and the processes involved. The return period of this discharge may range
from 1 to 100 years and in very special cases (large settlements, ancient
historic monuments, nuclear installations, etc.) may be substantially higher
(Jaeggi and Zarn, 1990).
The design of flood protection schemes nearly always involves the
computation of stage in two- or multistage channels. A considerable body
of basic research conducted in laboratory flumes (some of them up to 10 m
wide) and prototype channels has been carried out worldwide particularly
during the last 25 years. It has provided fairly comprehensive information
on stage–discharge curves, shear stress distribution and turbulence
characteristics in the main channel and flow over the flood plain(s) and the
rôle of the apparent shear plane between the main channel and flood
plain(s) (e.g. Keller and Rodi, 1988; Knight and Samuels, 1990; Ackers,
1993). Based on this research, guidelines for the design of straight and
meandering channels using zonal calculations and the coherence concept
(ratio of the single section conveyance – Section 8.2.2 – to the conveyance
obtained by summing the conveyances of the separate flow zones) have
been developed (Wark, James and Ackers, 1994).
The interaction of in- and overbank flow on sediment transport has
been investigated e.g. by Wormleaton, Sellin and Bryant (2004). A more
detailed discussion of the whole topic of hydraulics of multistage channels
is beyond the scope of this text; for further information see, e.g., Knight et
al.(1999) and Chadwick, Morfett and Borthwick (2004).
Fornavigation purposesthe main river improvement works are those
which provide sufficient depth and/or stabilize the river channel in a suit-
able form, and provide bank protection against wave action, particularly
on constricted waterways (Chapter 11).
The principal methods used to improve river channelsare river regu-
lation and dredging; on navigable rivers canalization, construction of
lateral canals and flow improvement by reservoir construction and opera-
tion have also to be considered. In planning river improvement works,
both the upstream situation and historical factors have to be taken into
account, as the river is an evolving system; a good design has to try to
assess this evolution even if the purpose of the improvement works is to
stabilize the situation at least over the life of the design. A typical example
of an upstream man-made influence is the effect of a reservoir construc-
tion on the downstream river morphology, which has to be taken into
account in the planning and execution of river training works. The
methods used for estimating this effect are spatial interpolation, use of
régime relationships (Section 8.3), use of one- or two-dimensional math-
ematical models or a combination of all these methods (Brierley and
Novak, 1983).
In river regulation or trainingthe river may be encouraged to pursue
its natural course or it may be straightened; the latter requires great
sensitivity and should be used only with caution and due regard to
amelia
(Amelia)
#1