B.3.3 Power Relationships Presented by Posada (1995)
Expanding earlier work by Nordin, Posada developed a sediment transport relation in 1995.
In the simplest form, the sediment transport in sand-bed rivers varies as a function of the
velocity to about the fifth power (Simons and Simons 1987). Using data from the Mississippi
River (85 sets of data) and the Amazon and Orinoco River Systems (114 sets of data),
Posada (1995) proposed a sediment discharge relation for sand-bed rivers as a function of
velocity,
qs = 30V 5
(B.5)
where:
qs
=
Unit sand discharge (metric tons/m/day)
V
=
Mean velocity (m/s)
This equation has an advantage for large, flat rivers in that velocity is easily measured and
conversely slope is very difficult to measure accurately. This is particularly true when dealing
with large, flat rivers such as the Mississippi, Padma, Amazon, and Orinoco Rivers. The
application of Equation B.5 can be improved by subdividing the cross sections in the reach of
channel being analyzed into similar parts, i.e., thalweg, etc. Also, better results are obtained
for coarse sand and gravel if the exponent is reduced to four (4), and for silt-bed rivers if the
exponent is increased to six (6).
Using Equation B.5, Posada computed the unit sand discharge for the data of the
Mississippi, Amazon, and Orinoco River Systems and compared the results to the measured
values. About 70 percent of the data for these river systems have a discrepancy ratio RD
between 0.5 and 2.0. However, the quality of this relation is limited when additional sets of
data from 23 river systems (2503 data points for sand-bed rivers) are included in the
analysis. The results are shown in Figure B.3.
The Pearson correlation coefficient of qs computed using Equation B.5 and qs measured is
0.66.
Figure B.3. Relation of unit sediment discharge qs from measured and
computed results using Equation B.5 for 2503 sets of field data.
B.17
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