Low-Impact Development Design Strategies

An Integrated Design Approach

Low-Impact Development Design Strategies

An Integrated Design Approach

Prepared by:

Prince Georges

County, Maryland

Department of

Environmental

Resources

Programs and

Planning Division

June 1999
Low-Impact Development:

An Integrated Design Approach

Low-Impact Development:

An Integrated Design Approach

June 1999

Prepared by:

Prince Georges County, Maryland

Department of Environmental Resource

Programs and Planning Division

9400 Peppercorn Place

Largo, Maryland 20774

Wayne K. Curry

County Executive

Samuel E. Wynkoop, Jr.

Director
Contents iii

Low-Impact Development: An Integrated Environmental Design Approach

Contents

Figures .................................................................................................. vi

Tables..................................................................................................viii

Preface .................................................................................................. ix

Chapter 1 - Introduction ...................................................................1-1

Low-Impact Development Goals .............................................1-2

Comparing Conventional Stormwater Management Site

Design with LID Site Design ..................................................1-4

How to Use This Manual ........................................................1-5

Chapter 2 - Low-Impact Development Site Planning .....................2-1

Introduction .............................................................................2-1

Fundamental LID Site Planning Concepts ............................2-2

Concept 1 - Using hydrology as the integrating

framework ...........................................................................2-2

Concept 2 - Thinking micromanagement ........................2-3

Concept 3 - Controlling stormwater at the source ...........2-4

Concept 4 - Remembering simple technologies ...............2-4

Concept 5 - Creating a Multifunctional Landscape

and Infrastructure ..............................................................2-5

The LID Site Planning Process ...............................................2-6

Identify Applicable Zoning, Land Use, Subdivision,

and Other Local Regulations ............................................2-6

Define Development Envelope and Protected Areas ......2-8

Reduce Limits of Clearing and Grading. ..........................2-8

Use Site Fingerprinting......................................................2-9

Use Drainage/Hydrology as a Design Element .................2-9

Reduce/Minimize Total Impervious Areas......................2-11

Develop Integrated Preliminary Site Plan ......................2-13

Minimize Directly Connected Impervious Areas ...........2-14

Modify/Increase Drainage Flow Paths ............................2-14
iv Contents

Low-Impact Development: An Integrated Environmental Design Approach

Compare Pre- and Postdevelopment Hydrology .............2-19

Complete LID Site Plan ...................................................2-19

References ..............................................................................2-20

Chapter 3 - Low-Impact Development Hydrologic Analysis ...........3-1

Introduction .............................................................................3-1

Regional Considerations ..........................................................3-1

Overview of Key Hydrologic Principles ..................................3-2

Summary of Comparison Between Conventional and LID

Stormwater Management Approaches....................................3-8

LID Hydrologic Considerations ............................................3-10

LID Hydrologic Tools.............................................................3-11

LID Hydrologic Evaluation ...................................................3-12

LID Hydrologic Evaluation Steps .........................................3-12

Hydrologic Evaluation Techniques .......................................3-14

LID Hydrologic Illustrations .................................................3-16

LID Runoff Volume and Peak Flow Management ..............3-17

Determination of LID Runoff Curve Number ......................3-17

Maintaining the Predevelopment Time of Concentration ..3-19

References ..............................................................................3-22

Chapter 4 - Low-Impact Development Integrated Management

Practices ...................................................................................4-1

Procedures for Selection and Design of IMPs ........................4-1

Integrated Management Practices..........................................4-8

Bioretention..............................................................................4-8

Dry Wells ................................................................................4-11

Filter Strips .............................................................................4-12

Vegetated Buffers...................................................................4-14

Level Spreaders ......................................................................4-14

Grassed Swales.......................................................................4-16

Rain Barrels ............................................................................4-18

Cisterns ...................................................................................4-19

Infiltration Trenches ..............................................................4-20

Other Environmentally Sensitive Management Practices .. 4-21

Monitoring ..............................................................................4-22

References ..............................................................................4-24
Contents v

Low-Impact Development: An Integrated Environmental Design Approach

Chapter 5 - Erosion and Sediment Control Considerations

for Low-Impact Development ..................................................5-1

Erosion and Sediment Control Steps ......................................5-1

References. ...............................................................................5-8

Chapter 6 - Low-Impact Development Public Outreach Program .6-1

Introduction .............................................................................6-1

Developing a Public Outreach Program.................................6-2

Step One: Define Public Outreach Program Objectives ......6-2

Step Two: Identify Target Audiences .....................................6-3

Step Three: Develop Outreach Materials..............................6-6

Step Four: Distribute Outreach Materials .............................6-8

Appendix A - Example LID Hydrologic Computation .................. A-1

Appendix B - Sample Maintenance Covenant ............................... B-1

Glossary ............................................................................................ G-1
vi Contents

Low-Impact Development: An Integrated Environmental Design Approach

Figures

Figure 1-1. Parking lot bioretention area .....................................1-1

Figure 1-2. Residential lot with LID features ..............................1-4

Figure 1-3. Major components of the LID approach ....................1-6

Figure 2-1. Hydrologically functional landscape .........................2-2

Figure 2-2. Frequency of small storms at San Francisco

International Airport ..................................................2-3

Figure 2-3. Relative cost as a function of distance from source 2-4

Figure 2-4. Bioretention cell .........................................................2-5

Figure 2-5. Some protected site features ......................................2-8

Figure 2-6. Impervious surface changes due to urbanization ......2-9

Figure 2-7. Increase in receiving stream impacts due to

site imperviousness....................................................2-10

Figure 2-8. Typical imperviousness ratios for conventional

and LID residential development design ................2-10

Figure 2-9. Length of pavement (imperviousness associated

with various road layout options) .............................2-11

Figure 2-10. Narrow road sections ................................................2-12

Figure 2-11. Integrated site plan ..................................................2-13

Figure 2-12. Roads placed along ridge lines preserve and

utilize the natural drainage system ..........................2-16

Figure 2-13. Low-impact development minimum lot grading

and 100-year buffer requirements ............................2-18

Figure 2-14. Vegetated swale ........................................................2-19

Figure 2-15. Site layouts with/without vegetation retention ......2-20

Figure 3-1. Hydrologic response of conventional IMPs ...............3-3

Figure 3-2. Relationship of the rainfall event recurrence

interval and rainfall volume, and its application

to stormwater management in Maryland ..................3-4

Figure 3-3. Approximate geographic boundaries for NRCS

rainfall distributions ....................................................3-5

Figure 3-4. Runoff variability with increased impervious

surfaces ........................................................................3-7

Figure 3-5. Groundwater in local, intermediate, or regional

setting ..........................................................................3-8

Figure 3-6. Comparison of the hydrologic response of

conventional IMPs and LID BMPs ...........................3-11

Figure 3-7. Prince Georges County, Maryland, example of

low-impact development analysis procedure ...........3-15

Figure 3-8. Customizing runoff CN for a low-impact

development site .......................................................3-17

Figure 3-9. Effect of low-impact development CN on the

postdevelopment hydrograph without

stormwater BMPs ...................................................... 3-19
Contents vii

Low-Impact Development: An Integrated Environmental Design Approach

Figure 3-10. Low-impact development hydrograph that has

a reduced CN and maintains the Tc without

conventional stormwater controls ............................3-20

Figure 4-1. Key steps in developing storm water plan using

LID practices...............................................................4-3

Figure 4-2. Bioretention area ........................................................4-9

Figure 4-3. Typical bioretention facility ......................................4-10

Figure 4-4. Typical dry well .........................................................4-11

Figure 4-5. Typical filter strip ......................................................4-13

Figure 4-6. Typical rock trench level spreader ...........................4-15

Figure 4-7. Example of dry swale.................................................4-16

Figure 4-8. Example of wet swale ................................................4-17

Figure 4-9. Typical rain barrel .....................................................4-18

Figure 4-10. Rain barrel application to LID .................................4-19

Figure 4-11. Cistern .......................................................................4-19

Figure 4-12. Median strip infiltration trench designs ..................4-20

Figure 4-13. Roof greening ............................................................4-22

Figure 5-1. A well-mulched site ...................................................5-5

Figure 5-2. Silt fence installation guidelines ...............................5-7
viii Contents

Low-Impact Development: An Integrated Environmental Design Approach

Tables

Table 2-1. Steps in LID Site Planning Process ............................2-6

Table 2-2. Common Zoning Components ....................................2-7

Table 2-3. Alternative Zoning Options .......................................2-8

Table 2-4. Permissible Velocities for Vegetated Channels .......2-15

Table 2-5. Alternative Road Layouts ........................................2-17

Table 3-1. Comparison of Conventional and LID

Stormwater Management Technologies ....................3-9

Table 3-2. Comparison of Model Attributes and Functions .....3-16

Table 3-3. Comparison of Conventional and LID Land

Covers ........................................................................3-18

Table 3-4. LID Planning Techniques to Reduce the

Postdevelopment Runoff Volume .............................3-18

Table 3-5. LID Techniques to Maintain

the Predevelopment Time of Concentration ...........3-21

Table 4-1. Site Constraints of IMPs .............................................4-4

Table 4-2. Hydrologic Functions of LID Integrated

Management Practices ...............................................4-6

Table 4-3. Reported Pollutant Removal Efficiency of IMPs .......4-7

Table 4-4. Bioretention Design Comonents ................................4-9

Table 4-5. Drywell Design Considerations ................................4-12

Table 4-6. Filter Strip Design Considerations ...........................4-14

Table 4-7. Grassed Swale Design Considerations ....................4-18

Table 4-8. Infiltration Trench Design Considerations ..............4-21

Table 4-9. Parameters to Report with Water-Quality Data for

Various BMPs.............................................................4-23

Table 5.1. Types of Mulches.........................................................5-5

Table 6.1. Educational Materials ................................................6-7
Preface ix

Low-Impact Development: An Integrated Environmental Design Approach

Preface

Low-impact development (LID) is a radically different approach

to conventional stormwater management. It is our belief that LID

represents a significant advancement in the state of the art in

stormwater management. LID enhances our ability to protect surface

and ground water quality, maintain the integrity of aquatic living

resources and ecosystems, and preserve the physical integrity of

receiving streams. Prince Georges County, Marylands Department

of Environmental Resources has pioneered several new tools and

practices in this field, which strive to achieve good environmental

designs that also make good economic sense. The purpose of this

manual is to share some of our experiences, and show how LID can

be applied on a national level.

The LID principles outlined in these pages were developed over

the last three years specifically to address runoff issues associated

with new residential, commercial, and industrial suburban development. Prince Georges County, which borders Washington, DC, is

rich with natural streams, many of which support game fish. Preserving these attributes in the face of increasing development pressure

was the challenge, which led to the development of LID techniques.

We describe how LID can achieve stormwater control through

the creation of a hydrologically functional landscape that mimics the

natural hydrologic regime. This objective is accomplished by:

 Minimizing stormwater impacts to the extent practicable.

Techniques presented include reducing imperviousness, conserving natural resources and ecosystems, maintaining natural

drainage courses, reducing use of pipes, and minimizing clearing

and grading.

 Providing runoff storage measures dispersed uniformly throughout a sites landscape with the use of a variety of detention,

retention, and runoff practices.
x Preface

Low-Impact Development: An Integrated Environmental Design Approach

 Maintaining predevelopment time of concentration by strategically routing flows to maintain travel time and control the

discharge.

 Implementing effective public education programs to encourage

property owners to use pollution prevention measures and

maintain the on-lot hydrologically functional landscape management practices.

LID offers an innovative approach to urban stormwater managementone that does not rely on the conventional end-of-pipe or inthe-pipe structural methods but instead uniformly or strategically

integrates stormwater controls throughout the urban landscape.

We wish to thank the US Environmental Protection Agency for

their encouragement and support of this document. In particular,

Robert Goo and Rod Frederick of EPAs Office of Water, Nonpoint

Source Control Branch. I would also like to acknowledge the

contributions of the many highly dedicated professionals who

contributed to the development of LID technology, especially Dr.

Mow-Soung Cheng and Derek Winogradoff of Prince Georges

County and the Tetra Tech project team led by Dr. Mohammed

Lahlou and including: Dr. Leslie Shoemaker, Michael Clar, Steve

Roy, Jennifer Smith, Neil Weinstein, and Kambiz Agazi.

It is my hope that the release of this manual will stimulate a

national debate on this promising form of stormwater management.

We are currently developing new LID principles and practices

directly applicable to such issues as urban retrofit, combined sewer

overflow, and highway design. This manual represents only the

beginning of a new paradigm in stormwater management. I hope

that you will take up the challenge and work with us to further

develop LID practices.

Larry Coffman, Director

Programs and Planning Division

Department of Environmental Resources

Prince Georges County, Maryland
Chapter 1

Introduction

Chapter 1

Introduction
Introduction 1-1

Low-Impact Development: An Integrated Environmental Design Approach

Introduction 1

The low-impact development (LID) approach combines a hydrologically functional site design with pollution prevention measures to

compensate for land development impacts on hydrology and

water quality. As shown in

Figure 1-1, a parking lot

bioretention area, LID techniques not only can function to

control site hydrology, but also

can be aesthetically pleasing.

In This Chapter

Introduction

Low-impact

Development Goals

How to Use This Manual

Chapter

Figure 1-1. Parking lot

bioretention area
1-2 Introduction

Low-Impact Development: An Integrated Environmental Design Approach

Low-Impact Development Goals

The primary goal of Low Impact Development methods is to

mimic the predevelopment site hydrology by using site design techniques that store, infiltrate, evaporate, and detain runoff. Use of these

techniques helps to reduce off-site runoff and ensure adequate groundwater recharge. Since every aspect of site development affects the

hydrologic response of the site, LID control techniques focus mainly

on site hydrology.

There is a wide array of impact reduction and site design techniques that allow the site planner/engineer to create stormwater

control mechanisms that function in a manner similar to that of

natural control mechanisms. If LID techniques can be used for a

particular site, the net result will be to more closely mimic the

watersheds natural hydrologic functions or the water balance between

runoff, infiltration, storage, groundwater recharge, and evapotranspiration. With the LID approach, receiving waters may experience

fewer negative impacts in the volume, frequency, and quality of runoff,

so as to maintain base flows and more closely approximate

predevelopment runoff conditions.

The goals of low-impact development are discussed and demonstrated throughout the manual. The list below highlights some of the

main goals and principles of LID:

• Provide an improved technology for environmental protection of

receiving waters.

• Provide economic incentives that encourage environmentally

sensitive development.

• Develop the full potential of environmentally sensitive site planning and design.

• Encourage public education and participation in environmental

protection.

• Help build communities based on environmental stewardship.

• Reduce construction and maintenance costs of the stormwater

infrastructure.

• Introduce new concepts, technologies, and objectives for

stormwater management such as micromanagement and multifunctional landscape features (bioretention areas, swales, and

conservation areas); mimic or replicate hydrologic functions; and

maintain the ecological/biological integrity of receiving streams.
Introduction 1-3

Low-Impact Development: An Integrated Environmental Design Approach

• Encourage flexibility in regulations that allows innovative engineering and site planning to promote smart growth principles.

• Encourage debate on the economic, environmental, and technical

viability and applicability of current stormwater practices and

alternative approaches.

LID is a comprehensive technology-based approach to managing

urban stormwater. Stormwater is managed in small, cost-effective

landscape features located on each lot rather than being conveyed and

managed in large, costly pond facilities located at the bottom of

drainage areas. The source control concept is quite different from

conventional treatment (pipe and pond stormwater management site

design). Hydrologic functions such as infiltration, frequency and

volume of discharges, and groundwater recharge can be maintained

with the use of reduced impervious surfaces, functional grading, open

channel sections, disconnection of hydrologic flowpaths, and the use

of bioretention/filtration landscape areas. LID also incorporates

multifunctional site design elements into the stormwater management

plan. Such alternative stormwater management practices as on-lot

microstorage, functional landscaping, open drainage swales, reduced

imperviousness, flatter grades, increased runoff travel time, and

depression storage can be integrated into a multifunctional site design

(Figure 1-2).

Specific LID controls called Integrated Management Practices

(IMPs) can reduce runoff by integrating stormwater controls throughout the site in many small, discrete units. IMPs are distributed in a

small portion of each lot, near the source of impacts, virtually eliminating the need for a centralized best management practice (BMP)

facility such as a stormwater management pond. By this process, a

developed site can be designed as an integral part of the environment

maintaining predevelopment hydrologic functions through the careful

use of LID control measures. IMPs are defined and described in

Chapter 4, Low-Impact Development Integrated Management Practices.

LID designs can also significantly reduce development costs

through smart site design by:

• Reducing impervious surfaces (roadways), curb, and gutters

• Decreasing the use of storm drain piping, inlet structures, and

• Eliminating or decreasing the size of large stormwater ponds.
1-4 Introduction

Low-Impact Development: An Integrated Environmental Design Approach

In some instances, greater lot yield can be obtained using LID

techniques, increasing returns to developers. Reducing site development infrastructure can also reduce associated project bonding and

maintenance costs.

Comparing Conventional Stormwater Management

Site Design With Lid Site Design

One paradigm has typically dominated site planning and

engineeringStormwater runoff is undesirable and must be removed

from the site as quickly as possible to achieve good drainage. Current

site development techniques result in the creation of an extremely

efficient stormwater runoff conveyance system. Every feature of a

conventionally developed site is carefully planned to quickly convey

runoff to a centrally located management device, usually at the end of

a pipe system. Roadways, roofs, gutters, downspouts, driveways, curbs,

pipes, drainage swales, parking, and grading are all typically designed

Figure 1-2

Residential lot with

LID features
Introduction 1-5

Low-Impact Development: An Integrated Environmental Design Approach

to dispose of the runoff in a rapid fashion. The magnitude of hydrologic

changes (increases in volume, frequency, and rate of discharge) are

amplified as natural storage is lost, the amount of impervious surfaces is

increased, the time of concentration is decreased, runoff travel times are

decreased, and the degree of hydraulic connection is increased. Typical

conventional site design results in developments devoid of natural

features that decrease travel times and that detain or infiltrate runoff.

Lack of these features often adversely affects the ecosystem.

In contrast, the principal goal of low-impact development is to

ensure maximum protection of the ecological integrity of the receiving

waters by maintaining the watersheds hydrologic regime. This goal is

accomplished by creatively designing hydrologic functions into the site

design with the intent of replicating the predevelopment hydrology

and thus having a significant positive effect on stream stability, habitat

structure, base flows, and water quality. It is well documented that

some conventional stormwater control measures can effectively

remove pollutants from runoff. Water quality, however, is only one of

several factors that affect aquatic biota or the ecological integrity of

receiving streams. Fish macroinvertebrate surveys have demonstrated

that good water quality is not the only determinant of biological

integrity. In fact, the poor condition of the biological communities is

usually attributed to poor habitat structure (cover, substrate, or

sedimentation) or hydrology (inadequate base flow, thermal fluxes, or

flashy hydrology). A conclusion that can be drawn from these studies

and from direct experience is that perhaps stormwater pond technology is limited in its ability to protect the watershed and cannot reproduce predevelopment hydrological functions. With this in mind, LID

can be a way to bridge this gap in protecting aquatic biota and provide

good water quality as well. This manual was developed to provide a

reference and a model for practitioners to use in experimenting with

and applying LID techniques across the nation.

How to Use This Manual

Low-impact development allows the site planner/engineer to use a

wide array of simple, cost-effective techniques that focus on site-level

hydrologic control. This manual describes those techniques and

provides examples and descriptions of how they work. It does not

discuss detailed site planning techniques for the conservation of

natural resources (trees, wetlands, streams, floodplains, steep slopes,

critical areas, etc.). Such site features/constraints are typically addressed as part of existing county, state, and federal regulations.

Compliance with the existing regulations is the starting point for

defining the building envelope and the use of LID techniques. Once
1-6 Introduction

Low-Impact Development: An Integrated Environmental Design Approach

the basic building envelope has been defined, LID techniques may

provide significant economic incentives to improve environmental

protection and expand upon the conservation of natural resources

areas. The manual has been formatted in a manner that allows the

designer to incorporate LID into a specific building envelope in a

logical step-by-step approach.

For ease of use and understanding, this document has been

divided into six chapters and appendices. A glossary is provided at

the end of the document. Figure 1-3 summarizes the major components of the LID approach.

Figure 1-3. Major

components of the

LID approach
Introduction 1-7

Low-Impact Development: An Integrated Environmental Design Approach

Chapter 1. Introduction

Chapter 2. Low-Impact Development Site Planning. The site

design philosophy and site planning techniques are described and

illustrated in this chapter.

Chapter 3. Low-Impact Development Hydrologic Analysis. This

chapter provides an overview and general description of the key

hydrologic principles involved in low-impact development, and

provides guidance on the hydrologic analysis required for the design of

LID sites.

Chapter 4. Low-Impact Development Integrated Management

Practices. Selection criteria and descriptions for specific LID IMPs are

provided along with fact sheets on IMPs.

Chapter 5. Erosion and Sediment Control Considerations for

Low-Impact Development. Erosion and sediment control and LID

principles are closely interrelated since LID technology can result in

improved erosion and sediment control. Chapter 5 addresses that

relationship.

Chapter 6. Low-Impact Development Public Outreach Program.

Chapter 6 explains why LID approaches require the education of

homeowners, landowners, developers, and regulators and offers

suggestions for conducting a successful public outreach program.

Appendix A. Example LID Hydrologic Computation

Appendix B. Sample Maintenance Covenant

Glossary.
Chapter 2

Low-Impact Development Site Planning

Chapter 2

Low-Impact Development Site Planning
LID Site Planning 2-1

Low-Impact Development: An Integrated Environmental Design Approach

Low-Impact Development Site Planning 2

Introduction

Site planning strategies and techniques provide the means to

achieve stormwater management goals and objectives; facilitate the

development of site plans that are adapted to natural topographic

constraints; maintain lot yield; maintain site hydrologic functions; and

provide for aesthetically pleasing, and often less expensive stormwater

management controls. Hydrologic goals and objectives should be

incorporated into the site planning process as early as possible.

The goal of LID site planning is to allow for full development of the

property while maintaining the essential site hydrologic functions. This

goal is accomplished in a series of

incremental steps, which are

presented in this chapter. These

steps include first minimizing the

hydrologic impacts created by the

site development through site

design and then providing

controls to mitigate or restore the

unavoidable disturbances to the

hydrologic regime. The hydrologic disturbances are mitigated

with the use of an at-source

control approach, in contrast to

the currently used end-of-pipe

control approach. The newer

approach results in the creation

of hydrologically functional

landscapes that preserve and

maintain the essential hydrologic

functions of the development site

and the local watershed.

In This Chapter

Introduction

Fundamental LID Site Planning Concepts

The LID Site Planning Process

Identify Applicable Zoning, Land Use,

Subdivision, and Other Local Regulations

Define Development Envelope and

Protected Areas

Use Drainage/Hydrology as a Design

Element

Reduce/Minimize Total Impervious Areas

Develop Integrated Preliminary Site Plan

Minimize Directly Connected Impervious

Areas

Modify/Increase Drainage Flow Paths

Compare Pre- and Post Development

Hydrology

Complete LID Site Plan

Chapter

Lot Yield

The total number of

buildable lots within

the development
2-2 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

Fundamental LID Site Planning Concepts

A few fundamental concepts that define the essence of low-impact

development technology must be integrated into the site planning

process to achieve a successful and workable plan. These concepts are

so simple that they tend to be overlooked, but their importance cannot

be overemphasized. These fundamental concepts include:

• Using hydrology as the integrating framework

• Thinking micromanagement

• Controlling stormwater at the source

• Using simplistic, nonstructural methods

• Creating a multifunctional landscape

These fundamental concepts are defined in the following sections.

Concept 1 - Using Hydrology as the Integrating Framework

In LID technology, the traditional approach to site drainage is

reversed to mimic the natural drainage functions. Instead of rapidly

and efficiently draining the site, low-impact development relies on

various planning tools and control practices to preserve the natural

hydrologic functions of the site. Planners may begin by asking, What

are the essential predevelopment hydrologic functions of the site, and

how can these essential functions be maintained while allowing full

use of the site? The application of low-impact development techniques results in the creation of a hydrologically functional landscape (Figure 2-1), the use of distributed micromanagement

practices, impact minimization, and reduced effective

imperviousness allowing maintenance of infiltration

capacity, storage, and longer time of concentration.

Integration of hydrology into the site

planning process begins by identifying and

preserving sensitive areas that affect

the hydrology, including streams and

their buffers, floodplains, wetlands,

steep slopes, high-permeability soils, and

woodland conservation zones. This process

defines a development envelope, with respect

to hydrology, which is the first step to minimizing

hydrologic impacts. This development envelope will

have the least hydrologic impact on the site while retaining important natural hydrologic features.

Hydrology

The movement of

water into and

across the site

Figure 2-1. Hydrologically

functional landscape
LID Site Planning 2-3

Low-Impact Development: An Integrated Environmental Design Approach

Potential site development and layout schemes are then evaluated to

reduce, minimize, and disconnect the total impervious area at the site.

Further analysis is then conducted on the unavoidable impervious areas to

minimize directly connected impervious surfaces. Bioretention areas,

increased flow paths, infiltration devices, drainage swales, retention areas,

and many other practices can be used to control and break up these

impervious areas. The end result is an integrated hydrologically functional

site plan that maintains the predevelopment hydrology in addition to

improving aesthetic values and providing recreational resources by adding

additional landscape features.

Concept 2 - Thinking Micromanagement

The key to making the LID concept work is to think small. This

requires a change in perspective or approach with respect to the size of

the area being controlled ( i.e., microsubsheds), the size of the control

practice (microtechniques), siting locations of controls, and the size

and frequency of storms that are controlled. Micromanagement

techniques implemented on small sub catchments, or on residential

lots, as well as common areas, allow for a distributed control of

stormwater throughout the entire site. This offers significant opportunities for maintaining the sites key hydrologic functions including

infiltration, depression storage, and interception, as well as a reduction

in the time of concentration. These micromanagement techniques are

referred to as integrated management practices (IMPs).

