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

Where relIable storm data Joes nUl exlS1, (he rrequen"y (;ulVe be(;umes the only data-basea LOol
to compare results. Basins where this is an issue are calibrated to minimize the sum of the
residual and the sum of the squared residual between the simul<iteP peak discharge and that of the
frequency curve for the 5- and 10- year flood. This ensure~e simulation ~, if unable to
exactly match the calibration points, will pass between them ~d be as close as possible. The
systematic records for most of these sites are eleven years. Short records increase the amount.of
sampling error when the frequency curve is e"trapolated much beyond twice the length of the
record. Given this, the 5- and I a-year floods should exhibit less variability than other points on
the frequency curve.

The multiple scenario simulation used to develop the limits of the frequency curve provide the
upper and lower limits of the 1 00 equally likely frequency curves. It is desirable to simulate
peak flows as close to the mean frequency curve as possible. The model simulation is deemed
good if the simulated peak falls within the bounds of the frequency ensemble for a given quantile
and questionable if the simulated peak is outside the ensemble but within the 95 percent
confidence limits. The model is deemed invalid for simulating a given quantile if the point falls
outside the 95 percent confidence limits.

4. Discussion of results

4.1 Paleoflood analysis

Table 2 summarizes the maximum floods believed to have occurred in the past 50 to 100 years
based on the PSI evidence collected at each site.

Table 2 - Estimated maximum discharges in the past 50 to 100 years.

Basin name Discharge (cfs)
[go Creek tributary nr Keota 245 - 390
Geary Creek tributary nr Rockport 375 - 670
Owl Creek tributary fir Rockport 3,400 ·

'Occurred within the systematic record.

Figure 7 illustrates the two cross-sections taken at Igo Creek tributary.

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Figure 7 - Cross-section plots of the (a) upstream and (b) downstream stations at [go Creek tributary.

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