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Chapter 4 - Experimental setup and test procedure
• The grain size distribution of the armoring layer (7) (surface samples) was analyzed after
every test at four locations: in the upstream and downstream scour at the inner and outer
sidewall (usually the same as the lowest and highest point of the cross-section) (§ 4.4.6).
Another sediment (volume) sample was taken in the end of the test at the outlet of the channel
in order to compare the initial grain size distribution with the one of the transported
sediment.
4.2.3 Boundary conditions
The flow was stabilized in the inlet box (§4.1.3). An additional element to quieten the flow was the
assembly of small pipes (Fig. 4.3), used for the preliminary tests. For the main tests the small pipes
were replaced by a floating polystyrene plate, which increased the uniformity of the flow conditions
at the inlet. The sediment supply was made approximately 50 cm downstream from the
beginning of the channel.
At the outfall, the bed level of the channel was controlled by a step at the outlet (see table below).
During the tests, the tilting gate at the exit was completely lowered leading to critical flow conditions
on the step. The steps at the inlet and outlet were adjusted to fit to the initially built in bed
slope (Table 4.1).
TESTS SLOPE* INLET STEP (A) OUTLET STEP (B)
A 0.50% 485 mm 370 mm.
B 0.50% 485 mm 370 mm .
C 0.70% 460 mm 300 mm*
D 0.35% 480 mm 400 mm.
E 0.50% 485 mm 370 mm.
Table 4.1:
Heights of the step (above channel bottom) at the inlet and outlet for the different tests
* built in initial longitudinal bed slope
* The step was fixed at 285 mm but leveled at
300 mm in the beginning of each test.
channel
bend
a
b
Since the Froude numbers were quite high, the water depths were close to critical flow depth.
Therefore the influence of the backwater curve (see Fig. 4.14) was limited to a short distance,
especially for the highest slope (< 4 mm at a distance of 1 m of the outlet); but it increased for the
intermediate and lowest slope. For the two highest slopes the difference between water depth and
normal water depth was less than 5 mm at the exit of the bend (6 m from the outlet). For the lowest
slope, the backwater curve can have an influence on the second scour.
page 80 / November 9, 2002
Wall roughness effects on flow and scouring