low impact management

LOW IMPACTDEVELOPMENT

AND SUSTAINABLESTORMWATERMANAGEMENT

LOW IMPACTDEVELOPMENT

AND SUSTAINABLESTORMWATERMANAGEMENT

Thomas H. Cahill, P.E.Consultant

Water Resources Engineering

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright © 2012 by John Wiley & Sons. All rights reserved.

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Library of Congress Cataloging-in-Publication Data:

Cahill, Thomas H., 1939–Low impact development and sustainable stormwater management / Thomas H. Cahill.

p. cm.Includes index.ISBN 978-0-470-09675-8 (cloth)

1. Urban runoff—Management. 2. Sustainable urban development. I. Title.TD657.C34 2012628′.212—dc23

2011037607

Printed in the United States of America

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CONTENTS

Prologue: Habitat, Sustainability, and StormwaterManagement xi

Acknowledgments xiii

1 Rainwater as the Resource 1

1.1 The Water Balance as a Guide for Sustainable Design / 11.2 The Water Balance by Region / 71.3 Arid Environments: The Southern California Model / 11

The Energy Demand for Water in Southern California / 131.4 The Altered Water Balance and Hydrologic Impacts / 16

Imperviousness / 16Increased Volume of Runoff / 20

1.5 The Impacts of Development on the Hydrologic Cycle / 24Reduced Groundwater Recharge / 24Reduced Stream Base Flow / 25Altered Stream Channel Morphology / 26Water Supply Impacts / 26

1.6 The Historic Approach: Detention System Design / 271.7 Stormwater Volume Methodologies / 30

2 Stormwater Hydrology and Quality 33

2.1 Overland Flow: The Beginning of Runoff / 332.2 Regional Hydrology / 35

Wetlands / 36First-Order Streams / 38

2.3 Stormwater Volume / 39

v

vi CONTENTS

2.4 The Water Quality Impacts of Land Development / 40Increased Pollutants in Urban Runoff / 43

2.5 The Chemistry of Urban Runoff Pollution / 442.6 Understanding Pollutant Transport in Stormwater / 47

Stormwater Quantity and Quality / 47Particulates / 48Solutes / 49

3 Land as the Resource 51

3.1 Historic Patterns of Land Development / 513.2 Sustainable Site Design / 583.3 Watershed Setting and Physical Context / 583.4 Smart Growth Issues / 59

Changes Related to Development / 593.5 Conflict Between Desired Land Use and Sustainability / 613.6 Physical Determinants of Land Development / 62

Geology / 62Physiography / 65Topography / 66Soil and Subsurface Conditions / 67

3.7 Urban Communities with Combined Sewer Overflows / 68End of the Sewer / 71Other Urban Infrastructure / 73

3.8 The Living Building and Zero Net Water Use / 74

4 The Planning Process for LID 79

4.1 Sustainable Site Planning Process with StormwaterManagement / 79

Guideline 1: Understand the Site / 79Guideline 2: Apply LID Conservation Design / 80Guideline 3: Manage Rainfall Where It Originates / 81Guideline 4: Design with Operation and Maintenance in

Mind / 83Guideline 5: Calculate Runoff Volume Increase and Water

Quality Impacts / 854.2 Overview of the Site Design Process for LID / 86

CONTENTS vii

5 The Legal Basis for LID: Regulatory Standards andLID Design Criteria 95

5.1 The Land–Water Legal Process / 95Common Law / 95Federal Water Quality Law / 96Federal Land Use Law / 97

5.2 The Evolution of Land Development Regulation / 985.3 The Regulatory Framework / 100

Pennsylvania Land Use Law / 101Pennsylvania Water Law / 102California Land Use Law / 103California Water Law / 104

5.4 Stormwater Management Regulations / 105Volume Control / 105Volume Control Criteria / 106Volume Control Guideline / 108Peak-Rate Control Guideline / 108Water Quality Protection Guideline / 109Stormwater Standards for Special Areas / 110Legal Implications of Green Infrastructure / 110

6 LID Design Calculations and Methodology 113

6.1 Introduction to Stormwater Methodologies / 1136.2 Existing Methodologies for Runoff Volume Calculations / 114

Runoff Curve Number Method / 114Small Storm Hydrology Method / 117Infiltration Models for Runoff Calculations / 119Urban Runoff Quality Management / 119

