Chapter 3 Flood Frequency Studies
45
watershed is below the applicable range of mean elevation for the equations. This
and the uncertainty and variance in the equations may account for the low value.
Considering the short period of flow data available, the uncertainty and variance in
the USGS regression equations, and that a calibrated rainfall-runoff model is
available, the analyst decided the best-estimate flow-frequency curve would be from
the frequency-storm method. Therefore, the peak flows computed by HEC-HMS
were adopted as the flow-frequency curve.
Table 18.
Comparison of results to fitted flow-frequency function and USGS
regression equations
HEC-SSP Quantiles
AEP
HEC-HMS
Computed
Peak Flow
(cfs)
USGS
Regression
Equation
Peak Flow
(cfs)
Expected
(cfs)
95%
Confidence
(cfs)
5%
Confidence
(cfs)
0.500 285
140 282
195
410
0.100 571
564 718
440
1,278
0.010 944 1,500 1,808 741
3,482
Computing future development frequency functions
Once the current development frequency function was adopted, the next step was to
develop the future development frequency function. The future development function
is developed using a hydrologic model. The hydrologic model must be correlated to
the adopted frequency curve. In so doing, changes in the frequency function can be
calculated by modifying the model and using the same precipitation events.
Modifications to the model may result from changes in land use or construction of
flood control projects. The two basic methods to correlate the hydrologic model are
to:
1. Calibrate the peak flow from the hydrologic model to match the desired
frequency.
2. Assign a frequency to the peak flow from the hydrologic model based on the
adopted frequency curve.
The adopted frequency curve in the example used the frequency storm method, so
the precipitation frequency and the resulting flow frequency were assumed to be the
same. This is the first method. Therefore, changes to the frequency curve due to
future development were calculated by modifying the watershed characteristics and
exercising the model with the same precipitation events used for the current
development condition. Both methods are described in more detail herein.
Method 1. The steps included below describe an approach that entails fitting a
watershed model to an adopted frequency function using loss values as the
calibration parameter. This process is schematically shown in
Figure 17
and requires
the following steps:
1. Adopt a frequency curve. Use one or more of the methods from
Table 17
to
develop the best-estimate frequency curve, see previous section.
2. On a parallel path, use IDF or DDF functions as boundary conditions in HEC-
HMS, as done in the example here. Use reasonable estimates of initial