FLOOD09640

• County: Weld
• Basin: South Platte
• Title: Supplementing Flood Frequency Analysis in Eastern Colorado using Rainfall Modeling
• Date: 8/2/1999
• Prepared For: CWCB
• Floodplain - Doc Type: Flood Documentation Report

with the average water surface elevaiion approxnnaiely u.JO ,eet below. Srauon (by exmblted a
large flood bar and some cut banks. The cut banks corresponded well with each other to form
the npper bound. The top of the flood bar defined the lower bound. A large lichen covered
boulder was at the crest of the flood bar. The stone was approximately 10 percent lichen
covered. This extent oflichen growth indicates the top of this flood bar has not been nnder water
in hundreds of years. The bounds were analyzed independently. Estimates based on section (a)
were in the range of 390 to 670 cfs. Estimates from station (b) were between 375 and 560 cfs.
The closeness of the estimated flows imparts high confidence in the estimates.

The PSI data proved a valuable tool in checking the data for quality and consistency at Owl
Creek tributary. Figure 9 shows the cross-sections taken at this site. Only the most upstrellm
and downstream sections are shown here. A complete set is included in the supplementary
material in Appendix C.

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Figure 9 - Cross-section plots of the (a) upstream and (b) downstream stations at Owl Creek tributary.

The largest flood in the systematic record is reported between 2,620 and 3,400 cfs in 1973
(Livingston and Minges, 1987; Cochran, et ai, 1983). The rating curve was also based pft)
indirect slope-area methods for the 640 cfs flood in 1972. The PSIs (namely the development '01'
the A horizon) at the gage location indicate the non-inundation surface to be approximately at a
stage reported for the 1973 flood. Using the critical flow assumption, the discharge at this
location and stage is approximately 2,150 cfs. The stage corresponding to the range of published
flows would require a Froude number in the range of 1.2 to 1.5. Jarrett (1990) showed that flow
in natural rivers is generally subcritical. Channels become unstable as the flow approaches a
critical state and natural process begin to limit development of supercritical flow
(UDFCD,1984).

60

Using the same datum as the original gage, a rating curve was developed using the critical flow
assumption. Comparison of the two rating curves indicates a consistent discrepancy of
approximately 60 percent. It is difficult to place the source of this error, although much may be
placed on the uncertainty associated with estimating the Manning's roughness coefficient used'in
the indirect slope-area calculations. Jarrett (in review) reported a reasonable range of uncertainty
of :t25 percent in the roughness coefficient. Unfortunately, the report containing the slope-area
calculations was not able to be located. Chow (1959) recommends a roughness coefficient of

13

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