Figure 2-2 presents a typical months rainfall in the San Francisco

Bay area, showing how small storms plus the first increment of the

bigger storms account for half of the total rainfall volume. These small

storms, because of their frequency and cumulative impacts, make the

largest contribution to total annual runoff volume and have the

greatest impact on water quality and receiving water hydrology.

Other advantages of micromanagement techniques include the

following:

• Provide a much greater range

of control practices that can

be used and adapted to site

conditions.

• Allow use of control practices

that can provide volume

control and maintain

predevelopment groundwater

Development

envelope

The total site areas

that affect the

hydrology (i.e., lots

to be developed,

streams, buffers,

floodplains,

wetlands, slopes,

soils, and

woodlands.

Interception

Water trapped on

vegetation before

reaching the ground

Figure 2-2. Frequency of

small storms at San

Francisco International

Airport (Source:

BASMAA, 1997)
2-4 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

recharge functions, thereby compensating for significant alterations of infiltration capacity.

• Allow on-lot control practices to be integrated into the landscape,

impervious surfaces, and natural features of the site.

• Reduce site development and long-term maintenance costs through

cost-effective designs and citizen participation and acceptance.

Concept 3 - Controlling Stormwater at the Source

The key to restoring the predevelopment hydrologic functions is to

first minimize and then mitigate the hydrologic impacts of land use

activities closer to the source of generation. Natural hydrologic

functions such as interception, depression storage, and infiltration are

evenly distributed throughout an undeveloped site. Trying to control

or restore these functions using an end-of-pipe stormwater management approach is difficult, if not impossible. Therefore, compensation

or restoration of these hydrologic functions should be implemented as

close as possible to the point or source, where the impact or disturbance is generated. This is referred to as a distributed, at-source

control strategy and is accomplished using micromanagement techniques throughout the site. The distributed control strategy is one of

the building blocks of low-impact development.

The cost benefits of this approach can be substantial. Typically, the

most economical and simplistic stormwater management strategies are

achieved by controlling runoff at the source. Conveyance system and

control or treatment structure costs increase with distance from the

source (Figure 2-3).

Concept 4 - Utilization of Simplistic, Nonstructural

Methods

Traditionally, most

stormwater management has

focused on large end-of-pipe

systems and there has been a

tendency to overlook the consideration of small simple solutions.

These simple solutions or systems

have the potential to be more

effective in preserving the

hydrologic functions of the

landscape and they can offer

significant advantages over

conventional engineered facilities

Figure 2-3. Relative cost

as a function of distance

from source (Source:

BASMAA, 1997)

Depression

storage

Small, water-holding

pockets on the land

surface
LID Site Planning 2-5

Low-Impact Development: An Integrated Environmental Design Approach

such as ponds or concrete conveyances. In some cases LID techniques

will need to be combined with traditional stormwater controls.

The use of LID techniques can decrease the use of typical engineering

materials such as steel and concrete. By using materials such as native

plants, soil and gravel these systems can be more easily integrated into the

landscape and appear to be much more natural than engineered systems.

The natural characteristics may also increase homeowner acceptance

and willingness to adopt and maintain such systems.

Small, distributed, microcontrol systems also offer a major technical advantage: one or more of the systems can fail without undermining the overall integrity of the site control strategy.

These smaller facilities tend to feature shallow basin depths and

gentle side slopes, which also reduce safety concerns. The integration

of these facilities into the landscape throughout the site offers more

opportunities to mimic the natural hydrologic functions, and add

aesthetic value. The adoption of these landscape features by the

general public and individual property owners can result in significant

maintenance and upkeep savings to the homeowners association,

municipality or other management entity.

Concept 5 - Creating a Multifunctional Landscape and

Infrastructure

LID offers an innovative alternative approach to urban stormwater

management that uniformly or strategically integrates stormwater

controls into multifunctional landscape features where runoff can be

micromanaged and controlled at the sources. With LID, every urban

landscape or infrastructure feature (roof, streets, parking, sidewalks,

and green space) can be designed to be multifunctional, incorporating

detention, retention, filtration, or runoff use.

The bioretention cell in

Figure 2-4 is perhaps the best

example of a multifunctional

practice and illustrates a

number of functions. First the

tree canopy provides interception and ecological, hydrologic, and habitat functions.

The 6-inch storage area

provides detention of runoff.

The organic litter/mulch

provides pollutant removal

Figure 2-4.

Bioretention cell
2-6 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

and water storage. The planting bed soil provides infiltration of runoff,

removal of pollutants through numerous processes, groundwater

recharge, and evapotranspiration through the plant material.

The opportunities, effectiveness, and benefits for control of runoff

through numerous small-scale multifunctional landscape features have

not been fully explored. To apply LID to any land use is simply a

matter of developing numerous ways to creatively prevent, retain,

detain, use, and treat runoff within multifunctional landscape features

unique to that land use.

The LID Site Planning

Process

Site planning is a

well-established process

consisting of several elements. The incorporation of

LID concepts into this process

introduces a number of new

considerations to better

mimic the predevelopment

hydrology and create a

hydrologically functional

landscape. These concepts

include considering hydrology as a design focus, minimizing imperviousness, disconnecting

impervious surfaces, increasing flow paths, and defining and siting

micromanagement controls. Table 2-1 provides a summary of the steps

involved in integrating the LID technology into the site planning

process. These steps are described below.

Identify Applicable Zoning, Land Use, Subdivision, and

Other Local Regulations

The planning process of a local governmental entity (county,

district, borough, municipality, etc.)zoning ordinances and comprehensive planningprovides a framework to establish a functional and

visual relationship between growth and urbanization. Zoning ordinances predesignate the use and physical character of a developed

geographic area to meet urban design goals. Common zoning components are summarized in Table 2-2. The zoning requirements are

intended to regulate the density and geometry of development,

specifying roadway widths and parking and drainage requirements, and

define natural resource protection areas.

Zoning

ordinances

Land use controls at

the county or

municipal level

designed to regulate

density, types, and

extent of

development

Table 2-1 Steps in LID Site Planning Process

Step 1 Identify Applicable Zoning, Land Use,

Subdivision and Other Local Regulations

Step 2 Define Development Envelope

Step 3 Use Drainage/Hydrology as a Design Element

Step 4 Reduce/Minimize Total Site Impervious Areas

Step 5 Integrate Preliminary Site Layout Plan

Step 6 Minimize Directly Connected Impervious Areas

Step 7 Modify/Increase Drainage Flow Paths

Step 8 Compare Pre and Post Development Hydrology

Step 9 Complete LID Site Plan
LID Site Planning 2-7

Low-Impact Development: An Integrated Environmental Design Approach

Identification of existing zoning ordinances and applicable subdivision regulations is not a new concept, but rather an established

element of current site planning practices. The LID site planning

process recognizes that in most instances, LID approaches need to

meet the local zoning requirement. However, typical conventional

zoning regulations are often inflexible and restrict development

options regarding certain site planning parameters. Consequently,

local planning agencies that wish to optimize the environmental and

economic benefits provided by the LID approach will want to consider

the adoption of environmentally sensitive and flexible zoning options

that facilitate the use of LID technology.

The LID approach employs a number of flexible zoning options to

meet the environmental objectives of a site without impeding urban

growth. The use of these options provides added environmental sensitivity to the zoning and subdivision process over and above what

conventional zoning can achieve. Alternative zoning options, such as

Table 2-2 Common Zoning Components

Subdivision

regulations

Local land use

controls specify how

large land parcels

are broken into

smaller pieces

Zoning Requirement Purpose

Land use restriction Separate residential, commercial and industrial uses

and/or specify the percentage mix of these uses

Lot Layout Requirement

Equal-sized or similarly

shaped lots

Provide consistency among residential use or

districts

Minimum lot sizes Provide consistency among residential uses or

districts

Frontage requirements Provide additional distinction among residential

zones; access

Fixed setbacks for front, back,

and side yards

Provide additional distinction among residential

and side yards provide consistency among

residential zones; control coverage by buildings.

Road Layout Requirements

Road width Ensure vehicular and pedestrian safety and avoid

rights-of-way public facility burdens

Road turnarounds Prevent undue fire safety hazards; provide

adequate fire safety vehicular access.

Sidewalks and pedestrian

walkways

Ensure vehicular and pedestrian safety and avoid

access public facility burdens.

Residential and commercial

development

Ensure vehicular and pedestrian safety and avoid

access public facility burdens.

Common or shared facilities Prevent environmental or safety hazards from

unmaintained facilities such as shared septic

systems or driveways.

Drainage and Grading

Curbs/gutters and storm

drains

Prevent undue burden of development on off-site

water, streets, and buildings

Stormwater quality and

quantity Structures

Prevent undue burden of development on off-site

water, streets, and buildings

Grading to promote positive

drainage

Prevent soil erosion problems due to drainage
2-8 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

those summarized in Table 2-3,

include overlay districts,

performance zoning, incentive

zoning, impervious overlay

zoning, and watershed-based

zoning to allow for the introduction of innovative development, site layout, and design

techniques.

Define Development

Envelope and Protected

Areas

After the zoning code and

subdivision regulations have

been analyzed, a development envelope can be prepared for the proposed site. This is done by identifying protected areas, setbacks, easements, topographic features and existing subdrainage divides, and other

site features. Site features to be protected are illustrated in Figure 2-5

and may include riparian areas such as floodplains, stream buffers, and

wetlands; woodland conservation zones and important existing trees;

steep slopes; and highly permeable and erosive soils. These features

can be mapped in an overlay mode.

Reduce Limits of Clearing and Grading

The limits of clearing and grading refer to the site area to which

development is directed. This development area will include all

impervious areas such as roads, sidewalks, rooftops, and pervious areas

such as graded lawn areas and open drainage systems. To minimize

hydrologic impacts on existing site land cover, the area of development

should be located in areas that are less sensitive to disturbance or have

lower value in terms of hydrologic function (e.g., developing barren

clayey soils will have less hydrologic impact than development of

forested sandy soils). At a minimum, areas of development should be

placed outside of sensitive area

buffers such as streams, floodplains, wetlands, and steep

slopes. Where practical and

possible, avoid developing areas

with soils which have high

infiltration rates to reduce net

hydrologic site impacts.

Figure 2-5. Some protected

site features

Table 2-3 Alternative Zoning Options

Zoning Option Functions Provided

Overlay District Uses existing zoning and provides

additional regulatory standard

Performance Zoning Flexible zoning based on general goals of

the site based on preservation of site

functions

Incentive Zoning Provides for give and take compromise on

zoning restrictions allowing for more

flexibility to provide environmental

protection

Imperviousness

Overlay Zoning

Subdivision layout options are based on

total site imperviousness limits

Watershed-based

Zoning

Uses a combination of the above

principles to meet a predetermined

watershed capacity or goal
LID Site Planning 2-9

Low-Impact Development: An Integrated Environmental Design Approach

Use Site Fingerprinting

Site fingerprinting (minimal disturbance techniques) can be used

to further reduce the limits of clearing and grading, thereby minimizing

the hydrologic impacts. Site fingerprinting includes restricting ground

disturbance by identifying the smallest possible area and clearly

delineating it on the site. Land-cover impacts can be reduced through

minimal disturbance techniques that include the following:

 Reduce paving and compaction of highly permeable soils.

 Minimizing the size of construction easements and material

storage areas, and siting stockpiles within the development envelope during the construction phase of a project.

• Siting building layout and clearing and grading to avoid removal of

existing trees where possible.

• Minimizing imperviousness by reducing the total area of paved

surfaces.

• Delineating and flagging the smallest site disturbance area possible

to minimize soil compaction on the site and restricting temporary

storage of construction equipment in these areas.

• Disconnecting as much impervious area as possible to increase

opportunities for infiltration and reduce water runoff flow.

• Maintaining existing topography and associated drainage divides

to encourage dispersed flow paths.

Use Drainage/Hydrology as a Design Element

Site hydrology evaluation and understanding are required to

create a hydrologically functional landscape. As illustrated in

Figure 2-6, urbanization and increased impervious areas greatly alter

Figure 2-6.

Impervious surface

changes due to

urbanization

Site

fingerprinting

Site clearing and

development using

minimal disturbance

of existing

vegetation and soils
2-10 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

the predevelopment hydrology

(USEPA, 1993; Booth and

Reinelt, 1993). This increase in

impervious areas has been

directly linked to increases in

impacts on receiving streams

(Figure 2-7) by numerous investigators (including Booth and

Reinelt, 1993; Horner et al.,

1994; Klein, 1979; May, 1997;

Steedman 1988). To reduce

these impacts created by land

development, LID site planning

incorporates drainage/hydrology

by carefully conducting hydrologic evaluations and reviewing spatial

site layout options.

Hydrologic evaluation procedures can be used to minimize the

LID runoff potential and to maintain the predevelopment time of

concentration. These procedures are incorporated into the LID site

planning process early on to understand and take advantage of site

conditions.

Spatial organization of the site layout is also important. Unlike

pipe conveyance systems that hide water beneath the surface and

work independently of surface topography, an open drainage system

for LID can work with natural landforms and land uses to become a

major design element of a site plan. The LID stormwater management drainage system can suggest pathway alignment, optimum

locations for park and play areas, and potential building sites. The

drainage system helps to integrate urban forms, giving the development an integral, more aesthetically pleasing relationship to the

natural features of the site. Not only does the integrated site plan

Figure 2-8. Typical

imperviousness ratios for

conventional and LID

residential development

design

Figure 2-7. Increases in

receiving stream impacts

due to site

imperviousness
LID Site Planning 2-11

Low-Impact Development: An Integrated Environmental Design Approach

complement the land, but it can also save on development costs by

minimizing earthwork and construction of expensive drainage structures.

Reduce/Minimize Total Impervious Areas

After, or concurrent with, the mapping of the development

envelope, the traffic pattern and road layout and preliminary lot layout

are developed. The entire traffic distribution network, (roadways,

sidewalks, driveways, and parking areas), are the greatest source of site

imperviousness, as shown in Figure 2-8. These changes in the impervious area alter runoff and recharge values and site hydrology (Figure

2-6). For LID sites, managing the imperviousness contributed by road

and parking area pavement is an important component of the site

planning and design process. Methods that can be used to achieve a

reduction in the total runoff volume from impervious surfaces are

presented below:

Alternative Roadway Layout. Traffic or road layout can have a

very significant influence on the total imperviousness and hydrology of

the site plan. Figure 2-9 illustrates that the total length of pavement

or imperviousness for various road layout options can vary from 20,800

linear ft for a typical gridiron layout to 15,300 linear ft for a loops and

lollipops layout. Selection of an alternative road layout can result in a

total site reduction in imperviousness of 26 percent.

Narrow Road Sections. Reduced width road sections are an

alternative that can be used to reduce total site imperviousness as

well as clearing and grading impacts. Figure 2-10 shows a typical

primary residential street road section and a typical rural residential

street road section (Prince Georges County, 1997). The right-ofway width for both sections is 60 feet. The widths of paving for the

primary residential section is 36 feet wide and the section includes

the use of curb and gutter. By using the rural residential road section

in place of the primary residential section, the width of paving can be

Figure 2-9. Length of

pavement

(imperviousness

associated with various

road layout options)

(Adapted from ULI,

1980)
2-12 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

Figure 2-10. Typical road

sections (Prince George’s

County, MD, 1997)

reduced from 36 to 24 feet, which represents a 33 percent reduction in

paved width. The rural section also eliminates the use of concrete curb

and gutter which reduces construction costs substantially and facilitates

the use of vegetated roadside swales.

Reduced Application of Sidewalks to One Side of Primary Roads.

Total site imperviousness can also be reduced by limiting sidewalks to

one side of primary roads. In some cases, sidewalks or pedestrian paths

can be eliminated on all other roads.

Reduced On-Street Parking. Reducing on-street parking requirements to one side, or even elimination of on-street parking altogether,

has the potential to reduce road surfaces and therefore overall site

imperviousness by 25 to 30 percent (Sykes, 1989). Two-sided parking

requirements are often unnecessary to provide adequate parking
LID Site Planning 2-13

Low-Impact Development: An Integrated Environmental Design Approach

facilities for each lot. For example, Sykes (1989) noted that allowing

parking on both sides of the street provides space for 4.5 to 6.5 cars

per residence.

Rooftops. Rooftops contribute to site imperviousness, and the

number of lots per acre (or lot coverage) generally determines the

sites rooftop impervious area. House type, shape, and size can affect

rooftop imperviousness. For example, more rooftop coverage is

generally required for ranch-type homes that spread out square footage

over one level. With this in mind, vertical construction is favored

over horizontal layouts to reduce the square footage of rooftops.

Driveways. Driveways are another element of the site plan that

can be planned to reduce the total site imperviousness. Some techniques that can be used include

 Using shared driveways whenever possible, but especially in

sensitive areas. This may require a subdivision waiver.

• Limiting driveway width to 9 feet (for both single and shared

driveways).

• Minimizing building setbacks to reduce driveway length.

• Using driveway and parking area materials which reduce runoff

and increase travel times such as pervious pavers or gravel.

Develop Integrated Preliminary Site Plan

After the development envelope has been delineated and the

total site imperviousness has been minimized, an integrated prelimiFigure 2-11. Integrated

site plan. Low-impact,

environmentally sensitive

development incorporates

a combination of all

natural resources

protection options into a

comprehensive, integrated

site design.
2-14 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

integrated site plan will provide a base for conducting the hydrologic

analysis to compare the pre- and postdevelopment site hydrology, and

to confirm that the overall objective of creating a hydrologically

functional site is being met. The procedures for conducting this

analysis and fine tuning the preliminary plan to arrive at a final plan

are described below. These procedures are aimed at disconnecting the

unavoidable impervious areas, as well as using techniques to modify

the drainage flow paths so that the postdevelopment time of concentration of stormwater runoff can be maintained as close as possible to

the predevelopment conditions.

Minimize Directly Connected Impervious Areas

After the total site imperviousness has been minimized and a

preliminary site plan has been developed, additional environmental

benefits can be achieved and hydrologic impacts reduced by disconnecting the unavoidable impervious areas as much as possible. Strategies for accomplishing this include

• Disconnecting roof drains and directing flows to vegetated areas.

• Directing flows from paved areas such as driveways to stabilized

vegetated areas.

• Breaking up flow directions from large paved surfaces.

• Encouraging sheet flow through vegetated areas.

• Carefully locating impervious areas so that they drain to natural

systems, vegetated buffers, natural resource areas, or infiltratable

zones/soils.

Modify/Increase Drainage Flow Paths

The time of concentration (Tc), in conjunction with the hydrologic site conditions, determines the peak discharge rate for a storm

event. Site and infrastructure components that affect the time of

concentration include

• Travel distance (flow path)

• Slope of the ground surface and/or water surface

• Surface roughness

• Channel shape, pattern, and material components

Techniques that can affect and control the Tc can be incorporated

into the LID concept by managing flow and conveyance systems

within the development site:

• Maximize overland sheet flow.

Level spreader

A stormwater outlet

designed to convert

concentrated runoff

to sheet flow

Sheet flow

Slow, shallow

stormwater runoff

over the land

surface

Open swale

Earthen channels

covered with a

dense growth of

hardy grass
LID Site Planning 2-15

Low-Impact Development: An Integrated Environmental Design Approach

• Increase and lengthen flow paths.

• Lengthen and flatten site and lot slopes.

• Maximize use of open swale systems.

• Increase and augment site and lot vegetation.

Overland Sheet Flow. The site should be graded to maximize the

overland sheet flow distance and to minimize disturbance of woodland

along the post-development Tc flow path. This practice will increase

travel times of the runoff and thus the time of concentration. Consequently, the peak discharge rate will be decreased. Flow velocity in areas

that are graded to natural drainage patterns should be kept as low as

possible to avoid soil erosion. Velocities in the range of 2 to 5 feet per

second are generally recommend. Table 2-4 provides recommended

velocities for various combinations of slopes, soils and vegetative cover

(SCS, 1983). Flows can be slowed by installing a level spreader along the

upland ledge of the natural drainage way buffer, or creating a flat grassy

area about 30 feet wide on the upland side of the buffer where runoff

can spread out. This grassy area can be incorporated into the buffer

itself. It may be unnecessary to set aside additional land to create this

area.

Table 2-4 Permissible Velocities for Vegetated Channels

Recommended

Permissible Velocity

Erosion Easily

Slope Range Resistant Soils Eroded Soils

No. Cover (percent) K< .3 fps K> .3 fps

1. Bermudagrass, Midland 0-5 6.0 5.0

and Coastal, Tufcote 5-10 5.0 4.0

over 10 4.0 3.0

2. Kentucky 31 Tall Fescue, 0-5 5.0 4.0

Kentucky Bluegrass 5-10 4.0 3.0

over 10 3.0 2.0

3. Grass-legume mixture 0-53 4.0 3.0

5-10 3.0 2.0

4. Red Fesuce, Redtop, 0-54 3.5 2.5

Lespedeza, sericea, Alfalfa

5. Annuals5, Common Lespedeza 0-55 3.0 2.0

Sundangrass, Small grain, Ryegrass

1 Common bermudagrass is a restricted noxious weed in Maryland.

2 Soil erodibility factor (K), < = less than, > = more than.

3 Do not use on slopes teepter than 10 percent, except for vegetated side slopes in combination with stone or

concrete or highly resistant vegetative center sections.

4 Do not use on slopes steeper than 5 percent except for side slopes in a combination channel as in 3 above.

5 Annuals are used on mild slopes or as temporary protection until permanent covers are established. Use on

slopes steeper than 5 percent is not recommended.

6 Good, dense vegatative cover is assumed.

Source: Maryland Standards and Specifications for Soil Erosion and Sediment Control (SCS), 1983.
2-16 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

Flow Path. Increasing flow path of surface runoff increases

infiltration and travel time. One of the goals of a LID site is to provide

as much overland or sheet flow as allowed by local jurisdictional codes

to increase the time it takes for rooftop and driveway runoff to reach

open swale drainage systems. To accomplish this, the designer can

direct rooftop and driveway runoff into bioretention facilities, infiltration trenches, dry wells, or cisterns that are strategically located to

capture the runoff prior to its reaching the lawn. In addition, strategic

lot grading can be designed to increase both the surface roughness and

the travel length of the surface runoff.

Site and Lot Slopes. Constructing roads across steep sloped

areas unnecessarily increases soil disturbance to a site. Good road

layouts avoid placing roads on steep slopes, by designing roads to

follow grades and run along ridge lines (see Figure 2-12). Steep site

slopes often require increased cut and fill if roads are sited using

conventional local road layout regulations. If incorporated into the

initial subdivision layout process, slope can be an asset to the development. The adjacent table provides suggestions on how to incorporate slope into lot layout and road design to minimize grading and

natural drainage way impacts.

Alternative road layout options use road plans that designate

length of cul-de-sacs and the number of branches of side streets off

collector streets based on the existing ridge lines and drainage patterns of a site:

 For areas with rolling terrain with dissected ridges use multiple

short branch cul-de-sacs off collector streets.

 For flat terrain use fluid grid patterns. Interrupt grid to avoid

natural drainage ways and other natural resources protection

areas.

Figure 2-12. Roads

placed along ridge

lines preserve and

utilize the natural

drainage system

(adapted from Sykes,

1989)
LID Site Planning 2-17

Low-Impact Development: An Integrated Environmental Design Approach

Slope of the

site

Site and Road Layout options

0 to 4 % Use with flat lots and streets parallel to the

contours. Use with rambler housing units.

4 to 8 % Use with sloped lots and streets parallel to

the contours. Use split-entry or walkout

housing units.

Use with streets perpendicular to the

contours with side-to-side split-level type

housing units.

8 to 11 % Use with sloped lots and streets

perpendicular to the contours.

Use with side-to-side split-level type

housing units.

> 11 % These areas are not easily used for

residential lots.

Adapted from Sykes, 1989.

Table 2-5. Alternative Road Layouts

Figure 2-13 illustrates low-impact development site grading

techniques for a site with low relief. Lot slopes are flattened to approach a minimum grade of 1 percent to increase infiltration and

travel time. For residential developments, low-impact development

practices should be applied to lot areas outside the building pad area as

shown. The building pad area is a 10 foot perimeter around the

building with a positive drainage slope of 4 percent. The designer is

responsible for ensuring that the slope of the lot does not cause

flooding during a 100-year event (i.e, 1-foot vertical and 25 foot

horizontal distance must by provided between the 100 year overflow

path and the dwelling unit). Soil compaction in the lot area should by

avoided to maximize the infiltration capacity of the soil. These infiltration areas can be hydraulically connected to impervious surfaces such

as rooftops and driveways to decrease travel times for these areas.

Open Swales. Wherever possible, LID designs should use multifunctional open drainage systems in lieu of more conventional storm

drain systems. To alleviate flooding problems and reduce the need for

conventional storm drain systems, vegetated or grassed open drainage

systems should be provided as the primary means of conveying surface

runoff between lots and along roadways (Figure 2-14). Lots should be

graded to minimize the quantity and velocity of surface runoff within

the open drainage systems. Infiltration controls and terraces can be
2-18 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

Figure 2-13. Lowimpact development

minimum lot

grading and 100yr

buffer requirements
LID Site Planning 2-19

Low-Impact Development: An Integrated Environmental Design Approach

used to reduce the quantity

and travel time of the surface

runoff as the need arises.

Site and Lot Vegetation.

Revegetating graded areas,

planting, or preserving existing

vegetation can reduce the peak

discharge rate by creating

added surface roughness as well

as providing for additional

retention, reducing the surface

water runoff volume, and

increasing the travel time

(Figure 2-15). Developers and engineers should connect vegetated

buffer areas with existing vegetation or forested areas to gain retention/detention credit for runoff volume and peak rated reduction.

This technique has the added benefit of providing habitat corridors

while enhancing community aesthetics.

Compare Pre- and Postdevelopment Hydrology

At this stage of the LID site planning process, most of the site

planning work is complete. Now the designer is ready to compare the

pre- and postdevelopment hydrology of the site, using the hydrologic

analysis procedures presented in Chapter 3. The hydrologic analysis

will quantify both the level of control that has been provided by the

site planning process and the additional level of control required

through the use of the integrated management practices (IMPs).

