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Chapter 6 - Analysis of the test results
6.7 Summary and discussion
6.7.1 Summary of observations without macro-roughness
a) Bed topography
The scour depth depends on the discharge and the overall energy slope of the channel. With
increasing values of theses parameters, the scour increases. It is interesting to observe that the
relative scour ( h s ⁄ h m ) decreases with an increasing discharge. Probably the relative scour is limited
to values between 3 and 5 if the width to depth ratio exceeds 10 (§ 6.2.1 a).
In the present study the maximum scour holes are located at about 30° to 35° and between the
end of the bend and the following one meter (§ 6.2.1 b). The first point bar ends after the first
scour and the second one upstream the second scour. The second scour hole seems to be
influenced by this point bar at its upstream end (§ 6.2.1 c).
The shape of the scour holes changes, too: the first one has an elliptic shape with its deepest point
located at about the average water depth from the outer bank, whereas the second one looks like a
drop flowing in the downstream direction along the outer side wall (§ 6.2.1 e).
The shape of the cross-section (in radial direction) presents a discontinuity developing between
about 10° and the first scour and between 70° and the second scour. The slope in the radial direction
towards the inner bank is rather flat. Towards the outer bank a pronounced line bend can be
observed and the bed slope steepens. The separation point between the two zones starts at the
outer bank and shifts - if we movein the downstream direction - towards the inner bank. A possible
explanation is the development of the secondary current starting at the outer bank and growing
in the bend. In the final scour, the cross-section is generally s-shaped (§ 6.2.1 f).
b) Water surface
Characteristic stationary waves can be observed in the bend 1 . Their amplitude can reach up to
50% (!) of the mean water depth. The wave length is between 3 and 5 times the mean water depth.
It is well known that the free water surface is inclined due to the centrifugal force. In the present
case, the water surface was super elevated by 10 to 15% of the mean water depth (§ 6.2.2).
Behind the point bars, return currents were observed. They were very weak and did not produce
systematically. In general they appeared if the height of the point bars was quite important.
Another interesting observation concerns floating objects introduced into the channel upstream
the bend. Independent of their radial position, they quit the channel more or less in the center
(§ 6.2.2).
h m
1. The local Froude numers along the outer side wall range up to 1.2 ... 1.4.
page 142 / November 9, 2002
Wall roughness effects on flow and scouring