6.3 RIVER TRAINING AND STABILIZATION
Various devices and structures have been developed to control river flow along a preselected
path and to stabilize the banks. Most have been developed through trial and error applications,
aided in some instances by hydraulic model studies. Specific functions of bank protection and
training works in relation to bridges and their approaches include: (1) stabilize eroding river
banks and channel location in the case of shifting streams; (2) economize on bridge lengths by
constricting the natural waterway; (3) direct flow parallel to piers and thereby minimize local
scour; (4) improve the hydraulic efficiency of a waterway opening, thereby reducing backwater
and scour and facilitating passage of ice and debris; (5) protect road approaches from stream
attack and prevent meanders from migrating into the approaches; (6) permit construction of a
well-aligned bridge crossing by diverting the channel from a skewed alignment; (7) reduce the
overall cost of a road project by diverting the channel away from the base of a valley slope,
thereby allowing a reduction in bridge length and height; (8) secure existing works, or to repair
damage and improve initial designs; and (9) protect longitudinal encroachments.
A comprehensive bank stabilization and channel rectification program to control a river reach
completely normally requires extensive work on concave banks in bends, minor work on
convex bars, and control work on both banks through bridge crossings.
To minimize attack by the stream on stabilization and rectification structures, the river is
shaped to an alignment consisting of a series of easy bends, with the flow directed from one
bend into the next bend downstream in such a way as to maintain a direction essentially
parallel to the channel flow line (see Section 5.8.4). Straight reaches and reaches of very small
curvature should be avoided, insofar as practicable, because there is a tendency for flows to
shift from side to side in such reaches. The optimum bend radius approximates that of
relatively stable bends in the general river reach.
6.3.1 Fixed Points
One of the essential requirements in designing a system of stabilization works is that
construction starts at a stable, fixed point on the bank and continues downstream to another
stable location or to some point below which the river can safely be left uncontrolled.
Construction of relatively short isolated stabilization work has often proved unsuccessful
because eventual changes in the direction of flow inherent in bank caving in the upstream
uncontrolled reach either will set up a direct attack against the isolated protective work and
severely damage or destroy it, or will shift the attack to some other nearby reach of bank,
requiring additional work and possible abandonment of the original work.
Revetments should be constructed on a smooth alignment, with no irregularities, in order to
avoid eddies set up by disturbances to the flow that can lead to local scour and subsequent
undermining of the revetment.
6.3.2 Radius of Curvature
The most appropriate radius of curvature for rectification and stabilization varies from river to
river and from reach to reach for a given river. It must be determined on the basis of relatively
stable natural bends for each stream (see Section 5.8.3).