Sediment transport investigations at Cochin Port using radiotracer technique

For the past several years, the dredged sediment is dumped at North and South dumping grounds of the Cochin Port. However, based on some recent investigations, it was suspected that the sediment ingress from the present dumping sites is one of the major sources of sediment input to the navigational channel. Hence two radiotracer investigations were conducted at two different sites. The sediment transport investigations involves selection and preparation of a suitable radioactive tracer, injection of the tracer on seabed at the proposed site, tracking of the tracer with waterproof radiation detectors, plotting the iso-activity contour maps and interpretation of iso-activity contours. Scandium-46 (half-life: 84 days, gamma energies: 1.12 MeV (100%) 0.89 MeV (100%) in the form of scandium glass powder is the most suitable radiotracer for tracing sediments on seabed over a period of few months. The tracer is prepared by incorporating 1 % of inactive scandium in the glass composition, grind the glass and mix the different grain size fractions to have the same grain size distribution as that of bed material. Scandium glass powder is then irradiated in a nuclear reactor to produce 46Sc. In the present investigations, about 3.7 Ci activity of Scandium-46 was mixed with 5 kg of dry sand and injected at both the disposal sites. The radiotracer was injected on to the seabed using a specially designed injection system. The movement of tracer at both sites was monitored for a period of three months using waterproof scintillation detector connected to a rate meter. The obtained radiotracer concentrations were corrected for natural background radiations and radioactive decay and two dimensional isocount contour maps were prepared. Maximum longitudinal spread of 400 m and 200 m were observed in North and South dumping sites, respectively, which implies that the sediment movement is not significant at these sites