ABSTRACT
Stream flow is a complex subject, whose proper study extends well beyond the scope of this book. The interaction between a bridge and stream may be divided into two parts.
There is, first, the effect of the bridge on the stream. The bridge typically causes some constriction of the waterway, resulting in afflux, or the elevation of flood levels upstream from the bridge. This afflux must be controlled, and the selection of bridge spans and the division of the bridge between embankments and open waterway is often determined by this factor. In addition, the reduction in waterway brings about an increase in the velocity of the stream beneath the bridge; this may cause scour, which is undesirable in itself and also leads to the deposition of material in the bed downstream. Moreover, the course of the stream may be diverted. To quote from Waterway Design (Austroads 1994:14), ‘…modification to perennial rivers’ [those with a permanent flow] ‘can be quite extensive, however the dynamic effects of rivers in arid and semi-arid regions… can be even more extensive and dramatic’. The disturbance caused by a bridge may cause major variations in the course of a river, such as one with meanders.
