GUIDELINES FOR DETERMINING FLOOD FLOW FREQUENCY Prepared by the U.S. Geological Survey Office of Water Data Coordination and the Interagency Advisory Committee on Water Data Soil Conservation Service, Forest Service, Science Education Administration, Corps of Engineers, National Weather Service, Housing and Urban Development,Bureau of Reclamation, Office of Surface Mining, Office of Water Research and Technology, Bureau of Indian Affairs, Bureau of Mines, Fish and Wildlife Service, National Park Service, Heritage, Conservation, and Recreation Service, Federal Highway Administration, Environmental Protection Agency, Federal Energy Regulatory Commission, Federal Emergency Management Agency, Nuclear Regulatory Commission, Tennessee Valley Authority, Water Resources Council 1982 BULLETIN 17B TABLE OF CONTENTS Foreword Hydrology Subcommittee Page Revisions to Bulletin 17 and 17A I. Introduction II. Summary     A. Information to be Evaluated     B. Data Assumptions     C. Determination of the Frequency Curve     D. Reliability Applications     E. Potpourri     F. Appendix III. Information to be Evaluated     A. Systematic Records     B. Historic Data     C. Comparisons with Similar Watersheds     D. Flood Estimates from Precipitation IV. Data Assumptions     A. Climatic Trends     B. Randomness of Events     C. Watershed Changes     D. Mixed Populations     E. Reliability of Flow Estimates V. Determination of Frequency Curve     A. Series Selection     B. Statistical Treatment         1. The Distribution         2. Fitting the Distribution         3. Estimating Generalized Skew         4. Weighting the Skew Coefficient         5. Broken Record         6. Incomplete Record         7. Zero Flood Years         8. Mixed Populations         9. Outliers         10. Historic Flood Data     C. Refinements to Frequency Curve         1. Comparisons with Similar Watersheds         2. Flood Estimates from Precipitation VI. Reliability Application     A. Confidence Limits     B. Risk     C. Expected Probablity VII. Potpourri     A. Non-conforming Special Situations     B. Plotting Position     C. Future Studies Appendices 1. References 2. Glossary and Notation 3. Table of K Values 4. Outlier Test K Values 5. Conditional Probability Adjustment 6. Historic Data 7. Two-Station Comparison 8. Weighting of Independent Estimates 9. Confidence Limits 10. Risk 11. Expected Probability 12. Flow Diagram and Example Problems 13. Computer Program 14. "Flood Flow Frequency Techniques" Report Summary ABSTRACT An accurate estimate of the flood damage potential is a key element to an effective, nationwide flood damage abatement program. Further, there is an acute need for a consistent approach to such estimates because management of the nation's water and related land resources is shared among various levels of government and private enterprise. To obtain both a consistent and accurate estimate of flood losses requires development, acceptance, and widespread application of a uniform, consistent, and accurate technique for determining flood-flow frequencies. In a pioneer attempt to promote a consistent approach to flood-flow frequency determination, the U.S. Water Resources Council in December 1967 published Bulletin No. 15, "A Uniform Technique for Determining Flood Flow Frequencies." The technique presented therein was adopted by the Council for use in all Federal planning involving water and related land resources. The Council also recommended use of the technique by State, local government, and private organizations. Adoption was based on the clear understanding that efforts to develop methodological improvements in the technique would be continued and adopted when appropriate. An extension and update of Bulletin No. 15 was publised in March 1976 as Bulletin No. 17, "Guidelines for Determining Flood Flow Frequency." It presented the currently accepted methods for analyzing peak flow frequency data at gaging stations with sufficient detail to promote uniform application. The guide was a synthesis of studies undertaken to find methodological improvements and a survey of existing literature on peak flood flow determinations. The present guide is the second revision of the original publication and improves the methodologies. It revises and expands some of the techniques in the previous editions of this Bulletin and offers a further explanation of other techniques. It is the result of a continuing effort to develop a coherent set of procedures for accurately defining flood potentials. Much additional study is required before the two goals of accuracy and consistency will be fully attained. All who are interested in improving peak floow-flow frequency determinations are encouraged to submit comments, criticism, and proposals to the Office of Water Data Coordination for consideration by the Hydrology Subcommittee. Federal agencies are requested to use these guidelines in all planning activities involving water and related land resources. State, local and private organizations are encouraged to use these guidelines also to assure more uniformity, compatibility, and comparability in the frequency values that all concerned agencies and citizens must use for many vital decisions. This present revision is adopted with the knowledge and understanding that review of these procedures will continue. When warranted by experience and by examination and testing of new techniques, other revisions will be published. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.