BRIDGES AND CULVERTS REDUCED IN SIZE AND COST BY USE OF CRITICAL FLOW TRANSITIONS

This Paper describes an innovative concept for designing channel transitions which will converge wide, shallow, sub-critical flows into high-velocity critical flows. The flows are then passed through deep, narrow throats that are cheaply culverted or bridged. The outflow is finally diverged, spread and decelerated before being passed downstream with minimum head losses. These 'minimum energy' or 'maximum discharge' designs were first used in Australia in 1961. They offer a means of passing large flows through much smaller, more efficient and more economical culverts and bridges, without the considerable upstream afflux that is the bane of conventional hydraulic structures. The design procedures have accurately predicted performances in the field. The usual problems of erosion, siltation, debris and energy dissipation are discussed and shown to be either absent or manageable. The concept is very useful in many instances, including that of increasing the capacity of many suitable floodplain bridges at minimal cost. Much experience is recorded, a worked example is presented, and both appropriate and unsuitable sites are described. (A)