Abstract
Flow measurements were performed in the altered Yeongsan estuary, Korea, in August 2011, to investigate changes in flow structure in the water column and turbulence characteristics very close to the bed. Comparison between the bottom turbulent kinetic energy (TKE) and suspended sediment concentration (SSC) was conducted to examine how discrete freshwater discharge affects the bottom sediment concentration. The discrete freshwater discharge due to the gate opening of the Yeongsan estuarine dam induced a strong two-layer circulation: an offshore-flowing surface layer and a landward-flowing bottom layer. The fine flow structure from the bed to 0.35 m above the bottom (mab hereafter) exhibited an upside-down-bell-shaped profile for which current speed was nearly uniform above 0.1 mab, with the magnitude of the horizontal and vertical flow speeds reaching 0.1 and 0.01 m/s, respectively. The bottom turbulence responded to the freshwater discharge at the surface layer and the maximum magnitude of the Reynolds stress reached up to 2 × 10−4 m2/s2 during the discharged period, which coincided with increased SSC in the bottom boundary layer. These results indicate that the surface freshwater discharge due to opening of the estuarine dam gate increases the SSC by the discharge-induced intensification of the turbulent flow in the bottom boundary layer.
Highlights
In estuaries, water currents are mainly driven by tide, wind, and freshwater discharge [1].Other factors affecting the flow structure include density stratification caused by temperature and salinity, seabed roughness due to the bottom surficial texture and bedforms, and artificial alterations such as dam construction
These results indicate that the surface freshwater discharge due to opening of the estuarine dam gate increases the suspended sediment concentration (SSC) by the discharge-induced intensification of the turbulent flow in the bottom boundary layer
The rapid increase of SSC in the bottom happened after the surface freshwater discharge and it gradually propagated to the upper layer, and since it happened at the same time or after the sudden increase of the bottom turbulent kinetic energy (TKE), the SSC near the bed was related to the intensified bottom turbulent flow that was the result of surface freshwater discharge due to the opening of the dam gate
Summary
Water currents are mainly driven by tide, wind, and freshwater discharge [1]. One factor which can induce a sudden response from an estuary is forced freshwater discharge because rapid currents can be formed in the surface with a velocity difference between the surface and bottom layers For example, Cho et al.currents [11] suggested that there exist four with layersa under low discharge conditions during the summer season, showing seaward flow in the surface velocity difference between the surface and bottom layers. During the summer season when the gate opens frequently, the freshwater discharge has become an important factor to change the current, temperature, and salinity distribution because of sudden and forced discharge through the surface layer [8,9,10,11,12].
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