Complete LID Site Plan

Completion of the LID site plan usually involves a number of

iterative design steps. Based on the results of the hydrologic evaluation, additional stormwater control requirements of the LID site are

identified. These requirements will be met using IMPs distributed

throughout the site. A trial-and-error iterative process is then used

until all the stormwater management requirements are met. In the

event the site requirements cannot be met with IMPs alone, additional

stormwater controls can be provided using conventional stormwater

techniques (e.g., detention ponds). Mixed use of LID measures and

conventional control is referred to as a hybrid system.

Figure 2-14.

Vegetated swale
2-20 LID Site Planning

Low-Impact Development: An Integrated Environmental Design Approach

Once the predevelopment hydrology objectives have been met,

the designer can complete the site plan by incorporating the typical

details, plan views, cross sections, profiles, and notes as required.

References

Bay Area Stormwater Management Agencies Association

(BASMAA). 1997. Start at the Source: Residential Site Planning and

Design Guidance Manual for Stormwater Quality Protection. Prepared by

Tom Richman and Associates, Palo Alto, California, 94301.

Booth, D.B., and L.E. Reinelt. 1993. Consequences of Urbanization Aquatic Systems-Measured Effects, Degradation Thresholds, and

Corrective Strategies. In Proceeding of Watershed 93.

Figure 2-15. Site

layouts with/

without

vegetation

retention
LID Site Planning 2-21

Low-Impact Development: An Integrated Environmental Design Approach

Horner, R.R., J.J. Skupien, E.H. Livingston, and H.E. Shaver.

1994. Fundamentals of Urban Runoff Management: Technical and Institutional Issues. Terrene Institute, Washington, DC.

Klein, R.D. 1979. Urbanization and Stream Quality Impairment.

Water Res. Bull. 15(4): 948:903

Soil Conservation Service (SCS). 1983. Maryland Standards and

Specifications for Soil Erosion and Sediment Control. Soil Conservation

Service, U.S. Department of Agriculture, College Park, Maryland.

May, C.W., E.B. Welch, R.R. Horner, J.R. Karr, and B.W. Mar. 1997.

Quality Indices for Urbanization in Puget Sound Lowland Streams. Prepared

for Washington Department of Ecology, Seattle, Washington, by

Department of Civil Engineering, University of Washington, Seattle, WA.

Natural Lands Trust. June 1997. Growing GreenerA Conservation Planning Workbook for Municipal Officials in Pennsylvania. Pennsylvania Department of Conservation and Natural Resources.

Pennsylvania State University. 1997. PA Blueprints. Department

of Landscape Architecture, Pennsylvania State University.

Soil Conservation Service (SCS). 1983. Maryland Standards and

Specifications for Soil Erosion and Sediment Control. Soil Conservation

Service, U.S. Department of Agriculture, College Park, Maryland.

Steedman, R.J. 1988. Modification and Assessment of an Index of

Biotic Integrity to Quantify Stream Quality in Southern Ontario. Can.

J. Fish. Aquat. Sci. 45:492-501.

Sykes, R.D. 1989. Chapter 3.1- Site Planning. University of

Minnesota.

Urban Land Institute (ULI). 1980. Village Homes, Project Reference File. The Urban Land Institute, Vol. 10 (April-June):8.

U.S. Environmental Protection Agency (USEPA). 1997. Urbanization and Streams: Studies of Hydrologic Impacts. Office of Water,

Washington, DC, 20460, 841-R-97-009, December 1997.

U.S. Environmental Protection Agency (USEPA). 1993. Guidance

Specifying Management Measures for Sources of Nonpoint Pollution in

Coastal Waters. U.S. Environmental Protection Agency, Office of

Water, Washington, DC.
Chapter 3

Low-Impact Development Hydrologic Analysis

Chapter 3

Low-Impact Development Hydrologic Analysis
Low-Impact Development Hydrologic Analysis 3-1

Low-Impact Development: An Integrated Environmental Design Approach

Low-Impact Development Hydrologic

Analysis 3

Introduction

Preserving or restoring the hydrologic functions of watersheds is a

fundamental premise of the LID approach. Consideration of hydrologic principles in all phases of site development is necessary to

maximize the effectiveness of planning and site design. Replication of

the natural or predevelopment site hydrology not only reduces downstream stormwater impacts, but also helps control or reduce localized

small-scale impacts.

The preservation of the predevelopment hydrologic regime of the

site can be evaluated through consideration of the runoff volume, peak

runoff rates, storm frequency and size, and water quality management.

LID controls the full range of storm events, including those storm

events smaller than the design storm.

This chapter reviews the basic hydrologic principles, LID hydrologic analysis concepts, methods

for hydrologic evaluations, and

compares conventional and LID

approaches in terms of their

effectiveness in controlling site

hydrology.

Regional Considerations

The United States is composed of a wide range of climatic,

geologic, and physiographic

conditions, which result in

regional provinces with widely

varying combinations of these

factors. Climate varies from arid

In This Chapter

Introduction

Overview of Key

Hydrologic Principles

Summary of Comparison

Between Conventional and

LID Approaches

LID Hydrologic

Considerations

LID Modification Tools

LID Hydrologic Evaluation

Chapter
3-2 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

regions with annual rainfall of 4 to 10 inches all the way to regions of

rainforest with annual precipitation of 100 inches. Geology includes

sedimentary coastal deposits through regions of piedmont, valley, and

ridge provinces to mountain terrain. Elevation ranges from sea level

and very low relief along the coastal areas (which include the largest

concentration of major cities and population), to areas of moderate

elevation and relief, such as the piedmont regions, to areas of very high

elevation, such as Denver and other areas in the Rocky Mountain

region.

It has been documented by EPAs Nationwide Urban Runoff

Program (USEPA, 1983) that although various regions of the country

display a wide range of the factors described above, they do have some

things in common. Any region of the country that is subject to urban

development will experience the range of hydrologic impacts previously described. The major difference between regions is likely to be

the relative importance or priority ranking for any one issue. A few

examples of these regional differences are described below.

A number of the rapidly developing areas of Florida, which are

heavily reliant on groundwater supplies, are experiencing a serious

lowering of the regional water table. This condition is due to a combination of increasing withdrawals and the loss of natural ground water

recharge as the naturally occurring permeable soils are converted to

impervious areas. This lowering of the water table together with the

associated increase in pollutants from urban runoff may be considered

the highest urban runoff priorities for these areas.

The rapidly developing areas of the Puget Sound lowlands are

experiencing a rapid degradation of the physical integrity of the

receiving streams in the areas that are developed (May, 1997). This

degradation and the associated loss of habitat that traditionally has

served as spawning grounds for a broad range of salmonids native to

this area are causing great concern in this region. Consequently, the

stream channel degradation associated with urban runoff may be

considered the highest urban runoff priority in this area.

The solution to these two examples, and to most urban runoff

control problems, is to try to mimic or maintain the predevelopment

site hydrology. This is precisely the objective of low-impact development.

Overview of Key Hydrologic Principles

Hydrology is the study of water and its movement through the

hydrologic cycle. Understanding how hydrologic components respond
Low-Impact Development Hydrologic Analysis 3-3

Low-Impact Development: An Integrated Environmental Design Approach

to land use changes and site development practices is the basis for

developing successful watershed and stormwater management programs. One way of interpreting the hydrologic response of a system is

through examination of a runoff hydrograph. A selection of typical

runoff hydrographs under various land use conditions is shown in

Figure 3-1.

 Hydrograph 1 represents the response to a given storm of a site in

a predevelopment condition (i.e., woods, meadow). A gradual rise

and fall of the peak discharge and volume define the hydrograph.

 Hydrograph 2 represents the response of a postdevelopment

condition with no stormwater management BMPs. This

hydrograph definition reflects a shorter time of concentration

(Tc), and an increase in total site imperviousness from the

predevelopment condition. The resultant hydrograph shows a

decrease in the time to reach the peak runoff rate, a significant

increase in the peak runoff and discharge rate and volume, and

increased duration of the discharge volume.

 Hydrograph 3 represents a postdevelopment condition with conventional stormwater BMPs, such as a detention pond. Although the

peak runoff rate is maintained at the predevelopment level, the

hydrograph exhibits significant increases in the runoff volume and

duration of runoff from the predevelopment condition, which is

depicted by the shaded hydrograph area in Figure 3-1.

Key elements of the hydrologic cycle and their relationship to

low-impact development technology are described below.

Figure 3-1.

Hydrologic response

of conventional

BMPs
3-4 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

Precipitation and Design Storm Events. Data for precipitation,

including both snow and rain, are used in site planning and

stormwater design. Precipitation occurs as a series of events characterized by different rainfall amount, intensity, and duration. Although

these events occur randomly, analysis of their distribution over a long

period of time indicates that the frequency of occurrence of a given

storm event follows a statistical pattern. This statistical analysis

allows engineers and urban planners to further characterize storm

events based on their frequency of occurrence or return period. Storm

events of specific sizes can be identified to support evaluation of

designs. Storms with 2- and 10-year return periods are commonly used

for subdivision, industrial, and commercial development design.

The 1- and 2-year storm events are usually selected to protect

receiving channels from sedimentation and erosion. The 5- and

10-year storm events are selected to provide adequate flow conveyance

design and minor flooding considerations. The 100-year event is used

to define the limits of floodplains and for consideration of the impacts

of major floods. Figure 3-2 provides a summary of the relationship of

the rainfall event recurrence interval and rainfall volume, and its

application to stormwater management in the state of Maryland.

There are numerous excellent texts and handbooks that describe the

use of rainfall data to generate a design storm for the design of drainage

systems (e.g., ASCE , 1994; Chow, 1964; SCS, 1972). For LID, a unique

approach has been developed to determine the design storm based on the

basic philosophy of LID ( Prince Georges County, MD, 1997).

Design storm

A specific size storm

event used to plan

for and design

stormwater controls.

Figure 3-2. Relationship of

the rainfall event recurrence

interval and rainfall

volume, and its application

to stormwater management

in Maryland (Source: CRC,

1996)
Low-Impact Development Hydrologic Analysis 3-5

Low-Impact Development: An Integrated Environmental Design Approach

Storm events commonly used for evaluation of designs differ for

the various climatic regions of the United States Summaries of typical

storm event characteristics (i.e., amount/intensity, duration, and

return period) are provided in national maps in Technical Paper 40

(Department of Commerce, 1963). In humid regions such as the

Mid-Atlantic states, the 2-year storm is approximately 3 inches of

rainfall and the 10-year storm is approximately 5 inches of rainfall.

The 2-year storm has a 50 percent probability of occurring in any

given year, while the 10-year storm has a 10 percent probability of

occurring in any given year. In dry areas, such as portions of Colorado

and New Mexico, the 2-year storm is approximately 1.5 inches of

rainfall and the 5-year storm is approximately 2.0 inches of rainfall.

 The required storage volume for peak runoff control is heavily

depended on the intensity of rainfall (rainfall distribution). Since the

intensity of rainfall varies considerably over geographic regions in the

nation, National Resource Conservation Service (NRCS) developed

four synthetic 24-hour rainfall distributions (I, IA, II, and III) from

available National Weather Service (NWS) duration-frequency data

and local storm data. Type IA is the least intense and type II the most

intense short-duration rainfall. Figure 3-3. shows approximate geographic boundaries for these four distributions.

Rainfall

abstraction

The physical process

of interception

evaporation,

transpiration,

infiltration, and

storage of

precipitation.

Represented as a

depth (inches) of

water over a site.

Figure 3-3. Approximate

geographic boundaries for

NRCS rainfall

distributions
3-6 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

Rainfall Abstractions. Rainfall abstractions include the physical

processes of interception of rainfall by vegetation, evaporation from

land surfaces and the upper soil layers, transpiration by plants, infiltration of water into soil surfaces, and storage of water in surface depressions. Although these processes can be evaluated individually, simplified hydrologic modeling procedures typically consider the combined

effect of the various components of rainfall abstraction.

The rainfall abstraction can be estimated as a depth of water

(inches) over the total area of the site. This depth effectively represents the portion of rainfall that does not contribute to surface runoff.

The portion of rainfall that is not abstracted by interception, infiltration, or depression storage is termed the excess rainfall or runoff.

The rainfall abstraction may change depending on the configuration of the site development plan. Of particular concern is the change

in impervious cover. Impervious areas prevent infiltration of water

into soil surfaces, effectively decreasing the rainfall abstraction and

increasing the resulting runoff. Postdevelopment conditions, characterized by higher imperviousness, significantly decrease the overall

rainfall abstraction, resulting not only in higher excess surface runoff

volume but also a rapid accumulation of rainwater on land surfaces.

The LID approach attempts to match the predevelopment condition by compensating for losses of rainfall abstraction through maintenance of infiltration potential, evapotranspiration, and surface storage, as well as increased travel time to reduce rapid concentration of

excess runoff. Several planning considerations combined with supplemental controls using LID integrated management practices (IMPs)

can be used to compensate for rainfall abstraction losses and changes

in runoff concentration due to site development. These practices are

described in Chapters 2 and 4 of this document.

Runoff. The excess rainfall, or the portion of rainfall that is not

abstracted by interception, infiltration, or depression storage, becomes

surface runoff. Under natural and undeveloped conditions, surface

runoff can range from 10 to 30 percent of the total annual precipitation (Figure 3-4). Depending on the level of development and the site

planning methods used, the alteration of physical conditions can result

in a significant increase of surface runoff to over 50 percent of the

overall precipitation. In addition, enhancement of the site drainage to

eliminate potential on-site flooding can also result in increases in

surface runoff. Alteration in site runoff characteristics can cause an

increase in the volume and frequency of runoff flows (discharge) and

velocities that cause flooding, accelerated erosion, and reduced

Runoff

The portion of

rainfall that is not

abstracted by

interception,

infiltration, or

depression storage.
Low-Impact Development Hydrologic Analysis 3-7

Low-Impact Development: An Integrated Environmental Design Approach

groundwater recharge and contribute to degradation of water quality

and the ecological integrity of streams.

Time of Concentration. Time of concentration (Tc) is an idealized

concept (Maidment, 1993) reflecting the response of a watershed to a

given storm event. The Tc has been defined as the time it takes water

from the most distant point (hydraulically) to reach the watershed

outlet (NEH-4, SCS, 1985). Although Tc varies, it is often used as a

constant. As the site imperviousness increases and the drainage

pathways are altered, the contribution of land areas to excess rainfall

water is likely to increase and the time to reach the downstream

outlets is shortened. Traditional stormwater management approaches

directed toward developing efficient drainage systems favor rapid

concentration of excess water and routing it off-site through a drainage system of curbs and gutters, inlet structures, and storm drain pipes.

Low-impact development relies on site planning tools and site-level

management techniques to maintain the predevelopment time of

concentration.

Figure 3-4. Runoff

variability with

increased

impervious surfaces

(FISRWG, 1998)

Time of

concentration

(Tc)

The time it takes for

surface runoff to

travel from the

farthest point of the

watershed to the

outlet.
3-8 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

Groundwater Recharge. A considerable percentage of the rainfall

abstraction infiltrates into the soil and contributes to groundwater

recharge. Groundwater may be part of a local, intermediate, or

regional water table, as illustrated in Figure 3-5. The local water table

is often connected to nearby streams, providing seepage to streams

during dry periods and maintaining base flow essential to the biological and habitat integrity of streams. A significant reduction or loss of

groundwater recharge can lead to a lowering of the water table and a

reduction of base flow in receiving streams during extended dry

weather periods. Headwater streams, with small contributing drainage

areas, are especially sensitive to localized changes in groundwater

recharge and base flow.

Summary of Comparison Between Conventional and

LID Stormwater Management Approaches

Stormwater management efforts that follow the historical design

storm approach focus on two elements:

1. Site Drainage. In conventional stormwater management design,

site drainage was accomplished by designing a very efficient site

drainage system. Curbs, gutters, and pipes are used and carefully

designed to quickly and efficiently drain any excess rainwater off

the site. This approach, although it provides excellent on-site

drainage, greatly alters the natural hydrologic regime of the site

and provides a higher pollutant transport capacity. In addition,

this approach does not address on-site water quality controls and

does not consider any of the LID site planning concepts.

2. Off-Site Flood Control. The total alteration of the natural site

hydrologic regime due to an efficient on-site drainage system

results in a significant increase in off-site flooding potential, as

Groundwater

recharge

The amount of

precipitation that

infiltrates into the

soil and contributes

to groundwater.

Figure 3-5.

Groundwater in local,

intermediate, or regional

setting
Low-Impact Development Hydrologic Analysis 3-9

Low-Impact Development: An Integrated Environmental Design Approach

Hydrologic Parameter Conventional LID

Onsite

Impervious Cover Encouraged to achieve effective drainage Minimized to reduce impacts

Vegetation/Natural

Cover

Reduced to improve efficient site drainage Maximized to maintain predevelopment

hydrology

Time of Concentration Shortened, reduced as a by-product of

drainage efficiency

Maximized and increased to

approximate predevelopment

conditions

Runoff Volume Large increases in runoff volume not

controlled

Controlled to predevelopment

conditions

Peak Discharge Controlled to predevelopment design storm

(2 year)

Controlled to predevelopment

conditions for all storms

Runoff frequency Greatly increased, especially for Small,

frequent storms

Controlled to predevelopment

conditions for all storms

Runoff duration Increased for all storms, because volume is

not controlled

Controlled to predevelopment

conditions

Rainfall Abstractions

(Interception, Infiltration,

Depression Storage)

Large reduction in all elements Maintained to predevelopment

conditions

Groundwater Recharge Reduction in recharge Maintained to predevelopment

conditions

Offsite

Water Quality Reduction in pollutant loadings but limited

control for storm events that are less than

design discharge

Improved pollutant loading reductions,

Full control for storm events that are less

than design discharge

Receiving Streams Severe impacts documentedChannel erosion and degradation

Sediment deposition

Reduced base flow

Habitat suitability decreased, or eliminated

Stream ecology maintained to

predevelopment

Downstream Flooding Peak discharge control reduces flooding

immediately below control structure, but

can increase flooding downstream through

cumulative impacts and superpositioning of

hydrographs

Controlled to predevelopment

conditions

well as high downstream environmental impacts associated with

increased peak flows and their frequency of occurrence, higher

storm flow volumes, and increased delivery of pollutant loads

(EPA, 1997). The traditional approach relies on designing treatment facilities targeted mainly to control peak flows for a given

storm size (i.e., 10-year storm). These facilities typically consist of

large stormwater ponds, strategically placed at the low point of the

site. Since environmental concerns are becoming an integral

component of stormwater management, it is assumed that such

facilities are providing some controls. Since these facilities are

designed for peak flow control and do not control those storm

events smaller than the design storm, this approach is often

referred to as the end of pipe control approach.

Table 3-1 summarizes how conventional stormwater management

and LID technology alter the hydrologic regime for on-site and off-site

conditions.

Table 3-1. Comparison of Conventional and LID Stormwater Management Technologies
3-10 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

LID Hydrologic Considerations

In a LID system the fundamental hydrologic processes are considered throughout the site planning process. An understanding of the

dynamics and interrelationships in the hydrologic cycle is used as a

guide to preserving the predevelopment hydrology.

The preservation of the predevelopment hydrology is evaluated by

comparison of pre- and postdevelopment conditions. The comparison

is facilitated by consideration of four fundamental measures-runoff

volume control, peak runoff rate control, flow frequency/duration

control, and water quality control. These four evaluation measures

are discussed further below.

Runoff Volume Control. As the imperviousness of the site is

increased, the runoff volume for a given storm increases. The ratio of

the corresponding runoff volume (in inches) to the total rainfall event

(in inches) is called the runoff coefficient. The typical site runoff

coefficient can be maintained at the predevelopment level by compensating for the loss of abstraction (interception, infiltration, depression

storage) through both site planning and design considerations.

Peak Runoff Rate Control. Low-impact development is designed

to maintain the predevelopment peak runoff discharge for all the

storms smaller than the selected design storm events. Use of site

planning tools (see Chapter 2) and preferred management practices

(Chapter 4) may control the peak runoff rate as well as the runoff

volume. If additional controls are required to reach the

predevelopment peak runoff rate, additional IMPs and supplemental

management techniques might be needed.

Flow Frequency/Duration Control. Since low-impact development is designed to emulate the predevelopment hydrologic regime

through both volume and peak runoff rate controls, the flow frequency

and duration for the postdevelopment conditions should be almost

identical to those for the predevelopment conditions (see Figure 3-6).

The potential impacts on the sediment and erosion and stream habitat

quality at downstream reaches can then be minimized.

Water Quality Control. Low-impact development is designed to

provide water quality treatment control for at least the first half-inch

of runoff from impervious areas using retention practices. In most LID

applications, the use of distributed control and retention throughout

the site will result in much higher levels of water quality treatment

control for a number of reasons. First the runoff volume controlled will

usually exceed the first half-inch of runoff, and frequently exceed two

inches of runoff volume, thereby treating a much greater volume of

LID hydrologic

considerations

Runoff volume

control

Peak runoff rate

control

Flow frequency/

duration control

Water quality

control
Low-Impact Development Hydrologic Analysis 3-11

Low-Impact Development: An Integrated Environmental Design Approach

LID hydrologic

modification

tools

Reduce/minimize

imperviousness

Disconnect

unavoidable

impervious surfaces

Preserve and protect

environmentally

sensitive site

features

Maintain time of

concentration (Tc)

Mitigate for

impervious surfaces

with PMPs

annual runoff. Also, this greater volume of runoff control will usually

be associated with decreases in both the time of concentration and

flow velocities which results in a reduction in the pollutant transport

capacity and overall pollutant loading. Low-impact development also

supports pollution prevention practices by modifying human activities

to reduce the introduction of pollutants into the environment.

LID Hydrologic Tools

To achieve the goal of preserving the predevelopment hydrologic

regime, a variety of LID site planning tools can be employed. The

following tools are used in a variety of combinations in LID design:

 Reduce/minimize imperviousness. Change in postdevelopment

hydrology can be minimized by reducing impervious areas and

preserving more trees and meadows to reduce the storage requirements to maintain the predevelopment runoff volume.

 Disconnect unavoidable impervious surfaces. Additional environmental benefits can be achieved and the hydrologic impacts

reduced by disconnecting unavoidable impervious surfaces as

much as possible.

 Preserve and protect environmentally sensitive site features. Site

features to be protected and preserved can include riparian areas,

floodplains, stream buffers, and wetlands; woodlands, conservation

zones, and valuable trees; steep slopes; and highly permeable and

erosive soils.

 Maintain time of concentration (Tc). Maintaining the

predevelopment Tc minimizes the increase of the peak runoff rate

Figure 3-6.

Comparison of the

hydrologic response of

conventional BMPs and

LID IMPs
3-12 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

after development by lengthening flow paths and reducing the

length of the runoff conveyance systems.

 Mitigate for impervious surfaces with IMPs. IMPs can provide

retention storage for volume and peak control, as well as water

quality control, to maintain the same natural initial abstraction

volume as the predevelopment condition.

 Locate the impervious areas on less pervious soil types.

 LID Hydrologic Evaluation

The purpose of the hydrologic evaluation is to determine the level

of control required to achieve the stormwater management goals for

LID sites. The required level of control may be achieved through

application of the various hydrologic tools during the site planning

process, the use of IMPs, and supplemental controls. The hydrologic

evaluation is performed using hydrologic modeling and analysis

techniques. The output of the hydrologic analysis provides the basis

for comparison with the four evaluation measures (i.e., runoff volume,

peak runoff, frequency, and water quality control).

LID Hydrologic Evaluation Steps

The hydrologic evaluation can be performed using various approaches and analytical techniques. Typically hydrologic evaluation

follows a series of steps resulting in defining the needs for hydrologic

control and management.

Step 1. Delineate the watershed and microwatershed areas.

Hydrologic evaluation requires delineation of the drainage area for the

overall study area or site and the subwatersheds contributing to key

portions of the site. Delineation may need to consider previously

modified drainage patterns, roads, or stormwater conveyance systems.

Step 2. Determine design storm(s). The design storms considered

in the analysis should be determined based on the basic LID philosophy identified (see Section A.6 on page A.21). Regulatory requirements for design storms may also be stipulated in local ordinances, and

these may limit or constrain the use of LID techniques or necessitate

that structural controls be employed in conjunction with LID techniques.

Step 3. Define modeling technique(s) to be employed. Data

gathering and analysis will depend on the specific type of model

selected. The model selected will depend on the type of watershed,

complexity of the site planning considerations, familiarity of the

LID hydrologic

evaluation steps

1. Delineate the

watershed and

microwatershed

areas

2. Define design

storms

3. Define modeling

techniques to be

employed

4. Compile

information for

predevelopment

conditions

5. Evaluate

predevelopment

conditions and

develop baseline

measures

6. Evaluate site

planning benefits

and compare with

baseline

7. Evaluate

integrated

management

practices (IMPs)

8. Evaluate

supplemental

needs
Low-Impact Development Hydrologic Analysis 3-13

Low-Impact Development: An Integrated Environmental Design Approach

agency with the model, and level of detail desired. Certain models use

simplified estimation methods whereas others provide detailed

process-based representation of hydrologic interactions.

Step 4. Compile information for predevelopment conditions.

Typical information needed includes area, soils, slopes, land use, and

imperviousness (connected and disconnected).

Step 5. Evaluate predevelopment conditions and develop baseline

measures. The selected modeling techniques are applied to the

predevelopment conditions. The results of the modeling analysis are

used to develop the baseline conditions using the four evaluation

measures.

Step 6. Evaluate site planning benefits and compare with baseline.

The site planning tools provide the first level of mitigation of the

hydrologic impacts. The modeling analysis is used to evaluate the

cumulative hydrologic benefit of the site planning process in terms of

the four evaluation measures. The comparison is used to identify the

remaining hydrologic control needs.

Step 7. Evaluate Integrated Management Practices (IMPs). The

hydrologic control needs may be addressed through the use of IMPs

(described in Chapter 4). This represents the second level of mitigation of the hydrologic impacts. After IMPs are identified for the site, a

second-level hydrologic evaluation that combines the controls provided by the planning techniques with the IMPs can be conducted.