6.3 Existing Methodologies for Peak-Rate/HydrographEstimates / 120

The Rational Method / 120The NRCS (SCS) Unit Hydrograph Method / 120

6.4 Computer Models / 121The HEC Hydrologic Modeling System / 121The SCS/NRCS Models: WinTR-20 and WinTR-55 / 121

viii CONTENTS

The Stormwater Management Model / 122The Source Loading and Management Model / 122Continuous Modeling / 123

6.5 Precipitation Data for Stormwater Calculations / 1236.6 Accounting for the Benefits of LID: Linking Volume and Peak

Rate / 1246.7 Recommended LID Stormwater Calculation Methodology / 124

Methods Involving No Routing / 125Methods Involving Routing / 126

6.8 Nonstructural BMP Credits / 127

7 Design of LID Systems 131

7.1 Nonstructural Measures / 131Impervious Surface Reduction / 131Limitation of Site Disturbance / 132Site Design with Less Space / 132

7.2 Structural Measures / 1337.3 Pervious Pavement with an Infiltration or Storage Bed / 134

Types of Porous Pavement / 134Description and Function / 136Pervious Bituminous Asphalt / 141Pervious Portland Cement Concrete / 141Pervious Paver Blocks / 141Reinforced Turf / 143Other Porous Surfaces / 144Potential Applications / 144Pervious Pavement Walkways (Concrete and Asphalt) / 144Rooftop and Impervious Area Connections / 144Water Quality Mitigation / 145

7.4 Bioremediation / 145Rain Garden: Design and Function / 146Primary Components of a Rain Garden System / 147

7.5 Vegetated Roof Systems / 152Design and Function / 154Design Elements of a Vegetated Roof System / 155Types of Vegetated Roof Systems / 155Dual Media with a Synthetic Retention Layer / 158Potential Applications / 158

CONTENTS ix

7.6 Capture–Reuse / 158Rain Barrels and Cisterns / 161Vertical Storage / 164

8 Structural Measures: Construction, Operation,and Maintenance 169

8.1 Porous Pavement Systems / 169Construction / 169Storage/Infiltration Bed Dimensions / 174Construction Staging / 174Operation and Maintenance / 176Vacuuming / 177Restoration of Porous Pavements / 178Cost of Porous Pavement / 178

8.2 Bioremediation Systems / 179Rain Gardens / 179Construction of a Rain Garden / 183Maintenance of Rain Gardens / 183Cost of Rain Gardens / 184Vegetated Roof Systems / 184Construction of a Vegetated Roof / 187Maintenance of Vegetated Roofs / 188Cost of Vegetated Roofs / 188

8.3 Capture–Reuse Systems / 188Construction / 188Volume Reduction / 191Peak-Rate Mitigation / 191Water Quality Mitigation / 191

Appendix A: The Stormwater Calculation Process 193

Appendix B: Case Studies 213

B.1 The Transition from Research to Practice / 213B.2 Manuals / 215B.3 LID Manual for Michigan (2008) / 219B.4 Models and Watershed Studies / 237B.5 Design and Construction Projects / 251

Index 283

PROLOGUE:HABITAT, SUSTAINABILITY,

AND STORMWATERMANAGEMENT

Over the past 4.5 million years, as our species has evolved from a simple mammalthat learned to walk upright, we have sought suitable habitat. For most of thatperiod, the form of this living space was quite simple, with only two basic criteria:protection from the weather, and a source of water. Within the past 12,000 years,our concept of what constitutes habitat has evolved and become infinitely morecomplex, although these basic criteria have remained unchanged. From caves orother natural shelters to structures built with local vegetation, the transformationto more elaborate buildings for habitat, commerce, worship, and recreation hastaken place in a relatively brief period of our existence as sentient creatures.

As we increased in numbers and the fabric of social structure evolved, ourperspective of the local environment did not change. The world and everything init was a gift from our god (or gods), and natural resources were to be exploited asnecessary to serve our needs. Even during the current period from the founding ofthe United States of America, the underlying dynamic of development was to con-quer the wilderness, clear the forest, fell the ancient trees, dam the mighty rivers,and till every possible acre for food productivity, especially along river valleysrich with deposited sediment. The gifts of nature were abundant and available,and the land belonged to each property owner, to use as he or she saw fit.

