban stormwater runoff are profiled in Table 1.1 and include:

Nutrients. Urban runoff has elevated concentrations of both phosphorus and nitrogen, which can enrich streams, lakes, reservoirs and estuaries (known as eutrophication). In particular, excess nutrients have been documented to be a major factor in the decline of Chesapeake Bay. Excess nutrients promote algal growth that blocks sunlight from reaching underwater grasses and depletes oxygen in bottom waters. Urban runoff has been identified as a key and controllable source.

Maryland has committed to reducing tributary nutrient loadings by 40% as part of the Chesapeake Bay restoration effort.

Suspended solids. Sources of sediment include washoff of particles that are deposited on impervious surfaces and the erosion of streambanks and construction sites. Both suspended and deposited sediments can have adverse effects on aquatic life in streams, lakes and estuaries.

Sediments also transport other attached pollutants.

Organic Carbon. Organic matter, washed from impervious surfaces during storms, can present a problem in slower moving downstream waters. As organic matter decomposes, it can deplete dissolved oxygen in lakes and tidal waters. Low levels of oxygen in the water can have an adverse impact on aquatic life.

Bacteria. Bacteria levels in stormwater runoff routinely exceed public health standards for water contact recreation. Stormwater runoff can also lead to the closure of adjacent shellfish beds and swimming beaches and may increase the cost of treating drinking water at water supply reservoirs.

Hydrocarbons. Vehicles leak oil and grease that contain a wide array of hydrocarbon compounds, some of which can be toxic at low concentrations to aquatic life.

Trace Metals. Cadmium, copper, lead and zinc are routinely found in stormwater runoff. These metals can be toxic to aquatic life at certain concentrations and can also accumulate in the sediments of streams, lakes and the Chesapeake Bay.

Pesticides. A modest number of currently used and recently banned insecticides and herbicides have been detected in urban streamflow at concentrations that approach or exceed toxicity thresholds for aquatic life.

Chapter 1. Introduction to the Manual ................................................Impact of Runoff

1.6

Chlorides. Salts that are applied to roads and parking lots in the winter months appear in stormwater runoff and meltwater at much higher concentrations than many freshwater organisms can tolerate.

Thermal Impacts. Impervious surfaces may increase temperature in receiving waters, adversely impacting aquatic life that requires cold and cool water conditions (e.g., trout).

Trash and Debris. Considerable quantities of trash and debris are washed through storm drain networks. The trash and debris accumulate in streams and lakes and detract from their natural beauty.

Table 1.1 Typical Pollutant Concentrations Found in Urban Stormwater

Typical Pollutants Found in Stormwater Runoff (Data Source)

Units

Average Concentration (1)

Total Suspended Solids (a) mg/l

Total Phosphorus (b)

0.30

Total Nitrogen (a)

2.0

Total Organic Carbon (d)

12.7

Fecal Coliform Bacteria (c)

MPN/100 ml

E. coli Bacteria (c)

MPN/100 ml

Petroleum Hydrocarbons (d)

3.5

Cadmium (e) ug/l

Copper (a)

Lead (a)

Zinc (e)

Chlorides (f) (winter only)

Insecticides (g)

0.1 to 2.0

Herbicides (g)

1 to 5.0

(1) these concentrations represent mean or median storm concentrations measured at typical sites and may be greater during individual storms. Also note that mean or median runoff concentrations from stormwater hotspots are 2 to 10 times higher than those shown here. Units = mg/l = milligrams/liter, µg/l = micrograms/liter, MPN = Most Probable Number.

Data Sources: (a) Schueler (1987) (b) Schueler (1995), (c) Schueler (1997), (d) Rabanal and Grizzard (1996)

(e) USEPA (1983) (f) Oberts (1995) (g) Schueler, (1996)

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1.1.2 Diminishing Groundwater Recharge and Quality

The slow infiltration of rainfall through the soil layer is essential for replenishing groundwater.

The amount of rainfall that recharges groundwater varies, depending on the slope, soil, and vegetation. Some indication of the importance of recharge is shown in Table 1.2 which shows Natural Resources Conservation Service (NRCS) regional estimates of average annual recharge volume based on soil type.

Table 1.2 NRCS Estimates of Annual Recharge Rates, Based on Soil Type

Hydrologic Soil Group (NRCS)

Average Annual Recharge Volume

“A” Soils

18 inches/year

“B” Soils

12 inches/year

“C” Soils

6 inches/year

“D” Soils

3 inches/year

Average annual rainfall is about 42 inches per year across Maryland.

