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Measurement technique and data acquisition
-300
( g )
Average velocity [mm/s]
-250
-200
-150
-100
Distance to
free water
surface
in mm
42
62
82
102
122
142
-50
0
0 20 40 60 80 100 120 140 160 180 200
Number of samples
Figure 4.22: Influence of the sampling time on the time-averaged velocity (in the tangential direction).
The vertical axis gives the computed average velocity over the number of samples (horizontal axis).
4.4.5 Other automatic recordings
The discharge during the tests was automatically regulated by an electromagnetic flow meter,
which is a part of the fixed laboratory equipment. The guaranteed precision of the pumps is of
1 % of the maximum flow scale corresponding to 2.5 l/s. Automatic recordings of the discharge
showed that the precision is even higher.
4.4.6 Manual readings
a) Water and bed-levels on the outer side wall
The water and bed levels on the outer side wall were recorded manually every 30 minutes during
the test every 2.5° (= 28 cm) starting at 1°. Towards the end of the test the frequency of readings
was reduced due to slower bed level changes. The water level changes were generally less important
than the bed level changes. The reading precision is of about 5 mm, which is mainly due to
the size of the grains making it rather difficult to determine an exact bed level.
b) Bed levels in the inlet and outlet reach
The bed level at the inlet (1.5 to 5.5 m) and at the outlet (2.0 to 5.0 m) was measured manually
every 1.0 m over the whole cross-section (every 10 cm). Therefore a metallic profile was put on
top of the channel. The distance between this profile and the bed surface was measured with a
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 89