For future generations, the beginning of the twenty-first century may wellbe considered the age of environmental enlightenment, when the extremes ofenergy and water exploitation that characterized the twentieth century have finallybeen recognized, and alternative strategies formulated. One of the most importantconcepts to have evolved in the past two decades is that of sustainability , whichin the context of land development means the ability to construct our neededfacilities without destroying the land and water systems that are essential elementsof our habitat. We are only beginning to comprehend that if we do not sustainthese natural resources for future generations, our communities will collapse

xi

xii PROLOGUE: HABITAT, SUSTAINABILITY, AND STORMWATER MANAGEMENT

within the near future. Countless examples of such failures can be drawn fromprevious societies, but nothing on the scale presently anticipated.

It serves no useful purpose to dwell on “doomsday” scenarios to illustrate thispotential collapse. This book develops simple and practical examples of designsthat change present practice without sacrificing any of the desired comforts ofa built environment. It is essentially a positive response to the issues at hand,intended to influence the current generation of engineers, architects, landscapearchitects, planners, and developers who will build our future habitats. This pro-cess will follow new methods and use new materials to create structures thatshelter us from the elements, assure a safe and sufficient supply of water, andprovide opportunities for our children to do the same. This concept is advocatedin the Living Building program developed by the Cascadia Division of the U.S.Green Building Council, and provides a template for the future of building, withzero net water (and energy) as the basic design goals.

Thomas H. Cahill

ACKNOWLEDGMENTS

This book is a compilation of information developed over a period of 49 years,especially the past 35, as a number of stormwater management concepts began toevolve into a body of practice. These concepts center on the reduction of runoffvolume, rather than simply runoff detention as has been the general methodof dealing with the impact of development since the 1970s. As site designsevolved, broader questions were raised, such as how and where we should (andshould not) build our structures. In time, stormwater management became partof a larger effort to rethink how we develop the land. A mix of disciplinescontributed to these concepts, including civil and water resource engineering,landscape architecture, planning, and architecture, and it is accurate to say thatthe process is still evolving.

A mix of talent, drawn largely from the staff of Cahill Associates (no longerin practice), contributed many of the ideas and designs included here. Mostimportant is Michele Adams, P.E., my daughter and partner, whose creativethinking is blended throughout the book and reflected in the quality of the work;Wesley R. Horner, P. P., the principal author of Chapter 4; Andrew Potts, P. E.,who wrote most of Chapter 6; Daniel Wible, P. E., who designed many of theexample projects illustrated throughout the book; and Tavis Dockwiler, L. A.,principal of Veridian Design, who has been my muse in the somewhat confusingworld of vegetation, following the role played initially by Carol Franklin, L. A.,of Andropogon Associates. Other contributors include Richard Watson, P. P., whocrafted much of Chapter 5, and Charles Miller, P. E., president of Roofmeadows,Inc., whose pioneering work in bringing the practice and construction of vegetatedroofs to the United States serves as an example of how good ideas succeed ifyou are tenacious.

Cahill Associates, Inc. (CA) was acquired by CH2M HILL (CH2) in 2008, andthe same team continued to work on then-contracted and new projects. The casestudies described in Appendix B were performed primarily under the guidance

xiii

xiv ACKNOWLEDGMENTS

of the CA firm, but one new study, the Allegheny Riverfront Vision Plan, wascarried out by CH2 under the project management of Courtney Marm, P. P.

A special thank you goes to my daughter Christine Steininger, who played acritical role in the final production of graphics for this book, with a skill levelfar beyond my own.

T. H. C.

1

RAINWATER AS THERESOURCE

1.1 THE WATER BALANCE AS A GUIDE FOR SUSTAINABLE DESIGN

In every portion of the planet, the cycle of water provides the same natural model:The water resource is replenished with each season and the land surface respondsto this cycle of abundance or drought with a vegetative system that flourishesand diminishes with the available rainfall. The hydrologic cycle is continuous,but it is by no means constant, and every human habitat must recognize andlive within the limits and constraints of this dynamic process. Over the past 4.5million years, our species has learned to live in balance with the water cycle; orif it changes over time, migrate to other environments.

