Flood Hydrology Manual A Water Resources Technical Publication by Arthur G. Cudworth, Jr. Surface Water Branch Earth Sciences Division FIRST EDITION 1989 United States Department of the Interior Bureau of Reclamation Denver Office As the Nation’s principal conservation agency, the Department of the Interior has responsibility for most of our nationally owned public lands and natural resources. This includes fostering the wisest use of our land and water resources, protecting our fish and wildlife, preserv- ing the environmental and cultural values of our national parks and historical places, and providing for the enjoyment of life through out- door recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also has a major respon- sibility for American Indian reservation communities and for people who live in Island Territories under U.S. Administration. UNITED STATES GOVERNMENT PRINTING OFFICE DENVER: 1992 For sale by the Superintendentof Documents, U.S. Government Printing Office, Washington, DC 20402, and the Bureau of Reclamation, Denver Offie, Denver FederalCenter, P.O. Box 25007, Denver, Colorado 80225, Attention: D-7923A 024-003-00170-8. PREFACE The need for a comprehensive Flood Hydrology Manual for the Bureau of Reclamation has become apparent as increasing numbers of civil en- gineers are called upon to conduct flood hydrology studies for new and existing Bureau dams. This is particularly important because of the in- creasing emphasis that has been placed on dam safety nationwide. In general, these engineers possess varying backgrounds and levels of ex- perience in the specialty area of flood hydrology. Accordingly, the pri- mary purpose of this manual is to provide the necessary background, relationships, criteria, and procedures to allow the engineer to conduct satisfactory flood hydrology studies. As a result, these studies should reflect greater consistency and reliability of results for most of the drain- age basins encountered in Bureau projects as well as those for other water resource construction agencies. These relationships, criteria, and procedures are based on detailed analyses of hydrologic and meteorologic data and studies of observed flood and severe rainfall events that have accumulated over the years. The information contained in this manual reflects the methodologies currently used by the Bureau in performing flood hydrology studies. These methodologies have been proven to provide satisfactory results for use in the planning, design, construction, and operation of the Bu- reau’s water control facilities. However, it would be inappropriate to infer that these methods are the final solutions to hydrologic problems because other methods are constantly evolving. It can reasonably be expected that, as additional data are collected and as new and more advanced techniques emerge, these methodologies will be improved upon and modified with the passage of time. It is also expected that such modifications and improvements will be fully incorporated into future editions of this manual. The Bureau of Reclamation is primarily responsible for the development of water resources in the 17 Western States to meet agriculture, munic- ipal and industrial, power, recreation, and environmental water supply requirements. In addition, with the recent emphasis on dam safety, the Bureau has been assigned the responsibility for safety of dams studies for Department of the Interior dams throughout the nation. The Bureau has no authority, as legislated by Congress, in the area of flood control except for a few unique, specifically authorized projects. Flood control is, and has been for the past 50 years, within the purview of the United States Army Corps of Engineers. Therefore, the flood hydrology discipline, as applied to Bureau activities, deals with two prin- cipal areas: (1) determination of the upper limit or probable maximum FLOOD HYDROLOGY MANUAL flood potential at a damsite so that dams whose failure would result in loss of human life or widespread property damage can be designed to safely accommodate this flood without failure, and (2) determination of more commonly occurring floods for use in the design of diversion dams; very low hazard storage dams; cross drainage facilities for the extensive canals, aqueducts, and roads associated with the delivery of water to users; and for diversion of flood waters that may occur during the construction of dams. It should be noted that the technical hydrologic procedures used in these two principal areas are essentially the same as those used in flood hydrology studies that support the planning, design, construc- tion, and operation of flood control projects. Recognizing the Congressionally authorized function of the Bureau, this manual concentrates on three major technical aspects of flood hydrology: (1) hydrometeorology related to probable maximum precipitation de- terminations, (2) probable maximum flood hydrograph determinations, and (3) statistics and probabilities relating to the magnitude and fre- quency of flood flows. Other important, but essentially nontechnical, aspects related to flood hydrology studies are also treated. Chapter 1, “Background and Historical Perspective,” provides a brief historical perspective of the flood hydrology discipline. Included in this discussion is a general overview of procedures and philosophies that have evolved and been applied over the years, from those used in the devel- opment of early water control works to the present. Chapter 2, “Basic Hydrologic and Meteorologic Data,” provides a dis- cussion of the basic hydrologic and meteorologic data that are available for analysis in the development of criteria for upper limit or probable maximum precipitation estimates, methods of determining rainfall-run- off relationships used in deriving probable maximum flood hydrographs, and in generating flood probability estimates. The methods and equip- ment currently used for collecting these data are briefly discussed to provide the reader with a general background on the mechanics involved. The important consideration of the reliability and accuracy of these data is included in subsequent chapters dealing with the specific uses of these data to provide the reader with an appreciation of the resulting accuracies of flood magnitude and frequency estimates. This chapter concludes with a more detailed description on assessing a drainage basin’s physical fea- tures as they impact the rainfall-runoff phenomena through field reconnaissances. Chapter 3, “Hydrometeorology,” begins with a general discussion on the theories underlying basic atmospheric processes. Emphasis is placed on those processes that are important in the development of regionalized or generalized criteria used in estimating probable maximum precipi- tation amounts that are, as shown in subsequent chapters, necessary in determining probable maximum flood hydrographs. This chapter then describes the procedures used in the analysis of observed events that lead ii PREFACE to the generation of criteria used for developing estimates of probable maximum precipitation and storm levels throughout the conterminous United States. The chapter concludes with remarks on continuing Fed- eral interagency activities leading to the development of hydrometeo- rological criteria by the Bureau of Reclamation, National Weather Service, Corps of Engineers, and the Soil Conservation Service. Chapter 4, “Flood Hydrograph Determinations,” presents the theory and procedures currently used by the Bureau in converting rainfall over a drainage basin into a hydrograph of flood runoff. The hydrologic cycle is discussed as it pertains to extreme runoff phenomena up to and in- cluding the probable maximum flood. The process of infiltration losses into the soil and their effect on the runoff hydrograph is presented in more detail. Discussion on both natural and developed basins is included because development has been shown to considerably modify the hy- drograph resulting from a given amount of precipitation. The chapter concludes with an example where a hydrograph representing the prob- able maximum flood event is generated for a relatively complex basin. Chapter 5, “Flood Routings Through Reservoirs and River Channels,” provides a discussion on procedures used by the Bureau in accomplishing the channel routing and combining of flood hydrographs. These pro- cedures are needed when studying large basins that have been divided into subbasins or include one or more reservoirs. Procedures for routing flood hydrographs through reservoirs are also included. Chapter 6, “Envelope Curves of Recorded Flood Discharges,” provides a discussion on the purpose and procedures used to develop envelope curves of experienced flood discharges, both peak and volume. These curves provide a check on the adequacy of probable maximum flood hydrograph estimates. Proper procedures for segregating the data by region, season, storm type, and topography so that curves represent hom- egeneous hydrologic and meteorologic conditions are discussed in detail. Chapter 7, “Statistics and Probabilities,” provides a comprehensive pres- entation of an extremely important area of flood hydrology. The chapter includes a discussion of the basic concepts encountered in the collection and statistical analysis of streamflow data with emphasis on theoretical frequency distributions. Examples of these distributions are provided and their use in