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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4.3.4 Comparative Analysis

A limited amount of information was available for performing a comparative analysis to
provide an understanding of historic stream stability. CDOT had information at the U.S. 40
bridge from their inspection program, and it was evident that the channel bed had adjusted both
up and down over the past 40 + years (Appendix I). The maximum bed degradation appears to
be about 7 to 8 ft in the vicinity of the piers. Their latest information indicated that in addition
to the degradation, a 7-ft scour hole had developed downstream of the west pier and was
threatening the footing.

No historic aerial photos were obtained to make a reasonable comparison of lateral channel
movement. The aerial photo taken during the 1997 flooding, indicates there has been a lot of
channel adjustment in the area upstream of Section 7. According to property owners, there is
an indication that the channel has migrated, or attempted to migrate westward. From
discussions with CDOT maintenance personnel, the channel has also eroded the east bank
upstream of the U.S. 40 bridge. This is supported by the previous stability efforts observed in
the field, but no data were available to quantify the extent of the migration.

4.3.5 Bank Instability

Along the lower Elk River, bank stability is a reflection of the soils present, the impact of
channel hydraulics on the toe of the bank, vertical loads, seepage forces to which the bank may
be subjected, the channel's adjustment to both natural and man-made controls, and past
dredging activity. Major failure mechanisms include block-type failures where more cohesive
materials are present, and sloughing of the bank due to undercutting and removal of material
from the toe area. Cattle trampling also factors into bank instability for the upper portions of
the study reach. From field observations, bank angles along the study reach appeared to range
between about 40 degrees to 90 degrees. Many of the steeper banks occurred at the outside of
meander bends, opposite either channel deposits or debris blockage, or opposite natural and
man-made controls.

4.3.6 Conclusions

No formal stability analysis was performed as part of this study. But from the geomorphic
analysis, it would appear that the channel is quite active along the lower Elk River. From the
limited survey information, bed slopes range from 0.002 Nft to 0.007 Nft. The steepest area
being in the vicinity of the sharp meander bend upstream of U.S. 40. With the large cobble
present in the bed along this reach, it would appear that the channel is in the process of self-
armoring. The scour occurring at the U.S. 40 bridge and evidence of headcutting just upstream
of the U.S. 40 bridge indicates the potential for some additional degradation to take place. This
could result in the undermining of bank stability measures that have been implemented to date
to protect both the bridge and private property. Also, active bank erosion is taking place
throughout the study reach. The areas of greatest concern are where existing structures, both
public and private, are being threatened by the shifting river.

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