Results of this hydrologic evaluation are compared with the

predevelopment conditions to verify that the discharge volume and

peak discharge objectives have been achieved. If not, additional IMPs

are located on the site to achieve the optimal condition.

Step 8. Evaluate supplemental needs. If after use of IMPs supplemental control for either volume or peak flow is still needed, selection

and listing of additional management techniques should be considered.

For example, where flood control or flooding problems are a key design

objective, or where site conditions, such as poor soils, or high water

table limits the use of IMPs, additional conventional end-of-pipe

methods, such as large detention ponds or constructed wetlands,

should be considered. In some cases these controls can be sized much

smaller than normal due to use of LID as part of the management

system. The hydrologic evaluation is used to compare the supplemental

management techniques and identify the preferred solutions.

The hydrologic evaluation steps are performed using an iterative

process. Numerous site planning and management configurations may
3-14 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

need to be evaluated to identify the optimum solutions. The concept of

low-impact development is to emphasize the simple and cost-effective

solutions. Use of hydrologic evaluations can assist in identifying these

solutions prior to detailed design and construction costs.

Prince Georges County, Maryland, has developed a detailed

illustration of an approach for conducting a hydrologic evaluation

based on the use of the SCS TR-55 method. A summary flow chart of

the hydrologic evaluation process is shown in Figure 3-7. A full

description of the application process is provided in Appendix A

(Prince Georges County, 1997).

Hydrologic Evaluation Techniques

A variety of models are available to simulate the rainfall-runoff

processes for watersheds. The selection of the appropriate modeling

technique will depend on the level of detail and rigor required for the

application and the amount of data available for setup and testing of

the model results. Four types of simulation models are briefly summarized below.

Hydrologic Simulation Program - FORTRAN (HSPF). The

HSPF model is a comprehensive package developed and maintained by

the U.S. Environmental Protection Agency for simulation of water

quantity and quality from mixed land use watersheds. The model uses

continuous simulation of rainfall-runoff processes to generate

hydrographs, runoff flow rates, sediment yield, and pollutant washoff

and transport. HSPF includes consideration of infiltration, subsurface

water balance, interflow, and base flow.

Storm Water Management Model (SWMM). SWMM is an urban

stormwater model developed and maintained by the U.S. Environmental Protection Agency. SWMM is applied to stormwater simulations

including urban runoff, flood routing, and flooding analysis. The

model provides continuous simulation, using variable timesteps, of

rainfall-runoff processes and associated pollutant washoff and transport. SWMM also includes flow routing capabilities for open channels

and piped systems.

HEC-1. The HEC-1 model was developed by the U.S. Army

Corps of Engineers Hydrologic Engineering Center (HEC). HEC-1 is

designed to simulate the surface runoff response of a river basin to

precipitation by representing the basin as an interconnected system of

hydrologic and hydraulic components. Each component provides

simulation of a rainfall-runoff process. The result of the modeling

process is the computation of streamflow hydrographs at desired

Hydrologic

evaluation

techniques

HSPF

SWMM

HEC-1

TR-55/TR-20

The rational

method
Low-Impact Development Hydrologic Analysis 3-15

Low-Impact Development: An Integrated Environmental Design Approach

Figure 3-7. Prince George’s County, Maryland, example of low-impact development analysis

procedure (Prince George’s County, 1997)
3-16 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

locations in the river basin. The depth-area option computes flood

hydrographs while preserving a user-supplied precipitation depth

versus area relation throughout the stream network.

TR-55/TR-20. The U.S. Department of Agriculture, Natural

Resources Conservation Service (NRCS), developed the TR-55/TR-20

model. TR-55 uses the runoff curve number method and unit

hydrographs to convert rainfall into runoff. TR-55 and TR-20 are

infiltration loss models that use the runoff curve number methods and

synthetic storm flow hydrograph development to predict peak volume

and flow rates for a given catchment area. The advantage of applying

TR-55 and TR-20 is the convenience of tables and input parameters

included for a wide range of soil and land use conditions. Also TR-55

and TR-20 models are widely used by field-level professionals.

The Rational Method. The rational method is a storm sewer

evaluation method based on the rational formula (Maidment, 1993).

The rational formula calculates the peak flow rate as a function of the

rainfall intensity (for a specific design return period and time of

concentration), the watershed area, and the runoff coefficient. The

rational method is frequently used in land development applications

due to its simplicity and ease of application.

Table 3-2 provides an overview of the attributes and functions of

the selected models.

LID Hydrologic Illustrations

To illustrate the hydrologic analysis techniques employed by

low-impact development, two examples from the Prince Georges

County Design Manual are discussed below (Prince Georges County,

Model Attribute

HSPF SWMM TR-55/TR-20 HEC-1 Rational

Sponsoring agency USEPA USEPA NRCS (SCS) CORPS (HEC) Method

Simulation type Continuous Continuous Single event Single event Single event

Water quality analysis Yes Yes None None None

Rainfall/runoff analysis Yes Yes Yes Yes Yes

Sewer system flow routing None Yes Yes Yes None

Dynamic flow routing equations None Yes Yes None None

Regulators, overflow structures None Yes None None None

Storage analysis Yes Yes Yes Yes None

Treatment analysis Yes Yes None None None

Data and personnel requirements High High Medium Medium Low

Overall model complexity High High Low High Low

Table 3-2 Comparison of Model Attributes and Functions
Low-Impact Development Hydrologic Analysis 3-17

Low-Impact Development: An Integrated Environmental Design Approach

Figure 3-8.

Customizing runoff

CN for a low-impact

development site

1997). The examples highlight the use of the LID tools in achieving

the runoff volume and peak flow objectives. The first example describes the control of runoff volume and peak flow using a TR-55

application. The second example describes methods used to control

the time of concentration to manage the peak flow rate.

LID Runoff Volume and Peak Flow Management

Calculation of the LID runoff potential is based on a detailed

evaluation of the existing and proposed land cover so that an accurate

representation of the potential for runoff can be obtained. This

calculation requires the investigation of parameters associated with a

low-impact development, such as the following:

 Land cover type

 Percentage and connectivity of impervious areas

 Soils type and texture

 Antecedent soil moisture conditions

Determination of LID Runoff Curve Number

The process for performing a hydrologic evaluation for a LID site

is illustrated through the use of a TR-55 application example (SCS,

1986). As illustrated in Figure 3-8, customizing the curve number

(CN) for a LID site allows the developer/engineer to take advantage of

and get credit for a variety of LID site planning practices, which

include in this case:
3-18 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

 Narrower driveways and roads (minimizing impervious areas)

 Maximizing tree preservation or forestation (tree planting)

 Site fingerprinting (minimal disturbance)

 Open drainage swales

 Preservation of soils with high infiltration rates (locate impervious

areas on low infiltration soils)

 Location of IMPs on high-infiltration soils

Table 3-3 shows the resulting

low-impact development CN land

cover compared with those of a

conventional development CN, as

found in Table 2.2a of TR-55

(SCS, 1986) for the example

1-acre lot.

Table 3-4 shows how LID site planning can

affect components of the CN, resulting in lower CN and more infiltration.

Figure 3-9 shows how hydrologic response is altered using LID

example techniques to reduce the impervious areas and the associated

runoff peak volume. Hydrograph 1 is the predevelopment condition,

and hydrograph 2 is the postdevelopment condition without controls.

Hydrograph 5 represents the resulting postdevelopment hydrograph

Table 3-4. LID Planning Techniques to Reduce the Postdevelopment Runoff

Volume

Table 3-3 Comparison of Conventional and

LID Land Covers

Conventional Land Covers

(TR-55 assumptions) LID Land Covers

20% impervious

80% grass

15% impervious

25% woods

60% grass

Suggested Options

Affecting Curve Number

Limit use of sidewalksReduce road length andwidthReduce driveway lengthand widthConserve natural resourcesareasMinimize disturbancePreserve infiltratable soilsPreserve natural depressionareasUse transition zonesUse vegetated swalesPreserve Vegetation

Land Cover Type ✔ ✔ ✔ ✔ ✔

Percent of Imperviousness ✔ ✔ ✔ ✔

Hydrologic Soils Group ✔ ✔

Hydrologic Condition ✔ ✔ ✔

Disconnectivity of

Impervious Area ✔ ✔ ✔

Storage and Infiltration ✔ ✔
Low-Impact Development Hydrologic Analysis 3-19

Low-Impact Development: An Integrated Environmental Design Approach

with a significant reduction in both postdevelopment peak rate and

volume, which can be achieved by just using LID site planning techniques to reduce CN values and without the benefit of IMP.

Maintaining the Predevelopment Time of

Concentration

The management of runoff volume, peak flow, and frequency

requires that the postdevelopment time of concentration (Tc ) be

maintained close to the predevelopment Tc. The travel time (Tt )

throughout individual lots and areas should be approximately the

same so that the Tc is representative of the drainage. This is critical

because low-impact development theory is based on a relatively

homogeneous land cover and distributed IMPs. To maintain the Tc,

low-impact developments use the following site planning techniques:

 Maintaining predevelopment flow path length by dispersing and

redirecting flows, generally through open swales and natural

drainage patterns.

 Increasing surface roughness (e.g., reserving woodlands, using

vegetated swales).

 Detaining flows (e.g., open swales, rain gardens).

Figure 3-9. Effect of

low-impact

development CN on

the postdevelopment

hydrograph without

stormwater BMPs
3-20 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

 Minimizing disturbance (minimizing compaction and changes to

existing vegetation).

 Flattening grades in impacted areas.

 Disconnecting impervious areas (e.g., eliminating curb/gutter and

redirecting downspouts).

 Connecting pervious and vegetated areas (e.g., reforestation,

forestation, tree planting).

To maintain predevelopment Tc, an iterative process that analyzes

different combinations of the above appropriate techniques may be

required. These site planning techniques are incorporated into the

hydrologic analysis computations for postdevelopment Tc to demonstrate an increase in postdevelopment Tc above conventional techniques and a corresponding reduction in peak discharge rates.

Figure 3-10 illustrates

the hydrologic response to

maintaining equal

predevelopment and

postdevelopment TCs.

Hydrograph 1 is the

predevelopment condition. Hydrograph 5, as

previously described,

shows the benefits of

using LID techniques to

reduce impervious areas

and the associated runoff

peak volume.

Hydrograph 6 represents the effects of using LID techniques to

maintain the Tc. This effectively shifts the postpeak runoff time to

that of the predevelopment condition and lowers the peak runoff rate.

Maintaining the same Tc in a small watershed can be mainly

accomplished by maintaining or raising the Mannings roughness n

for the initial overland (sheet) flow at the top of the watershed and

increasing the flow path length to the most hydraulically distant point

in the drainage area. After the transition to shallow concentrated

flow, additional gains in Tc can be accomplished by:

 Decreasing the slope

 Increasing the flow length

 Directing the flow over pervious areas.

Figure 3-10. Low-impact

development hydrograph that

has a reduced CN and

maintains the Tc without

conventional stormwater

controls
Low-Impact Development Hydrologic Analysis 3-21

Low-Impact Development: An Integrated Environmental Design Approach

In LID sites, the volume of flow in closed channels (pipes) should

be minimized to the greatest extent possible. Swales and open channels should be designed with the following features:

 Increase surface roughness to retard velocity.

 Maximize sheet flow conditions.

 Use a network of wider and flatter channels to avoid fast-moving

channel flow.

 Increase channel flow path.

 Reduce channel gradients to decrease velocity ( many local

jursidictions have a minimum slope requirement of 2 percent; 1

percent may be considered on a case-by-case basis).

 The channel should flow over pervious soils whenever possible to

increase infiltration so that there is a reduction of runoff to

maximize infiltration capacity.

Table 3-5 identifies LID techniques and objectives to maintain the

predevelopment Tc.

Detailed guidance and computational examples are provided in

the Appendix A, Example LID Hydrologic Computations, which has

been adapted from the Prince Georges County LID Hydrologic

Analysis Manual (Prince Georges County, 1997).

Low Impact Development Technique

Low-Impact Development Objective

On-lot bioretentionWider and flatter swalesMaintain sheet flowClusters of trees andshrubs in flow pathProvide tree conservation/transition zonesMinimize storm drainpipesDisconnect imperviousareasSave treesPreserve existingtopographyLID drainage andinfiltration zones

Minimize disturbance ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔

Flatten grades ✔ ✔ ✔ ✔ ✔

Reduce height of slopes ✔ ✔ ✔

Increase flow path (divert and redirect) ✔ ✔ ✔ ✔ ✔ ✔

Increase roughness n ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔

Table 3-5. LID Techniques to Maintain the Predevelopment Time of

Concentration
3-22 Low-Impact Development Hydrologic Analysis

Low-Impact Development: An Integrated Environmental Design Approach

References

American Society of Civil Engineers (ASCE). 1994. Design and

Construction of Urban Stormwater Management Systems. ASCE Manuals

and Reports of Engineering Practice, No.77. Prepared by the Urban

Water Resources Research Council of the American Society of Civil

Engineers and the Water Environment Federation, Reston, VA.

Chesapeake Research Consortium (CRC). 1996. Design of

Stormwater Filtering Systems. Prepared for the Chesapeake Research

Consortium, Inc., Solomons, Maryland by, The Center for Watershed

Protection, Silver Spring, Maryland.

Chow, V.T. 1964. Handbook of Applied Hydrology. McGraw-Hill,

Inc., New York.

Department of Commerce. 1963. Rainfall Frequency Atlas of the

United States for Durations from 30 minutes to 24 hours and Return

Periods from 1 to 100 Years. Technical Paper 40. U.S. Department of

Commerce, Washington, D.C.

Federal Interagency Stream Restoration Working Group. 1998.

Stream Corridor Restoration: Principles, Processes, and Practices.

Leopold , L.B. 1968. Hydrology for Urban Land Planning: A Guidebook on the Hydrologic Effects of Land Use. U.S. Geological Survey

Circular 554.

Leopold, L.B., M.G. Wolman, and J.P. Miller. 1964. Fluvial Processes

in Geomorphology. Dover Publications, Inc., Mineola, New York.

Maidment, D.R. 1993. Handbook of Hydrology. McGraw-Hill, Inc.

New York.

Maryland Department of the Environment (MDE). 1998. Maryland Stormwater Design Manual. Vol. 1. Water Management Administration, Maryland Department of the Environment, Baltimore, Maryland.

May, C.W., E.B. Welch, R.R. Horner, J.R. Karr and B.W. Mar.

1997. Quality Indices for Urbanization in Puget Sound Lowland Streams.

Prepared for Washington Department of Ecology, Seattle, Washington,

by Department of Civil Engineering, University of Washington,

Seattle, Washington.

Prince Georges County, Maryland. 1999. Prince Georges County

Low Impact Development Hydrologic Analysis. Prince Georges County

Department of Environmental Resources, Maryland.
Low-Impact Development Hydrologic Analysis 3-23

Low-Impact Development: An Integrated Environmental Design Approach

Prince Georges County, Maryland. 1997. Low-Impact Development

Design Manual, Department of Environmental Resources, Prince

Georges County, Maryland.

SCS. 1986. Urban Hydrology for Small Watersheds. Technical

Release 55, US Department of Agriculture, Soil Conservation Service,

Engineering Division, Washington, DC.

SCS. 1985. National Engineering Handbook. Section 4, Hydrology

(NEH-4). Soil Conservation Service, US Department of Agriculture,

Washington, DC.

U.S. Environmental Protection Agency (USEPA). 1997. Urbanization and Streams: Studies of Hydrologic Impacts. Office of Water,

Washington, DC. 841-12-97-009. December 1997.

U.S. Environmental Protection Agency (USEPA). 1983. Results of

the Nationwide Urban Runoff Program. Vol. I. Final report. Water

Planning Division, Washington, DC.
Chapter 4

Low-Impact Development Integrated Management

Chapter 4

Low-Impact Development Integrated Management
Low-Impact Development Integrated Management Practices 4-1

Low-Impact Development: An Integrated Environmental Design Approach

Low-Impact Development Integrated

Management Practices 4

Low-impact development technology employs microscale and

distributed management techniques, called integrated management

practices (IMPs), to achieve desired postdevelopment hydrologic

conditions. The site planning process (Chapter 2) has identified how

fundamental design techniques can be used to minimize the hydrologic

effects of development. The hydrologic analysis (Chapter 3) demonstrates how to quantify the predevelopment and postdevelopment

conditions under various design scenarios. This chapter presents the

third step in the LID processidentifying and selecting IMPs. Detailed descriptions of the IMPs are included.

Procedures for Selection and Design of IMPs

Site planning techniques can significantly reduce the hydrologic

impacts of development. Once site-planning techniques have been

exercised, additional modifications are likely to be required to match

the predevelopment hydrograph. Measures used to evaluate the

hydrologic impact include the

runoff volume and the peak flow

condition. The shaded portion of

Figure 3-10 illustrates the

remaining control that might

be required to meet the development hydrology goal. IMPs can

be used to provide that additional

hydrologic control of peak

discharge and runoff volume.

LID IMPs are used to satisfy

the storage volume requirements

calculated in Chapter 3. They

are the preferred method because

In This Chapter

Introduction

Procedures for selection

and design of IMPs

Suitability criteria/factors

Integrated management

practices (IMPs)

Chapter

IMPs addressed

in this chapter

Bioretention

Dry wells

Filter/buffer strips

Grassed swales

Rain barrels

Cisterns

Infiltration trenches
4-2 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

they can maintain the predevelopment runoff volume and can be

integrated into the site design. The design goal is to locate IMPs at the

source or lot, ideally on level ground within individual lots of the

development. Management practices that are suited to low-impact

development include:

 Bioretention facilities

 Dry wells

 Filter/buffer strips and other multifunctional landscape areas

 Grassed swales,

bioretention swales, and

wet swales

 Rain barrels

 Cisterns

 Infiltration trenches

The process for selection

and design begins with the

control goals identified using

the hydrologic techniques

described in Chapter 3. The

steps identify the opportunities for supplemental controls

and guide the designer

through the selection and

design process (Figure 4-1):

Step 1: Define hydrologic control required.

Step 2: Evaluate site constraints.

Step 3: Screen for candidate practices.

Step 4: Evaluate candidate IMPs in various configurations.

Step 5: Select preferred configuration and design.

Step 6: Supplement with conventional controls, if necessary.

Fundamental questions addressed in

the IMP selection and design process

What are the goals for reduction of the volume and

peak flow conditions after development?

What are site constraints for selection of IMPs?

What types of IMPs are appropriate for my site?

How many IMPs do I need to plan for?

How much will it cost to install and maintain these

practices?

Will IMPs be sufficient to meet the goals and

regulatory requirements?
Low-Impact Development Integrated Management Practices 4-3

Low-Impact Development: An Integrated Environmental Design Approach

Step 1: Define Hydrologic Controls Required

The goal of the LID approach is to mimic the predevelopment

hydrologic regime of the site and thus maintain the predevelopment

runoff volume, peak runoff rates, and frequency. These control

objectives were defined and addressed, to the degree possible, through

site planning techniques described in Chapter 2.

The remaining need for control must be identified based on the

hydrologic goals identified in Chapter 3. This is illustrated in Figure 3-9.

Hydrologic functions such as infiltration, frequency and volume of

discharges, and groundwater recharge become essential considerations

when identifying and selecting IMPs. Following the procedures

described in Chapter 3, the hydrologic functions can be quantified

with respect to the various design parameters, which include runoff

volume, peak discharge, frequency and duration of discharge, groundwater recharge, and water quality parameters. When these design

parameters are quantified for predevelopment conditions, they define

or quantify the hydrologic controls required for a specific site.

Figure 4-1.

Key steps in developing

stormwater plan using

LID practices
4-4 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Step 2: Evaluate Site Opportunities and Constraints

Each site has unique characteristics and opportunities for control.

The LID concept encourages innovation and creativity in the management of site planning impacts. In this step the site should be evaluated

for opportunities and constraints. Opportunities are locations where

physical conditions like available space, infiltration characteristics,

and slopes are amenable to IMP installation. These same conditions

might also constrain the use of IMPs. Table 4-1 provides a summary of

potential site constraints of IMPs.

Table 4-1. Site Constraints of IMPs

Bioretention Dry Well Filter/Buffer Strip

Swales: Grass,

Infiltration, Wet Rain Barrels Cistern Infiltration Trench

Space

Required

Minimum surface

area range:

50 to 200 ft2

Minimum width:

5 to 10 ft

Minimum length:

10 to 20 ft

Minimum depth:

2 to 4 ft

Minimum surface

area range:

8 to 20 ft2

Minimum width:

2 to 4 ft

Minimum length:

4 to 8 ft

Minimum depth:

4 to 8 ft

Minimum length

of 15 to 20 ft

Bottom width:

2 ft minimum,

6 ft maximum

Not a factor Not a factor Minimum surface

area range:

8 to 20 ft2

Minimum width:

2 to 4 ft

Minimum length:

4 to 8 ft

Soils Permeable soils

with infiltration

rates > 0.27

inches/hour are

recommended. Soil

limitations can be

overcome with use

of underdrains

Permeable soils

with infiltration

rates > 0.27

inches/hour are

recommended

Permeable soils

perform better,

but soils not a

limitation

Permeable soils

provide better

hydrologic

performance, but

soils not a

limitation.

Selection of type

of swale, grassed,

infiltration or wet

is influenced by

soils

Not a factor Not a factor Permeable soils with

infiltration rates >

0.52 inches/hour are

recommended

Slopes Usually not a

limitation, but a

design

consideration

Usually not a

limitation, but a

design

consideration.

Must locate

downgradient of

building and

foundations

Usually not a

limitation, but a

design

consideration

Swale side slopes:

3:1 or flatter

Longitudinal

slope: 1.0%

minimum;

maximum based

on permissible

velocities

Usually not a

limitation, but

a design

consideration

for location of

barrel outfall

Not a factor Usually not a

limitation, but

a design

consideration. Must

locate downgradient of

buildings and

foundations

Water Table/

Bedrock

2- to 4-ft clearance

above water table/

bedrock

recommended

2- to 4-ft

clearance above

water table/

bedrock

recommended

Generally not a

constraint

Generally not a

constraint

Generally not

a constraint

2- to 4-ft clearance

Proximity to

build

foundations

Minimum distance

of 10 ft

downgradient from

buildings and

foundations

recommended

Minimum

distance of 10 ft

downgradient

from buildings

and foundations

recommended

Minimum

distance of 10 ft

downgradient

from buildings

and foundations

recommended

Minimum

distance of 10 ft

downgradient

from buildings

and foundations

recommended

Not a factor Minimum distance

of 10 ft downgradient from

buildings and

foundations

recommended

Max. Depth 2- to 4-ft depth

depending on soil

type

6- to 10-ft depth

depending on

soil type

Not applicable Not applicable Not applicable 6- to 10-ft depth

depending on soil

type

Maintenance Low requirement,

property owner can

include in normal

site landscape

maintenance

Low requirement Low requirement,

routine landscape

maintenance

Low requirement,

routine landscape

maintenance

Low

requirement

Moderate to high
Low-Impact Development Integrated Management Practices 4-5

Low-Impact Development: An Integrated Environmental Design Approach

Suitability Criteria/Factors

The site designer should consider or evaluate the following factors

when selecting LID IMPs.

Space/Real Estate Requirements. The amount of space required

for stormwater management controls is always a consideration in

the selection of the appropriate control. LID IMPs, because they

are integrated into and distributed throughout the sites landscape,

typically do not require that a separate area be set aside and

dedicated to stormwater management.

Soils. Soils and subsoil conditions are a very important

consideration in every facet of LID technology, including the site

planning process, the hydrologic considerations, and the selection

of appropriate IMPs. The use of micromanagement practices, as

well as the use of underdrains to provide positive subdrainage for

bioretention practices, helps to overcome many of the traditional

soil limitations for the selection and use of IMPs.

Slopes. Slope can be a limiting factor when the use of the larger

traditional stormwater controls is considered. With the application

of the distributed micromanagement IMPs, however, slope is

seldom a limiting factor; it simply becomes a design element that

is incorporated into the hydrologically functional landscape plan.

Water Table. The presence of a high water table calls for special

precautions in every aspect of site planning and stormwater

management. The general criterion is to provide at least 2 to 4 feet

of separation between the bottom of the IMP and the top of the

seasonally high water table elevation. Also, the potential for

contamination should be considered, especially when urban

landscape hotspots are involved.

Proximity to Foundations. Care must be taken not to locate

infiltration IMPs too close to foundations of buildings and other

structures. Considerations include distance, depth, and slope.

Maximum Depth. By their nature, the micromanagement practices

that make up the LID IMPs do not require much depth, and thus

this factor is not usually a major concern. Bioretention cells, for

example, usually allow only 6 inches of ponding depth, and 2 to 4

feet of depth for the planting soil zones.

Maintenance Burden. Maintenance costs for traditional

stormwater controls are significant and have become a

considerable burden for local governments and communities.

Maintenance costs can equal or exceed the initial construction

cost. In comparison, many of the IMPs require little more than

normal landscaping maintenance treatment. Additionally, this cost

is typically the responsibility of the individual property owner

rather than the general public. Communities are advised to retain

the authority to maintain their sites if they fail to function as

designed.
4-6 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

As previously discussed, one of the key concepts to making LID

technology work is to think small with respect to the size of the area

being controlled (microsubsheds) and the size of the practice

(micropractices). This combination allows the designer to incorporate many of the LID practices into the landscape and to overcome

potential site constraints with respect to available space, soils,

slopes, and other factors in a way that would not be possible with the

larger conventional methods.

Step 3: Screen for Candidate Practices

Based on the evaluation of site opportunities and constraints, a

comparison with the available practices is made. IMPs that are

inappropriate or infeasible for the specific site are excluded from

further consideration. Screening should consider both the site

constraints (Table 4-1) and the hydrologic and water quality functions identified in Table 4-2.

Table 4-2 provides an assessment of the hydrologic functions of the

preferred LID management practices. Table 4-3 provides a summary of

the reported water quality benefits provided by the LID IMPs.

It is important to recognize that LID stormwater management is

not simply a matter of selecting from a menu of available preferred

practices. Rather, it is an integrated planning and design process.

