Fundamentals of Urban Runoff Management TechNICAL AND INstItutIONAL Issues 2nd Edition | 2007 Earl Shaver Richard Horner Joseph Skupien Chris May Graeme Ridley C H A pter 2 Water Quantity Impacts of Urban Land Use Reality vs. Theory .....................................................................................................................................17 The Rainfall-Runoff Process ................................................................................................................... 20 Runoff Estimation: Typical Parameters .................................................................................................21 Rainfall .............................................................................................................................................21 Time ................................................................................................................................................ 28 Drainage Area ................................................................................................................................30 Soils ................................................................................................................................................ 31 Land Cover ..................................................................................................................................... 34 Runoff Estimation: Methods and Models .............................................................................................36 Runoff Volume Methods ...............................................................................................................36 Peak Runoff Rate Methods ...........................................................................................................36 Runoff Hydrograph Methods .......................................................................................................36 Impacts of Land Use Change .................................................................................................................38 Summary and Conclusions .................................................................................................................... 41 References ............................................................................................................................................... 41 chapter 2: Water Quantity Impacts of Urban Land Use 2-15 C H A pter 2 Water Quantity Impacts of Urban Land Use Urban runoff is a by-product of the land’s surface slope, and the texture, density, and permeability interaction with rainfall. Since, by definition, of the surface and subsurface soils. Conditions that affect urban runoff remains on and moves along the stormwater runoff also include the thickness and quality land’s surface, it is the most visible of the many of the surface cover and the amount of water already forms into which rainfall is converted. This stored both on the surface and within the soil profile. chapter provides the technical fundamentals of Conversely, stormwater runoff also affects the sur- the rainfall-runoff … process. It also describes faces upon which it is created and/or that it flows across. ways that land development alters this process These effects include both the deposition of pollutants and quantifies some of the adverse impacts. captured from the atmosphere by the falling rain and the mobilization and removal of pollutants previously So began Chapter 2 of the 1994 edition of Fun- stored on the surfaces. The most readily visible effects damentals of Urban Runoff Management. And while it are erosion and sedimentation, where forces created by still can serve as the opening paragraph of this new the moving runoff become large enough to dislodge, Chapter 2, our technical knowledge of both urban suspend, and transport soil particles and associated runoff hydrology and the effects of land use change has pollutants downstream. This process continues until grown considerably in the intervening years. As a result, slower velocity areas are encountered, whereupon the the technical content of this new chapter goes beyond particles drop out of the runoff and back onto the the original version, including new and updated topics. surface. Depending on the type and character of the However, in presenting this technical information, the surface cover, this process of dislodging soil particles and chapter’s goal remains the same: to present the infor- mobilizing pollutants can be aided by the impact of the mation not as an end in itself, but so as to assist in the falling raindrops themselves. Further erosion, sedimenta- development of urban runoff management programs. tion, and pollutant loading can occur downstream in The arrival of the EPA’s Stormwater Phase II Final swales, channels, streams, and rivers, depending on the Rule in 1999, which requires municipalities and other rate, depth, velocity, and duration of the runoff flowing entities to develop such programs by 2003, highlights in them. the value of such assistance. The volume of stormwater runoff produced by a From the above, three key conclusions can already rain event, the rates, velocities, and depths at which be reached: it flows, and the pollutants that it carries depend on • Since the volume, rate, and velocity of runoff several factors. In addition to the quantity, intensity, from a particular rain event will depend upon the and duration of the rain itself, the resultant runoff will characteristics of the surfaces on which the rain be determined by the characteristics, condition, and falls, changes to these surfaces can significantly relative areas of the various surfaces on which it falls. change the resultant runoff volume, rate, and As explained in detail in the following sections, these velocity. Changes normally associated with land characteristics include the type of surface cover, the development and urbanization that increase 2-16 fundamentals of urban runoff management impervious cover and decrease soil permeability used to expand readers’ knowledge beyond the pages can significantly increase runoff. of this book. • Since pollutant mobilization and soil erosion It is important to note that, as our understanding of are the direct result of excessive runoff rates urban runoff processes and impacts continues to grow, and durations, changes in land surfaces can also so does the scope and requirements of the programs significantly increase both surface and channel we’ve developed to manage them. Following along and, erosion rates and runoff pollutant loadings. at times, inspiring this growth has been an increasing emphasis on and understanding of runoff fundamentals. • In developing urban runoff management It is this greater understanding that has allowed us to programs, the greater the knowledge of the progress from relatively simple runoff quantity controls rainfall-runoff process, the more effective the in the 1970s to the integrated quantity and quality resultant program will be. programs of today. It has also allowed us to expand While the details of the rainfall-runoff process are the scope and applicability of both our mathematical highly complex and much remains to be learned about models and the various measures and practices we can them, the fundamentals are readily understandable, now use to implement their findings. For example, the particularly when presented in a direct, concise man- growing use of nonstructural measures and low-impact ner. That is the goal of this chapter. Equipped with the development practices essentially began with a detailed information presented here, those involved in develop- re-examination of the fundamental principles of the ing urban runoff management programs at all levels, as hydrologic cycle which, in turn, became the basis for well as those responsible for complying with them, can their design and implementation. Therefore, it is hoped base their efforts on a sound understanding of the basic that the runoff fundamentals presented in this chapter hydrologic processes at the core of their program. will continue to inspire and direct the development of This chapter provides readers with basic information urban runoff programs with ever greater scopes, goals, on the rainfall-runoff process. It also highlights some and accomplishments. of the important unknowns and uncertainties of the process and recommends ways to acknowledge and account for them in computation methods and program Reality vs. Theory requirements. Using this information, the chapter also provides information on the adverse impacts land use In most complex technical matters, differences exist change and urbanization can have on runoff quantity between reality and theory. That is because theories and the damaging consequences of excessive increases developed to explain or simulate reality can only go in runoff rates, volumes, and velocities. so far. Typically, there are aspects of reality that are not Next, the chapter utilizes this rainfall-runoff infor- entirely understood and, therefore, are either ignored or mation to illustrate how various practices can either simplified in the theory. Recognizing these differences avoid or control such impacts. This broad approach is important when developing and implementing a not only helps ensure that decisions made during the technology-based regulatory program such as one that development of an urban runoff management program manages urban runoff. The “real” runoff processes that are based on an informed understanding of runoff occur