INSPECTION OF SITE AND RECOMMENDED RULE SHOULD BE ELABORATED BEFORE SELECTION OF SITE FOR CULVERT OR A SMALL BRIDGE.

It is necessary to have inspection of the suitable site where a culvert or small bridge is a choice and the inspection should also include notes on the channel conditions from which the silt factor and the co-efficient of rugosity can be estimated, so required for the flood discharge. The best way to calculate maximum discharge from data collected by personal inspection of the existing structure is necessary including the local enquiry for useful information, namely, marks indicating the maximum flood level, the afflux, the tendency to scour, the probable maximum discharge, the likelihood of collection of brushwood during floods, and many other particulars. It should be seen whether the existing structure is too large or too small or whether it has other defects. All these should be carefully recorded on the data records of the selection of site preferred by the Engineer in charge. The following points should particularly be noticed where there is any choice of site :-- Which must be situated on a straight reach of the stream, sufficiently below the bends;(2) Which is so far away from the confluence of large tributaries as to be beyond their disturbing influence. (3) Which has well defined banks ;(4) Which makes approach roads feasible on the straight and (5) Which offers a square crossing as far as possible. In siting small bridges and culverts, due consideration should be given to the geometrics of the approach alignment and the latter should essentially govern the selection of the site unless there are any specific problems of the bridge design. The recommended rule required for the flood discharge can be estimated in three different ways and the values obtained should be compared. The highest of these values should be adopted as the design discharge provided it does not exceed the next highest discharge by more than fifty percent, If it does , restrict it to that limit. The necessity for this elaborate procedure for fixing discharge arises for sizeable structures and as regards the small culverts discharge may be taken as the discharge determined from the run -off formulae.