DESIGN PARAMETERS FOR SMALL DRAINAGE AND PIPE CULVERTS

Shiva Nand ji, It is a matter of concern to be brought on your kind notice as discussed on the online meeting scheduled for 23-8-2026 11.00 am to 1.00 pm., that Drains and culverts are designed to take the run off from the drainage area resulting from a storm which is considered to produce the greatest momentary run off that can occur once a year. Exceptionally heavy storms of rare occurance are permitted to cause flooding for the reasoning that the damage done by such flooding doesn't justify the provision of large drains or culverts at heavy extra costs capable of taking the heaviest flow likely to occur at any time. Since the intensity of rain fall is not uniform through out the storm period, hence it is essential to follow critical (or design) intensity for a catchment is that maximum intensity which can occur in a time interval equal to the concentration time. For the design of minor bridges and storm water drains, it will be sufficient to take, one hour rainfall and for greater accuracy this has to be modified further with the time of concentration to arrive at the critical (or design) intensity. As far construction is concerned in the case of small drainage crossings pipe culverts are often found in practice to be the most economical and easily constructed. These culverts can be easily enlarged subsequently to take more discharge by the addition of one or more pipes, and the cement concrete pipes are commonly used RCC pipes are more economical for sizes above 45 cm and under heavy loads. Pipe culverts should be laid on firm bedding and if the soil furnishes a poor support the pipes should be bedded in a layer of concrete;in the case of cause ways all pipes are embedded in concrete however solid foundations may not be provided in good soils as some amount of settlement as a whole in the pipes can be tolerated. Where a defined channel does not exist, and 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 (box) functioning with the inlet submerged. The discharge through a circular opening is much more than through a rectangular opening of the same cross sectional area, specially when running full, circular openings give about 25 to 30 percent more discharge:-- Er Fateh chand Guleria, Director RTI welfare Association registered number HPCD, 3552,Bilaspur Himachal pradesh phone number, 9459334377