FLOOD00328

• County: Eagle
• Community: Eagle
• Stream Name: Eagle River
• Title: Hydrology Report Roaring Fork and Eagle Rivers
• Date: 12/1/1997
• Prepared For: Eagle County
• Prepared By: J.F. Sato & Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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There are two options for determining flood frequency values to be used for the current study of the
Roaring Fork River. One option is to perform a complete reanalysis of individual streamflow
stations in the region and then perform a new regional analysis of data for those stations. Performing
such a comprehensive study is beyond the scope of this task order. The other option is to use results
of available computations and apply them in the most technically sound manner. The following
paragraphs contain a discussion of that option.

Flood frequency computations were also made for these two stations with peak flow records from
1978 to the present being deleted. This computation reflects flood frequency computations from
streamflow records of lengths similar to those available when previous hydrologic studies were
made. The purpose of this comparison was to determine the impact of using shorter records
compared to records now available. This analysis showed that the longer streamflow records
resulted in 100-year discharges that were slightly lower than when determined using the shorter
record: Aspen 4 percent lower and Glenwood Springs 3 percent. Extending the records for these
two gages by 19 years did not produce a change in the results that could be considered significant.

A plot of peak annual discharge versus time was made to determine whether there was any obvious
trend in the data. A plot of these values is shown in Figure 2 for the Aspen gage and Figure 3 for
the GIenwood Springs gage. There is a wide scatter of data, but in general, peak flows during the
earlier record at Aspen seemed to be a little higher than more recent peaks. Similarly, earlier peak
flows at Glenwood Springs also may be a little smaller during more recent years. There may be a
slight trend due to developments such as construction of Ruedi Dam with a storage capacity of
102,300 acre-feet in 1968, transmountain diversions and increasing water use. A trend in the data
is not adequately defined to suggest changes should be made in flood frequency analysis.

The results of JFSA's flood frequency analysis for the Roaring Fork River were also compared with
results of previous computations compiled in Table 1. Table 3 contains a list of 100-year peak
discharges for the Roaring Fork that were plotted versus their drainage areas. A plot of those data
in logarithmic paper is shown in Figure 4. Computations of the 100-year flood for Glenwood
Springs by Simmons, Li and Associates plots higher than the value determined in the current study
(21,200 versus 18,100). Theirresults for the reach of the Roaring Fork upstream from the Crystal
River area also higher than those presented by the COE for the reach between Sopris Creek and the
Garfield County line (12,600 versus 10,800).

The COE value computed for the reach upstream from the Fryingpan River plots close to a line
connecting Aspen and Glenwood Springs computed 100-year values. Their other two points for
reaches downstream from the Fryingpan River plot below the curve, but this may reflect a reduction
in contributing drainage areas because of some control of runoff by Ruedi Reservoir.

Recommended Peak Discharges for the Roaring Fork River Reach Flood frequency analyses
of short and long term records for the Roaring Fork River show that there is no significant change
in results due to using longer streamflow records, The COE results are more consistent with current
flood frequency studies than the results of studies by Simmons, Li and Associates. Results of the

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