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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3.4

Table 3.1 Elk River and West Fork Discharge Distributions

Description Total Discharge Elk River West Fork
Channel Flow Break Out Flow
5-year 4,770 cfs 4,360 cfs 410 cfs
100-vear 6,190 cfs 5,635 cfs 555 cfs
500-year 6,750 cfs 6,130 cfs 620 cfs
June 1997 5,370 cfs 4,900 cfs 470 cfs
Low Flow 500 cfs 500 cfs o cfs

MODEL CALIBRATION

The hydraulic model was calibrated to high water mark data from the 1997 flood. Six high
water marks were provided within the study reach at Sections I through 6 (Table 3.2).
Calibration efforts focused on the high water marks at Sections 3, 4, 5, and 6. These locations
are upstream of U.S. 40, and were along the segments where stabilization issues are to be
addressed. Because of the limited data available to document the actual overflows to the West
Fork, calibration of the hydraulic model was achieved by two methods. This approach
provided a range of overflow values that may have occurred during 1997 flood event. The
procedure for calibrating the model includes adjusting the 1997 discharge value according to
the composite rating curve developed for overflows to the West Fork near Section 8.

Table 3.2 - High Water Mark Data

High Water Mark Elevations
River Section Elevation
(ft)
1.0 6527.80
2.0 6529.17
3.0 6530.82
4.0 6532.79
5.0 6536.57
6.0 6538.24

The first calibration model concentrated on matching high water marks at Sections 4 and 5,
since these locations appeared to be the most sensitive with respect to the lateral stability of the
channel. Manning's 'n' values of 0.025 for the channel and 0.040 for immediate overbanks
were used. From the total 5,370 cfs, it was determined that 4,900 cfs would remain within the
channel, and 470 cfs would overflow to the West Fork. Water surface elevations from this
model are within 0.2 feet of the high water marks at Sections 4 and 5 (Figure 3.3). The water
surface elevation at Section 6 was 1.3 feet higher than the high water mark provided, but the
section had been truncated at the west bank. Realistically, it appears that flow between the Elk
River and the West Fork may not actually be separate until downstream of Section 6.

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