Unfortunately, over the past century, our modern society has not followed thisprocess in the building of our current communities. As our numbers increasedand spread across the land surface, we began to exploit rather than sustain ourland and water resources. During the past century, our control of energy sourcesallowed us to neglect the principle of sustaining our habitat, and we gave littlethought as to how we built our modern cities, disregarding the local environmentand the natural limits of each place. Guided by a false confidence that we couldconquer any constraint or natural limitation, we have stripped and sculpted theland to fit our perceived image of how we can best situate our structures. Wehave exploited the available water resources, without careful consideration ofwhere we live in terms of natural topography and hydrology.

Low Impact Development and Sustainable Stormwater Management, First Edition. Thomas H. Cahill.© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

1

2 RAINWATER AS THE RESOURCE

Shallowinfiltration

Runoff

Deepinfiltration

Groundwater discharge to lakes,streams, & oceans

Evaporation

Transpiration

Figure 1-1 The hydrologic cycle.

The hydrologic cycle or water balance serves as a model for understandingthe concept of sustainability of our water resources (Figure 1-1). The challengeof sustainability is to draw upon elements of this cycle to serve our needswithout significantly disrupting the balance. With careful land use planning andwater resource management, every available drop of rain can be used and reusedto meet our needs without destroying the quality or affecting the character ofnatural streams and rivers. Many of our uses, such as drinking supply, can belargely recycled with the proper waste system design, and many other uses canbe reduced in quantity if they are largely “consumptive” uses, such as irrigationof artificial landscapes. Consumptive demands of cultivation can also be reducedby methods such as drip irrigation, and energy systems can be designed thatdo not consume fresh water in the cooling process. All modern water suppliesrequire energy, and most energy systems affect water. Similar to the land–waterdynamic, the energy–water interrelationship requires that any system changesconsider both resources.

The principle of water balance is best understood in the context of a measurableland area—watershed, drainage basin, or land parcel—that quantifies the watercycle. The rain that falls on the land surface over a period of time defines themagnitude of the resource and the quantity required to sustain the cycle. Thepotential demands on this balance imposed by our land development process canthen be applied to this model as an initial step in understanding how the cycleshould guide our activity on the land.

Perhaps the easiest way to understand the concept of the water balance isto consider a small unit area (Figure 1-2), an acre or hectare, and measure themovement of rainfall through this tiny portion of the planet. The flow begins (orcontinues) with rainfall, shown in the figure as the annual average for a temperateclimate, the mid-Atlantic region of eastern North America. Whereas the annualamount of rainfall varies greatly from place to place across the United States(Table 1-1) and can also experience significant seasonal differences (Table 1-2),the hydrologic cycle remains a constant.

THE WATER BALANCE AS A GUIDE FOR SUSTAINABLE DESIGN 3

Rainfall38” Evapo-

Baseflow10.5” or 780

gpd/acre

Fracturedbedrock

AquiferRunoff

6.6”

Infiltration

Soil

Transpiration21”

Figure 1-2 The hydrologic cycle on an undeveloped unit area (in./yr).

Table 1-1 Annual Rainfall in Major U.S. Cities

City Annual Rainfall (in.) Annual Snowfall (in.)

Albany, NY 38.6 64.4Anchorage, AL 16.1 70.8Atlanta, GA 50.2 2.1Austin, TX 33.6 0.9Boston, MA 42.5 42.8Charlotte, NC 43.5 5.6Chicago, IL 36.3 38.0Denver, CO 15.8 60.3Duluth, MN 31.0 80.6Honolulu, HI 18.3 0Houston, TX 47.8 0.4

Source: [1].

It is also possible to structure a more complicated model of this dynamicprocess (Figure 1-3), realizing that the water movement through all elementsis continuous, while some elements, such as the soil mantle, act as short-termstorage units, holding or releasing moisture from year to year. Other processes,such as evapotranspiration, vary greatly from season to season, and by location,throughout the year. Thus even this more complex graphic fails to fully describethe water balance cycle.


Table 1-2 Seasonal Variation of Rainfall in Regional Watersheds

Major River Rainfall (in.)