The site planning process described earlier is a necessary and essential component of the LID stormwater management concept. The

preferred practices by themselves might not be sufficient to restore

the hydrologic functions of a site without the accompanying site

planning procedures described in Chapter 2.

Table 4-2. Hydrologic Functions of LID Integrated Management Practices (IMPs)

Suitability

criteria/factors

Soils

Slopes

Water table

Proximity to

foundations

Maximum depth

Maintenance burden

PMP

Hydrologic

Functions Bio Ret

Dry

Well

Filter/

Buffer

Swale

Grass

Rain

Barrel Cistern

Infilt.

Trench

Interception H N H M N N N

Depression Storage H N H H N N M

Infiltration H H M M N N H

G.W. Recharge H H M M N N H

Runoff Volume H H M M L M H

Peak Discharge M L L M M M M

Runoff Frequency H M M M M M M

Water Quality H H H H L L H

Base Flow M H H M M N L

Stream Quality H H H M N L H

H = High M = Moderate L = Low N = None
Low-Impact Development Integrated Management Practices 4-7

Low-Impact Development: An Integrated Environmental Design Approach

Table 4-3 Reported Pollutant Removal Efficiency of IMPs

Step 4: Evaluate Candidate IMPs in Various Configurations

After the candidate IMPs are identified, they are deployed as

appropriate throughout the site and the hydrologic methods described

in Chapter 3 are applied to determine whether the mix of IMPs meets

the hydrologic control objectives identified in Step 1. Typically, on the

first design attempt the hydrologic control objectives are not met

precisely but instead are overestimated or underestimated. An iterative process might be necessary, adjusting the number and size of IMPs

until the hydrologic control objectives are optimized. An example

LID hydrologic computation that illustrates this procedure is provided

in the Appendix.

Step 5: Select Preferred Configuration and Design

The iterative design process typically identifies a number of

potential configurations and mixes of IMPs. The designer has the

option to use more or fewer bioretention structures, rain barrels,

cisterns, dry wells, infiltration trenches, vegetated swales, and other

practices. Design factors such as space requirements, site aesthetics,

and construction costs can all be factored into the decision-making

process to arrive at an optimum or preferred configuration and mix of

IMPs that provide the identified level of hydrologic control at a reasonable

cost.

Step 6: Design Conventional Controls if Necessary

If for any reason the hydrologic control objectives developed for a

given site cannot be achieved using IMPs, it might be necessary to add

some conventional controls. Sometimes site constraints like

low-permeability soils, the pressure of a high water table or hard rock,

PMP TSS Total P Total N Zinc Lead BOD Bacteria

Bioretention - 81 43 99 99 - -

Dry Well 80-100 40-60 40-60 80-100 80-100 60-80 60-80

Infiltration Trench 80-100 40-60 40-60 80-100 80-100 60-80 60-80

Filter/Buffer Strip 20-100 0-60 0-60 20-100 20-100 0-80 -

Vegetated Swale 30-65 10-25 0-15 20-50 20-50 - Neg.

Infiltration Swale 90 65 50 80-90 80-90 - -

Wet Swale 80 20 40 40-70 40-70 - -

Rain Barrel NA NA NA NA NA NA NA

Cistern NA NA NA NA NA NA NA

Source: CRC, 1996; Davis et al. 1997; MWCG, 1987; Urbonas & Stahre, 1993; Yousef et al., 1985;

Yu et al., 1992; Yu et al., 1993.
4-8 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

or very intensive land uses such as commercial or industrial sites can

preclude the use of sufficient IMPs to meet the hydrologic design

objectives, particularly the peak discharge criteria. In these situations

it is recommended that IMPs be used to the extent possible and then

that additional conventional controls such as detention or retention

practices (i.e. ponds) be used to meet the remaining hydrologic design

objectives. An example computation that illustrates how to determine

when additional conventional controls are required is provided in the

Appendix.

Integrated Management Practices (IMPs)

LID IMPs are designed for on-lot use. This approach integrates

the lot with the natural environment and eliminates the need for large

centralized parcels of land to control end-of-pipe runoff. The challenge

of designing a low-impact site is that the IMPs and site design strategies must provide quantity and quality control and enhancement,

including

 Groundwater recharge through infiltration of runoff into the soil.

 Retention or detention of runoff for permanent storage or for later

release.

 Pollutant settling and entrapment by conveying runoff slowly

through vegetated swales and buffer strips.

In addition, LID also provides an added aesthetic value to the

property, which increases a sense of community lifestyle.

 Multiple use of landscaped areas. In some cases, the on-lot or

commercial hydrologic control also can satisfy local government

requirements for green or vegetated buffer space.

Placing controls in series provides for the maximum on-lot

stormwater runoff control (i.e., the maximum mitigation of site development impacts on the natural hydrology). This type of design control is

known as a hybrid and is effective in reducing both volume and peak

flow rate. Examples of specific IMPs are described below.

Bioretention

Bioretention is a practice to manage and treat stormwater runoff

by using a conditioned planting soil bed and planting materials to filter

runoff stored within a shallow depression. The bioretention concept

was originally developed by the Prince Georges County, Maryland,

Department of Environmental Resources in the early 1990s as an

LID Functions

Include

Groundwater

recharge

Retention or

detention of runoff

Pollutant settling

Aesthetic value

Multiple use

Bioretention

A practice using

landscaped areas on

lots to hold and

infiltrate stormwater
Low-Impact Development Integrated Management Practices 4-9

Low-Impact Development: An Integrated Environmental Design Approach

alternative to traditional BMP

structures (ETA, 1993). The

method combines physical

filtering and adsorption with

biological processes. The system

can include the following components, as illustrated in Figures 4-2

and 4-3: a pretreatment filter

strip of grass channel inlet area, a

shallow surface water ponding

area, a bioretention planting

area, a soil zone, an underdrain

system, and an overflow outlet

structure.

Design Considerations. The

major components of the bioretention system all require careful design

considerations. These major components include

 Pretreatment area (optional)  In situ soil

 Ponding area  Plant material

 Ground cover layer  Inlet and outlet controls

 Planting soil  Maintenance

The key design consideration for these components are summarized in Table 4-4. Detailed design guidance can be obtained from the

Prince Georges County Bioretention Manual (ETA, 1993).

Figure 4-2.

Bioretention area

Table 4-4. Bioretention Design Components

Pretreatment area Required where a significant volume of debris or

suspended material is anticipated such as parking lots and

commercial areas. Grass buffer strip or vegetated swale

are commonly used pretreatment devices

Ponding area Typically limited to a depth of 6 inches

Groundcover area 3 inches of mature mulch recommended

Planting soil Depth = 4 feet

Soil mixtures include sand, loamy sand, and sandy loam

Clay content ≤ 10%

In-situ soil Infiltration rate ≥ 0.5 inches/hour w/o underdrains

Infiltration rate ≤ 0.5 inch/hour underdrain required

Plant materials Native species, minimum 3 species

Inlet and outlet controls Non erosive flow velocities (0.5 ft/sec)

Maintenance Routine landscape maintenance

Hydrologic design Determined by state or local agency
4-10 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Figure 4-3. Typical bioretention facility
Low-Impact Development Integrated Management Practices 4-11

Low-Impact Development: An Integrated Environmental Design Approach

Dry Wells

Small excavated

trenches backfilled

with stone, designed

to hold and slowly

release rooftop

runoff

Dry Wells

A dry well consists of a small excavated pit backfilled with aggregate,

usually pea gravel or stone. Dry wells function as infiltration systems used

to control runoff from building rooftops. Another special application of

dry wells is modified catch basins, where inflow is a form of direct surface

runoff. Figure 4-4 shows a typical detail of a dry well.

Dry wells provide the majority of treatment by processes related to

soil infiltration, including adsorption, trapping, filtering, and bacterial

degradation.

Design considerations. The key design considerations for dry wells

are summarized in Table 4-5. Detailed design guidance can be

obtained in Maryland Standards and Specifications for Infiltration Practices (MDDNR, 1984); Maintenance of Stormwater Management Structures, a Departmental Summary (MDE, 1986); and Maryland Stormwater

Design Manual (MDE, 1998).

Figure 4-4. Typical

dry well
4-12 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Filter Strips

Bands of closegrowing vegetation,

usually grass,

planted between

pollutant source

areas and a

downstream

receiving waterbody

Filter Strips

Filter strips are typically bands of close-growing vegetation, usually

grass, planted between pollutant source areas and a downstream

receiving waterbody (Figure 4-5). They also can be used as outlet or

pretreatment devices for other stormwater control practices. For LID

sites, a filter strip should be viewed as only one component in a

stormwater management system.

Design Considerations. The key design considerations for filter

strips are summarized in Table 4-6. Detailed design guidance is

provided in Maryland Standards and Specifications for Infiltration Practices (MDDNR, 1984), Design of Stormwater Filtering Systems, (CRC,

1996), and Maryland Stormwater Design Manual (MDE, 1998).

Table 4-5. Dry Well Design Considerations

Design storms Determined by local or state agencies. Guidance provided

in Prince Georges County LID Manual is recommended

Soil permeability ≥ 0.27  0.50 inches /hour

Storage time Empty within 3 days

Backfill Clean aggregate ≥ 11/2, ≤ 3, surrounded by engineering

filter fabric

Runoff filtering Screens should be placed on top of roof leaders, grease,

oil floatable organic materials and settable solids should

be removed prior to entering well

Outflow structures Overland flow path of surface runoff exceeding the

capacity of the well must be identified and evaluated. An

overflow system leading to a stabilized channel or

watercourse including measures to provide non-erosive

flow conditions must be provided

Observation well Must be provided, 4-inch PVC or foot place constructed

flush with ground surface, cap with lock

Depth of well 3 to 12 feet

Hydrologic design Determined by state or local agency. Maryland Design

Manual is recommended

Water quality See Table 4.3 for performance data

Maintenance Periodic monitoringquarterly at first and annually

thereafter
Low-Impact Development Integrated Management Practices 4-13

Low-Impact Development: An Integrated Environmental Design Approach

Figure 4-5. Typical filter

strip (CRC, 1996).
4-14 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Vegetated Buffers

Vegetated buffers are strips of vegetation, either natural or planted,

around sensitive areas such as waterbodies, wetlands, woodlands, or highly

erodible soils. In addition to protecting sensitive areas, vegetated strips

help to reduce stormwater runoff impacts by trapping sediment and

sediment-bound pollutants, providing some infiltration, and slowing and

dispersing stormwater flows over a wide area.

Level Spreaders

A level spreader typically is an outlet designed to convert concentrated runoff to sheet flow and disperse it uniformly across a slope to

prevent erosion. One type of level spreader is a shallow trench filled

with crushed stone. The lower edge of the level spreader must be

exactly level if the spreader is to work properly. Figure 4-6 shows a

typical rock-filled trench level spreader detail.

Design Considerations. Sheet flow, or overland flow, is the movement of runoff in a thin layer (usually less than 1 inch in depth) over a

wide surface, which begins when water ponded on the surface of the

Table 4-6. Filter Strip Design Considerations

Design storm Determined by state or local agency. Recommended

guidance in Prince Georges County, Maryland, LID

Manual (PGC, 1997) and Maryland Stormwater Design

Manual (MDE, 1998)

Drainage area Maximum drainage area to filter strips is limited by the

overland flow limits of 150 feet for pervious surfaces and

75 feet for impervious surfaces

Slope Minimum slope = 1.0%

Maximum slope = determined by field conditions

Flow Should be used to control overland sheet flow only.

Discharge should not exceed 3.5 cubic feet per second

range

Length and size The size of the filter strip is determined by the required

treatment volume. A minimum length of 20 feet is

recommended

Water quality The pollution removal effectiveness of the filter strip is

summarized in Table 4.3

Maintenance Routine landscape maintenance required
Low-Impact Development Integrated Management Practices 4-15

Low-Impact Development: An Integrated Environmental Design Approach

land becomes deep enough to overcome surface retention forces.

Level spreaders can be used to convey sheet flow runoff from lawn

areas within graded areas to bioretention facilities and transition

areas.

They can also be used to deliver runoff from parking lots and other

impervious areas to infiltration areas. The receiving area of the outlet

must be uniformly sloped and not susceptible to erosion. Particular

care must be taken to construct the outlet lip completely level in a

stable, undisturbed soil to avoid formation of rilling and channeling.

Erosion-resistant matting might be necessary across the outlet lip,

depending on expected flows. Alternative designs to minimize erosion

potential include hardened structures, stiff grass hedges, and segmenting of discharge flows into a number of smaller, adjacent spreaders.

Sheet flow should be used over well-vegetated areas, particularly

lawns, to achieve additional retention and increase the time of

concentration.

Figure 4-6. Typical rock

trench level spreader
4-16 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Grassed Swales

Traditionally, swale designs were simple drainage and grassed

channels that primarily served to transport stormwater runoff away

from roadways and rights-of-way. Today designers can design these

channels to optimize their performance with respect to the various

hydrologic factors. Two types of grassed swales are being used for this

purposethe dry swale, which provides both quantity (volume) and

quality control by facilitating stormwater infiltration (Figure 4-7), and

the wet swale, which uses residence time and natural growth to reduce

Figure 4-7. Example of dry

swale. Dry swales are used

at low density residential

projects or for very small

impervious areas
Low-Impact Development Integrated Management Practices 4-17

Low-Impact Development: An Integrated Environmental Design Approach

peak discharge and provide water quality treatment before discharge to

a downstream location (Figure 4-8). The wet swale typically has water

tolerant vegetation permanently growing in the retained body of water.

These systems are often used on highway designs.

Design Considerations. The key design considerations for grassed

swales are summarized in Table 4-7. Detailed design guidance is provided in Maryland Standards and Specifications for Infiltration Practices

(MDDNR, 1984), Design of Stormwater Filtering Systems (CRC, 1996),

and Maryland Stormwater Design Manual (MDE, 1998).

Figure 4-8. Example of wet

swale. Wet swales are ideal for

treating highway runoff in low

lying or flat terrain areas
4-18 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Design Storm Determined by state or local agency. Refer to guidance

provided by the Prince Georges County LID Design Manual

and the Maryland Stormwater Design Manual (MDE, 1998).

Local condition may necessitate adjustment of the

recommendations in the guidance documents.

Channel Capacity Swale must be sized to convey the peak discharge of the

design storm

Soils The permeability (infiltration rate) of the soils will determine

whether a dry or wet swale can be used. It is recommended

that soils used for dry swales have infiltration rates of 0.27 

0.50 inches per hour.

Channel Shape Trapezoidal or parabolic shape recommended

Bottom Width 2 foot minimum, 6 foot maximum

Side Slopes 3:1 or flatter

Channel Longitudinal

Slope

1.0 % minimum, 6.0 % maximum

Flow Depth 4.0 inches for water quality treatment

Mannings n value 0.15 for water quality treatment (depth < 4 ) 0.15  0.03

for depths between 4 and 12 0.03 minimum for depth 12

Flow Velocity 1.0 fps for water quality treatment - 5.0 fps for 2 year storm

fps for 10 year storm

Length of channel Length necessary for 10 minute residence time

Water Quality The pollutant removal effectiveness of grassed swales is

summarized in Table 4-3

Maintenance Routine landscape maintenance required.

Table 4-7. Grassed Swale Design Considerations

Figure 4-9. Typical rain

barrel

Rain Barrels

Rain barrels are low-cost, effective, and easily maintainable

retention devices applicable to both residential and commercial/

industrial LID sites. Rain barrels operate by retaining a predetermined

volume of rooftop runoff (i.e., they provide permanent storage for a

design volume); an overflow pipe provides some detention beyond the

retention capacity of the rain barrel. Figure 4-9 and Figure 4-10 show
Low-Impact Development Integrated Management Practices 4-19

Low-Impact Development: An Integrated Environmental Design Approach

a typical rain barrel. Rain barrels

also can be used to store runoff

for later reuse in lawn and garden

watering

Design Considerations.

Rainwater from any type of roofing

material can be directed to rain

barrels. To be aesthetically

acceptable, rain barrels can be

incorporated into the lots landscaping plan or patio or decking

design. Rain barrels placed at each

corner of the front side of the

house should be landscaped for

visual screening. Gutters and downspouts are used to convey water from

rooftops to rain barrels. Filtration screens should be used on gutters to

prevent clogging of debris. Rain barrels should also be equipped with a

drain spigot that has garden hose threading, suitable for connection to a

drip irrigation system. An overflow outlet must be provided to bypass

runoff from large storm events. Rain barrels must be designed with

removable, child-resistant covers and mosquito screening on water entry

holes. The size of the rain barrel is a function of the rooftop surface area

that drains to the barrel, as well as the inches of rainfall to be stored. For

example, one 42-gallon barrel provides 0.5 inch of runoff storage for a

rooftop area of approximately 133 square feet.

Cisterns

Stormwater runoff cisterns are roof water management devices that

provide retention storage volume in underground storage tanks. On-lot

storage with later reuse of stormwater also provides an opportunity for

water conservation and the possibility of reducing water utility costs.

Design Considerations. Cisterns are applicable to

residential, commercial, and industrial LID sites. Due to

the size of rooftops and the amount of imperviousness of

the drainage area, increased runoff volume and peak

discharge rates for commercial or industrial sites may

require larger-capacity cisterns. Individual cisterns can be

located beneath each downspout, or storage volume can

be provided in one large, common cistern.

Premanufactured residential use cisterns come in sizes

ranging from 100 to 1,400 gallons (Figure 4-11). Cisterns

should be located for easy maintenance or replacement.

Figure 4-11. Cistern. Image

courtesy of Pow Plastics,

Ltd., Devon, England

Figure 4-10. Rain barrel

application to LID
4-20 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

Infiltration Trenches

An infiltration trench is an excavated trench that has been

back-filled with stone to form a subsurface basin. Stormwater runoff is

diverted into the trench and is stored until it can be infiltrated into

the soil, usually over a period of several days. Infiltration trenches are

very adaptable IMPs, and the availability of many practical configurations make them ideal for small urban drainage areas (Figure 4-12).

They are most effective and have a longer life cycle when some form of

pretreatment is included in their design. Pretreatment may include

techniques like vegetated filter strips or grassed swales (Figure 4-7).

Care must be taken to avoid clogging of infiltration trenches, especially during site construction activities.

Design Considerations. The key design considerations for the

infiltration trench are summarized in Table 4-8. Detailed design

guidance is provided in Maryland Standards and Specifications for

Infiltration Practices (MDDNR, 1984), Maintenance of Stormwater

Management Structures: A Departmental Summary (MDE, 1986); and

Maryland Stormwater Design Manual (MDE, 1998).

Figure 4-12. Median strip

infiltration trench design

(adapted from MWCOG,

1987).
Low-Impact Development Integrated Management Practices 4-21

Low-Impact Development: An Integrated Environmental Design Approach

Other Environmentally Sensitive Management

Practices

Low-Impact Development is a relatively new concept. It is anticipated that over the next few years many additional integrated management practices and improvements to the LID approach will be introduced as local agencies and designers begin to experiment with the use

of the practice. A number of interesting developments are currently

underway that may prove useful in future application. However the

information available on these techniques is still somewhat limited.

Rooftop Greening. Rooftop greening is a technique being developed in Germany by Strodthogff & Behrens which consists of the use

of pre-cultivated vegetation mats( Figure 4-13 which are reported to

provide the following benefits:

 improve air quality ( up to 85% of dust particles can be filtered out

of the air)

 cooler air temperatures and higher humidity can be achieved

through natural evaporation.

 30-100% of annual rainfall can be stored, relieving stormdrains

and feeder streams.

Table 4-8. Infiltration Trench Design Considerations

Design Storm Determined by state or local agency. Guidance provided by the

Prince Georges County LID Design Manual and the Maryland

Stormwater Design Manual is recommended. Local condition

may necessitate adjustment of the recommendations in the

guidance document.

Soil Permeability > 0.27  0.50 inches per hour

Depth 3  12 feet

Storage Time Empty within 3 days

Backfill Clean aggregate > 11/2, < 3, surrounded by engineering

filter fabric

Runoff Filtering

Outflow Structures Overland flow path of surface runoff exceeding the capacity of

the trench must be identified and evaluated. An overflow

system leading to a stabilized channel or watercourse

including measures to provide non-erosive flow conditions

must be provided.

Observation Well Must be provided, 4 PVC on footplate, constructed flush with

ground surface, cap with lock.

Hydrologic Design Determined by state or local agency. Maryland Stormwater

Design Manual is recommended

Water Quality See Table 4.3 for performance data

Maintenance Periodic monitoring; Quarterly during first year, annual

thereafter.
4-22 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

 Visible green roofs provide a more aesthetic landscape.

Conservation Design for Stormwater Management. Conservation

design is a design approach to reduce stormwater impacts from land

development and achieve multiple objectives related to land use. This

approach has been jointly developed by the Delaware Department of

Natural Resources and Environmental Control and Environmental

Management Center of the Brandywine Conservancy.

Monitoring

Another and the final component of LID design includes the

development of appropriate pre and post development monitoring

protocols to document the effectiveness of individual IMPs as well at

the overall LID approach. Effective stormwater monitoring, whether

physical, chemical or biological is very difficult and expensive, and

consequently the design of a monitoring program will have to be

approached very carefully.

Providing guidance on a specific monitoring program is beyond the

scope of this document. However, some general guidance can be

provided.

Monitoring programs aimed at evaluating the effectiveness of a

given management practice (IMP can adapt the monitoring approaches currently being used for BMPs. Table 4-9 provides a listing of

parameters that should be reported with water quality data for various

BMPs (Urbonas, 1995). In addition to a comprehensive discussion of

Figure 4-13.

Roof Greening
Low-Impact Development Integrated Management Practices 4-23

Low-Impact Development: An Integrated Environmental Design Approach

Table 4-9. Parameters to Report with Water-Quality Data for Various BMPs

Parameter

(1)

Retention

Pond

(2)

Extended

Detention

Basin

(3)

Wetland

Basin

(4)

Wetland

Channel

(5)

Sand

Filter

(6)

Oil and

Sand

trap

(7)

Infiltration

and

Percolation

(8)

Tributary watershed area     

Total % tributary watershed is

impervious     

Percent of impervious area hyd.

Connected     

Gutter, sewer, swale, ditches, in

watershed     

Average storm runoff volume     

50th percentile runoff volume     

Coefficient of variation of runoff

volumes     

Average daily base flow volume     

Average runoff interevent time     

50th percentile interevent time     

Coefficient of variation of runoff

volumes     

Average storm duration     

50th percentile storm duration     

Coefficient of variation of storm

durations     

Water temperature     

Alkalinity, hardness and pH     

Sediment setting velocity distribution,

when available     

Type and frequency of maintenance     

Inlet and outlet dimensions and details     

Solar radiation, when available  

Volume of permanent pool   

Permanent pool surface area   

Littoral zone surface area 

Length of permanent pool   

Detention (or surcharge) volume     

Detention basins surface area     

Length of detention basin     

Brim-full emptying time     

Half-brimful emptying time     

Bottom stage volume 

Bottom stage surface area 

Forebay volume     

Forebay length     

Wetland type, rock filter present  

Percent of wetland surface at P 0.3 and

P 0.6 depths  

Meadow wetland surface area  

Plant species and age of facility   

2-year flood peak velocity  

Depth high ground water or

impermeable layer  
4-24 Low-Impact Development Integrated Management Practices

Low-Impact Development: An Integrated Environmental Design Approach

monitoring considerations is provided in the publication, Stormwater

NPDES related Monitoring Needs (ASCE, 1994).

Monitoring programs aimed at an overall evaluation of LID

designs will be more difficult to design, particularly where cause and

effect relationships in urban ecosystems are involved. Monitoring

programs will need to be tailored to each specific sites requirement,

and will likely require a mix of physical, chemical, and biological

considerations. Guidance for undertaking this work can be found in

the following publications: 1) Stormwater NPDES Related Monitoring

Needs, (ASCE, 1994: Effects of Watershed Development & Management on Aquatic Ecosytems , (SCE, 1996): and Urban Quality

Monitoring and Assessment Approaches in Wisconsin, (Bannerman,

1998).

References

American Society of Civil Engineers(ASCE), 1996, Effects of

Watershed Developments on Aquatic Ecosystems, Proceedings of

Engineering Foundation Conference, ed. By L.A. Roesner, Snowsbird,

Utah, ASCE, New York, NY.

American Society of Civil Engineers (ASCE), 1994, Stormwater

NPDES related Monitoring Needs, Proceedings of an Engineering

Foundation Conference, ed. By H.C.Tomo, Mount Crested Butte,CO,

ASCE, New. York, NY.

Bannerman, Roger, 1998 Urban Water Quality Monitoring and

Assessment Approaches in Wisconsin, paper presented at a Regional

Conference on Minimizing Erosion, Sediment, and Stormwater

impacts; Protection and Enhancement of Aquatic Resources in the

21st Century, University of Delaware, Newark, DE, 19716.

Chesapeake Research Consortium (CRC). 1996. Design of

Stormwater Filtering Systems. Prepared by the Center for Watershed

Protection, Silver Spring, Maryland.

Davis, A.P., M. Shokoubian, Sharma, and C. Minanci. 1998.

Optimization of Bioretention Design for Water Quality and Hydrologic

Characteristics. Final report. Environmental Engineering Program,

Department of Civil Engineering, University of Maryland, College

Park, Maryland.

Engineering Technologies Associates, Inc. (ETA). 1993 Design

Manual for Use of Bioretention in Stormwater Management. Prepared for

Prince Georges County, Maryland, Department of Environmental

Resources.
Low-Impact Development Integrated Management Practices 4-25

Low-Impact Development: An Integrated Environmental Design Approach

Maryland Department of Natural Resources (MDDNR). 1984.

Maryland Standards and Specifications for Infiltration Practices. Department of Natural Resources, Water Resources Administration,

Stormwater Management Division.

Maryland Department of the Environment (MDE). 1986. Maintenance of Stormwater Management Maintenance Structures: A Departmental Summary. Maryland Department of the Environment, Sediment and

Stormwater Division, Annapolis, Maryland.

