For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/.Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2018, Guidelines for determining flood flow frequency-Bulletin 17C: U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. PrefaceThis series of manuals on Techniques and Methods (TM) describes approved scientific and data-collection procedures and standard methods for planning and executing studies and laboratory analyses. The material is grouped under major subject headings called "books" and further subdivided into sections and chapters. Book 4 is on hydrologic analysis and interpretation and section B is on surface water.The unit of publication, the chapter, is limited to a narrow field of subject matter. These publications are subject to revision because of experience in use or because of advancement in knowledge, techniques, or equipment, and this format permits flexibility in revision and publication as the need arises. Chapter B5 of book 4 (TM 4-B5) deals with flood flow frequency analysis at gaged sites using the Expected Moments Algorithm. The use of extreme flood data represented by interval and censored data types, including historical, paleoflood, botanical evidence, is emphasized.This revision is adopted with the knowledge and understanding that review of these procedures will be ongoing. Updated methods will be adopted when warranted by experience and by examination and testing of new techniques. AcknowledgmentsThese revised Guidelines were written by a team assembled from the Hydrologic Frequency Analysis Workgroup (HFAWG), under the auspices of the Subcommittee on Hydrology (SOH) of the Advisory Committee on Water Information (ACWI). Input from other members of HFAWG and of the SOH is gratefully acknowledged. HFAWG and SOH Work Group Members and participants in this revision are listed in appendix 2. The authors would also like to acknowledge the contributions of the U.S. Geological Survey (USGS) colleague reviewers, whose comments and suggestions materially improved this report. We also appreciate the technical advice and assistance from Dr. William H. Asquith (USGS).Lastly, our colleague and friend, Dr. Timothy A. Cohn, authored or coauthored many of the papers that form the foundation of this update to the flood ...

Uniform flood frequency guidelines in the United States recommend the use of the log Pearson type 3 (LP3) distribution in flood frequency investigations. Many investigators have suggested alternate models such as the generalized extreme value (GEV) distribution as an improvement over the LP3 distribution. Using flood-flow data at 383 sites in the southwestern United States, we explore the suitability of various flood frequency models using L-moment diagrams. We also repeat the experiment performed in the original Water Resource Council report (Bulletin 17B, issued in 1982), which led to the LP3 mandate. All our evaluations consistently reveal that the LP3, GEV, and the two-and three-parameter lognormal models (LN2 and LN3) provide a good approximation to flood-flow data in this region. Other models such as the normal, Pearson, and Gumbel distributions are shown to perform poorly. Recent research indicates that regional index-flood procedures should be more accurate and more robust than the type of at-site procedures evaluated here. Nevertheless, this study reveals that index-flood procedures need not be restricted to the GEV distribution because the LN2, LN3, and LP3 distributions appear to be suitable alternatives, at least in the southwestern United States.

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The growth and development of the United States has been dependent on the availability of water resources. In colonial times, springs, shallow wells, streams, and rainwater collected in cisterns provided water for domestic and livestock uses. These supplies were subject to the uncertainties of droughts and were vulnerable to contamination. Major population centers developed along rivers and streams that provided water for public supply and transportation. Urban growth in the Eastern United States, particularly after 1800, caused the quality of many city water supplies to deteriorate noticeably. Shallow groundwater supplies often were contaminated by household privies that commonly were located near family wells. Rainwater stored in cisterns was subject to contamination by accumulations of soot, dust, and street debris that Introduction

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