environmental constraints. In the upstream reaches the main problem is
the short-term and seasonal variation of flow, high velocity, channel insta-
bility and shoal formation. In the middle and lower reaches it is often
necessary to raise river banks and carry out works reducing the channel
width, e.g. groynes, longitudinal training walls etc. Dredging, using
mechanical or suction dredgers, is the most effective means of estuarine
river regulation, but its impact is often only temporary.
An efficient river training system will try to maintain and improve
the natural sequence of bends of a meandering river, while preserving suf-
ficient depth (e.g. for navigation) at low flows and suppressing unduly
sharp bends and excessive velocities. This is mainly achieved by groynes,
jetties, longitudinal dykes, and embankments and ground sills (see Section
8.7.2).
Even today, the guidelines for river regulation follow the ‘laws’ for-
mulated by Fargue from his experience with the River Garonne in the
latter part of the 19th century (laws of deviation, greatest depth, trace,
angle, continuity and slope), which in principle advocate the avoidance of
sharp discontinuities in river planform and longitudinal section, and the
following of natural river forms and meanders appropriate to the river
concerned (Section 8.3).
The normalization of the River Rhine upstream of Mannheim,
carried out in the 19th century, is shown in Fig. 8.8: this was only partially
successful because of the use of curves of too large a radius, and excessive
shortening of the river (Jansen, van Bendegom and van den Berg, 1979).
One must realize that the construction of cut-offs, which drastically
shorten the river meander, increases the channel slope and can result in
upstream erosion and downstream sediment deposition unless accompan-
ied by carefully designed local training works, to preserve the tractive
344 RIVER ENGINEERING
Fig. 8.8 Normalization of the Rhine (Jansen, van Bendegom and van den
Berg, 1979)
