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Analysis of the sediment transport
The higher discharges (Figure 6.17 center and bottom) show an almost constant transport rate
(a slightly decreasing tendency can be figured out for the case without macro-roughness). This is
mainly due to the circumstance that the tests were performed as a series with increasing discharge.
The bed was only levelled after the first test (Q = 150 l/s). The next tests continued with the bed
topography of the previous test. Since the discharges were high enough to provoke bed changes
this was no problem, but as far as the sediment transport is concerned, the bed was already
armored; therefore the sediments introduced at the inlet were transported over the armoring layer
to the outlet.
In the presence of macro-roughness, a natural river, subject to a constant sediment transport rate
will compensate the reduced transport capacity in the bend by steepening the slope in the bend:
Upstream the bend, the water depth increases locally. Therefore the transport capacity is reduced
and sediments accumulate in the inlet reach. The overall channel slope increases, till it is sufficient
to pass the bedload across the bend. The increased slope leads to higher velocities in the exit
reach, inducing some additional erosion after the curve.
This phenomenon was observed during the preliminary tests, for which the sediment transport
rate was kept constant (Figure 6.18 and Appendix 3.3.3). For a discharge of 170 l/s respectively
210 l/s the overall bed slope increased by about 40%, respectively 50% from the initial value
(0.50%).
Difference to initial bed level [mm]
100
0
-100
-200
-300
-400
Comparison of the bed levels with and without macro-roughness - Q = 212.5 l/s
Q=212.5 l/s without mr, avg
Q=212.5 l/s with mr, avg
Q=212.5 l/s, Difference, avg
Bend
-500
-100
0 5 10 15 20 25
Distance to the inlet [m]
Figure 6.18: Changes of the average bed level for the preliminary tests (with and without ribs) at a constant sediment
transport rate
500
400
300
200
100
0
Difference between without / with mr
[mm]
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 133