Maryland Department of the Environment (MDE). 1998. Maryland Stormwater Design Manual. Prepared for Water Resources Administration, Maryland Department of the Environment, by the Center for

Watershed Protection, Inc., Environmental Quality Resources, Inc.,

and Loiderman Associates, Inc.

Metropolitan Washington Council of Governments (MWCOG).

1987. Controlling Urban Runoff: A Practical Manual for Planning and

Designing Urban BMPs. Metropolitan Washington Council of Governments, Department of Environmental Programs, Washington, DC.

Urbonas, Ben, R., 1995 Recommended Parameters to Report with

BMP Monitoring Data, Journal of Water Resources Planning and

Management Division, Vol. 121, No.1, Jan-Feb, 1995, ASCE, New

York, NY.

Urbonas, B., and P. Stahre. 1993. Best Management Practices and

Detention for Water Quality, Drainage and CSO Management.

Prentice-Hall, Inc. New Jersey.

Young, G.K., S. Stein, P. Cole, T. Kammer, F. Graziano, and F. Bank.

1996. Evaluation and Management of Highway Runoff Water Quality.

Publication No. FHWA-PD-96-032. Federal Highway Administration,

Office of Environment and Planning.

Yousef, Y., M. Wanielista, H. Harper, D. Pearce and R. Tolbert.

1985. Best Management PracticesRemoval of Highway Contaminants by

Roadside Swales. Final report. University of Central Florida, Florida

Department of Transportation, Orlando, Florida.

Yu, S.L., S. Barnes, and V. Gerde. 1993. Testing of Best Management

Practices for Controlling Highway Runoff. FHWA/VA 93-R16. Virginia

Transportation Research Council.

Yu, S.L., M. Kasnick, and M. Byrne. 1992. A Level Spreader/

Vegetative Buffer Strip System for Urban Stormwater Mangement. In

Integrated Storm Water Management, pp. 93-104, ed. R. Field. Lewis

Publishers, Boca Raton, Florida.
Chapter 5

Erosion and Sediment Control Considerations

for Low-Impact Development

Chapter 5

Erosion and Sediment Control Considerations

for Low-Impact Development
Erosion and Sediment Control Considerations for Low-Impact Development 5-1

Low-Impact Development: An Integrated Environmental Design Approach

Erosion and Sediment Control

Considerations for Low-Impact

Development

5

Erosion and sediment control and stormwater management are

closely interrelated. The application of LID concepts and the associated emphasis on minimizing the areas disturbed, as well as breaking

up drainage areas into small manageable subcatchment areas, is in

total harmony with the basic principles of erosion and sediment

control. The designer will find that the application of LID technology

can easily result in improved erosion and sediment control without

significant additional effort.

Erosion and Sediment Control Steps

The following five basic common sense steps govern the development and implementation of a sound erosion and sediment control

plan for any land development activity.

Step One: Planning. Plan the operation to fit the existing site

features, including topography,

soils, drainage ways, and natural

vegetation.

Step Two: Scheduling of

Operations. Schedule grading

and earthmoving operations to

expose the smallest practical area

of land for the shortest possible

time. If possible, schedule land

disturbance activities during dry

seasons or periods.

Step Three: Soil Erosion

Control. Apply soil erosion

In This Chapter

Introduction

Erosion and Sediment

Control Steps

Chapter

Erosion and

Sediment Control

Steps

1. Planning

2. Scheduling of

operations

3. Soil erosion

control

4. Sediment control

5. Maintenance

Erosion and Sediment Control Considerations for Low-Impact Development
5-2 Erosion and Sediment Control Considerations for Low-Impact Development

Low-Impact Development: An Integrated Environmental Design Approach

prevention and control practices as a first line of defense against

off-site damage.

Step Four: Sediment Control. Apply sediment control practices as

a second line of defense against off-site damage.

Step Five: Maintenance. Implement a thorough maintenance

program before, during, and after development is completed.

The following sections describe in more detail how these steps are

used in controlling erosion and sedimentation in an LID setting.

Step One: Planning. The first step in controlling erosion and

sediment is to plan the development to fit the site features, including;

topography, soils, drainage ways, and natural vegetation. It should be

observed that this step is very similar to the planning guidelines

provided for low impact development in Chapters 2 and 3 of this

design manual. In other words, by following the planning guidelines

set forth in Chapters 2 and 3 of this manual, the site planner or

designer will also be implementing the first step of erosion and sediment

control. Not surprisingly, the two processes are similar. Listed below

are key considerations of the planning element.

Topography. The primary considerations are slope steepness and

slope length. Because of the effect of runoff, the longer and steeper

the slope, the greater the erosion potential. The percent of slope can

be determined from the site topography. Areas of similar steepness can

be identified and grouped together to produce a slope area map. Slope

gradients can be grouped into three or more general ranges of soil

erodibility as presented below:

0% - 7 % Low erosion hazard

7% - 15 % Moderate erosion hazard

15 % or over High erosion hazard

Within these slope gradient ranges the greater the slope length,

the greater the erosion hazard. Therefore, in determining potential

critical areas the site planner should be aware of excessively long

slopes. As a general rule, the erosion hazard will become critical if

slope lengths exceed the following values:

0% - 7 % 300 feet

7% - 15 % 150 feet

15 % or over 75 feet

Step One

Plan the development

to fit the site

features:

• topography

• drainage ways

• soils

• vegetation
Erosion and Sediment Control Considerations for Low-Impact Development 5-3

Low-Impact Development: An Integrated Environmental Design Approach

Drainage ways. Natural drainage patterns that exist on the site

should be identified to plan around these critical areas where water

will concentrate. Where possible, natural drainage ways should be

used to convey runoff over and off the site to avoid the expense and

problems of constructing an artificial drainage system. These natural

drainage ways should be protected with vegetative buffers whenever

possible.

Man-made ditches, diversions, and waterways will become part of

the erosion problem if they are not properly stabilized. Care should

also be taken to be sure that increased runoff from the site will not

erode or flood the existing natural drainage system.

Soils. Major soil considerations from an erosion and sediment

control standpoint include erodibility, permeability, depth to water

table and bedrock, and soils with special hazards including shrink/

swell potential or slippage tendencies.

Erodibility is a term that describes the vulnerability of a soil to

erosion. The average particle size and gradation (texture), percentage

of organic matter, and soil structure influence soil erodibility. The

most erodible soils generally contain high proportions of silt and very

fine sand. The presence of clay or organic matter tends to decrease

soil erodibility. Clays are sticky and tend to bind soil particles together,

which along with organic matter helps to maintain stable soil structure.

By combining the soils information with information on the

topography, drainage, and vegetation on the site, the planner can

determine the critically erodible and sensitive areas that should be

avoided if possible during construction.

Natural Vegetation. Ground cover is the most important factor in

terms of preventing erosion. Any existing vegetation that can be

saved will help prevent erosion. Vegetative cover shields the soil

surface from raindrop impact while the root mass holds soil particles in

place. Vegetation also can filter sediment from runoff. Thus grass

buffer strips can be used to remove sediment from surface runoff.

Vegetation also slows the velocity of runoff and helps maintain the

infiltration capacity of a soil. Trees and unique vegetation protect the

soil as well as beautifying the site after construction. Where existing

vegetation cannot be saved, the planner should consider staging of

construction, temporary seeding, or temporary mulching.

Soil considerations

 Erodibility

 Permeability

 Depth

 Constraints

Natural Vegetation

• Protects soil surface

• Filters sediment

• Reduces runoff

velocity
5-4 Erosion and Sediment Control Considerations for Low-Impact Development

Low-Impact Development: An Integrated Environmental Design Approach

Step Two: Scheduling of Operations. The second erosion and

sediment control step is to expose the smallest practical area of land

for the shortest possible time. The reason behind this step is rather

simple-1 acre of exposed land will yield less sediment than 2 acres of

exposed land, and an area exposed for 3 months will yield less sediment than an area exposed for 6 months.

The clearing, grubbing and scalping of excessively large areas of

land at one time is an unnecessary invitation to sediment problems.

As previously described in Chapter 2, these initial earth-disturbing

activities should be kept to a bare minimum. On the areas where

disturbance takes place, the site designer should consider staging of

construction, temporary seeding, and/or temporary mulching as a

technique to reduce erosion. Staging of construction involves stabilizing one part of the site before disturbing another. In this way the

entire site is not disturbed at once and the time without ground cover

is minimized. Temporary seeding and mulching involves seeding or

mulching areas that would otherwise lie open for long periods of time.

The time of exposure is limited and therefore the erosion hazard is

reduced.

Step Three: Soil Erosion Control Practices. The third important

principle is to apply soil erosion control practices on disturbed areas as

a first line of defense against off-site damage. Control does not begin

with the perimeter sediment trap or basin. It begins at the source of

the sediment, the disturbed land area, and extends down to the control

structure.

Soil particles become sediment when they are detached and moved

from their initial resting place. This process, which is called erosion, is

accomplished for the most part by the impact of falling raindrops and

the energy exerted by moving water and wind, especially water. A

reduction in the rate of soil erosion is achieved by controlling the

vulnerability of the soil to erosion processes or the capability of moving

water to detach soil particles. In humid regions this is accomplished

through the use of soil stabilization and runoff control practices.

Soil stabilization practices include a variety of vegetative, chemical,

and structural measures used to shield the soil from the impact of

raindrops or to bind the soil in place, thus preventing it from being

detached by surface runoff or wind erosion. Representative soil

stabilization practices include the following:

 Vegetative stabilization, both temporary and permanent

 Topsoiling

Step Two

Expose the smallest

practical area for the

shortest possible time.

Step Three

Apply soil erosion

practices as a first line

of defense
Erosion and Sediment Control Considerations for Low-Impact Development 5-5

Low-Impact Development: An Integrated Environmental Design Approach

 Erosion control mattings

(Figure 5-1)

 Mulching

 Tree protection

The use of mulch to achieve

temporary stabilization is gaining

increased attention and recognition. Ongoing research efforts

are confirming the fact that

mulching is a very effective

method of reducing runoff as well

as removing pollutants from

runoff. Table 5-1 displays types of

mulches.

Runoff control practices, in contrast, include a number of measures

designed to reduce the amount of runoff generated on a construction

site, prevent off-site runoff from entering the disturbed area, or slow

the runoff moving through and exiting the disturbed area.

Stormwater runoff is the principal cause of soil erosion.

Stormwater runoff control is achieved through the proper use of

vegetative and structural practices, and construction measures that

control the location, volume and velocity of runoff. Proper

stormwater handling for erosion control can be accomplished in one

or a combination of the following ways:

Table 5-1. Types of Mulches

Step Four

Apply sediment control

practices as a second

line of defense against

off-site damage

Figure 5-1.

Erosion control

mattings

Mulch Benefits Limitations

Chipped wood Readily available; inexpensive;

judged attractive by most

High nitrogen demand; may inhibit

seedlings; may float off-site in surface

runoff

Rock May be locally available and

inexpensive

Can inhibit plant growth; adds no

nutrients; suppresses diverse plant

community; high cost where locally

unsuitable or unavailable

Straw or hay Available and inexpensive; may add

undesirable seeds

May need anchoring; may include

undesirable seeds

Hydraulic mulches Blankets soil rapidly and

inexpensively

Provides only shallow-rooted grasses,

but may outcompete woody vegetation

Fabric mats Relatively durable (organic) or very

durable (inorganic); works on steep

slopes

High costs; suppresses most plant

growth; inorganic materials harmful to

wildlife

Commercial compost Excellent soil amendment at

moderate cost

Limited erosion-control effectiveness;

expensive over large areas
5-6 Erosion and Sediment Control Considerations for Low-Impact Development

Low-Impact Development: An Integrated Environmental Design Approach

 Reduction and detention of the runoff

- staging operations

- grading and shaping of soil surfaces

- manipulation of slope length and gradient

 Interception and diversion of runoff

- diversion berm or dike

- reverse benches

- drainage swales

- vegetation buffers

 Proper handling and disposal of concentrated flow

- vegetative swales

- downdrain structures

- outlet stabilization

Step Four: Sediment Control Practices. The fourth step is to

apply sediment control practices as a second line of defense against

offsite damage. Even with the best erosion control plan, some sediment will be generated and controlling it is the objective of this step.

Whereas erosion control practices are designed to prevent soil particles from being detached, sediment control involves using practices

that prevent the detached particles from leaving the disturbed area

and reaching the receiving waterways. This goal is accomplished by

reducing the capacity of surface runoff to transport sediment and by

containing the sediment on site.

Sediment control practices are designed to slow the flow of water

by spreading, ponding, or filtering. By so doing, the capacity of the

water to transport sediment is reduced, and sediment settles out of

suspension. Commonly used control practices include (1) the preservation or installation of vegetated buffer areas downslope of the

disturbed area to slow and filter the runoff, (2) the construction of

small depressions or dikes to catch sediment (particularly

coarse-textured material) as close to its point of origin as possible, and

(3) the construction of sediment traps or basins at the perimeter of the

disturbed area to capture additional sediment from the runoff.

The amount of sediment removed from the runoff is mostly

dependent upon (1) the speed at which the water flows through the

filter, trap, or basin; (2) the length of time the water is detained; and

(3) the size, shape, and weight of the sediment particles.

Currently, the most frequently used approach to sediment control

is simply to direct all surface runoff into a large sediment basin, which

Sediment removal

is dependent upon

 Water flow rates

 Length of time

water is detained

 Size, shape and

weight of sediment

particles
Erosion and Sediment Control Considerations for Low-Impact Development 5-7

Low-Impact Development: An Integrated Environmental Design Approach

is later cleaned out and converted to a stormwater management pond.

Although this approach is arguably the simplest and lowest cost

method to control sediment, it often fails to address the other principles described above and thus may not represent the best way to

prevent and control sediment.

One of the underlying concepts of LID technology involves

breaking up the drainage areas of a given site into very small catchment areas to disconnect

hydraulically connected

areas and to provide

opportunities to increase

the time of concentration and thus reduce

peak discharges. Accordingly, this approach

will benefit sediment

control efforts by diffusing surface flow into

many directions and

providing more flexibility

in the use of a variety of

sediment control practices.

This approach will provide more opportunity to use silt fences

(Figure 5-2) and small traps, such as the stone outlet trap and the

rip-rap outlet trap, to control small catchment areas generally in the

range of 1 to 3 acres in size. It will also allow more opportunity to

integrate the use of vegetative buffers in sediment control. When

bioretention practices are planned for stormwater management, they

can first be used as a small temporary trap by excavating the top 2 feet

of soil. Then after the site is stabilized the trap and accumulated silt

can be removed and the bioretention cell can be installed. It should

be noted that the bottom of the bioretention cell should be two (2)

feet below the invert of sediment trap. Also, no long term controls are

to be placed in use prior to completion of construction and permanent

stabilization of all disturbed areas.

Step Five: Inspection and Maintenance. The final important

control step is to implement a thorough inspection and maintenance

program. This step is vital to the success of an erosion and sediment

control program. A site cannot be controlled effectively without

thorough, periodic checks of all erosion and sediment control practices.

Figure 5-2. Silt fence

installation guidelines

Step Five

Implement a thorough

maintenance and

follow-up operation
5-8 Erosion and Sediment Control Considerations for Low-Impact Development

Low-Impact Development: An Integrated Environmental Design Approach

When inspections reveal problems, modifications, repairs, cleaning, or

other maintenance operations must be performed expeditiously.

Particular attention must be paid to water-handling structures

such as diversions, sediment traps, grade control structures, sediment

basins, and areas being revegetated. Breaches in the structures or

areas being revegetated must be repaired quickly, preferably before the

next rainfall.

References

Federal Interagency Stream Restoration Work Group (FISRWG).

1998. Stream Corridor Restoration: Principles, Processes, and Practices.

Maryland Department of the Environment. 1994. 1994 Maryland

Standards for Erosion and Sediment Control. Prepared by the Water

Management Administration, Maryland Department of the Environment, in association with the Soil Conservation Service and the State

Soil Conservation Committee.

Prince Georges County Soil Conservation District. 1994. Soil

Erosion & Sediment Control/Pond Safety Reference Manual.

USDA Soil Conservation Service (USDA-SCS). 1967. Soil Survey,

Prince Georges County, Maryland. Prepared by USDA Soil

Conservation Service in cooperation with the Maryland Agricultural

Experiment Station.
Chapter 6

Low-Impact Development Public Outreach Program

Chapter 6

Low-Impact Development Public Outreach Program
Low-Impact Development Public Outreach Program 6-1

Low-Impact Development Public Outreach Program

Low-Impact Development Public

Outreach Program 6

Introduction

Using LID approaches in new development can help achieve overall

stormwater and pollution reduction goals. It has become more important for municipalities to be more creative in the ways they manage

stormwater. LID approaches offer creative ways to control stormwater

runoff, while at the same time achieving multiple development objectives. Several potential advantages include reducing the scale of

maintenance costs to levels affordable by the property owner and the

transfer of maintenance costs to the property owner. In addition, state

and local governments may be able to decrease property acquisition

costs due to a decreased need for structural stormwater controls.

A critical component to the success of LID approaches is the

proper maintenance of installed IMPs by the property owners, or

other designated entity. In addition information should be provided

to commercial and residential property owners/managers about

effective pollution prevention

practices. The developer and

local public agency/authority

must effectively communicate

the benefits of low-impact

development as well as its

maintenance responsibilities to

potential and existing property

owners. Proper maintenance

practices for LID properties

include maintaining vegetative

buffers and removing trash and

other debris from the outflow

points. Property owners must

also be educated about the

In This Chapter

Introduction

Developing a Public

Outreach Program

Step One: Define Public

Outreach Program

Objectives

Step Two: Identify Target

Audiences

Step Three: Develop

Outreach Materials

Step Four: Distribute

Outreach Materials

Chapter

LID IMP

Maintenance

 Maintain

vegetated

buffers

 Remove trash

and debris
6-2 Low-Impact Development Public Outreach Program

Low-Impact Development Public Outreach Program

necessity of not disturbing, compacting, or eliminating IMPs.

Pollution prevention practices that can support LID approaches

include careful use of fertilizers on landscaped areas, parking lot

sweeping, judicious mowing practices that allow the runoff to

slowly percolate into the ground, and general water conservation

habits. It is much more cost-efficient to prevent the pollutants

from entering the stormwater than it is to remove the pollutants

once they are in the system.

This chapter describes the components needed to ensure a

successful low-impact development public outreach program. It is

based on successful efforts by Prince Georges County, Maryland.

Developing a Public Outreach Program

Effective public outreach programs for LID properties must be

tailored not only for each site, but for specific audiences. One cannot

develop or distribute a single brochure on maintaining IMPs to

property owners. The key to effective outreach is to target a message

to a specific audience and have them respond to that message. There

are four key steps to follow in developing effective public outreach

materials for LID properties:

• Step One: Define public outreach objectives.

• Step Two: Identify the target audiences.

• Step Three: Develop materials for those audiences.

• Step Four: Distribute outreach materials.

Each of these steps is reviewed below.

Step One: Define Public Outreach Program Objectives

The first step in developing a public outreach program is to

clearly identify the objectives. Are you trying to educate a potential

property owner about maintenance requirements of the IMPs on the

property? Do you want to make commercial property owners aware of

the potential cost savings of LID stormwater controls? The objectives

identified will determine what messages are developed and how the

outreach materials are distributed.

The LID education/awareness program accomplishes several

objectives, including the following:

• Creating a marketing tool for developers to attract environmentally conscious buyers.
Low-Impact Development Public Outreach Program 6-3

Low-Impact Development Public Outreach Program

• Promoting stewardship of our natural resources by empowering

citizens to take initiatives on environmental protection measures.

• Promoting more aesthetically pleasing development by creating

more landscaped areas.

• Educating property owners on effective pollution prevention

practices.

• Educating residential and commercial property owners on the

potential cost savings of using LID approaches.

• Encouraging a greater sense of community due to the unique

environmental character of LID designs.

• Ensuring proper maintenance of installed IMPs.

To help define objectives and to take advantage of the vast amount

of public outreach information available, it is helpful for the developer

to coordinate the public outreach program with the review agencies.

This effort should begin during the site planning phase. Once the

potential IMPs are identified, the developer should meet with the

regulatory agency to gain an understanding of the construction and

maintenance requirements of the IMPs until they are transferred to the

property owner or homeowners association.

The program and planning phase will help identify the relevant

target audiences to receive the outreach materials, provide the developer with existing informational materials and identify additional

materials that can be developed and possible distribution mechanisms

for the materials.

Step Two: Identify Target Audiences

For each LID property, whether it is residential, commercial, or

industrial, there are different audiences that the developer needs to

reach with public outreach information-potential buyers, new property

owners, builders and construction site managers, homeowner associations and existing property owners. Specific messages must be tailored

to each of these audiences based on the kind of property in question.

Each of these audiences is discussed in more detail below, along with

recommended messages for the audiences.

Potential Buyers

Potential buyers make up a primary target audience for outreach of

LID benefits and maintenance requirements. For residential properties,
6-4 Low-Impact Development Public Outreach Program

Low-Impact Development Public Outreach Program

the developer has the opportunity to promote the green aspects of

low-impact development. Not only can the developer promote the

extensive effort to preserve natural resources on the site, but also the

measures (such as reforestation and landscaping practices) that were

conducted on each lot. Those same measures will increase the

aesthetic appeal, value, and habitat potential of the property. This

message also works to some degree on commercial properties, by

conveying the message that customers appreciate shaded areas in

parking lots and the aesthetics of landscaped areas around developments.

Potential buyers must also be made aware of their individual

responsibilities, as well as community responsibilities, for the upkeep

and improvement of the property. For residential properties, the maintenance of on-site IMPs by the individual owner is a unique concept.

Although the anticipated amount of maintenance is small, the owner

must be made aware of the importance of the upkeep of plant materials

and making sure that drainage structures are unimpaired. It must also be

impressed on the property owners that these systems should not just be

considered another part of their yard that they can freely landscape.

The concept of maintenance of IMPs by the owner of commercial

properties is similar to conventional developments. The difference is

that instead of a large centralized facility that requires an infrequent, but

large-scale, maintenance effort (e.g., mucking, mowing, reseeding,

cleaning, and pumping), there may be smaller facilities distributed

throughout the site. The smaller sites may require more frequent

maintenance, such as trash removal and replanting, but the long-term

capital costs are less.

The maintenance materials given to the potential owner at this

phase do not have to be detailed, but they must clearly convey the basic

requirements for the potential IMPs located on each lot and within the

community/commercial property.

Builders and Site Construction Managers

Builders and site construction managers need to be made aware

of planned IMPs on the property. During the construction phase,

the local regulatory inspectors will verify the procedures used to

protect IMP facility locations, limits of clearing and grading, and

adherence to construction practices. To avoid potential problems

during construction that might require extensive remedial actions to

ensure the success of a IMP facility, the developer should make the

builder and site construction manager aware of the appropriate

phasing and construction practices. The education program should
Low-Impact Development Public Outreach Program 6-5

Low-Impact Development Public Outreach Program

include information on clearing and grading restrictions, timing of

revegetation, sedimentation removal, and maintenance after construction. Experience with bayscapes has shown that a critical

element that is often neglected is follow-up care of the LID vegetation directly after installation of the system. Without proper watering and care, these systems can fail due to plant mortality.

New Property Owners

The developer, or seller, must allow the new property owner to

examine and then accept any conditions that have to be met with

the acquisition of the land. LID sites may require legal information

and instruments to ensure that the facilities will be properly maintained. These may include easements, covenants, or homeowners

association requirements, or other applicable instruments depending

on the type of development. The developers attorney will typically

develop these documents. The maintenance requirements for

easements and covenants can be developed from brochures, fact

sheets, and example documents, which are available from Prince

Georges County. A sample maintenance covenant is provided in

Appendix B. The requirements and wording to be included in the

documents must be approved by the local regulatory agency. The

documents that are to be conveyed must be complete and detailed.

They should show maintenance schedules, equipment requirements,

and lists of replacement plants for vegetated IMPs.

Existing Property Owners

Once the property owner has been made aware of the proper

procedures for maintenance of IMPs, it is the responsibility of

the community and property owner to implement these procedures. After the initial property transfer, the developer assigns

someone, either a representative of the developer or of the

homeowners association, to monitor and train the new property owners on proper maintenance procedures. This will

help ensure that the facilities are kept up while other units

are being sold and will ensure consistent operation of the

facilities. Procedures include not only maintaining vegetation and keeping structures in good condition, but also

employing pollution prevention practices. Local authorities should take enforcement actions on maintenance

issues only when there is a public nuisance or safety issue, or

clear intent to destroy or functionally alter the LID system. The

best enforcement mechanisms are the understanding of the importance of the IMP maintenance functions and that the owner has
6-6 Low-Impact Development Public Outreach Program

Low-Impact Development Public Outreach Program

pride in the community. It is considered advisable for local

governments to have the requisite authority to take action and

the mechanisms should be clearly identified before LID methods

are adopted for private land owners.

Industrial and Commercial Property Owners

LID techniques are also applicable to industrial and commercial settings. Fact sheets in Chapter 4 and case studies in Chapter

5 explain LID techniques for stormwater management that can

help to control and manage runoff from industrial sites including

parking lots and industrial material storage areas. Local

stormwater management agencies must work with commercial and

industrial property owners both to retrofit existing sites with LID

technologies and to incorporate LID approaches into the site

planning process. In many instances, LID approaches may even

save industrial and commercial property owners money by

• Requiring less land for stormwater management.

• Incorporating on-site infiltration into existing parking lot

designs.

• Reducing the amount of piping and engineering required to

convey stormwater.

• Lowering ongoing maintenance costs.

• Reducing the amount of grading and land disturbance when

developing new sites.

Step Three: Develop Outreach Materials

Once the target audiences are identified, the appropriate materials can be developed. When identifying different target audiences it is

important to consider the best formats for the audience. For example,

homeowners may read a fact sheet sent to their residence about not

mowing vegetative buffers, but commercial and industrial properties

may benefit from a training session with accompanying materials to

explain maintenance requirements for the IMPs. Many of the materials developed by Prince Georges County, Maryland, to support the

implementation of LID in residential settings can be modified for

industrial applications.

