FLOOD02750

• Document ID: 125
• County: Routt
• Stream Name: Elk River
• Basin: Yampa/White
• Water Division: 6
• Title: Streambank Stabilization Limited Watershed
• Date: 11/1/1999
• Prepared For: CWCB
• Prepared By: ICON Engineering, Inc.
• Watershed Pro - Doc Type: Educational/Technical/Reference Information

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A second calibration model was created. In this model, Manning's 'n' values were changed to
0.035 for the channel and 0.05 for the immediate overbanks in Sections 4 and 5. The
contraction and expansion coefficients at the U.S. 40 and railroad bridges were also increased
to 0.3 and 0.5. Water surface elevations from this model are within 0.7 feet of the high water
mark for Section 6, and within 0.4 feet of the high water marks for Sections 3, 4, and 5. This
second model did not correlate well with the composite rating curve developed to determine
overflows to the West Fork. Therefore, it was only used to provide a range of possible
overflows to the West Fork.

Results of the calibration efforts indicate that between 470 cfs and 1670 cfs may have spilled
from the west bank of the Elk River during the June 1997 flood event. Since the first
calibration model provides a more conservative estimate of depth and velocities for stability
consideration, and since it correlates with the rating curve for the overflows to the West Fork, it
was used as the base model for all other storm events. A summary of the results is shown in
Table 3.3. Copies of the HEC-RAS output for the multiple profile and calibration models are
provided in Appendices G and H, respectively. The resulting 100-year water surface profile is
shown in Figure 3.4.

Table 3.3 - Summary of Hydraulic Analysis Results
Discharge Elevation Avg. Velocity Comments
River Station (cfs) (ft) (ft/sec)
0.5 4360.0 6528.0 6.0 5-year
5635.0 6528.5 6.6 100-year
6130.0 6528.7 6.8 500-year
4900.0 6528.2 6.2 June 1997
500.0 6525.5 2.6 Low Flow
1.0 4360.0 6529.1 6.0 5-year
5635.0 6529.7 6.7 100-year
6130.0 6529.9 6.9 500-year
4900.0 6529.4 6.3 June 1997
500.0 6526.7 2.6 Low Flow
2.0 4360 6530.22 7.39 5-year
5635.0 6530.9 8.3 100-year
6130.0 6531.1 8.6 500-year
4900.0 6530.4 7.9 June 1997
500.0 6527.0 2.9 Low Flow
3.0 4360.0 6531.3 8.0 5-year
5635.0 6532.1 8.9 1 OO-yea r
6130.0 3532.3 9.2 500-yea r
4900.0 6531.6 8.5 June 1997
500.0 6527.9 2.9 Low Flow

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