The various factors affecting site selection for bridge are :

1) Width of River :

The stream at the bridge site should be well defined and as narrow as possible.

The smaller the width of river, the lesser will be the cost of bridge and its maintenance.

2) A straight reach :

There should be a straight reach of stream at the bridge site.

On curved reach of stream, construction and mainteance of bridge is difficult and costly.  

3) Foundations :

Suitable, unyielding and non-crodable material for foundation should be available at a shallow depth for abutments and piers of a bridge. 

For soft foundation soils, depth of foundation required will be more, thus increasing cost of substructure.

4) River Banks :

The bridge site should have firm, permanent, straight and high river banks. 

Such type of river banks are not affected during heavy floods. 

5) Square alignment :

It should be feasible to have straight approach roads and square alignment, i.e. flow of water should be nearly perpendicular to the centrel-line of bridge. 

Square alignment result in easiness and economy in bridge construction.

6) Materials and Labour :

There should be easy availability of labour, construction materials and transport facilities in the vicinity of bridge site. 

Transportation of materials and labour from a long distance may result in increase in cost of construction of a bridge.

7) Velocity of flow :

The flow of water in the stream at the bridge site should be in a steady regime condition. 

It should be free from whirls and currents. 

The velocity of flow at the bridge should be non-scouring and non-silting.

8) Free board : 

Sufficient free board should be available for the passage of boats, ships under the bridge superstructure if the river is used for navigation purpose.

9) Minimum obstruction to waterway :

There should be minimum obstruction of natural waterway so as to have minimum afflux.

Due to reduction of the natural waterway, water level rises on the upstream side of the stream. 

This is known as afflux.

10) Absence of scouring and silting :

There should be no scouring and silting at bridge site, because scouring and silting may increase the maintenance cost of a bridge.

11) No confluence of large tributaries : 

There should be no confluence of large tributaries in the vicinity of bridge site.

12) Adequate vertical height :

The bridge site should be such that sufficient depth of water is available underneath the bridge for navigational use.

There should be no need for costly river training works in the vicinity of bridge of bridge site.

The bridge site should be such that no excessive work is to be carried out inside water for constructing foundations.

There should be no adverse environmental input.

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