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APPLICABILITY AND LIMITATIONS By J,B. Atkins The regression equations in National Flood 6. In some instances, the maximum flood envelope Frequency (NFF) Program are applicable and represen- value might be less than some T-year computed tative of data used to derive them, Because the user of peak discharges for a given watershed, The T- NFF is responsible for the assessment and interpreta- year peak discharge is that discharge that will tion of the computed frequency results, the following be exceeded as an annual maximum peak dis- limitations of NFF should be observed: charge, on average, every T years. The user I. The rural equations in NFF should only be used should carefully determine which maximum flood-region contains the watershed being ana- for rural areas and should not be used in urban areas unless the effects of urbanization are lyzed (fig. 3) and is encouraged to consult insignificant. Crippen and Bue (1977) and (or) Crippen (1982) for guidance and interpretation. 2. NFF should not be used where dams, flood- t detention structures, and other man-made 7. NFF allows the weighting of estimated and works have a significant effect on peak dis- observed peak discharges for frequency curve charges. calculations. However, because very few 500- t 3. The user is cautioned that the magnitude of the year peak discharges estimates have been pub- lished, NFF does not allow the user to enter standard errors can be larger than the reported observed values for the 500-year peak dis- errors if the equations in NFF are used to esti- charge. The user should evaluate the weighted i mate flood magnitudes for streams with curve thoroughly; it is possible for the 500-year I explanatory variables outside the ranges identi- peak discharge to be less than some of the other , fied in NFF, less extreme T-year peak discharges. 4, Drainage area must always be determined, as NFF requires a value. Although a hydrologic region 8. The user should be cautioned that some hydro- might not include drainage area as a variable in logic regions do not have prediction equations the prediction equation to compute a frequency for peak discharges as large as the lOO-year curve, NFF requires the use of a watershed's peak discharge. The user is responsible for the drainage area for other computations, such as assessment and interpretation of any interpo- determining the maximum flood envelope dis- lated or any extrapolated T-year peak dis- charge from Crippen and Bue (\ 977) and (or) charges. Examination of plots of the frequency Crippen (1982), and weighting of flood- curves computed by NFF is highly desirable. frequency curves for watersheds in more than 9. Hydrographs of flood flows, computed by proce- one region, dures in NFF, are not applicable to watersheds 5. Frequency curves for watersheds contained in whose flood hydrographs are typically derived more than one region cannot be computed if from snowmelt runoff, or watersheds that typi- the regions involved do not have correspond- cally exhibit double-peaked hydrographs. Fur- ing T-year equations. Failure to observe this thermore, the flood hydro graph estimation limitation of NFF will lead to erroneous procedure might not be applicable to water- results. Frequency curves are weighted by the sheds in the semiarid/arid regions of the Nation percentage of drainage area in each region because the procedure is based on data from within a given state, No provision is provided Georgia (Inman, 1987), Future versions ofNFF for weighting frequency curves for watersheds will include flood hydrograph estimation pro- in two different Slates. cedures for different regions of the country. 20 Nationwide Summary 01 U.S. Geological Survey Regional Ragresalon Equatlonalor Estimating Magnitude and Frequency of Floods lor Ung.ged Sit.., 1993