The Influence of Suspended Sediments on the Plume of a Small Mountainous River

Abstract

A comprehensive field experiment was conducted to monitor the plume of the Tsengwen River, a monsoon-regulated small mountainous river located on a wave-dominated micro-tidal coast in southern Taiwan. The field work included the deployment of a tripod near the river mouth that measured the salinity, temperature, tide, waves, current, and turbidity at 1 m above the bed. Shipboard profiling of the salinity, temperature, and turbidity was also conducted. In addition to the ground survey, a Landsat TM image taken during the time of the field work was converted to a map for suspended sediment distribution in the study area. Our results show that the expansion and contraction of the river plume is largely controlled by the river discharge, tidal phase, and the wind field. Horizontal advection, downward settling, and the resuspension are the three mechanisms that control the near-bed suspended sediment concentration near the river mouth. Both forms of hypopycnal and hyperpycnal plumes from the river have been documented. Our study shows the type of plume of small rivers not particularly rich in suspended sediments on wave-dominated coasts have greater variability and is subject to the influence of the suspended sediment dynamics. In the presence of weakened runoff and absence of runoff, stratification can become unstable in the farfield when vertical movements of suspended sediments are present. In summary, the plumes observed in this study represent three types of plumes in terms of relative dominance between the river and nearshore suspended sediment dynamics.