Q. Estimation of design flood discharge?
These methods are also in vogue for estimating maximum discharge. Discharge estimated using the above three methods may be compared with design discharge based on the concept of return period and if the latter is significantly smaller, modification in design discharge figure may be made as considered necessary.
Where a formula has been established or graphs or charts prepared for discharge estimation based on actual hydrological and meteorological considerations, the values given by them may be adopted.
In case of new bridges, a temporary stream gauging station should be set up at or near the bridge site for as long a period prior to the design stage as practicable and gauge and discharge data collected. The highest flood level obtained in the past should be determined by local inquiries. A stage discharge curve should be drawn using the observed data and discharge corresponding to highest known flood level determined by extrapolation of the curve. Preparation and extrapolation of gauge discharge or stage-discharge curve has been dealt with in Chapter 10 on measurement of river water levels and discharge.
Where stage-discharge data cannot be obtained through field observations, a few points along the course of the river indicating the highest flood marks should be determined by local inquiries. The flood slope should be calculated from these marks and the discharge determined by Slope Area method.