WSP03331

• File Number: 8210.430
• Description: Colorado River Basin Organizations-Entities - Gunnison - UVWUA - AB Lateral
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 7/1/2000
• Author: DOI
• Title: AB Lateral Hydropower Project - Supplemental Environmental Impact Report
• Water Supply Pro - Doc Type: Report/Study

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Affected Environment & Consequences

3-16

. Widening, where the river widens so that fluid shear stress acting on the bed drops to
or below the critical level.

The point at which flow can initiate bed material movement is called incipient motion,
and was defined in two ways for this study. The first defines the critical particle size as
the size at which any material is mobilized. This value is based upon a Shields value ('tc)
of 0.03. A second definition assumed a Shields value of 0.045, which is typically used to
define the level at which measurable bed-material movement begins. Table 3-11
indicates the percentage of time these values would be exceeded for the No Action and
Development alternatives.

Table 3.11. Percent of Time Incipient Motion Occurs

Alternative
E-l E-2
48.2% 47.8%
0,6% 0.6%

Shields Value
0.03
0.045

Critical flow
614
2028

A
7.3%
0,0%

E.3
47.3%
0.4%

The values shown in Table 3-11 indicate that critical flows are exceeded under all
alternatives, and stream adjustment is likely to occur. The Sponsors then determined how
this adjustment would occur. For gravel bed rivers with easily erodible banks such as the
Uncompahgre, channel adjustment is more likely to result in widening than in bed
lowering (degradation). However, comments expressed the concern that the introduction
of Project-related flows would result in massive channel degradation. Therefore, the
Sponsors evaluated the ability of the river to degrade if there was no bank erosion and no
channel widening (j.e, all stream energy was directed towards degradation).

This evaluation was done using a sediment continuity model.3 This model uses the
Parker bedload function, which is the current state of the art model for gravel bed rivers,
to assess bedload movement (Sigma, 1995b), Bed-level changes were calculated on a reach
by-reach basis, with the model moving sediment through the river based upon each
reach's transport capacity. The location of the reaches is shown in Figures 3-2 and 3-3.

The continuity model indicated that the potential for bed degradation is minimal even
assuming all energy is directed at the stream bottom instead of the banks. In two of the
14 reaches (representing 2.5 of 27 total miles), the bed could degrade slightly. These
reaches are immediately downstream of the Selig and Garnet Diversion Dams. Initial
degradation would be 2.5 to 3 inches per year (1.5 to 2 inches per year for the 750-cfs
alternative). Because of the nature of the bed material, an armor layer can be expected to
develop very quickly, which would lirnit such degradation (ME! 1995b). In reality, the
effect will be less or not occur at all, since the river will tend to widen rather than degrade
(see below).

3

Development of the continuity model was coordinated with representatives from the Corps of
Engineers.

AB Lateral Hydropower Project

July 2000

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