HYDRAULICS OF THE PIPE AND BOX CULVERTS FLOWING FULL AND FIXING OF THIS LEVEL IS THE FIRST STEP IN THE DESIGN AND DEPENDS ON THE VENT WAY.

Where a defined channel does not exist and the natural velocity of flow is very low, it is economical to design a culvert as consisting of a pipe or a number of pipes of circular or rectangular section functioning with the inlet submerged. As the flood water starts heading up at the inlet, the velocity through the barrel goes on increasing. This continues till the discharge passing through the culvert equals the discharge coming towards the culvert. When this state of equilibrium is reached the upstream water level does not rise any higher. For a given design discharge the extent of upstream heading up depends upon the vent way of the culvert. The latter has to be so chosen that the heading up should not be higher than a predetermined safe level, the criterion for safety being that the road embankment should not be overtopped, nor any property damaged by submergence. The fixing of this level is the first step in the design. As for as the surface level of the tail race is concerned it is essential that the HFL in the outfall channel near the exit of the culvert should be known. This may be taken as the HFL prevailing at the proposed site of the culvert before the construction of the road embankment with same allowance for the concentration of flow caused by the construction of the culvert. The opening head when the culvert flows full, in case the tail race water surface is below the crown of the exit and in case it is above that operating head, H is the height of the upstream water level measured from the surface level in the tail race or from the crown of the exit of the culvert whichever is higher. As for as the velocity generated by the operating head H is concerned it is utilized in (1) Supplying the energy required to generate the velocity of flow through the culvert, (2) forcing water through the inlet of the culvert, and (3) overcoming the frictional resistance offered by the inside wetted surface of the culvert. If the velocity through the pipe is v, the head expended in generating it is (v) square /2g .