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<br />I. Introduction <br /> <br />In December 1967, Bulletin No. 15, "A Uniform Technique for Determining <br />Flood Flow Frequencies," was issued by the Hydrology Committee of the Water <br />Resources Council. The report recommended use of the Pearson Type III <br />distribution with log transformation of the data (log-Pearson Type III <br />distribution) as a base method for flood flow frequency studies. As pointed <br />out in that report, further studies were needed covering various aspects of <br />flow frequency determinations. <br />+ In March 1976, Bulletin 17, "Guidelines for Determining Flood Flow <br />Frequency" was issued by the Water Resources Council. The guide was an <br />extension and update of Bulletin No. 15. It provides a more complete guide <br />for flood flow frequency analysis incorporating currently accepted technical <br />methods with sufficient detail to promote uniform application. It was <br />limited to defining flood potentials in terms of peak discharge and exceedance <br />probability at locations where a systematic record of peak flood flows is <br />available. The recommended set of procedures was selected from those currently <br />used or described in the literature, based on studies conducted for this <br />purpose at the Center for Research in Water Resources of the University of <br />Texas at Austin (summarized in Appendix 14) and on studies by the Work Group <br />on Flood Flow Frequency. <br />Bulletin 17A revises and expands on the procedures described in the <br />original publication. It is the latest result of a continuing effort to <br />develop a coherent set of procedures for accurately defining flood potentials <br />at gaged locations. + <br />Major problems are encountered when developing guides for flood flow fre- <br />quency determinations. There is no procedure or set of procedures that can be <br />adopted which, when rigidly applied to the available data, will accurately <br />define the flood potential of any given watershed. Statistical analysis alone <br />will not resolve all flood frequency problems. As discussed in subsequent <br />sections of this guide, elements of risk and uncertainty are inherent in any <br />flood frequency analysis. User decisions must be based on properly applied <br />procedures and proper interpretation of results considering risk and uncer- <br />tainty. Therefore. the judgment of a professional experienced in hydrologic <br />analysis will enhance the usefulness of a flood frequency analysis and promote <br />appropriate application. <br /> <br />1 ' <br /> <br />/ <br />