Sediment transport on macrotidal flats in Garolim Bay, west coast of Korea: significance of wind waves and asymmetry of tidal currents

Abstract

A self-recording instrument, named Tidal Sediment Dynamics Observational System (TISDOS), was built to monitor transport characteristics of nearbed sediments on tidal flats. It was deployed on a tidal flat in the semi-enclosed Garolim Bay, west coast of Korea, over a 15-day period between 5 and 20 January 2002 to examine sediment-transport processes during winter seasons. The measurements involved brief durations of high waves allowing for observation of wave effects upon the sediment transport on the tidal flat. Time series of various hydrodynamic parameters (water depth, current velocities, wave height, suspended sediment concentration, and bed level) from point measurements show characteristic interrelationships between parameters on both temporal and spatial scales. The tidal flat is dominated by flooding currents up to 2 times stronger than ebb currents. The current speeds measured simultaneously at two stations along a cross-shore transect varied in harmony with water depth, reaching the maximum during spring tide that was steadily decreased onshore. The onshore decrease in current speed was compatible with a fining textural trend from sand on the lower flat to mud toward the upper flat. Both the maximum water depth and current speed during individual tidal cycles also show semi-diurnal asymmetry that was highlighted during spring tide. Waves were of critical importance in resuspending bed material and thus yielding higher suspended sediment concentrations. On the middle flat, the suspended sediment concentrations were highest, exceeding 400 mg/l at 0.5 m above the seabed during large waves (relative wave height, 0.33) under weakest neap currents. In this wavy climate, the suspended sediment concentration increased over time during ebb, in strong contrast with a gradual decrease through time after mid-flood peaks under tidal currents without waves. The daily vertical flux of suspended sediments trapped in a plastic bottle also indicates the significance of wave effects in terms of enhancing resuspension of bottom sediments. As a whole, the suspended silts as well as seabed sands should be transported onshore by dominant flooding currents despite occasional offshore increase in suspension load by waves.