Investigation into the Dispersion Coefficients for Natural Rivers by Using a Radiotracer Method

The mixing characteristics of natural rivers were investigated by analyzing their radioisotope concentration curves for transverse dispersion coefficients. The radioisotope was instantaneously injected into a river as a point source by means of an underwater glass-vial crusher. The detection was made with 60 2inch Nal(TI) scintillation detectors bound to 6 transverse lines at a downstream position. The multi-channel data acquisition systems, specifically fabricated by the research group for field studies, collected and processed the detection signals transmitted from the detectors. Transverse dispersion coefficient was determined by a moment method on the basis of field measurement data. As a result, the averaged transverse coefficients were 0.25 for S-River and 0.24 for H-River. This result was compared with the conventional values calculated with the empirically proposed formulas and its physical meaning is discussed.