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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ENGINEERING, tNC. 7105-6 South Alton Wa~, Erglewood, CO 50lle Phone (303) eel-050e / Fax. (303) eel-4019
November 15, 1999

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Design

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Management

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Mr, Larry Lang
Colorado Water Conservation Board
721 Centennial Bnilding
1313 Sherman Street
Denver, CO 80203

RE: Draft Submiltal- Elk River Stream bank Stabilization and Limited Watershed Hydrology Study

Dear Mr, Lang:

Presented herein is the draft version of the above referenced study, This report was prepared for the
Colorado Water Conservation Board (CWCB) in accordance with our contract dated September 14. 1999.
as received by our office on November 8.1999,

The objectives of this report were to evaluate two primary areas of concern: I.) Examine the existing
stability problems of the lower 0,9 mile reach of the Elk River, including investigation into cost effective
stabilization means and; 2,) Conduct a limited watershed hydrology study of the overall basin, Our
fmdings are summarized below,

Lower 0.9 Mile Stabilitv Issues:

1. Our geomorphic analysis indicates that the lower reach of the Elk River is subject to both vertical
and lateral movement with potential negative impacts to both public and privately owned
properties,
2, Localized scour under the V,S, Highway 40 bridge has resulted in scour holes up to seven feet
deep, Without construction of stabilization measures, the Elk River will likely continue to
increase the severity of the meander directly upstream of the bridge, This will increase the
approach angle that is currently 30 to 40 degrees, and thereby increase the potential scour, If left
unchecked, the channel instability places the bridge at risk,
3, Similarly, erosion further upstream of the bridge will continue to attack the stream banks,
Stabilization efforts near the bridge may also exacerbate bank instability immediately upstream,
Continued bed degradation and erosion along the toe of the banks will result in additional mass
failure of bank material, While some natural shifting of the river is expected, there is a need to
address stability where the bank migration threatens privately owned improvements,
4, Based upon the survey data available, the channel bed has a variable slope, and coupled with
scour conditions observed at the V,S, Highway 40 bridge, there is a vertical instability that could
create headcutting, While the channel appears to be in the process of self-armoring, to a degree,
potential vertical degradation necessitates placement of grade control measures to limit channel
bottom adjustments, This will also require that bank improvements be adaptable to some vertical
movement of the channel bed,
5, Hydraulic modeling indicates a range of stream velocities that are generally very high throughout
a wide range of flow values, At key locations the high velocities greatly limit viable alternatives
for bank protection,
6. Due to the high velocities, it is our recommendation that riprap toe protection along the channel
bank be placed along the most critical bend lengths and that compensation for any loss of channel
flow capacity be achieved through partial removal of opposite bar deposits, Flow depths from
major flood events will exceed the current bank heights in many areas, and will continue to do so
after stabilization measures have been implemented, Since the channel velocities remain fairly
constant over a high range of flows, the final height of the riprap toe protection is largely a
function of balancing the risk of failure with the cost of improvements, The length and number of
river bends to be protected is also a function of available funding. Protection measures detailed in