FLOOD-FREQUENCY ANALYSES MANUAL OF HYDROLOGY: PART 3 FLOOD-FLOW TECHNIQUES by T. Dalymple Prepared by the U.S. Geological Survey Geological Survey Water-Supply Paper 1543-A Methods and Practices of the Geological Survey 1960 TABLE OF CONTENTS Abstract Introduction Flood frequency at a gaging station     Two kinds of flood series         Annual floods         Partial-duration series         Relation between the two series     Flood-frequency curves         Discharge curves         Stage curves         Flood-volume curves     Listing of data         Annual-flood peaks         Partial-duration series peaks         Flood volumes     Historical data     Plotting positions         Methods         Plotting historical data     Plotting paper     Fitting frequency graphs     Single-station analysis Regional flood frequency     Basic frequency curve         Homogeneity test     Mean annual flood         Physiographic factors         Meteorologic factors         Composite factors     Computation procedure         Selection of stations         Selection of base period         Adjustment of records to base period         Preliminary frequency curve         The mean annual flood         Homogeneity test         Computation of median flood ratios         Definition of regional frequency curve         Estimation of mean annual flood         Summary of procedure     Special treatment for large streams Plotting positions in frequency analysis, by W.B. Langbein     Median     Mean     Mode Characteristics of frequency curves based on a theoretical 1,000-year record, by M.A. Benson     Base data     Mean annual flood     Short periods of record         Periods used         Drawing of frequency curves         Arithmetic versus graphical mean annual flood         Reliability of mean annual-flood values         "Working range" of mean annual floods         Reliability of flood magnitudes     Composite flood-frequency curves         Periods combined         Results from composite graphs         Effect of number of stations combined         Median versus average ratios         Theoretical distribution of composite curves     Conclusions Selected references Index    FIGURES 1. Flood data listing 2. Tabulation of flood-volume data 3. Flood-frequency curve, rectangular scale 4. Flood-frequency curve, logarithmic scale 5. Partial-duration series flood-frequency curve 6. Annual flood volume-frequency curves 7. Youghiogheny and Kiskiminetas River basins, showing location of gaging stations 8. Bar graph showing period of record of maximum annual peaks at gaging stations 9. Tabulation of flood data, Big Piney Run near Salisburg, Pa. 10. Correlation of annual floods, Big Piney Run versus Stony Creek 11. Preliminary frequency curve, Big Piney Run near Salisburg, Pa. 12. Homogeneity test chart 13. Regional frequency curve 14. Variation of mean annual flood with drainage area 15. Determination of mean annual flood from a short record 16. Flood-frequency curve for a 100-square-mile drainage basin 17. Variation of mean annual flood with distance along stream 18. Relation of flood frequencies to drainage area 19. Theoretical frequency curve 20. Frequency curves for 10-year periods 21. Frequency curves for 25-year periods 22. Frequency curves for 50-year periods 23. Frequency curves for 100-year periods 24. Recurrence intervals of 2.33-year flood 25. Computing range for mean annual flood 26. Recurrence intervals of 10-year flood 27. Composite frequency curves based on ten 10-year periods 28. Composite frequency curves based on ten 25-year periods 29. Composite frequency curves based on ten 50-year periods 30. Theoretical distribution of composite curves ABSTRACT This report describes the method used by the U.S. Geological Survey to determine the magnitude and frequency of momentary peak discharges at any place on a stream, whether a gaging-station record is available or not. The method is applicable to a region of any size, as a river basin or a State, so long as the region is hydrologically homogeneous. The analysis provides two curves. The first expresses the flood discharge-time relation, showing variation of peak discharge, expressed as a ratio to the mean annual flood, with recurrence interval. The second relates the mean annual flood to the size of drainage area along, or to the size area and other significant basin characteristics. A frequency curve may be defined for any place in the region by use of these two curves. The procedure is: (a) measure the drainage area and other appropriate basin characteristics from maps; (b) from the second curve, select the mean annual flood corresponding to the proper drainage area factors; (c) from the first curve, select ratios of peak discharge to mean annual flood for selected recurrence intervals, as 2, 10, 25, and 50 years; and (d) multiply these ratios by the mean annual flood and plot the resulting discharges of known frequency to define the frequency curve. Two reports not previously given general circulation are included as sections of this report. These are "Plotting Positions in Frequency Analysis" by W.B. Langbein, and "Characteristics of Frequency Curves Based on a Theoretical 1,000-Year Record" by M.A. Benson. 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.