Very near bottom turbulence and flow structure change by the forced-freshwater discharge in estuary

초록

Measurement of turbulence very near the seabed (less than 30 cm above bottom) in Yeongsan estuary, Korea, was made in August 23-28, 2011, to investigate of the fine vertical structure change of the turbulent flow parameters such as the mean and turbulent flow, Reynolds stress, turbulent kinetic energy (TKE), etc. And the comparison between the bottom TKE and the suspended sediment concentration (SSC) was also carried out to see how the turbulent flow affects the sediment distribution near bottom. The flow structure of horizontal component from the bed to 0.35 meters above bottom (mab) showed the upside-downed bell shape and maximum speed was less than 15 cm/s. The vertical component was, however, almost 1 cm/s in maximum, and showed peak at 0.1 mab and gradually decreased and then keep almost same magnitude. When the freshwater was discharged, the bottom flow rapidly responded by showing sudden change of direction. The magnitude of Reynolds stress (-<u’v’> and -<v’w’>) was about 2 cm2/s2 in maximum and TKE showed less than 10 cm2/s2 with upside-downed half bell shape. The stress and TKE component were directly responded by the surface freshwater discharging, and when the TKE was rapidly increased, the SSC was also increased in the bottom boundary layer, indicating that the active turbulent flow help suspending the bottom sediment. A hypothesis was suggested that the surface freshwater discharge by dam gate opening could increase the SSC of the bottom boundary layers by high turbulent flow activity.