SEDIMENT TRANSPORT ON MACROTIDAL FLATS IN GAROLIM BAY, WEST COAST OF KOREA: ROLES OF WAVES AND TIDAL CURRENTS

Garolim Bay is one of the largest bays in the west coast of Korea, submerged during the Holocene transgression. The macro-tidal setting of the bay has led to the development of vast tidal flats with tidal channels bifurcating toward the bay head. A monsoonal climate creates strong northwesterly winds during winter, frequently resulting in wavy conditions in the bay. Measurements of bed level on the tidal flats show deposition during summer and erosion during winter, although the seasonal sedimentation rates are less than a few cm. A self-recording instrument, named TISDOS (Tidal Sediment Dynamics Observational System), was built to monitor hydrodynamic behavior of tidal sediment transport and deployed on a tidal flat in Garolim Bay over 15-day period between 5 and 20 January 2002. The observation involved rough wavy climates allowing for analyses of wave effects on sediment transport on the tidal flat as well as the role of tidal currents. Time-series data acquired at three stations along a cross-shore transect show characteristic interrelationships between parameters (water depth, current, wave, suspended sediment concentration, and bed level) on both temporal and spatial scales. The current speeds varied in harmony with water depth, showing spring/neap tidal cycle and semi-diurnal asymmetry. Flood currents were consistently stronger than ebb currents during the measurement period. Suspended sediment concentrations (SSC) gradually decreased after mid-flood peaks in calm conditions, implying deposition of suspended sediment and dominant onshore transport by stronger flooding currents. The SSC increased significantly with wave height, indicating a critical role of waves in resuspending bed material. In the wavy climate, the SSC rather increased over time during ebb. This is suggestive of limited deposition and enhanced offshore transport of suspended sediment under wavy conditions. The differential roles of tidal currents and waves in sediment transport are probably responsible for the seasonal deposition/erosion cycles of the tidal flats.