Anyone using NFF?

[JEmH]

I've noticed that a lot of reviewing agencies (I work in Pennsylvania and surrounding states, USA) are now accepting the use of the National Flood Frequency Program for determining peak stream flows. (For designing a bridge into a residential development, for example.) Depending on where you are in the US, there might only be a couple of input values. For instance, in Pennsylvania, Region B (Northwest corner of the state), the only inputs are drainage area, area controlled by lakes/swamps/reserviors, and the old C&B flood region. That seems pretty "quick and dirty." I'm curious as to how many people are using this method and what they think of it.

 

[RWF7437]

I have used it often, but only for flood studies which are subject to review by FEMA. Many existing flood studies done by others use either the NFF program or the equivalent local regression equations. The flows estimated by NFF are quite high when compared to flows estimated by other methods, in my experience. Also, the range of basin size allowed in the NFF program is limited to the range for which there exist stream flow measurements (records ). This makes it unusable for very small or very, very large basins.

good luck

 

[blueoak]

I use it as a ballpark and for quick estimates (before I bought my house I checked to see if I was above the 500-yr)

I nice part of it is the references though. Some of the referenced reports give better guidance if you are interested in other calculations like time of conc. compared to lag or Kp or other items.

 

[LHA]

I've never heard of it, but I practice in PA also.

Try it, then backcheck it against PSU-IV, available for free download here:

http://www.dot.state.pa.us/hh/software.htm

PSU-IV is widely accepted, like PennDOT and PaDEP and others. You could just use that one, it is also a statistical regression, not an actual "calculation" algorhythm, like SCS. So, if you don't like the lack of input parameters, you may not trust it any better. I only use regressions for very large watershed, that is what they are designed for.

Engineering is the practice of the art of science - Steve

 

[RWF7437]

By the way, it is not true that NFF is designed only for very large watersheds. Here is a quote from the user manual:
"
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Applicability and Limitations

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By J.B. Atkins and K.G. Ries III
The regression equations in the National Flood Frequency (NFF) Program are applicable and representative of the data used to derive them. Because the user of NFF is responsible for the assessment and interpretation of the computed frequency results, the following limitations of NFF should be observed.


The rural equations in NFF should be used only for rural areas and should not be used in urban areas unless the effects of urbanization are insignificant, or if the rural equations contain independent variables, such as basin development factor, percentage of impervious area, percentage of urban development, or an urbanization index that account for the effects of urbanization.
NFF should not be used where dams, flood-detention structures, and other man-made works have a significant effect on peak discharges.
The user is cautioned that the magnitude of the standard errors will be larger than the reported errors if the equations in NFF are used to estimate flood magnitudes for streams with explanatory variables near or beyond the ranges identified in NFF.
Drainage area must always be determined, as NFF requires a value. Although a hydrologic region may not include drainage area as a variable in the prediction equation to compute a frequency curve, NFF requires the use of a watershed's drainage area for other computations, such as determining the maximum flood-envelope discharge from Crippen and Bue (1977) and (or) Crippen (1982), and weighting of flood-frequency curves for watersheds in more than one region.
Frequency curves for watersheds contained in more than one region cannot be computed if the regions involved do not have corresponding T-year equations. Failure to observe this limitation of NFF will lead to erroneous results. Frequency curves are weighted by the percentage of drainage area in each region within a given State. No provision is provided in the software for weighting frequency curves for watersheds in different States.
In some instances, the maximum flood-envelope value might be less than some T-year computed peak discharges for a given watershed. The T-year peak discharge is the discharge that will be exceeded as an annual maximum peak discharge, on average, every T years. The user should carefully determine which maximum flood region contains the watershed being analyzed (fig. 3), and is encouraged to consult Crippen and Bue (1977) and (or) Crippen (1982) for guidance and interpretation.
The NFF Program allows the weighting of the logarithms of the estimated and observed peak discharges for streamgaging stations using the equivalent years of record of the regression estimate and the number of years of observed record as the weighting factors. If NFF has determined the 500-year flood for the site of interest by extrapolation, then the equivalent years of record of the 100-year regression equation and the extrapolated 500-year flood are used in the weighting calculation. If the equivalent years of record are not available for the 2- through 200-year floods, NFF cannot compute weighted estimates, and it uses the observed peak discharges as the final estimates.
The NFF Program allows the weighting of regression estimates for ungaged sites with estimates based on the flow per unit area of an upstream or downstream streamgaging station to determine improved estimates for the ungaged site. The drainage area for the ungaged site should be within 0.5 and 1.5 times the drainage area for the streamgaging station; otherwise, only the regression estimates should be used.
Some hydrologic regions do not have prediction equations for peak discharges as large as the 100-year peak discharge. The user is responsible for the assessment and interpretation of any interpolated or any extrapolated T-year peak discharges. Examination of plots of the frequency curves computed by NFF is highly desirable.
Hydrographs of flood flows, computed by procedures in NFF, are not applicable to watersheds whose flood hydrographs are typically derived from snowmelt runoff, or to watersheds that typically exhibit double-peaked hydrographs. Furthermore, the flood-hydrograph estimation procedure might not be applicable to watersheds in the semiarid/arid regions of the Nation because the procedure is based on data from Georgia (Inman, 1987). "

 

[JEmH]

lha: Right. The equation for PSU-IV in much of Pennsylvania takes the same form as that of NFF. As far as easy to use regression goes, I've always liked PSU-IV for two reasons. First, the skew and standard deviation contour map makes me feel like someone really thought about this. Second, the manual clearly explains the "confidence" and lets you adjust this if you feel it appropriate. I like the honesty about how accurate the method is.
I do find it interesting that NFF consistantly gives me higher flows than PSU-IV.