7196

• UCREFRP Catalog Number: 7196
• Author: O'Brien, J. S.
• Title: Hydraulic and Sediment Transport Investigation Yampa River Dinosaur National Monument 1983 Final Report.
• USFW Year: 1984.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

48
The Helley-Smith sampler is sensitive to local hydraulic conditions on
the bed. ,If the cobbles protrude above the sand bed level, sand will
pass under the sampler nozzle resting on the top of cobbles (Photo 3).
Numerous observations were made on the phenomena of sand transport
over and sand scour from a cobble bed. This information is important to
understanding the processes required to sustain a cobble bed relatively
free of sand over a range of hydraulic conditions. Sand moves as
bedload through the cobble bed material in a series of interrupted waves
and strings. As expected the sand rolled and saltated in and around the
cobbles, scouring upstream of a cobble and being deposited in the wake
of the cobble.
The sand bed was nearly level throughout the flume at a depth of
one half to one cobble diameter below the average cobble bed height.
When a small sand wave progressed through the test section, the sand bed
was approximately the same level as the average cobble height. When
equilibrium conditions were established in the flume to simulate those
conditions at the cobble bars in the field (including maximum bedload
transport rates), the bed surface was approximately 75% cobbles and 25%
sand (Photo 4). Where sand waves were found, the bed was 80 to 90% sand
with only the large cobbles exposed. Even more prominant than the waves
were strings of sand, parallel to the flow direction. These were
observed in the field depicting high rates of bedload transport.
For the second test of the flume study, a six inch bed of sand was
laid over the cobble substrate. During succeeding flume runs the sand
bed was removed. Various bedforms were observed for different runs at
different locations in the flume test section. In the lower regime,
ripples and large dunes were noted; in the upper regime, the bed planned
out with high transport rates (Photo 5). The sand creating the bedforms
was significantly coarser than-the original sand laid on the cobble bed.
With several high velocity runs, the layer of sand and all the bedforms
were removed and the cobble bed was once. again exposed. Sand was
removed progressively from upstream to downstream.
Sand is scoured out from between the cobbles as a result of
turbulent bursts in the interstices.- Vortex events were irregular and
observed to be strongest where several cobbles, stacked on top of each
other, protruded above the average height of the cobble bed. A scour
pit developed beneath these stacks of cobbles, but was occasionally
filled in when a dune bedform passed (Photo 6). The sand could be
scoured to maximum depth slightly greater than one median cobble
diameter below the surface. Velocities in the cobble,- interstices were
too small to entrain the larger sand sizes long enough to secure passage
between the cobbles and back into the main flow zone.
Figure 23 shows the rate of sand removal from the cobbles. A state
of equilibrium was reached when the sand level was approximately one
cobble diameter below the cobble surface. This level fluctuated with a
passage of a sand wave by the point of observation. The three stations
all displayed a similar response, reaching an equilibrium level approx-
mately 160 minutes after the test was initiated. It is postulated that