The seasonal and fortnightly erosion and sedimentation pattern as well as the bonding characteristics of the mud deposit at the estuarine turbidity maximum (ETM) zone of the macrotidal Chikugo River estuarine channel were studied during 2005-2008 using the periodical topographic surveys and mud sampling along with continuous monitoring of water level, turbidity and flow velocity. The results revealed that the estuary was influenced by the tidal forcing during the dry season. This tidal forcing accelerated the net landward sediment transport and sedimentation takes place in the estuarine channel. On the contrary, the river discharge dominated the estuary during the rainy season which eroded those mud deposit in the estuarine channel and exported to the downstream and the sandy base layer appeared at the bottom. During the dry season, a mud deposit of 1.5-1.8 m thick was formed which was characterized by 90% of mud (fine silt and clay) and 10% of fine sand. The high viscosity ratio and loss on ignition of this mud deposit revealed that its consolidation begins at the early stages of deposition due to thixotropy and the presence of organic matter. This consolidation process in the natural environment will be several times higher than that of the disturbed mud, which will help the sediment surface to maintain stability against erosion even at high velocities. Moreover, the consolidation of the mud deposit has a strong impact on the seasonal changes in estuarine morphology other than external forcing such as river discharge, tides, wind and availability of sediments.
Keywords: Chikugo River estuary; ETM; Fluid mud; Morphodynamics; Sediment transport; Thixotropy.
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