FLOW-RATE MEASUREMENT OF MOUNTAINOUS RIVERS BY RADIO-ACTIVE IS' PLS ----A FIELD STUDY

Abstract

A number of multipurpose, irrigation and hydropower projects are being planned and investigated in. the Himalayan region. For proper planning and rational design of these projects and also to know the quantum of water as a limited natural resource, accurate measurement of stream flow rate is essential. Conventional methods, besides being time consuming, c bersome and fs to isfacto y 1 within certain limits, fail to give accurate and reliable results specially in hilly terrain where most of these projects are located. For stream flow rate measurements, dilution methods (with non-radioactive and radioactive tracer) are being used for last decade. With the advert of radioactive tracers which can be easily detected at very low concentrations, the scope of radiometric methods has increased manyfold. The success of river flow gauging by radioactive tracer technique mainly depends on its proper mixing over entire cross-section of flow. To study application of radiometric methods for flow rate measurement, field experiments been conducted on various mountainous rivers using tritiated water as a radioactive tracer. Instantaneous injection method has been used in the experiments followed by continuous sampling" technique till the radioactive tracer 'cloud' passed in the measuring section. The discharge was obtained and obser-ved by conventional method as well. The results of conventional ;;; and radiometric method differ by 5 %' to 1 % on either side. The tracer dispersion has been studied by collecting samples at various stations on both the bank, some time in the centre of river cross-section, downstream of injection point. Proper mixing has been assumed at the station where the area under concentration-time curves on two banks differ by less than 2 e. The observed mixing length has been compared with that obtained by emperical formulae suggested by Rimmar, Full and Andres. large variations in the observed and calcu-lated values . show that there i s a need for developing a dependable criterion for predicting the mixing length and other flow parameters for estimation of discharge of hilly rivers.