In developing outreach materials, the developer should

remember that the target audience must be shown why this

information is important to them. This ties back to the
Low-Impact Development Public Outreach Program 6-7

Low-Impact Development Public Outreach Program

objectivescost savings, increased property values, reduction of

pollutant runoff, etc.

To help the developer conduct effective outreach, local regulatory agencies can help prepare brochures, manuals, and fact sheets.

Table 6-1 identifies the outreach materials developed by Prince

Georges County, Maryland, in support of its LID program. The

table categorizes this information into critical areas, as well as

showing general information on design and construction and pollution prevention. The developer may use this information directly or

Table 6-1 Educational Materials

Application

Document

Design and 

Construction

Pollution Prevention

Program PlanningPotential buyersSettlement

Site visits

Bioretention Manual ?

State Infiltration Manual ? ?

Low-Impact Development Manual ?? ?

SWM Manual ?? ?

Bioretention Fact Sheet ? ??

Pollution Prevention Fact Sheet ?

Countys Pollution Laws ?

NPDES Fact Sheet ?

Bayscapes Brochure ???

Car Care Brochure ? ?

Lawn Care Brochure ? ?

County Information and Service Numbers ???

Household Hazardous Waste ? ??

Water Conservation ? ?

Stream Teams ???

Community Cleanup ? ?

Homeowners Drainage Manual ???

Low-Impact Maintenance Manual ? ?

Reporting Pollution Prevention Fact Sheets ? ??

Glossary of Stormwater Terms ?

Integrated Pest Management ? ?

Wildlife Habitat Improvement

Pollution Prevention Manual
6-8 Low-Impact Development Public Outreach Program

Low-Impact Development Public Outreach Program

use it as a basis for customized brochures or legal documents tailored

for the specific development.

Pollution Prevention Materials

In addition to specific information regarding the maintenance

requirements for LID properties, it is important to provide materials

on pollution prevention practices that residential, commercial, and

industrial property owners can implement to reduce the amount of

pollutants going into the stormwater. Dozens of fact sheets and

brochures on pollution prevention practices are available.

Basic education programs can be considered a nonstructural

IMP that should be implemented for everyone. Too much

pollution enters streams, rivers and lakes through carelessness

or ignorance. Many people will adopt new methods or use

alternative materials if they are simply informed of techniques

that can reduce the impacts on receiving waters. Industry

employees can learn to properly handle and store materials

and dispose of industrial wastes through in-house training

courses, videotape presentations, and interactive seminars.

Local libraries and government agencies, such as the

Cooperative Extension Service and the Industrial Extension Service, are good sources of educational materials.

Residential property owners should know the proper way to

dispose of litter, yard waste, used motor oil, and other household

wastes. Industries, municipalities, and homeowners can also learn

how to use fertilizer and pesticides correctly to maintain their lawns

and gardens without polluting nearby streams and rivers.

Step Four: Distribute Outreach Materials

There are several points in the property transfer process at which

the developer can distribute outreach materials:

Construction of IMPs. Developers can provide the builder and

construction site managers with outreach materials to ensure that

the planned IMPs are not disturbed during the building phase.

Potential Buyers. Potential property owners can be made aware of

the benefits as well as the responsibilities of owning a LID property

when they first express interest in the property.

At Settlement. Educational materials outlining maintenance

procedures, as well as legal instruments such as covenants and

easements, can be presented at settlement.
Low-Impact Development Public Outreach Program 6-9

Low-Impact Development Public Outreach Program

Site Visits. Periodic site visits by the developer and/or

homeowners associations and local government should be made to

ensure that the IMPs are being properly maintained. Educational

materials can be distributed at this time to reinforce the maintenance requirements and benefits.

Homeowner Association Meetings. Developers can make

presentations and answer questions about LID maintenance requirements at homeowners association meetings. These meetings also

offer a good opportunity to distribute information on pollution

prevention practices.

By implementing a strong public outreach program the developer

can increase the effectiveness of the IMPs installed on the property

and promote LID approaches as the preferred alternative to conventional stormwater practices.
Appendices Appendices
Appendix A-1

Low-Impact Development: An Integrated Environmental Design Approach

Appendix A

Example Low-Impact Development

Hydrologic Computation*

Contents

Introduction..................................................................................... A-3

Data Collection ............................................................................... A-3

Determinating the LID Runoff Curve Number .............................. A-3

Development of the Time of Concentration (Tc)........................... A-8

Low-Impact Development Stormwater Management

Requirements................................................................................... A-8

Determination of Design Storm Event.......................................... A-21

Figures

Figure A.1 Low-Impact Development Analysis Procedures...........A-4

Figure A.2 Comparison of Land Covers Between Conventional

and LID CNs. ................................................................A-6

Figure A.3 Approximate Geographic Boundaries for NRCS

Rainfall Distributions. ...................................................A-8

Figure A.4 Procedure to Determine Percentage of Site Area

Required for IMPs to Maintain Predevelopment

Runoff Volume and Peak Runoff Rate ........................A-11

Figure A.5 Comparison of Retention of Storage Volumes Required

to Maintain Peak Runoff Rate Using Retention and

Detention ................................................................... A-14

* Adapted from Prince George’s County, Maryland, Low Impact

Development Hydrologic Analysis, 1999
A-2 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

Figure A.6 Storage Volume Required to Maintain Peak

Runoff Rate ................................................................ A-15

Figure A.7 Comparison of Storage Volume for Various Tcs......... A-16

Tables

Table A.1 Representative LID Curve Numbers ............................A-5

Table A.2 Representative Percentages of Site Required for

Volume and Peak Control ...........................................A-17

Exhibits

A. Storage Volume Required to Maintain the Pre-Development

Runoff Volume Using Retention Storage .................................A-25

B. Storage Volume Required to Maintain the Pre-Development

Peak Runoff Rate Using 100% Retention Storage...................A-27

C. Storage Volume Required to Maintain the Pre-Development

Peak Runoff Rate Using 100% Detention Storage ..................A-29
Appendix A-3

Low-Impact Development: An Integrated Environmental Design Approach

A.1 Introduction

The Appendix provides a detailed example of an LID hydrologic

computation based on the use of the SCS TR-55 hydrologic model.

This example computation is adapted from the Low-Impact Development, Hydrologic Analysis Prince Georges County, Maryland (1999).

The hydrologic analysis of low-impact development is a sequential

decision-making process that can be illustrated by the flow chart

shown in Figure A.1. Several iterations may occur within each step

until the appropriate approach to reduce stormwater impacts is determined. The procedures for each step are described below. Supporting

design charts have been developed to determine the amount of storage

required to maintain the existing volume and peak runoff rates to

satisfy typical storm water management requirements at different

geographic areas in the nation (Types I, IA, II and III storms). A few

representative examples of these charts are provided in Exhibits A, B,

and C.

A.2 Data Collection

The basic information used to develop the low-impact development site plan and used to determine the runoff curve number (CN)

and time of concentration (Tc) for the pre- and postdevelopment

condition is the same as conventional site plan and stormwater

management approaches.

A.3 Determining the LID Runoff Curve Number

The determination of the low-impact development CN requires a

detailed evaluation of each land cover within the development site.

This will allow the designer to take full advantage of the storage and

infiltration characteristics of low-impact development site planning to

maintain the CN. This approach encourages the conservation of more

woodlands and the reduction of impervious area to minimize the needs

of IMPs.

The steps for determining the low-impact development CN are as

follows:

Step 1: Determine percentage of each land use/cover.

In conventional site development, the engineer would refer to

Figure 2.2.a of TR-55 (SCS, 1986) to select the CN that represents

the proposed land use of the overall development (i.e., residential,

commercial) without checking the actual percentages of impervious

area, grass areas, etc. Because low-impact design emphasizes minimal
A-4 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

Figure A.1. Low-impact development analysis procedure
Appendix A-5

Low-Impact Development: An Integrated Environmental Design Approach

site disturbance (tree preservation, site fingerprinting, etc.), it is

possible to retain much of the pre-development land cover and CN.

Therefore, it is appropriate to analyze the site as discrete units to

determine the CN. Table A.1 lists representative land cover CNs used

to calculate the composite custom low-impact development CN.

Step 2: Calculate composite custom CN.

The initial composite CN is calculated

using a weighted

approach based on

individual land covers

without considering

disconnectivity of the

site imperviousness.

This is done using

Equation A.1. This

weighted approach is

illustrated in Example A.1.

j

j j

c A A A

CN A CN A CN A

CN

+ +

+ + = ...

...

1 2

1 1 2 2 Eq. A.1

Where:

CNc = composite curve number;

Aj

 = area of each land cover; and

CNj = curve number for each land cover.

Overlays of SCS Hydrologic Soil Group boundaries onto homogeneous land cover areas are used to develop the low-impact development CN. What is unique about the low-impact development custommade CN technique is the way this overlaid information is analyzed as

small discrete units that represent the hydrologic condition, rather

than a conventional TR-55 approach that is based on a representative

national average. This is appropriate because of the emphasis on

minimal disturbance and retaining site areas that have potential for

high storage and infiltration. This custom-made CN technique is

documented in Example A.1.

This approach provides an incentive to save more trees and

maximize the use of HSG A and B soils for recharge. Careful planning

can result in significant reductions in post-development runoff volume

and corresponding IMP costs.

Land Use/Cover Curve Number for Hydrologic Soils Groups1

A B CD

Impervious Area 98 98 98 98

Grass 39 61 74 80

Woods (fair condition) 36 60 73 79

Woods (good condition) 30 55 70 77

1Figure 2.2a, TR-55 (SCS, 1986).

Table A.1. Representative LID Curve Numbers
A-6 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

Step 3: Calculate low-impact development CN based on

the connectivity of site impervious area.

When the impervious areas are less than 30 percent of the

site, the percentage of the unconnected impervious areas within

the watershed influences the calculation of the CN (SCS, 1986).

Disconnected impervious areas are impervious areas without any

direct connection to a drainage system or other impervious surface.

For example, roof drains from houses could be directed onto lawn

areas where sheet flow occurs, instead of to a swale or driveway. By

increasing the ratio of disconnected impervious areas to impervious areas on the site, the CN and resultant runoff volume can be

reduced. Equation A.2 is used to calculate the CN for sites with

less than 30 percent impervious area.

(98 ) (1 0.5 ) 100CN R

P

CN CN p

imp

c p × − × − 













 = + Eq. A.2

where:

R = ratio of unconnected impervious area to total impervious

 area;

CNc

 = composite CN;

CNp = composite pervious CN; and

Pimp = percent of impervious site area.

Example A.1 uses steps 1 through 3 to compare the calculation

of the curve number using conventional and low-impact development techniques using the percentages of land cover for a typical 1-

acre residential lot from Figure A.2.

Figure A.2. Comparison of

land covers between

conventional and LID CNs
Appendix A-7

Low-Impact Development: An Integrated Environmental Design Approach

Example A.1

Detailed CN Calculation

Given:

One-acre residential lot

Conventional CN: 68 (From TR-55 Table 2.2a-Runoff curve numbers

for urban areas (SCS, 1986)) Table 2.2a assumes HSG B, 20% imperviousness with a CN of 98 and 80% open space in good condition.

Custom-made LID CN: CN for individual land covers based on

Table 2.2a. Assume 25% of the site will be used for reforestation/

landscaping (see Figure A.2) HSG B.

Procedure:

Step 1: Determine percentage of each land cover occurring on

site and the CN associated with each land cover.

Land

% of Coverage

HSG CN Site (ft2)

Land Use (1) (2) (3) (4)

Impervious (Directly Connected) B 98 5 2,178

Impervious (Unconnected) B 98 10 4,356

Open Space (Good Condition, Graded) B 61 60 26,136

Woods (Fair Condition) B 55 25 10,890

Step 2: Calculate composite custom CN (using Equation A.1).

43,560

98× 4,356 + 98× 2,178 + 61× 26,136 + 55×10,890 CNc =

CNc = 65

Step 3: Calculate low-impact development CN based on the

connectivity of the site imperviousness (using Equation A.2).

CN p = 61 26 136 55 10 890 × +×

37 026

, ,

,

CN p = 59 2.

R = 10

15

R = 0 67 .

( ) CN ( ) R

P

CN CN p

imp

c p × − × − × 













 = + 98 1 0.5

100

( )( ) 98 59.2 1 0.5 0.67

100

15 59.2 × − × − × 

 



 CNc = +

= 63.1(use 63) CNc

LID custom CN of 63 is less than conventional CN of 68

(predevelopment CN is 55).
A-8 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

A.4 Development of the Time of Concentration (Tc)

The pre- and postdevelopment calculation of the Tc for lowimpact development is exactly the same as that described in the TR-55

(SCS, 1986) and NEH-4 (SCS, 1985) manuals.

A.5 Low-Impact Development Stormwater

Management Requirements

Once the CN and Tc are determined for the pre- and

postdevelopment conditions, the stormwater management storage

volume requirements can be calculated. The low-impact development

objective is to maintain all the predevelopment volume, predevelopment

peak runoff rate, and frequency. The required storage volume is calculated using the design charts in Exhibits A (page A-25), B (page A-27),

and C (page A-29) for different geographic regions in the nation.

As stated previously, the required storage volume for peak runoff

control is heavily depended on the intensity of rainfall (rainfall distribution). Since the intensity of rainfall varies considerably over geographic

regions in the nation, National Resource Conservation Service (NRCS)

developed four synthetic 24-hour rainfall distributions (I, IA, II, and III)

from available National Weather Service (NWS) duration-frequency

data and local storm data. Type IA is the least intense and type II the

most intense short-duration rainfall. Figure A.3. shows approximate

geographic boundaries for these four distributions.

Figure A.3. Approximate

geographic boundaries for

NRCS rainfall

distributions
Appendix A-9

Low-Impact Development: An Integrated Environmental Design Approach

The remaining low-impact development hydrologic analysis

techniques are based on the premise that the post-development Tc is

the same as the pre-development condition. If the post-development

Tc does not equal the pre-development Tc, additional low-impact

development site design techniques must be implemented to maintain

the Tc.

Three series of design charts are needed to determine the

storage volume required to control the increase in runoff volume

and peak runoff rate using retention and detention practices. The

required storages shown in these design charts are presented as a

depth in hundredths of an inch (over the development site area).

Equation A.3 is used to determine the volume required for IMPs.

Volume = (depth obtained from the chart)

 x ( development size)/100 Eq. A.3

It is recommended that 6-inch depth be the maximum depth for

bioretention basins used in low-impact development.

The amount, or depth, of exfiltration of the runoff by infiltration

or by the process of evapotranspiration is not included in the design

charts. Reducing surface area requirements through the consideration

of these factors can be determined by using Equation A.4.

Volume of site area for IMPs = (initial volume) x (100  x) / 100 Eq. A.4

where: x = % of the storage volume infiltrated and/or reduced by

evaporation or transpiration. x% should be minimal (less than 10% is

considered).

Stormwater management is accomplished by selecting the appropriate IMP, or combination of IMPs, to satisfy the surface area and

volume requirements calculated from using the design charts as

described below. The design charts to be used to evaluate these

requirements are:

• Chart Series A: Storage Volume Required to Maintain the

Predevelopment Runoff Volume Using Retention Storage

(Exhibit A).

• Chart Series B: Storage Volume Required to Maintain the

Predevelopment Peak Runoff Rate Using 100% Retention

(Exhibit B).

• Chart Series C: Storage Volume Required to Maintain the

Predevelopment Peak Runoff Rate Using 100% Detention

(Exhibit C).
A-10 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

These charts are based on the following general conditions:

• The land uses for the development are relatively homogeneous

throughout the site.

• The stormwater management measures are to be distributed

evenly across the development, to the greatest extent possible.

• The rainfall (design storm event) is based on 1-inch increments.

Use linear interpolation for determining intermediate values.

The procedure to determine the IMP requirements is outlined in

Figure A.4 and described in the following sections.

Step 1: Determine storage volume required to maintain

predevelopment volume or CN using retention storage.

The post-development runoff volume generated as a result of the

post-development custom-made CN is compared to the

predevelopment runoff volume to determine the surface area required

for volume control. Use Chart Series A: Storage Volume Required to

Maintain the Predevelopment Runoff Volume using Retention Storage. The procedure for calculating the site area required for maintaining runoff volume is provided in Example A.2. It should be noted that

the practical and reasonable use of the site must be considered. The

IMPs should not restrict the use of the site, unless the regulatory

authority decides that the sensitivity of the receiving water body

requires such restrictions.

The storage area found, is for runoff volume control only;

additional storage may be required for water quality control. The

procedure to account for the first 1⁄2-inch of runoff from impervious

areas, which is the current water quality requirement, is found in

Step 2.

Step 2: Determine storage volume required for water

quality control.

The surface area, expressed as a percentage of the site, is then

compared to the percentage of site area required for water quality

control. The volume requirement for stormwater management quality

control is based on the requirement to treat the first 1⁄2 inch of runoff

(approximately 1,800 cubic feet per acre) from impervious areas. This

volume is translated to a percent of the site area by assuming a storage

depth of 6 inches. The procedure for calculating the site area required
Appendix A-11

Low-Impact Development: An Integrated Environmental Design Approach

Figure A.4. Procedure to determine percentage of site area required for IMPs to maintain predevelopment

runoff volume and peak runoff rate.
A-12 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

Example A.2

Determining Site Area Required to Maintain Volume (CN) Using

Chart Series A: Storage Volume Required to Maintain the

Predevelopment Runoff Volume Using Retention Storage

Given:

Site Area is 18 acres

Existing CN is 60

Proposed CN is 65

Design storm is 5 inches

Design depth of IMP is 6 inches

Solution:

Use Chart Series A: Storage Volume Required to Maintain Runoff

Volume or CN.

0.35 inch of storage over the site is required to maintain the runoff

volume.

Therefore: if 6-inch design depth is used, 1.1 acres (18 acres x

0.35 / 6) of IMPs distributed evenly throughout the site are required to

maintain the runoff volume, or CN.

Additional Considerations:

1) Account for depths other than 6 inches:

Site of IMP Area = 1.1 acres, if 6-inch depth is used

Depth of IMPs = 4 inches

Site of IMP Area = 1.1 x 6 in./4 in.

Site of IMP Area = 1.65 acres

2) Account for infiltration and/or evapotranspiration (using

Equation A.4)

If 10% of the storage volume is infiltrated and/or reduced by

evaporation and transpiration.

Site of IMP Area = (storage volume) x (100 - X) / 100

Site of IMP Area =1.1 x (100-10)/100

Area for IMP Storage = 1.0 acre
Appendix A-13

Low-Impact Development: An Integrated Environmental Design Approach

for quality control is provided in Example A.3. The greater number,

or percent, is used as the required storage volume to maintain the

CN.

From the results of Example A.3, 0.1 of storage is required for

water quality using retention; from Example A.2, 0.35 of storage is

required to maintain the runoff volume using retention. Since the

volume required to maintain the runoff volume is larger, in this case

0.35 of storage over the site should be reserved for retention IMPs.

Step 3: Determine storage volume required to maintain

peak stormwater runoff rate using 100 percent retention.

The percentage of site area or amount of storage required to

maintain the predevelopment peak runoff rate is based on Chart Series

B: Percentage of Site Area Required to Maintain Predevelopment Peak

Runoff Rate Using 100% Retention (Exhibit B). This chart is based on

the relationship between storage volume, ∀

∀

s

r

, and discharge, i

o

Q

Q ,

to maintain the predevelopment peak runoff rate.

Where: Vs = volume of storage to maintain the predevelopment

peak runoff rate using 100% retention;

Vr = postdevelopment peak runoff volume;

Qo = peak outflow discharge rate; and

Qi = peak inflow discharge rate.

Example A.3

Calculation of Volume, or Site Area, for Water Quality Control

Given:

Site area is 18 acres

Impervious area is 3.6 acres (20%)

Depth of IMP is 6 inches

Solution:

The water quality requirement is to control the first 1⁄2 inch

of runoff from impervious areas (18 acres x 20%) x 0.5in. / 18

acres = 0.1 inch storage for water quality 0.1 inch is less than

0.35 inch (from example A.2). Therefore, use storage for runoff

volume control to meet the water quality requirement.
A-14 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

The relationship for retention storage to control the peak runoff

rate is similar to the relationship for detention storage. Figure A.5 is an

illustration of the comparison of the storage volume/discharge relationship for retention and detention. Curve A is the relationship of

storage volume to discharge to maintain the predevelopment peak

runoff rate using the detention relationship from Figure 6-1 (SCS,

1986) for a Type II 24-hour storm event. Curve B is the ratio of

storage volume to discharge to maintain the predevelopment peak

runoff rate using 100 percent retention. Note that the volume required to maintain the peak runoff rate using detention is less than the

requirement for retention. This is graphically demonstrated in

Figure A.6.

• Hydrograph 2 represents the response of a postdevelopment

condition with no stormwater management IMPs. This hydrograph

definition reflects a shorter time of concentration (Tc), and

increase in total site imperviousness than that of the

predevelopment condition. This resultant hydrograph shows a

decrease in the time to reach the peak runoff and discharge rate

and volume, and increased duration of the discharge volume.

• Hydrograph 8 illustrates the effect of providing additional detention storage to reduce the postdevelopment peak discharge rate to

predevelopment conditions.

Figure A.5. Comparison

of retention of storage

volumes required to

maintain peak runoff

rate using retention and

detention.
Appendix A-15

Low-Impact Development: An Integrated Environmental Design Approach

∀1 is the storage volume required to maintain the

predevelopment peak discharge ratio using 100% detention storage.

The combination of ∀1 and ∀2 is the storage volume required to

maintain the predevelopment peak discharge rate using 100%

retention storage.

The following calculations apply to Design Chart Series B:

• The Tc for the postdevelopment condition is equal to the Tc for

the predevelopment condition. This equality can be achieved by

techniques such as maintaining sheet flow lengths, increasing

surface roughness, decreasing the amount and size of storm drain

pipes, and decreasing open channel slopes. Chapter 2 of this

manual provides more details on these techniques.

• The depth of storage for the retention structure is 6 inches. For

other depths, see Example A.2.

If the Tc is equal for the predevelopment and postdevelopment

conditions, the peak runoff rate is independent of Tc for retention and

detention practices. The difference in volume required to maintain the

predevelopment peak runoff rate is practically the same if the Tcs for

the predevelopment and postdevelopment conditions are the same.

These concepts are illustrated in Figure A.7. In Figure A.7, the

difference in the required IMP area between a Tc of 0.5 and a Tc of 2.0

Figure A.6. Storage volume

required to maintain peak

runoff rate
A-16 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

is minimal if the predevelopment and postdevelopment Tcs are

maintained.

Step 4: Determine whether additional detention storage is

required to maintain the predevelopment peak runoff rate.

The storage volume required to maintain the predevelopment

runoff volume using retention, as calculated in Step 1, might or might

not be adequate to maintain both the predevelopment volume and

peak runoff rate. As the CNs diverge, the storage requirement to

maintain the volume is much greater than the storage volume required

to maintain the peak runoff rate. As the CNs converge, however, the

storage required to maintain the peak runoff rate is greater than that

required to maintain the volume. Additional detention storage will be

required if the storage volume required to maintain the runoff volume

(determined in Step 1) is less than the storage volume required to

maintain the predevelopment peak runoff rate using 100 percent

retention (determined in Step 3).

The combination of retention and detention practices is defined as

a hybrid IMP. The procedure for determining the storage volume

required for the hybrid approach is described in Step 5.

Table A.2 illustrates the percentage of site area required for

volume and peak control for representative curve numbers. Using a 5-

inch type II 24-hour storm event and 6 design depth, with a

predevelopment CN of 60, the following relationships exist:

• For a post-development CN of 65, 5.9 percent of the site area

(column 4) is required for retention practices to maintain the

Figure A.7. Comparison

of storage volumes for

various Tcs.
Appendix A-17

Low-Impact Development: An Integrated Environmental Design Approach

Table A.2. Representative Percentages of Site Required for Volume and Peak Control

Runoff Curve No. % of Area Needed for BMP

Type of

24-Hour

Storm

Event

(1)

Existing

(2)

Proposed

(3)

Volume Control

Using 100%

Retention

Chart Series A

(4)

Peak Control

Using 100%

Retention

Chart Series B

(5)

Peak Control

Using 100%

Detention Chart

Series C

(6)

Hybrid

Design

(Eq. 4.6)

(7)

Percent of

Volume

Retention

for Hybrid

Design

(Eq. 4.5)

(8)

50

55

60

65

70

80

1.7

4.0

6.9

10.4

19.3

1.6

3.4

6.2

9.3

18.0

0.9

2.4

4.5

7.3

15.8

1.7

4.0

6.9

10.4

19.3

100

100

100

100

100

60

65

70

75

90

2.9

6.3

10.5

27.5

3.9

6.7

10.0

24.9

2.3

4.4

7.1

18.7

3.6

6.6

10.5

27.5

80

96

100

100

70

75

80

85

90

4.1

8.9

14.6

21.2

5.9

9.7

13.9

18.7

3.4

5.8

8.8

12.6

5.3

9.5

14.6

21.2

77

94

100

100

3"

75

80

85

90

4.8

10.5

17.1

7.5

11.8

16.6

4.2

7.0

10.2

6.6

11.4

17.1

73

91

100

50

55

60

65

70

80

4.8

10.1

16.0

22.4

36.7

6.9

11.1

15.6

20.6

32.8

4.0

6.9

10.4

14.5

23.9

6.3

10.9

16.0

22.4

36.7

77

93

100

100

100

60

65

70

75

90

5.9

12.3

19.1

42.9

9.5

14.6

19.8

37.2

5.3

8.4

12.0

25.3

8.3

13.9

19.6

42.9

71

88

97

100

70

75

80

85

90

6.9

14.3

22.2

30.7

13.2

18.9

24.5

30.5

7.2

10.7

14.3

18.2

10.9

17.4

23.8

30.7

63

82

93

100

5"

75

80

85

90

7.4

15.3

23.8

15.0

20.6

26.7

8.1

11.6

15.2

12.3

18.9

25.7

60

81

92

50

55

60

65

70

80

7.6

15.6

23.9

32.5

50.5

12.3

18.6

25.0

31.4

44.5

6.8

10.7

15.1

19.6

30.0

10.7

17.7

24.7

32.5

50.5

71

88

97

100

100

60

65

70

75

90

8.3

16.9

25.8

53.7

16.6

23.2

29.9

49.7

9.0

13.2

17.3

30.7

13.6

21.2

28.7

53.7

61

80

90

100

70

75

80

85

90

8.9

17.9

27.2

36.7

20.4

26.8

33.4

42.3

10.9

14.7

18.9

23.0

16.1

23.8

31.5

39.2

55

75

87

94

7"

75

80

85

90

9.1

18.4

27.9

22.1

28.6

35.3

11.5

15.6

19.8

17.1

25.1

32.9

53

73

85
A-18 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

predevelopment volume. To maintain the predevelopment peak

runoff rate (column 5), 9.5 percent of the site is required.