Groundwater is a critical water resource across the State. Not only do many residents depend on groundwater for their drinking water, but the health of many aquatic systems is also dependent on its steady discharge. For example, during periods of dry weather, groundwater sustains flows in streams and helps to maintain the hydrology of non-tidal wetlands (Figure 1.3). Because development creates impervious surfaces that prevent natural recharge, a net decrease in groundwater recharge rates can be expected in urban watersheds. Thus, during prolonged periods of dry weather, stream flow sharply diminishes. In smaller headwater streams, the decline in stream flow can cause a perennial stream to become seasonally dry.

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Figure 1.3 Decline in Stream Flow Due to Diminished Groundwater Recharge

After development, stream flow declines during extended periods of dry weather because of the diminished recharge of groundwater.

Urban land uses and activities can also degrade groundwater quality if stormwater runoff is directed into the soil without adequate treatment. Certain land uses and activities are known to produce higher loads of metals and toxic chemicals and are designated as stormwater hotspots.

Soluble pollutants, such as chloride, nitrate, copper, dissolved solids and some polycyclic aromatic hydrocarbons (PAH’s) can migrate into groundwater and potentially contaminate wells.

Stormwater runoff should never be infiltrated into the soil if a site is a designated hotspot.

1.1.3 Degradation of Stream Channels

Stormwater runoff is a powerful force that influences the geometry of streams. After development, both the frequency and magnitude of storm flows increase dramatically.

Consequently, urban stream channels experience more bankfull and sub-bankfull flow events each year than they had prior to development (see Figure 1.4).

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Figure 1.4 Increased Frequency of Flows Greater than the Critical Discharge Rate in a Stream Channel after Development

Development greatly increases the frequency that a stream exceeds the critical discharge rate (the discharge rate associated with bankfull flow) that corresponds to the onset of channel erosion and enlargement.

As a result, the stream bed and banks are exposed to highly erosive flows more frequently and for longer periods. Streams typically respond to this change by increasing cross-sectional area to handle the more frequent and erosive flows either by channel widening or down cutting, or both.

This results in a highly unstable phase where the stream experiences severe bank erosion and habitat degradation. In this phase, the stream often experiences some of the following changes:

rapid stream widening increased streambank and channel erosion decline in stream substrate quality (through sediment deposition and embedding of the substrate) loss of pool/riffle structure in the stream channel degradation of stream habitat structure

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The decline in the physical habitat of the stream, coupled with lower base flows and higher stormwater pollutant loads, has a severe impact on the aquatic community. Recent research has shown the following changes in stream ecology:

decline in aquatic insect and freshwater mussel diversity decline in fish diversity degradation of aquatic habitat

Traditionally, Maryland has attempted to provide some measure of channel protection by imposing the two-year storm peak discharge control requirement, which requires that the discharge from the two-year post development peak rates be reduced to pre development levels. However, recent research and experience indicate that the two-year peak discharge criterion is not capable of protecting downstream channels from erosion. In some cases, controlling the two-year storm may actually accelerate streambank erosion because it exposes the channel to a longer duration of erosive flows than it would have otherwise received.

1.1.4 Increased Overbank Flooding

Flow events that exceed the capacity of the stream channel spill out into adjacent floodplains.

These are termed “overbank” floods and can damage property and downstream drainage structures.

While some overbank flooding is inevitable and even desirable, the historical goal of drainage design in most of Maryland has been to maintain pre development peak discharge rates for both the two and ten-year frequency storms after development, thus keeping the level of overbank flooding the same over time. This prevents costly damage or maintenance for culverts, drainage structures, and swales.

Overbank floods are ranked in terms of their statistical return frequency. For example, a flood that has a 50% chance of occurring in any given year is termed a “two-year” flood. The two-year storm is also known as the “bankfull flood,” as researchers have demonstrated that most natural stream channels in the State have just enough capacity to handle the two-year flood before spilling into the floodplain. In Maryland, about 3.0 to 3.5 inches of rain in a 24-hour period produces a two-year or bankfull flood. This rainfall depth is termed the two-year design storm.

Similarly, a flood that has a 10% chance of occurring in any given year is termed a “ten-year flood." A ten-year flood occurs when a storm event produces about 4.5 to 5.5 inches of rain in a 24 hour period. Under traditional engineering practice, most channels and storm drains in Maryland are designed with enough capacity to safely pass the peak discharge from the ten-year design storm.

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Urban development increases the peak discharge rate associated with a given design storm because impervious surfaces generate greater runoff volumes and drainage systems deliver it more rapidly to a stream. The change in post development peak discharge rates that accompany development is profiled in Figure 1.5.

Figure 1.5 Change in Hydrograph following Development

The impervious surfaces and conveyance systems of developed sites result in an earlier and higher peak discharge rate.

1.1.5 Floodplain Expansion

The…