Region Basin and City Spring Summer Fall Winter Total

Northeast Delaware River 11 11.3 10.6 9.7 42.5Philadelphia, PA 26% 26% 25% 23%

Northwest Willamette River 13.8 3.5 15.7 27.2 60.2Portland, OR 23% 6% 26% 45%

Southeast St. Johns River 10 10 19.1 12.2 51.2Jacksonville, FL 19% 19% 37% 25%

Southwest Santa Anna River 4 0.4 2.7 7.5 14.6Los Angeles, CA 27% 4% 15% 51%

The development of plants on the planet surface long preceded the mammalsfrom which we evolved, and plants have fulfilled their part in the hydrologic cyclefor several billion years. On those land surfaces that evolve a natural vegetativecover, especially woodlands, the trees and grasslands utilize the input of rainfallto live by photosynthesis [2], drawing the infiltrating moisture from the soil (ordirectly from the atmosphere) and transforming the water into oxygen and organicmatter, a process described by the reaction

(6H2O + 6CO2) + (sunlight, 48 mol) = 6O2 + C6H12O6

This simple miracle of plant life is carried out by the role of chlorophyll in thevegetation. In addition to producing the oxygen by which all species live, thisprocess maintains the critical balance of CO2 in the atmosphere for the benefitof all animal life forms, including the human species. While the air we breatheis comprised primarily of 78% nitrogen with slightly less than 21% oxygen, therole of minor gases (argon, 0.93%; CO2, 0.038%) and water (1%) is critical inmaintaining the temperature at a relatively constant level over time. The rapidincrease in CO2 over the past century has played an important and causal rolein global warming, specifically as the result of burning fossil fuels [3, 4]. Thus,the importance of sustaining surface vegetation, especially trees, during the landdevelopment process cannot be overstated (Figure 1-4), as it compensates forthis human impact [5, 6]. It should be noted that terrestrial vegetation providesonly a portion of the photosynthetic production, with marine plankton actuallygenerating more of the balance on a global basis.

On a naturally vegetated land surface, about half of the rain that falls isreturned to the atmosphere by the evapotranspiration process. The balance ofinfiltrating rainfall, not utilized by the vegetation or evaporated from the surfaceby sunlight and air currents, infiltrates or percolates slowly (or quickly) into thesoil mantle. A portion of this rain drains deep into the soil and weathered rocksurface, eventually reaching the zone of saturation, described as the water table(Figure 1-5), and becomes groundwater.

THE WATER BALANCE AS A GUIDE FOR SUSTAINABLE DESIGN 5

Precipitation45″

Depressionstorage Evaporation

Evapo-transpiration

Soilmoisture

Groundwaterreservoir

Groundwaterinflow

Groundwateroutflow

Net

Infiltration

1″

36″ 37″

22″

0″

0″ 0″

15″ 15″

22″

2″

15″(Base flow)

OceanStreamflow

Surfacerunoff

1″

1″ 1″

21″ 21″

1″

8″ 8″

Figure 1-3 The hydrologic cycle or water balance model for a watershed in southeasternPennsylvania: the Brandywine model project, 1984.

As each raindrop is added to this groundwater, it begins to move in thedirection of available energy, created by the inexorable pull of gravity. Sincethe easiest pathway for displacement is through the soil (and fractures in therock) following the surface of the land, this water eventually travels down-hill, emerging as a seep or spring, flowing over the surface to a swale orsteam channel. Actually, as each raindrop enters the groundwater, it displaceswater from the low end of the saturated zone. A single raindrop may actu-ally take weeks or months to complete the journey from where it falls onthe land surface to the point of discharge downgradient, as it returns to thesurface.


Evapo-transpiration

Evapo-transpiration

InterceptionInterception

Drip from foliageDrip from foliage

Reduced energy/erosive force

Reduced energy/erosive force

Stemflow

Evaporation fromsurface

Temp surface ponding

Stemflow

Evaporation fromsurface

Percolation into groundwater

Uptake by root system forevapotranspiration

Temp surface ponding Infiltration into soil

Figure 1-4 The perfect LID measure for stormwater management: a tree.

