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Analysis of the final scour
6.2 Analysis of the final scour
Subsequently, the final bed topography is analyzed. In particular the characteristics of the scour
holes and of the point bars are discussed. The influence of the macro-roughness is discussed at the
end of each of the following paragraphs.
6.2.1 Analysis of the final bed topography
Parameters related to the bed topography like the scour depth, the location of the scour holes and
point bars, the shape of the scour hole and the volume of the scour are studied hereafter.
a) Depth of the scour holes
First, the scour depth in the first and second scour was analyzed. The scour depth given in Appendixes
4, 7 and 8 is defined as the distance between free water surface and the final channel bed
topography (Fig. 3.6). Some parameters influencing the maximum scour depth are discussed in the
present paragraph.
The first characteristic influencing the scour depth is the discharge. With increasing discharge, the
erosive power of the flow grows. Figure 6.1 (top) shows this phenomenon. If we have a look at
the plot at the bottom, we can make another interesting observation: with increasing discharge,
the relative scour depth 1 decreases. A possible explanation is that important discharges induce
high mean water depths and that the growth of the absolute scour is less important than ∆h m ,
which reduces the relative scour. In a formula describing the maximum scour, the discharge frequently
enters as a velocity or in dimensionless form as a Froude number or as (average) water
depth.
Secondly, the slope of the channel in stream direction influences the scour depth 2 . Despite a poor
correlation, Figure 6.2 clearly shows the tendency that the relative scour depth gets bigger with
increasing energy slope. This phenomenon is particularly important for the first scour. The second
scour hole is probably much more influenced by the flow conditions after the first scour than by
the channel slope.
1. In this chapter, the ratio h s
⁄ h m
(scour depth to mean water depth) is called relative scour.
2. This parameter is not independent from the discharge but linked by well known laws like the
ones of Strickler, Weissbach-Darcy, a.s.o.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 115