Therefore, additional detention storage or a hybrid approach

(calculated in column 7) is required.

• For a postdevelopment CN of 90, 42.9 percent of the site area

(column 4) is required for retention practices to maintain the

predevelopment volume. To maintain the predevelopment peak

runoff rate (column 5) 37.2 percent of the site is required. Therefore, the storage required to maintain the runoff volume is also

adequate to maintain the peak runoff rate. However, 42.9 percent

of the site for IMPs may not be a practical and reasonable use of

the site. Refer to Step 7, hybrid approach, for a more reasonable

combination of retention and detention storage.

Step 5: Determine storage required to maintain

predevelopment peak runoff rate using 100 percent

detention. (This step is required if additional detention

storage is needed.)

Chart Series C: Storage Volume Required to Maintain the

Predevelopment Peak Runoff Rate Using 100% Detention is used to

determine the amount of site area to maintain the peak runoff rate

only. This information is needed to determine the amount of detention storage required for hybrid design, or where site limitations

prevent the use of retention storage to maintain runoff volume. This

includes sites that have severely limited soils for infiltration or retention practices. The procedure to determine the site area is the same

as that of Step 3. Using Chart Series C, the following assumptions

apply:

• The Tc for the post-development condition is equal to the Tc for

the predevelopment condition.

• The storage volume, expressed as a depth in hundredths of an

inch (over the development site), is for peak flow control.

These charts are based on the relationship and calculations from

Figure 6.1 (Approximate Detention Basin Routing for Rainfall Types I,

IA, II and III) in TR-55 (SCS, 1986).

Step 6: Use hybrid facility design (required for additional

detention storage).

When the percentage of site area for peak control exceeds that

for volume control as determined in Step 3, a hybrid approach must

be used. For example, a dry swale (infiltration and retention) may

incorporate additional detention storage. Equation A.5 is used to
Appendix A-19

Low-Impact Development: An Integrated Environmental Design Approach

determine the ratio of retention to total storage. Equation A.6 is

then used to determine the additional amount of site area, above

the percentage of site required for volume control, needed to

maintain the predevelopment peak runoff rate.

 ( 4 ( ) ) ( )

50 100 100 100 2

100

100 100

x D D R D R

R D

 ́ -" + " +  ́ " - "  ́ "

" - " = Eq. A.5

where

∀R = Storage Volume required to maintain predevelopment

runoff volume (Chart Series A)

∀R100 = Storage Volume required to maintain predevelopment

peak runoff rate using 100% retention (Chart Series B)

∀D100 = Storage Volume required to maintain predevelopment

peak runoff rate using 100% detention (Chart Series C)

x = Area ratio of retention storage to total storage

and the hybrid storage can be determined as:

H = ∀R x (100 ÷ x) Eq. A.6

Equations A.5 and A.6 are based on the following assumptions:

• x% of the total storage volume is the retention storage required to

maintain the predevelopment CN calculated from Chart Series A:

Storage Volume Required to Maintain Predevelopment Volume

using Retention Storage.

• There is a linear relationship between the storage volume required

to maintain the peak predevelopment runoff rate using 100%

retention and 100% detention (Chart Series B and C)

The procedure for calculating hybrid facilities size is shown in

Example A.4.

Step 7: Determine hybrid amount of IMP site area

required to maintain peak runoff rate with partial volume

attenuation using hybrid design (required when retention

area is limited).

Site conditions, such as high percentage of site needed for retention storage, poor soil infiltration rates, or physical constraints, can

limit the amount of site area that can be used for retention practices.

For sites with poor soil infiltration rates, bioretention is still an acceptable alternative, but an underdrain system must be installed. In this

case, the bioretention basin is considered detention storage.
A-20 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

When this occurs, the site area available for retention IMPs is

less than that required to maintain the runoff volume, or CN. A

variation of the hybrid approach is used to maintain the peak runoff

rate while attenuating as much of the increased runoff volume as

possible. First, the appropriate storage volume that is available for

Example A.4:

Calculation of Additional Storage Above Volume Required to

Maintain CN and Maintain Predevelopment Peak Runoff Rate Using

Hybrid Approach

Given:

• 5-inch Storm Event with Rainfall Distribution Type II

• Existing CN = 60

• Proposed CN = 65

• Storage volume required to maintain volume (CN) using retention

storage = 0.35 inch (from Chart Series A)

• Storage volume required to maintain peak runoff rate using 100%

retention = 0.62 inch (from Chart Series B)

• Storage volume required to maintain peak runoff rate using 100%

detention = 0.31 inch (from Chart Series C)

Step 1: Solve for x (ratio of retention to total storage) using

Equation A.5:

( ) c = ( ) -  ́- + +  ́ -  ́ 50

62 31

31 31 4 62 31 35 2

. . . . (. . ) .

χ = 68

Therefore: 0.35 inch of storage needed for runoff volume control is

68% of the total volume needed to maintain both the predevelopment

volume and peak runoff rates.

Step 2: Solve for the total area to maintain both the peak runoff

rate and volume using Equation A.6. Therefore, the difference

between 0.35 inch and 0.51 inch is the additional detention area

needed to maintain peak discharge.

H = 0.35 x  ́ 100

68

H = 0.51 inch

Therefore , the difference between 0.35 inch and 0.51 inch is the

additional detention area needed to maintain peak discharge.
Appendix A-21

Low-Impact Development: An Integrated Environmental Design Approach

runoff volume control (∀R′) is determined by the designer by

analyzing the site constraints. Equation A.7 is used to determine the

ratio of retention to total storage. Equation A.8 is then used to

determine the total site IMP area in which the storage volume

available for retention practices (∀R′) substitutes the storage

volume required to maintain the runoff volume.

( ) ( ) ( ) ¢ = " -" c  ́ -" + " +  ́ " - "  ́" ¢ 50

4

100 100

100 100

2

100 100

R D

D D RD R 8 Eq.

A.7

Where ∀R′ = storage volume acceptable for retention IMPs. The

total storage with limited retention storage is:

H′ = ∀R′ x (100 ÷ x ′) Eq. A.8

where H′ is hybrid area with a limited storage volume available for

retention IMPs.

Example A.5 illustrates this approach.

A.6 Determination of Design Storm Event

Conventional stormwater management runoff quantity control is

generally based on not exceeding the predevelopment peak runoff rate

for the 2-year and 10-year 24-hour Type II storm events. The amount

of rainfall used to determine the runoff for the site is derived from

Technical Paper 40 (Department of Commerce, 1963). For Prince

Georges County, these amounts are 3.3 and 5.3 inches, respectively.

The 2-year storm event was selected to protect receiving channels

from sedimentation and erosion. The 10-year event was selected for

adequate flow conveyance considerations. In situations where there is

potential for flooding, the 100-year event is used.

The criteria used to select the design storm for low-impact

development are based on the goal of maintaining the

predevelopment hydrologic conditions for the site. The determination of the design storm begins with an evaluation of the

predevelopment condition. The hydrologic approach of low-impact

development is to retain the same amount of rainfall within the

development site as that retained by woods (or meadows, if they

were the natural historical landscape), in good condition, and then

to gradually release the excess runoff as woodlands would release it.

By doing so, we can emulate, to the greatest extent practical, the

predevelopment hydrologic regime to protect watershed and natural

habitats. Therefore, the predevelopment condition of the lowimpact development site is required to be woods in good condition.
A-22 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

Example A.5:

Calculation of Percentage of Site Area Required to Maintain the

Peak Runoff Rate Using the Hybrid Approach of Retention and

Detention

Given:

• 5-inch storm event with rainfall distribution Type II

• Existing CN = 60

• Proposed CN = 65

• Storage volume required to maintain volume (CN) = 0.35 inch

(From Chart Series A)

• Storage volume required to maintain peak runoff rate using 100%

retention = 0.62 inch (from Chart Series B)

• Storage volume required to maintain peak runoff rate using 100%

detention = 0.31 inch (from Chart Series C)

• Only half of the required site area is suitable for retention practices, remainder must incorporate detention.

(∀R′ = 0.35 x 0.50 = 0.18 inch)

Step 1: Determine appropriate amount of overall IMP area

suitable for retention practices. Half of area is appropriate (given

above). Use Equation A.7:

( ) ¢ = ( ) - c  ́- + +  ́ -  ́

50

62 31

31 31 4 62 31 18 2

. . . . (. . ) .

χ' = 41.2%

Therefore, 0.35 inch of site area available for runoff volume

control is 41.2% of the total volume needed for maintaining the

predevelopment peak runoff rate.

Step 2: Solve for the total area required to maintain the peak

runoff rate using Equation A.8.

H 0 18 ¢ =  ́ 100

41 2 . .

H' = 0.43 inch

Therefore, totally 0.43 inch of the site is required to maintain the

predevelopment peak runoff rate but not the runoff volume. Of the 0.43

inch storage, 0.18 inch of the storage is required for retention volume.
Appendix A-23

Low-Impact Development: An Integrated Environmental Design Approach

This requirement is identical to the State of Marylands definition of

the predevelopment condition. The CN for the predevelopment

condition is to be determined based on the land cover being woods

in good condition and the existing HSG. The design storm is to be

the greater of the rainfall at which direct runoff begins from a woods

in good condition, with a modifying factor, or the 1-year 24-hour

storm event. The rainfall at which direct runoff begins is determined using Equation A.9. The initial rainfall amount at which

direct runoff begins from a woodland is modified by multiplying this

amount by a factor of 1.5 to account for the slower runoff release

rate under the wooded predevelopment condition.

P = 0.2 x ÷

÷

ø

ö

ç

ç

è

æ -10 1000

CNc Eq. A.9

where P is rainfall at which direct runoff begins.

It should be noted that this assumption will need to be adjusted

for communities with different climatic conditions such as the arid

southwest or the great plains.

A three-step process, illustrated in Example A.6, is used to

determine the design storm event.

Step 1: Determine the predevelopment CN.

Use an existing land cover of woods in good condition overlaid

over the hydrologic soils group (HSG) to determine the composite site

CN.

Step 2: Determine the amount of rainfall needed to initiate

direct runoff.

Use Equation A.9 to determine the amount of rainfall (P) needed

to initiate direct runoff.

Step 3: Account for variation in land cover.

Multiply the amount of rainfall (P) determined in Step 2 by a

factor of 1.5.

Example A.6 demonstrates this approach.
A-24 Appendix

Low-Impact Development: An Integrated Environmental Design Approach

Example A.6:

Determination of Design Storm

Step 1: Determine the predevelopment CN based on woods (good

condition) and HSG.

Given:

Site condition of 90% HSG soil type B and 10% HSG soil type C,

CNc

 = 0.9 (55) + 0.1 × (70)

CNc ≥ 56.5 ≈ 57 use 57

Step 2: Determine the amount of rainfall to initiate direct runoff

using Equation A.9.

÷

ø

ö ç

è

æ =  ́ -10

57

1000 P 0.2

P = 1.5 inches

Step 3: Multiply the amount of rainfall by a factor of 1.5.

Design rainfall = P x 1.5

Design rainfall = 1.5 inches x 1.5

Design rainfall = 2.25 inches
Appendix A-25

Low-Impact Development: An Integrated Environmental Design Approach

Exhibit A

Storage Volume Required to Maintain the

Predevelopment Runoff Volume

Using Retention Storage
A-26 Appendix

Low-Impact Development: An Integrated Environmental Design Approach
Appendix A-27

Low-Impact Development: An Integrated Environmental Design Approach

Exhibit B

Storage Volume Required to Maintain the

Predevelopment Peak Runoff Rate

Using 100% Retention Storage
A-28 Appendix

Low-Impact Development: An Integrated Environmental Design Approach
Appendix A-29

Low-Impact Development: An Integrated Environmental Design Approach

Exhibit C

Storage Volume Required to Maintain the

Predevelopment Peak Runoff Rate

Using 100% Detention Storage
A-30 Appendix

Low-Impact Development: An Integrated Environmental Design Approach
Sample Maintenance Covenant B-1

Low Impact Development: An Integrated Environmental Design Approach

Appendix B - Sample Maintenance

Covenant

DECLARATION OF COVENANTS

For Storm and Surface Water Facility, and

Integrated Management System Maintenance

THIS DECLARATION OF COVENANTS, made this

_________________ day of ________________, 20___, by

____________________________________________________________________________

hereinafter refered to as the Covenantor(s) to and for the benefit of

(governing bodystate, county, city, etc.) and its successors and assigns

hereinafter referred to as the (State, County, City, etc.).

WITNESSETH:

WHEREAS, the (State, County, City) is authorized and required to

regulate and control the disposition of storm and surface waters within

the County’s Stormwater Management District set forth in (cite governing laws or regulations): and

WHEREAS, Covenantor(s) is (are) the owner(s) of a certain tract

or parcel of land more particularly described as:

__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

being all or part of the land which it acquired by deed dated ________

___________________________ from _________________________

______________________________________________________________________________________

grantors, and recorded among the Land Records of (governing body), in

Liber _______________________ at Folio _____________________

such property being hereinafter referred to as the “the property”; and

WHEREAS, the Covenantor(s) desires to construct certain improvements on its property which will alter the extent of storm and surface

water flow conditions on both the property and adjacent lands: and

WHEREAS, in order to accommodate and regulate these anticipated

changes in existing storm and surface water flow conditions, the

Covenantor(s) desires to build and maintain at its expense, a storm and
B-2 Sample Maintenance Covenant

Low Impact Development: An Integrated Environmental Design Approach

surface water management facility and system more particularly described and shown on plans titled _____________________________

____________________________________________________________________________________________________________________________________________________________________________

and further identified under approval number _____________________

__________________; and _________________________________.

WHEREAS, the (State, County, City, etc.) has reviewed and approved these plans subject to the execution of this agreement.

NOW THEREFORE, in consideration of the benefits received

by the Covenantor(s), as a result of the (State, County, City) approval

of his plans. Covenantor(s), with full authority to execute deeds, mortgages, other covenants, and all rights, title and interest in the property

described above do hereby covenant with the (State, County, City) as

follows:

1. Covenantor(s) shall construct and perpetually maintain, at its sole expense, the above-referenced storm and surface management facility and system in strict accordance with the plan approval

granted by the (State, County, City).

2. Covenantor(s) shall, at its sole expense, make such

changes or modifications to the storm drainage facility and system as

may, in the (State, County, City) discretion, be determined necessary

to insure that the facility and system is properly maintained and continues to operate as designed and approved.

 3. The (State, County, City), its agents, employees and

contractors shall have the perpetual right of ingress and egress over

the property of the Covenantor(s) and the right to inspect at reasonable times and in reasonable manner, the storm and surface water facility and system in order to insure that the system is being properly

maintained and is continuing to perform in an adequate manner.

4. The Covenantor(s) agrees that should it fail to correct any defects in the above-described facility and system within ten

(10) days from the issuance of written notice, or shall fail to maintain

the facility in accordance with the approved design standards and with

the law and applicable executive regulation or, in the event of an emergency as determined by the (State, County, City) in its sole discretion,

the (State, County, City) is authorized to enter the property to make

all repairs, and to perform all maintenance, construction and reconstruction as (State, County, City) deems necessary. The (State, County,

City) shall then assess the Covenantor(s) and/or all landowners served

by the facility for the cost of the work, both direct and indirect, and

applicable penalties. Said assessment shall be a lien against all properties served by the facility and may be placed on the property tax bills of

said properties and collected as ordinary taxes by the (State, County,

City).
Sample Maintenance Covenant B-3

Low Impact Development: An Integrated Environmental Design Approach

5. Covenantor(s) shall indemnify, save harmless and defend the (State, County City) from and against any and all claims, demands, suits, liabilities, losses, damages and payments including attorney fees claimed or made by persons not parties to this Declaration against

the (State, County, City) that are alleged or proven to result or arise

from the Covenantor(s) construction, operation, or maintenance of the

storm and surface water facility and system that is the subject of this

Covenant.

6. The covenants contained herein shall run with the land

and the Covenantor(s) further agrees that whenever the property shall

be held, sold and conveyed, it shall be subject to the covenants, stipulations, agreements and provisions of this Declaration, which shall apply

to, bind and be obligatory upon the Covenantor(s) hereto, its heirs, successors and assigns and shall bind all present and subsequent owners of

the property served by the facility.

7. The Covenantor(s) shall promptly notify the (State,

County, City) when the Covenantor(s) legally transfers any of the

Covenantor(s) responsibilities for the facility. The Covenantor(s) shall

supply the (State, County, City) with a copy of any document of transfer,

executed by both parties.

8. The provisions of this Declaration shall be severable and

if any phrase, clause, sentence or provisions is declared unconstitutional,

or the applicability thereof to the Covenantor is held invalid, the remainder of this Covenant shall not be affected thereby.

9. The Declaration shall be recorded among the Land

Records of (Governing Body) at the Covenantor(s) expense.

10. In the event that the (State, County, City) shall determine at its sole discretion at future time that the facility is no longer

required, then the (State, County, City) shall at the request of the

Covenantor(s) execute a release of this Declaration of Covenants which

the Covenantor(s) shall record at its expenses
B-4 Sample Maintenance Covenant

Low Impact Development: An Integrated Environmental Design Approach

IN WITNESS WHEREOF, the Covenantor(s) have executed

this Declaration of Covenants as of this_______day

of___________________, 20_____.

ATTEST: FOR THE COVENANTOR(S)

_________________________ _________________________

 (Signature) (Signature)

_________________________ _________________________

 (Printed Name) (Printed Name and Title)

STATE OF_____________________ :

COUNTY OF __________________ :

On this________day of______________, 20___, before me,

the undersigned officer, a Notary Public in and for the State and

County aforesaid, personally appeared _________________________,

who acknowledged himself to be__________________________,

of___________________________, and he as such authorized to do

so, executed the foregoing instrument for the purposes therein contained by signing his name as___________________________for

said_______________________________.

WITNESS my hand and Notarial Seal

My commission expires_________________ __________________

 Notary Public

Seen and approved

__________________________________

(Governing Body)
Glossary Glossary
Glossary G-1

Low-Impact Development: An Integrated Environmental Design Approach

Glossary

Bioretention: On-lot retention of stormwater through the use of

vegetated depressions engineered to collect, store, and infiltrate

runoff.

IMP: Best Management Practice; a practice or combination of

practices that are the most effective and practicable (including

technological, economic, and institutional considerations) means of

controlling point or nonpoint source pollutants at levels compatible

with environmental quality goals.

Buffer: A vegetated zone adjacent to a stream, wetland, or

shoreline where development is restricted or controlled to minimize

the effects of development.

Cluster Development: Buildings concentrated in specific areas

to minimize infrastructure and development costs while achieving

the allowable density. This approach allows the preservation of

natural open space for recreation, common open space, and preservation of environmentally sensitive features.

Curbs: Concrete barriers on the edges of streets used to direct

stormwater runoff to an inlet or storm drain and to protect lawns

and sidewalks from vehicles.

Design storm: A rainfall event of specific size, intensity, and

return frequency (e.g.,. the 1-year storm) that is used to calculate

runoff volume and peak discharge rate.

Detention: The temporary storage of stormwater to control

discharge rates, allow for infiltration, and improve water quality.

Dry Well: Small excavated trenches filled with stone to control

and infiltrate rooftop runoff.

EPA: Environmental Protection Agency.
G-2 Glossary

Low-Impact Development: An Integrated Environmental Design Approach

Erosion: The process of soil detachment and movement by the

forces of water.

Filter Strips: Bands of closely-growing vegetation, usually grass,

planted between pollution sources and downstream receiving

waterbodies.

Greenway: A linear open space; a corridor composed of natural

vegetation. Greenways can be used to create connected networks of

open space that include traditional parks and natural areas.

Groundwater: Water stored underground in the pore spaces

between soil particles or rock fractures.

Habitat: An area or type of area that supports plant or animal life.

Hydrology: The science dealing with the waters of the earth,

their distribution on the surface and underground, and the cycle

involving evaporation, precipitation, flow to the seas, etc.

IMP: Intregrated management practice. A LID practice or

combination of practices that are the most effective and practicable

(including technological, economic, and institutional considerations) means of controlling the predevelopment site hydrology.

Impervious Area: A hard surface area (e.g., parking lot or

rooftop) that prevents or retards the entry of water into the soil, thus

causing water to run off the surface in greater quantities and at an

increased rate of flow.

Imperviousness Overlay Zoning: One form of the overlay

zoning process. Environmental aspects of future imperviousness are

estimated based on the future zoning build-out conditions. Estimated impacts are compared with watershed protection goals to

determine the limit for total impervious surfaces in the watershed.

Imperviousness overlay zoning areas are then used to define subdivision layout options that conform to the total imperviousness limit.

Incentive Zoning: Zoning that provides for give-and-take

compromise on zoning restrictions, allowing for more flexibility to

provide environmental protection. Incentive zoning allows a developer to exceed a zoning ordinances limitations if the developer

agrees to fulfill conditions specified in the ordinance. The developer

may be allowed greater lot yields by a specified amount in exchange

for providing open spaces within the development.
Glossary G-3

Low-Impact Development: An Integrated Environmental Design Approach

Infiltration: The downward movement of water from the land

surface into the soil.

Level Spreader: An outlet designed to convert concentrated

runoff to sheet flow and disperse it uniformly across a slope to

prevent erosion.

Low-Impact Development: The integration of site ecological

and environmental goal and requirements into all phases of urban

planning and design from the individual residential lot level to the

entire watershed.

Nonpoint Source Pollution: Water pollution caused by rainfall

or snowmelt moving both over and through the ground and carrying

with it a variety of pollutants associated with human land uses. A

nonpoint source is any source of water pollution that does not meet

the legal definition of point source in section 502(14) of the Federal

Clean Water Act.

NPDES: National Pollutant Discharge Elimination System; a

regulatory program in the Federal Clean Water Act that prohibits

the discharge of pollutants into surface waters of the United States

without a permit.

Open Space: Land set aside for public or private use within a

development that is not built upon.

Overlay Districts: Zoning districts in which additional regulatory standards are superimposed on existing zoning. Overlay districts

provide a method of placing special restrictions in addition to those

required by basic zoning ordinances.

Performance Zoning: Establishes minimum criteria to be used

when assessing whether a particular project is appropriate for a

certain area; ensures that the end result adheres to an acceptable

level of performance or compatibility. This type of zoning provides

flexibility with the well-defined goals and rules found in conventional zoning.

Permeable: Soil or other material that allows the infiltration or

passage of water or other liquids.

Planned Unit Development (PUD) Zoning: Planned unit

development provisions allow land to be developed in a manner that

does not conform with existing requirements of any of the standard

zoning districts. The PUD allows greater flexibility and innovation
G-4 Glossary

Low-Impact Development: An Integrated Environmental Design Approach

than conventional standards because a planned unit is regulated as

one unit instead of each lot being regulated separately.

Rain Barrels: Barrels designed to collect and store rooftop

runoff.

Recharge Area: A land area in which surface water infiltrates

the soil and reaches the zone of saturation or groundwater table.

Riparian Area: Vegetated ecosystems along a waterbody

through which energy, materials, and water pass. Riparian areas

characteristically have a high water table and are subject to periodic

flooding.

Runoff: Water from rain, melted snow, or irrigation that flows

over the land surface.

SCS: U.S. Department of Agriculture Soil Conservation Service;

renamed the Natural Resources Conservation Service (NRCS).

Site Fingerprinting: Development approach that places development away from environmentally sensitive areas (wetlands, steep

slopes, etc.), future open spaces, tree save areas, future restoration

areas, and temporary and permanent vegetative forest buffer zones.

Ground disturbance is confined to areas where structures, roads,

and rights-of-way will exist after construction is complete.

Subdivision: The process of dividing parcels of land into smaller

building units, roads, open spaces, and utilities.

Swale: An open drainage channel designed to detain or infiltrate stormwater runoff.

Urbanization: Changing land use from rural characteristics to

urban (city-like) characteristics.

Urban Sprawl: Development patterns, where rural land is

converted to urban uses more quickly than needed to house new

residents and support new businesses. As a result people become

more dependent on automobiles and have to commute farther.

Sprawl defines patterns of urban growth that include large acreage

of low-density residential development, rigid separation between

residential and commercial uses, residential and commercial development in rural areas away from urban centers, minimal support for

nonmotorized transportation methods, and a lack of integrated

transportation and land use planning.
Glossary G-5

Low-Impact Development: An Integrated Environmental Design Approach

USGS: United States Geological Survey, an agency within the

Department of the Interior.

Watershed: The topographic boundary within which water

drains into a particular river, stream, wetland, or body of water.

Watershed-based Zoning: Zoning that achieves watershed

protection goals by creating a watershed development plan, using

zoning as the basis (flexible density and subdivision layout specifications), that falls within the range of density and imperviousness

allowable for the watershed to prevent environmental impacts.

Watershed-based zoning usually employs a mixture of zoning practices.

Wet pond: A stormwater management pond designed to detain

urban runoff and always contain water.

Zero-lot-line Development: A development option in which

side yard restrictions are reduced and the building abuts a side lot

line. Overall unit-lot densities are therefore increased. Zero-lot-line

development can result in increased protection of natural resources,

as well as reduction in requirements for road and sidewalk.

Zoning: Regulations or requirements that govern the use,

placement, spacing, and size of land and buildings within a specific

area.