ERDC/CHL CHETN-VII-4
June 2002
consideration. In large sand bed rivers the general rule is that the sand waves are anything but
consistent in size and celerity both longitudinally and laterally. Inevitably many assumptions and
extrapolations must be made in order to say anything of the bed-load transport rate of the entire
section. This is the main difference with the ISDOT method. It was conceived as a control volume
type of consideration in order to circumvent the necessity of dealing with the spatially varying wave
celerities and or dune profile statistics. The second possible reason why this has not been considered
as practical before is the change in technology. The quality and quantity of detailed bathymetric data
required to make this method work was not available even just a few years ago.
PRELIMINARY RESULTS: The initial results are encouraging. Since no one is able to actually
measure bed-load transport rates in large sand bed rivers, the question arises as to how accurate this
method might be. There are at least two possibilities for determining whether or not the computed
values are even meaningful. One would be to compare with mechanically measured values on the
same or similar rivers. A second way would be to compare with accepted analytic transport
functions. Both comparisons were made.
Data taken on the Nile River in Egypt in 1991 (van Rijn and Gaweesh 1994) using the Delft-Nile
Sampler was used as a comparison for mechanically collected data. These data were used because
the hydraulic and sediment characteristics of the Nile River in the vicinity of the measurements were
very close to those of Pool 8. The average bed-load transport was determined by averaging the data
for six measurement sites on the Nile recorded in Table 3, page 1374, (van Rijn and Gaweesh 1994).
The result was .0175 kg/sec/m (0.0118 lb/sec/ft) of width. For Pool 8 using the multibeam data and
the ISDOT method, a value of .0041 kg/sec/m (0.003 lb/sec/ft) was calculated (149 metric tons/day
(164 tons/day)). Thus the measured value is four times that of the ISDOT value.
Three transport functions were selected and run using average channel parameters and the sediment
characteristics of Pool 8. These functions were: Einstein's bed-load function, Toffaleti's function
(Toffaleti 1968), and van Rijn's function. In tons per day (bed-load portion of the bed material load),
the respective values were: 2255, 312, and 1688 metric tons (2481, 344, and 1857 tons). The
Toffaleti procedure results were slightly more than twice the value computed using the ISDOT
method. The other two methods were substantially higher. Further investigation into which
transport function gives more reasonable results for large sand bed rivers is being considered.
FUTURE PLANS: For both mechanical and analytic methods, the ISDOT method seems to
underpredict the bed-load transport. Without even considering these other two methods, it was
already guessed that the results of the present calculations would be a lower value than the
maximum. This has to do with the time-step between successive bathymetric measurements.
During some of the future trips the need to take the "snapshots" at shorter and more regular intervals
will be addressed. In addition, a joint measurement field campaign is being planned with the Delft
Hydraulics Laboratory and University of Utrecht researchers in which both measurement techniques
will be used side by side. Additionally, the wave celerity method is still being considered and will
be discussed with researchers at the University of Iowa. Finally, future cooperation is anticipated
with the University of Iowa and the Agricultural Research Service (ARS) National Sedimentation
Laboratory researchers for a proof of concept flume study.
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