Q. Constriction of waterway at bridges?
Constriction of waterway at bridges is effected by extending the approach banks to cover spill area or, in addition, part of active channel. Such encroachment causes obstruction to flow. Guide bunds are then required to be provided for guiding flow smoothly through the bridge.
The course of main channel may change on account of progressive movement of meanders or formation and movement of islands in braided rivers. In the process, attack may develop on bridge approaches. Under such conditions it becomes obligatory to provide guide bunds in order to pass the river discharge axially with as uniform flow distribution through the bridge as possible. Guide bunds also prevent the river cutting into the bridge approaches, causing breaches and forming deep scour at abutments. These objectives are achieved by giving guide bunds suitable shape, providing adequate length, by adding curved heads and by giving suitable protection to exposed faces against river attack.