On the occurrence of low concentration hyperpycnal river plumes in a small mountainous river-canyon system

This study aimed to understand the mechanisms controlling the formation of hyperpycnal river plumes in small mountainous rivers. The suspended sediment concentration (SSC) and grain size distribution in the Gaoping River were obtained by field observations and used as the basis for numerical simulations. The SSC of the Gaoping River during Typhoon Soudelor was lower than 20 g/L according to the real-time SSC estimated using the calibration curve between turbidity and SSC. The calculated median grain size of the suspended sediments was 15.15 μm, which indicated that the silt accounted for a large proportion of the flow. The observed grain-size distribution and SSC were incorporated into a numerical model to examine the conditions under which hyperpycnal river plumes occur. The model results revealed a three-dimensional process near the river mouth: cross-canyon currents converge near the head of the canyon, and the topography of the submarine canyon further confines the path and width of the hyperpycnal river plume. The descending plume continues to move through the steep slope of the submarine canyon. In addition, the simulation results showed that the SSC of the hyperpycnal river plumes increases as the grain size of riverine sediment decreases. However, if the sediment grain size is smaller than 8 μm, most of the suspended sediment remains on the sea surface in a surface plume.