부산대학교 도서관

Continuous near bottom measurements of wave characteristics, bottom velocity and turbidity were performed using an acoustic Doppler velocimeter (Sontek ADV Ocean Hydra) integrated with turbidity meter (YSI OBS-3+) and a pressure wave gauge (PTR Group). An experimental autonomous submersible camera system (by the Center for Biorobotics) was used to monitor the motion of particles in the bottom boundary layer (BBL). The ADV measured flows consisted of wind induced currents, wave induced orbital motions and turbulence. Maximum of wind induced currents reached up to 10–15 cm/s, while the maximum near bed orbital motions peaked over 40 cm/s. From the comparison of ADV, turbidity and wave characteristics it followed that turbidity was clearly depending on the wave energy. It means only long and quite high waves generating bottom orbital velocities (calculated from the wave gauge data and/or measured using ADV) over 20 cm/s were able to resuspend bottom sediment and induce some increase in turbidity values — 5–12 NTU in stormy days. Sediment fluxes were estimated using measuring data, sediment characteristics and BBL model. It followed from the modeling results that the height of all BBL varied from 1 cm to 10 cm, and the height of the bottom-most layer (skin friction layer) varied from 0.01 cm to 1 cm. Running the model has shown also that the currents have a significant impact to the sediment resuspension only if their speed reaches to 15–20 cm/s occurring very seldom in Estonian sea areas. Absolute majority of measurements showed an error below 5% using the wave skin friction shear velocity instead of the total skin friction velocity, i.e. excluding the existence of a skin friction sublayer. Captured video clips were split into frames and pairs of images were analyzed using particle image velocimetry software by MatPIV toolbox for MATLAB. The velocity and vorticity vector fields of bottom boundary layer were visualized.