Bends can be divided into free (surface), limited (entrenched) and
forced (deformed), with ratios of radius of curvature to width ranging
from about 3 (forced) to about 7.5 (limited) (5 for free bends). In free and
limited bends the depth gradually increases to a maximum downstream of
the apex of the bend; bends are characterized by spiral flow and triangular
sections, with the maximum depth and velocity at the concave bank, and
maximum sediment transport at the convex bank and the talweg (line of
maximum depth) deviating from the river centreline, as shown in Fig. 8.2.
Crossings are relatively straight reaches between alternate bends and
are approximately rectangular in section (Fig. 8.3).
Meandering rivers usually have a ratio of channel to valley length
greater than 1.5 (Petersen, 1986) with the meander length (the distance
between vertices of alternate bends) about 10 times the stream width. The
ratio of meander length to width varies between 2 and 4 (for further
details see, for example, Jansen, van Bendegom and van den Berg (1979)).
In rivers the mean cross-sectional velocity varies, from about 0.5 m s^1
at low flows to 4.0 m s^1 at floods, but there are exceptions to these values.
The maximum velocity in a section usually exceeds the mean by 25–30%.
River régime is concerned with the channel geometry. A river in
alluvium is considered to be in régime if its channel is stable on a long-
term average. Short-term changes will occur with changes of discharge and
sediment transport, and the concept of ‘stability’ here clearly differs from
the one defined by the critical limiting tractive force implying no motion of
sediment on the bed and banks.
The cross-section and longitudinal slope for a régime channel will
primarily be a function of discharge, with the width B, depth yand slope S
328 RIVER ENGINEERING
Fig. 8.2 Flow in a bend
