WSP09211

• File Number: 8220.101.10
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powell-Glen Canyon Adaptive Management-
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/2005
• Author: Wiele-Torizzo
• Title: Modeling of Sand Deposition in Archaeologically Significant Reaches of the Colorado River in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

00629

Modelling of ,and deposilion in Colorado River 369

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condition at the inlet appropriate for the discharge and sand supply. The model was
run until the sand coverage reached near-equilibrium with the sand influx. For the
cases with low and intermediate sand supply, corresponding sand boundary condi-
tions were used to model the initial sand coverage. The highest sand supply, how-
ever, is produced by relatively brief events (David J. Topping, US Geological Survey,
oral communication, 1999), such as local tributary flooding. As a result, a bed in
equilibrium with the intermediate sand supply was judged a more reasonable ante-
cedent condition for the high sand supply than a bed configuration in equilibrium
with the high sand supply.
The effects of the different combinations of water and sand discharge on sand
deposition are considered in the following sections. The final results are taken from
the end of the simulation. The length of the simulation varies as a result of differ-
ences in the real time required for the model run. The length of the model runs
depends on tbe time step, which is determined witbin the model on tbe basis of the
rate at which deposition or erosion occurs. As a result, events with rapid changes in
bed morphology, such as would occur with the high sand supply, generally progress
in the model much more slowly in real time than events in which there is little change
in bed morpbology. Nevertheless, direct comparisons between model results under
different conditions can be made at different times because most of the changes
within a reach occur rapidly. The events with the shortest elapsed simulation time (as
a result of small time steps causing slow rates of advance in time) tend to be those
with tbe most rapid changes and, therefore, reach near-equilibrium in the least
amount of simulated time.
Altbough maximum sand deposition is generally favourable for tbe restoration
and preservation of tbe riparian environment, deposition within the main cbannel is
of little long-term value because tbe sand storage is sbort-lived. Reduction in sand
supply, by the cessation of tributary flooding (which is the only mecbanism currently
capable of producing high sand concentrations), will lead to the rapid erosion of
main channel deposits. For the purposes of this study, we emphasize near-shore
environments and deposits above the 708 mJ Is stage. In the two upstream reacbes,
most of the deposition that would be likely to fill gully mouths occurs in recirculation
zones, and so these environments are considered in detail. In the two downstream
reaches, in whicb deposition is not dominated by recirculation zones, deposits along
the channel sides above the 708mJ/s stage are considered. The depositional patterns
and volumes with the intermediate sand supply are generally bracketed hy tbe results
with the high and low sand supplies. Consequently, only the model predictions with
the high and low sand supplies are shown in the figures.

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14.6 The four modelliDg sites

14.6.1 Above Lava Chuar reach

14.6.1.1 lnitilll conditio...

The Above Lava Cbuar reach contains an expansion along river left (i.e. left side of
the river looking downstream) in tbe upper part of the reach (A) that shows no

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A'
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