Of course, not all infiltrating rainfall follows an identical pathway of movementin the subsurface, and the complex layering of the soil in different horizons, eachwith a very different permeability, can make this journey lengthy and circuitous.Where highly impervious layers exist in the soil mantle, infiltrating rain willmove across this surface, again following the energy gradient. The underlyingbedrock also influences the speed and direction of groundwater movement, inboth the unsaturated zone and deep below the water table. If the underlyingrock is comprised of soluble carbonates, it includes open solution channels orsubsurface flow pathways that formed hundreds of millions of years ago and nowprovide an underground river network, carrying the rainfall many miles from the

THE WATER BALANCE BY REGION 7

Averagegroundwater table

and stream base flow

Aquifer

SoilWater table (normal conditions) Stream Bedrock

Figure 1-5 Groundwater recharge feeds the local surface waters and sustains base flow.

initial point of infiltration. In coastal watersheds, the groundwater may dischargedirectly to estuary systems, never reappearing on the land surface.

In some physiographic regions, a fraction of the infiltrating rain enters intodeeper aquifers and does not reappear at the surface, but may remain storedfor centuries. In active seismic regions, geothermal sources may actually bringsome of the deep water to the surface. This vertical flow of groundwater maycomprise a portion of surface systems, such as the Snake River tributary in theColumbia River system, originating from the “hot spot” that forms the geysersof Yellowstone (Figure 1-6). However, for most of the developed regions of theUnited States, the simple model illustrated by Figure 1-2 is a valid representationof this complex water balance.

While the full hydrologic cycle includes water movement on a global basis,the consideration of stormwater management is limited to the freshwater portionof the total resource, a fraction of the world’s water (about 2.5%). Most of thatfresh water is currently contained in ice (although the future is quite uncertain)and represents 77.2%, with an additional 22.6% contained in the subsurfaceas groundwater, leaving only 0.32% in surface rivers and lakes, 0.18% as soilmoisture, and 0.04% in the atmosphere, for a total “available” water resource of0.54% [8]. All of the following discussion is concerned with this sum of thesesmall portions, although it amounts to the trillions of gallons of water that sustainthe human biosphere.

1.2 THE WATER BALANCE BY REGION

Although the most obvious measure of the water resources available in a givenregion of the country is the average annual rainfall received, this statistic canbe deceiving if we do not recognize the potential variability in this measure,


Figure 1-6 Deep groundwater is discharged by geothermal vents (Yellowstone). (From [7].)

especially in arid regions such as the Southwest, where the extremes of “wet”and “dry” years can result in a system that experiences a crisis under both cycles.It is the extremes of the cycle that create the greatest stress in every community,and the duration of individual droughts or flood-creating rainfall periods thatmeasure how well or poorly we have built our communities.

Most large river basins in the United States have experienced significant humanalteration or structural intervention over the past two centuries. It is interesting toconsider the net effect of human activities on the regional watershed, although wehave no baseline (pre-disturbance) flow data to compare with current conditions.However, we can compare the net runoff generated in these large systems with therainfall experienced within the watershed (Table 1-3). Also shown is a referencecity, usually situated at the downstream reach of the river basin. In the SantaAnna basin draining to Los Angeles, the inflow from three diversion canalsaffects these statistics significantly.

THE WATER BALANCE BY REGION 9

Table 1-3 Water Balance by Region and River Basin

Region and River Basin Basin MeanPercent and Reference Size Discharge Rainfall (in) Runoff

of Area City (m2) (ft3/sec) Alteration City Basin (in.)

NE 60% DelawarePhiladelphia,PA

12,757 14,902 NYCdiversion

42 42.5 25.6

NW 63% WillamettePortland, OR

11,478 32,384 Dams/lakes 37 60.2 38.2

SE 23% St. JohnsJacksonville,FL

8,702 7,840 Lakes 52 51.6 11.8

SW 2.3% Santa AnnaLos Angeles,CA

2,438 60 Diversions 15 13.4 0.3

Source: Derived from [9].

Whatever the average annual rainfall or variability of this volume in a givenlocation, the design of structures or systems to convey or mitigate the impactsof this volume (and flow rate) of surface runoff have always focused on individ-ual storm events. These “design storms” are events during which the intensity,duration, and amount of rainfall produce the most severe impacts.

We remember the most extreme rainfall events, especially when they are theresult of cyclonic storm patterns produced in both the Atlantic and Pacific oceansthat approach the mainland in the form of hurricanes or cyclones. We evenidentify them by name when they reach a given magnitude or anticipated windspeed, assigning a category of intensity that can change during the approach.Most recent memory cannot help to identify hurricane Katrina (Figure 1-7), whichdevastated the Gulf coast in September 2005, but other names and memories areshared by communities throughout the country. Most periods of prolonged rainfalldo not receive this recognition or nomenclature, but have produced dramaticflooding impacts in large and small watersheds.

