7087

• UCREFRP Catalog Number: 7087
• Author: Elliot, J. G., J. E. Kircher and P. V. Guerard.
• Title: Sediment Transport in the Lower Yampa River, Northwestern Colorado.
• USFW Year: 1984.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

in the size ranges of: sand and gravel, silt. and clay, 0.062 to 0.25 mm, and
0.25 to 1.0 mm; therefore, one sediment transport equation for each of these
sediment categories was sufficient for subsequent analyses. The relations
were significantly different for bedload discharge and for the discharge of
sediment coarser than 1.0 mm. However, bedload and the discharge of sediment
coarser than 1.0 mm were not highly correlated with streamflow; therefore,
determining separate sediment-discharge equations for rising and recessional
periods was not reasonable for these two sediment categories.
Occasionally it may be necessary to estimate the total sediment discharge
of a river--for example, when reliable bedload discharge measurements are
unavailable. Several computational techniques have been developed to estimate
sediment discharge for different sizes of bed material. Total sediment
discharge at Deerlodge Park was estimated using the Modified Einstein proce-
dure, and these estimated sediment discharges were compared to measured
sediment discharges. The Modified Einstein procedure computes total sediment
discharge at a cross section for a river primarily having a sand bed. This
procedure uses measured hydraulic variables, the mean concentration and
particle-size distribution of measured suspended sediment, and the size
distribution of material in the bed. The procedure consists of computing the
sediment discharge for several ranges of particle sizes by applying different
methods of computation for the small particle sizes than for the large parti-
cle sizes (Colby and Hembree, 1955). Estimated sediment loads for Deerlodge
Park were calculated with the Modified Einstein procedure using a computer
program written by Stevens (1978). Calculated total sediment discharges, and
sediment discharges in various size categories are presented in table 14 in
the Supplemental Data section at the end of this report. Sediment discharges
estimated by the Modified Einstein procedure were greater than measured
sediment discharges for virtually all observations in every sediment size
class. Differences in sediment discharges were largest in the coarse sand-
size ranges. Some of the estimated sediment discharges in the 0.5-mm to
1.0-mm, and 1-mm to 2-mm size ranges were much greater than the measured
sediment discharges; whereas, in the 4-mm and larger range the estimated
sediment discharges were less than the measured sediment discharges.
Disparity between measured sediment discharges and estimated sediment
discharges could be a result of undersampling by the Helley-Smith sampler or
from overestimation by the Modified Einstein procedure. Measured sediment
discharge could be understated because material finer than 0.25 mm (the mesh
size of the Helley-Smith sampler) is not totally accounted for in the area of
flow sampled by the Helley-Smith sampler. Also, the correction factor applied
in suspended sediment computations for the percentage of streamflow actually
sampled may be too great if dune bedforms are present (D. W. Hubbell, U.S.
Geological Survey, oral commun., 1984). Measured bedload transport rates are
subject to error from several sources. Although all bedload samples at
Deerlodge Park were collected at the same intervals and on the same cross
section, the exact location of the bedload sampler with respect to bedforms
was never known. Logistical considerations limited the duration of bedload
sampling, and as such, temporal variations in bedload discharge could not be
entirely accounted for. Another source of disparity in bedload measurements
could be in the hydraulic efficiency of the sampler design (Hubbell, 1964).
The ratio of sampler nozzle entrance size to exit size affects the hydraulic
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