Q. How to set Location of the bridge?
A bridge is normally located where the river section has minimum width so that the structure becomes shorter and economical. As far as possible it is aligned normal to the river. If satisfying these requirements involves a long detour, some compromise is made in siting a crossing. Care is also taken to avoid, if possible, sites where frequent changes occur in river course, tendency for aggradation or degradation is manifest and problems of bank erosion or difficult foundation conditions are required to be faced.
Approach banks in case of constricted bridges are generally aligned in line with the bridge axis and normal to the guide bunds. Approaches deviating towards downstream direction add to their safety from river loops approaching dangerously close while deviation in upstream direction exposes them more to river attack.
Location of the bridge with respect to Khadir width also determines the length and position of approach bank exposed to river attack. When bridge is constructed near one of the ends of the Khadir width and when the khadir edge consists of stiff material resisting erosion, cost of one guide bund can be saved. Longer length of the approach at other end however gets exposed and becomes vulnerable to river attack in this case. On the other hand central location of the bridge reduces length of approach open to river attack, minimises possibility of extreme obliquity of approach of the river towards the bridge but requires provision of guide bunds on both the flanks.
In general, bridge location and alignment are fixed to ensure normal approach of the river, equitable distribution of flow across bridge section and minimum possibility of river attack on approach banks.