STUDY ON THE SURFACING OF FINE SEDIMENT IN THE TONE RIVER ESTUARY

Abstract

Fine bed sediment in a river estuary is sometimes flushed by a freshet, and it affects bio-chemical condition in the estuary. The process of the phenomenon has not been much clarified yet because of the difficulty of measurements. This paper discusses the phenomenon by paying attention to black patterns of the water surface appearing on aerial photos taken in the Tone River estuary during a freshet. Sediment data suggests that the patterns correspond to deoxidized fine sediment raised from the channel bed. From correlations of the patters with bed topography and flow field obtained from stereo analysis of the aerial photos, the process is considered as follows: A turbulent spot caused by a local depression of channel bed triggers sediment pick up and intensifies vertical mixing, raising the sediment to the water surface. The transverse shear between the vertically mixed zone and the surrounding flow generates further turbulence, resulting in the expansion of the zone of vertical mixing and sediment rising.