Streams Not Wholly Alluvial
In streams which are not wholly alluvial, consisting of bed of pebbles, rock, etc. the liner waterway is kept equal to the actual surface width of the stream, measured from edge to edge along the high flood level line.
Streams with Rigid Boundaries
The bridge should span from bank to bank, measured along the high flood level line.
Streams which Overflow the Banks
In streams which overflow the banks, the waterway is kept equal to the Lacey's regime width and the river is provided with guide bunds to train the flow.
The number of spans and the span length are decided on the basis of the well-known thumb rule that the cost of the superstructure equals the cost of the substructure and foundations. Roughly, the span is 1.5 times the height of the pier from the bottom of the foundation to the top of the pier.
Clearance is provided above the high flood level upto the levels where bearings are fixed for accommodating any trees floating along the river. The clearance depends upon the discharge, and varies from 0.15 m for small discharge (upto 0.3 cu m per sec) to 1.5 m for discharge over 3000 cu m per sec.