Page 365 - Soil and water contamination, 2nd edition

352 Soil and Water Contamination

1000
a = 2.62 6642 6642 6642
b = 0.33

Suspended solids concentration (mg l -1 ) 100

10

Uncorrected rating curve
Corrected rating curve
Obervations
1
1000 10000
Discharge (m 3 s -1 )
Figure 18.11 Corrected and uncorrected sediment rating curves for the river Rhine near Lobith, the Netherlands
(source: Waterbase, 2006).

which can largely be explained by the hysteresis behaviour of the Q–C relationship and the
different concentration responses between hydrological events as discussed above.
According to Morgan (1995), the a coefficient of the rating curve (Equations 18.1–
18.3) represents the erodibility of the soils. Large values of the a coefficient occur in areas
characterised by intensively weathered materials that can easily be eroded and transported.
The b coefficient represents the erosive power of the river, with large values being indicative
for rivers where an increase in discharge brings about a strong increase in erosive power and
sediment transport capacity . Asselman (2000) argued that it is more appropriate to use the
steepness of the rating curve, which is a combination of the a and b values, as a measure
of soil erodibility and erosivity of the river. Steep rating curves with a small value for a and
a large value for b are characteristic for river sections with little sediment transport during
low discharge. An increase in discharge results in a large increment of suspended sediment
concentrations. This suggests that during high discharge periods the river’s power to erode
material is large or that important sediment sources become available. Flat rating curves
usually occur in river sections with intensively weathered materials or loose sedimentary
deposits that are transportable during low discharges.
Figure 18.12 shows some rating curve s for different locations in the catchment of the
river Rhine (Asselman, 2000). The steepest rating curves are found in the river’s tributaries,
suggesting that there is a limited amount of fine sediment that can be eroded from the bed
at low discharge. Once a certain discharge threshold has been exceeded, sediment supply to
the river increases, and sediment can be picked up from the river bed, resulting in a rapid
increase in suspended sediment concentrations. The presence of weirs in most tributaries
also results in steep rating curves. During low discharge, much suspended sediment settles
behind the weirs, but it is flushed out during periods of high discharge, causing suspended
sediment concentrations to rise sharply. Along the river Rhine , the steepness of the rating
curves decreases in downstream direction. The flattest rating curve is found near Rees,
indicating that near the German–Dutch border large quantities of fine material are available
for transport at low discharge.

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