8251

• UCREFRP Catalog Number: 8251
• Author: Rakowski, C. L. and J. C. Schmidt.
• Title: The Geomorphic Basis of Colorado Squawfish Nursery Habitat in the Green River Near Ouray, Utah.
• USFW Year: 1996.
• USFW - Doc Type: #93-1070,
• Copyright Material: NO

Draft Final Completion Report to UDWR for Contract #93-1070. Amendment 3

reach (Andrews and Nelson. 1989). Consequently, the roughness due to bars. bedforms, and saltating grains was

distributed throughout the channel rather than being restricted to the location where the roughness element extracts

momentum from the flow.

The Engelund- Hansen (1967) sediment transport equation was used for these Green River simulations. The
Engelund-Hansen relationship was developed for dunes in sand-bedded rivers. but was shown to be valid over the range
of flows including dunes. transition. standing waves, anti-dunes, and chute-and-pool flows (Engelund and Hansen,
1967). During this study. only ripples and dunes were observed in the sedimentary structures of the bank-attached bar
and on the fathometer traces, and these structures were composed of medium-sized sand (Appendix A). Unlike Yalin's
(1963) transport equation, the Engelund-Hansen equation for suspended load did not require additional bedform
information such as amplitude and wavelength. This approach was necessary for the Green River where a variety of

bedforms, from ripples to dunes superposed on bar forms, were present across the channel. The shear stress values,

calculated at all points of the matrix, were then used in the sediment transport equation to determine areas of erosion and
deposition. It was assumed that the modeled reach is in sedimentary equilibrium, that is, exactly as much sediment was
transported out of the reach as was transported into the reach.
While the Andrews and Nelson (1989) model was a physically-based 3-dimensional numerical model there
were several conditions for which this model did not function well (Nelson. 1994, pers. comm.). These conditions
include:

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1) when any part of the bar was emergent and where split flow around an island existed, and
2) low-flow conditions where areas of recirculation or stagnation existed.
Nelson (1994, pers. comm.) also noted that calibration of the model is necessary for each river reach to which it is
applied.
As a consequence of the first condition. only flows greater than the elevation of the highest elevation bar were
modeled. The second condition prevented the prediction of the locations of backwater habitat at low discharge. Thus,
the model was run only to predict the rate and style of bar building.

Model Modifications

Nelson (1994, pers. comm.) made modifications to the model to allow its use for the cross sections and

topography surveyed in 1993. Changes to the boundary conditions were necessary because the original version of the