ANALYSIS OF BEDFORM AND BOIL BASED ON OBSERVATIONS IN BRAHMAPUTRA RIVER

抄録

 This paper introduces one of the prominent observed phenomena, and describes it in terms of textbook hydraulics knowledge. The authors conducted field measurements in Brahmaputra river where bedform and boil were observed and analyzed the effects of those on the spatial-temporal distribution of suspended sediment. Dense measurements were conducted in an area where boils were observed, for comparison dense measurements were also carried out in an adjacent area where no boils were observed. It was found that bedform had occurred in the area where boils were observed, whereas, no bedform was observed in the area where boils did not exist. Using ADCP backscatter as surrogate for Sediment Concentrations (SC), results show higher SC exists where boils are observed, which suddenly decreases downstream of the boil. ADCP observation of the boil shows higher vertical velocities during upward flow at the downstream of the bedform crest. Thereafter, a downwards flow was also observed. The rapid settling of SC in the downstream of boil is linked with downwards flow due to boil. Results shows that the force generated by the boil lifts the larger particles, and this strength is larger than the one estimated as τ*.