PROGRAMS AND PLANS--New WRC Guidelines on Flood-Frequency Analysis In Reply Refer To: December 17, 1975 EGS-Mail Stop 415 SURFACE WATER BRANCH TECHNICAL MEMORANDUM NO. 76.07 SUBJECT: PROGRAMS AND PLANS--New WRC Guidelines on Flood-Frequency Analysis The purpose of this memorandum is to inform field offices of the current status of recommended techniques for flood-frequency analysis. Sometime after the first of the year, the Water Resources Council will issue a new report entitled, "Guidelines for Determining Flood-Flow Frequencies." That report will supersede WRC Bulletin 15 (December 1967) as the standards by which Federal agencies are to define flood-frequency curves from a gaging station record. The new guidelines provide a hierarchy of procedures rather than a direct "cookbook" technique. Within this hierarchy are various alternatives that may be employed in the several analytical steps necessary to define a frequency curve. The guidelines suggest that a frequency curve defined quickly by simple techniques may be satisfactory for some uses, but that a very detailed analysis may be justified for some complex or costly projects. Inherent in this concept is an assumption that the more complete or detailed the analysis the more reliably defined will be the frequency curve. The Surface Water Branch has been considering how Water Resources Division analysts should react to the available alternatives, has prepared some written guidelines that are being tested, and is having the computer programs modified to provide analytical efficiency. A basic tenet of the Branch is that the analyst will be allowed as much freedom as possible in selecting and following the analytical options. A minimum level of analytical effort will be specified but the analyst is encouraged to utilize as many of the optional routines as he judges may be necessary, useful, and practical for his region, his data, and his project. It should be recognized that a more detailed and complete analysis might be justified for a HUD Flood Insurance Study than for a regional regression analysis. More details will be provided subsequently, but minimum analytical standards will include: (a) use of a log-Pearson Type III probability distribution with skew coefficient determined by weighting of a regional (map) and computed value, (b) test for the existence of low outliers and frequency curve adjustments for outliers as required, (c) adjustment for any known historic peak data, (d) examination of computer output and of the computed station frequency curve errors, and (e) weighting of computed station frequency curves with regional regression estimates to obtain an improved estimate. When the new guidelines are used to compute frequency curves, the limitations on curve extension described in Surface Water Branch Technical Memorandum No. 73.16 can be liberalized. It will then be permissible to present in a regional flood-frequency report the lOO-year flood estimates for all sites having ten or more years of flood record. Coordinates of two frequency curves should be provided for each site. One set of coordinates defines for the station the log-Pearson Type III frequency curve based upon weighted skew and is provided to document input for regional analysis. The second set of coordinates defines the curve computed by combining the station frequency curve with regional estimates to form a "weighted estimate" for the site. The WRC requirement to coordinate lOO-year flood estimates with other concerned agencies still exists. Project planning must provide two to three weeks for other agency review and should consider that the more detailed the frequency analysis, the more acceptable will be the defined station frequency curves. For ongoing projects, each district will have to judge the desirability of switching to the new analytical techniques. If this switch would unduly hinder the completion of a project, there is no need to redo the analysis, but the general acceptability of curves defined by the new guidelines may justify some extra analytical effort. Guidance should be sought from the Regional staff. All future starts in frequency analysis should use the new guidelines. Although the printed copies of the WRC report may not be available for some time, a xerox copy of the report will be available for short-term loan. Interim guidelines on the minimum level of effort are being tested and will be provided as soon as possible. Each District should acquire and maintain expertise in applying the new analytical guidelines, and should assure the validity and completeness of the flood-peak data file. Public announcement of the new guidelines quite possible will spur increased interest in frequency determinations. Because of the complexity of the methodology, little analytical capability will exist outside of a few Federal agencies. Harry H. Barnes, Jr. Chief, Surface Water Branch WRD Distribution: A, FO-L