some groynes with short longitudinal dykes to protect the head against
erosion and aid sediment deposition between the groynes; in mountain
streams it is advisable to offset the small dyke slightly into the space
between the groynes.
Longitudinaldykes(training walls) are usually more economical than
groynes and – if properly positioned – equally or even more effective. The
material used is again rubble, stone, or fascine work (on soft river beds).
Training walls may be single – on one side of the channel – or double. An
example of the combined use of groynes and dykes in river training is shown
in Fig. 8.16; note the open end of the training wall and its connection with
the old river bank. High water dykes are used in flood protection (Fig. 8.11)
and their design and construction has been discussed in Chapter 2; due
consideration must be given to their protection against erosion by high-
velocity flows and undermining by scour from a meandering river.
In some instances a series of stream deflectors (vanes)constructed of
wood panels or metal (e.g. floating drums with sheet metal vanes), placed
at a suitable angle (often almost parallel to the bank) and depth, can be
used to either divert an eroding flow from the river bank or, on the other
hand, to induce bed erosion and local deepening of the flow. Details of
348 RIVER ENGINEERING
Fig. 8.14 River training by groynes (after Kinori and Mevorach, 1984)