The statistic of rainfall that has the most common usage in defining severerainfall events is the 100-year storm, which is the rainfall that occurs duringa 24-hour period with a frequency of once in 100 years. This figure cannot,however, convey the full impact on a local watershed of more severe and intenserainfalls. For example, in July 2004 the Rancocas Creek in southern New Jerseywas visited by a rainfall pattern [10] that dumped some 13 in. in some portions ofthis small (250 m2) watershed (which has a 100-year rainfall frequency of 7.2 in.),in a pattern that was anything but uniform. The net result was the destruction ofsome 22 small earthen dams, built for various purposes, and significant propertydamage (but no loss of life).

This type of localized event can be visited on any portion of the country,regardless of our statistics and classification of storms, and is repeated all toofrequently all across the globe. While the total rainfall is a given period and


Figure 1-7 Hurricane Katrina strikes the U.S. Gulf coast.

the intensity of that precipitation have much to do with the resulting impact,the hydrologic response of any given watershed is also a function of land coverconditions, especially vegetation, and season, with frozen ground producing someof the most severe runoff conditions during early spring in mountainous regions.

If we were to measure all of the rainfalls at a given location over a century, wewould find that the vast majority were of very small magnitude (Figure 1-8). Thepie chart in the figure shows rainfall distribution for southeastern Pennsylvania,with a total annual rainfall of 44 in./yr. The relative distribution is the samefor most other regions, with most of the storms less than 3 in. in total rainfall,and offers insight as to the defining statistic for a stormwater volume reductionmanagement strategy.

While the traditional focus of concern has been the extremes of rainfallor drought, the major portion of our precipitation actually occurs in smaller,more frequent events. In fact, in almost every major physiographic or climato-logic region, the 2-year-frequency rainfall serves as the defining statistic for thestormwater management designs that are outlined in this book. This rainfall and

ARID ENVIRONMENTS: THE SOUTHERN CALIFORNIA MODEL 11

3″–4″3%

2″–3″11%

1″–2″31%

0″–1″54%

> 4″1%

Figure 1-8 Frequency and magnitude of rainfall events, southeastern Pennsylvania. Mostrainfall occurs in small storms, less than the 2-year frequency.

Table 1-4 Two-Year-Frequency Rainfall Event

Two-Year RainfallU.S. Region City (in. in 24 hr)

Northeast West Chester, PA 3.3Northwest Seattle, WA 3.2Southeast Chapel Hill, NC 3.6Southwest Los Angeles, CA 2.9Central Minneapolis, MN 2.5

that of all the storms of lesser magnitude represents about 95% of the total rainfallvolume over a prolonged period of decades, and so better defines the efficiencyof any proposed mitigation measure. Since this statistic has great significanceas a basis for the design of most of the measures described in this book, it isimportant to compare the variation in this type of rain event in different portionsof the country. Table 1-4 shows the 2-year-frequency rainfall in major regions,and the values are quite similar. Figure 1-9 illustrates the intensity of this type ofstorm over a 24-hour period (as well as the 100-year rainfall) for a mid-Atlanticwatershed. This is described by an S-curve, developed by the Soil ConservationService of the U.S. Department of Agriculture during the 1960s [11]. Of course,nature never cooperates with our assumptions concerning climate conditions suchas rainfall patterns, but this type of distribution is assumed because it will producethe most extreme runoff conditions.

1.3 ARID ENVIRONMENTS: THE SOUTHERN CALIFORNIA MODEL

To be sustainable, low-impact design (LID) must consider all human demandson the hydrologic cycle that result from the land development process. Thismeans that we begin our site planning with the issue of water supply, the single


8

7

6

5

4

3

2

1

00 4

Rai

nfal

l (in

.)

8 12Time (hr)

16 20 24

3.3 in.

7.3 in.

Figure 1-9 The S curves of assumed rainfall intensity and distribution; 2- and 100-year-frequency storms in southeastern Pennsylvania.

most critical aspect of site development. When this basic need is satisfied, weconsider the return of this water to the cycle, containing all of the pollutantswe have added during our use. The need for increasingly efficient pollutantremoval processes during the past century has resulted largely from the increasein population and density in our land development, as we realized that the sewagefrom one community is the water supply for downstream residents.

