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Chapter 3 - Theoretical considerations
3.1 Introduction
3.1.1 Bed constitution and material
a) Constituting sediment layers in a gravel bed
In a natural mountain river, the bed material is usually formed of gravel, big stones and blocks. A
sediment sample has different characteristics depending on where in the river it is taken. A sample
taken in the sub layer will be constituted of coarse and fine sediments whereas a sample in the
armoring layer (upper layer) will be mainly composed of coarse grains due to the wash out of fine
sediments. If the sample is taken on a point bar (e.g. at the inner bank of a river bend) the sediments
will be much finer than the sediments of the sub layer. The grains of the armoring layer are
oriented in an overlaying pattern. (Fig. 3.1)
Grain size
distribution
Suspended material layer
Sediment transport layer
Surface / armoring layer
Sub / bed layer, substrate
Figure 3.1: Layers in a coarse gravel bed river submitted to armoring
b) Sieving and analysis of the grain size distribution
The geometry of a gravel grain can be approached by an ellipsoid (Fig. 3.2). The longest axis is
defined as a, the longest dimension perpendicular this axis b and the shortest main dimension c.
Generally b is considered as the main characteristic, since it is the dimension that is most perpendicular
to the flow direction and therefore determinant for the erodibility of the bed.
For practical sieving considerations, the axis b limits the passing through a sieve with circular
openings. If one uses a sieve with quadratic openings (as in the present study), the diameter
remaining on the sieve is systematically too small. In general, the grain diameter remaining on a
page 22 / November 9, 2002
Wall roughness effects on flow and scouring