8.4.5 Sediment transport
The sediment concentration in rivers varies enormously with time and
between continents, countries, and even catchments, e.g. from an average
15 000 ppm at the mouth of the Hwang Ho River to 10 ppm in the Rhine
delta (de Vries, 1985) (Section 4.5).
Sediment data are essential for the study of the morphological prob-
lems of a river. From the existing sediment transport equations and the
hydraulic parameters of the river it is possible to estimate the rates of sedi-
ment transport; however, the computed results could lead to large errors
and differ by several orders of magnitude. Thus, whenever possible, actual
sampling is the more reliable method of measuring sediment transport
rates in a river.
Suspended sediment (concentration) samples can most simply be col-
lected by spring-loaded flap valve traps, or by samplers consisting of a col-
lecting pipe discharging into a bottle. Continuous or intermittent pumped
samplers are also being used. Point-integrating or depth-integrating sedi-
ment samplers, with nozzles oriented against and parallel to the flow and
samplers shaped to achieve a true undistorted stream velocity at the
intake, are used for measuring suspended sediment discharge. The
methods used for measuring suspended sediment concentration and dis-
charge are summarized in ISO 4363/1993. The US series of integrating
samplers (particularly the US P-61) developed by the US Geological
Survey are frequently used (e.g. Jansen, van Bendegom and van den Berg,
1979). Sediment from the bed is collected for further analysis of size,
shape, etc. by various types of grabs (ISO 4364/1977).
Bedload transport measurement can, in principle, be carried out as
follows:
- using a sediment sampler, i.e. a device placed temporarily on the
bed, and disturbing the bedload movement as little as possible;
- using other methods consisting of (i) surveying sediment deposits at
river mouths or, in smaller streams, collected in trenches, (ii) differ-
ential measurement between normally suspended sediment load and
total load, including the bedload brought temporarily into suspen-
sion in a river section with naturally or artificially increased turbu-
lence (turbulence flumes), (iii) dune tracking, (iv) remote sensing,
(v) tracers, and (vi) acoustic dectectors.
Quantitative measurements of bedload is extremely difficult and
there is probably no universally satisfactory method, although some rea-
sonably well functioning samplers have been developed. Their efficiency
(i.e. the ratio of actually measured sediment transport to that occurring
without the presence of the sampler) has to be tested in the laboratory for
336 RIVER ENGINEERING