Stormwater has been regarded as a nuisance, to be drained away from ourdevelopments as quickly as possible following a rainfall. In arid environments,the value of rainfall to support our continued occupation of habitat is an unuti-lized resource, especially when the available water supply is limited and runoffis discharged to coastal waters. Nowhere is this lack of water resource manage-ment more apparent than in southern California. In these coastal watersheds, landdevelopments draining to the Pacific coast and to inland waters and reservoirshave generated significant increases in stormwater runoff volume, which in turnhas contributed to the discharge of pollutants into receiving waters, degradedaquatic habitat, affected the recreational use of these waters, and interfered withtheir use as water supply. Through implementation of LID practices, these pol-lutant discharges can be reduced significantly, so that the quality of these coastaland inland waters can be restored and sustained.

But the potential of LID goes well beyond reducing the volume of pollutedstormwater runoff. This rainfall can also be understood as a lost resource in thesemiarid environment of southern California, where increasing demand for freshwater requires costly importation of water supplies to sustain ever-growing com-munities. Interestingly, the quantity of water imported into southern California isalmost equal to the net loss of stormwater runoff to coastal waters (Table 1-5),referred to as the salt sink .

ARID ENVIRONMENTS: THE SOUTHERN CALIFORNIA MODEL 13

Table 1-5 Water Balance in Southern California, 2000 (1,000 Acre-Feet)

Supply Use (Demand)

Precipitation 7,500 Evapotranspiration 7,441Imported 2,991 Consumptive use 1,819Depletion of groundwater 1,245 Outflow to the salt sink 2,498

Total supply 11,752 Total use 11,758

Note: The runoff lost to the ocean is almost equal to the required import.

Could LID practices such as capture and reuse of stormwater runoff signif-icantly reduce the need for importation of water supply to the region? Thesepractices offer the possibility of working to redress at least some of the watercycle imbalances that confront southern California communities, but will requirea significant rethinking of existing stormwater and water supply system designs.

Central to the concept of watershed sustainability is the water cycle and itsbalance, very roughly defined as the matching of water inputs (“supply”) to wateroutputs (“demands”). Any watershed, physiographic region, or land area that canbe well defined in terms of water supply and use, both natural and human, can alsobe evaluated in terms of water cycle. Analysis of the existing water balance forsouthern California demonstrates that the natural water resources are insufficientto meet the demands of the existing 19 million residents in the 11,000-square mileregion from Ventura to San Diego, with 5 million additional residents projectedto arrive by 2020 [12].

The deficit in natural water resources has been met over the years by threeaqueducts (Figure 1-10), which convey imported water hundreds of miles to theregion. Table 1-5 provided a simplification of this water resource balance for theyear 2000, with both supply and demand (use) summarized from more detailedstatistics. In 2000, rainfall provided only 72% of the total water supply use ordemand, again with the quantity of runoff to the ocean [2,498 thousand acre-feet(TAF)] close to the water importation (2,991 TAF).

The depletion of groundwater is especially troublesome in this “balance.”Although a number of large aquifer systems lie beneath the surface of the regionand are constantly being replenished by recharge from surface sources, theircapacity has been exceeded during most years. The Los Angeles area receivesover 40% of their current water supply from these aquifers, utilizing spreadinggrounds to recharge both runoff and recycled effluent, based on the conceptof “conjunctive use.” However, year after year, the groundwater reservoirs arefurther depleted, despite our plans.

The Energy Demand for Water in Southern California

It requires over 10,200 kilowatthours (kWh) for every million gallons (MG)of water imported into southern California, 40 times greater than the nationalaverage and 20% of total residential energy usage for the region, as shown in


Figure 1-10 South coast hydrologic region of southern California. The imported waterequals the runoff lost to the ocean.

Table 1-6. This can be compared with the national average of 250 kWh/MG.Any significant reduction in importation of water to the region can be expectedto translate into significant energy savings.

What does energy demand have to do with LID? When we consider the appli-cation of sustainability concepts to land development, we must include the impacton both water and energy, and in this region the shortage of both resources iscritical. If we can capture the rainfall and at the same time reduce the amount of

low impact management

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