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Chapter 5 - Test results
and 210 l/s), the first scour started developing at about 60 to 70° and moved quickly upstream
towards its final position. This upstream “walk” took less than half an hour. At the same time the
scour deepened.
The second scour developed quite rapidly, too, almost proportionally to the development of a
point bar at the inner bank. Depositions at the inner bank started at about 60°. The fine sediments
fractions moved over the point bar and were deposited at its downstream end until the end of the
point bar was finally located close to 90°.
Another interesting observation can be made concerning the location of the point bar, compared
to the position of the maximum scour. The end of the upstream point bar was located downstream
the first scour whereas the end of the second point bar was located upstream the second
scour. This may be an explanation of the growth of the second scour being proportional to the
point bar formation (Appendix 4).
The scour seems to be dominated by different phenomena: In the first scour, the armoring probably
plays a controlling role, whereas the second scour seems to be governed by another phenomenon.
Both scour holes oscillated around their mean position until they stabilized at their final location.
The phenomenon is very sensitive to small changes in the upstream or downstream part of the
channel. If the sediment transport rate at the inlet was somewhat too high, causing an increase of
the inlet bed slope, the scour process reactivated respectively increased a few minutes later first in
the upstream and then in the downstream scour hole. At the opposite, if the outlet conditions in
the channel were slightly modified (during some tests the tilting gate at the outlet came up 1 , so that
the flow depth at the outlet was higher than the critical depth), the second scour started decreasing
slowly but almost instantaneously. This influence of the backwater curve on scouring was also
observed by REINDL (1994). Due to a continuous control of the bed and water levels, all these
fluctuations were stabilzed which allowed to obtain good results.
b) Armoring
The first scour hole and the straight inlet reach armored quickly at the beginning of the test. Afterwards,
the sediments were mainly transported over this armoring layer.
The armoring layer in the second scour needed much more time to be formed than the one in the
first scour. This was due to the fact that the armoring propagated from upstream to downstream.
The second scour started armoring, once the first one was armored.
c) Particular events and phenomena during the tests
During the performed tests, an important number of irregularities occurred related to the different
elements of the quite complete experimental setup. In order to track irregularities is the history of
the tests, Appendix 2.4 gives the main problems encountered during the tests as well as special
1. As soon as this phenomenon was detected, the tilting gate was pushed down again and fixed to
the ground. If the gate came up towards the end of the test, the run was continued some more time
to make sure that the final test result was not influenced by this event.
page 110 / November 9, 2002
Wall roughness effects on flow and scouring