DESIGN FLOOD FOR BRIDGES CARRYING WATER

For the purpose of design flood, C.D. (Cross Drainage) structures/aqueducts are classified into very large and minor C.D. structures in the existing Indian Standards. The design flood for very large C.D. structures/aqueducts is considered as MPF whereas for minor C.D. structures, the design flood is 10-25 year flood. In case of important structures, MRF is to be increased by 10 to 30 percent depending on the catchment area varying from 500 square km to 25,000 square km and above. There is no apparent provision in the existing Codes for structures which do not fall under "very large" or "minor" category. The present codal position as far as design discharge is concerned seems to be conservative and there is a reasonable scope to review them to effect economy without sacrificng safety. The codal provisions for design discharge for road bridges which should be comparable to design discharge for aqueducts also appear to be different and need to be rationalised. Suggestions for modifying the design flood criteria to cover all types of structures are proposed in the paper. (A) For the covering abstract see IRRD 860938.