Morphology, sedimentology and tidal channel processes on a high-tide-range mudflat, west coast of South Korea

Abstract

Most of the west coast of Korea is fronted by broad intertidal sand and mudflats which have formed in a high-tide-range environment that is subjected periodically to intense winter-season surges from monsoonal winds. The lack of a large source of sediments and the absence of a protective barrier at their seaward margins makes these tidal flats unusual, if not unique. Field studies, conducted in 60 km 2 of an embayed region just south of Inchon, were undertaken for the dual purpose of describing tidal channels and morphology of surrounding tidal flat surfaces, and assessing the role of tidal channels in processes of sedimentation. This paper discusses morphologic variability, channel geometry, and processes of intertidal sedimentation; the following companion paper addresses internal hydraulics of channel flow and the effects of observed internal motions on the erosion, transport, and deposition of sediments. Results show a highly variable morphology with locally dense drainage channel networks. North-south transects revealed that the intertidal surface was actually terraced by step-like changes in elevation at three of the channels, producing 2–3 m of intertidal relief over a distance of only a few kilometers. Whereas the northern, terraced tidal flat was characterized by tightly meandering tributary channels and sediments that were 80–90% mud, the southern region contained coarser sediments and lacked tributary channels altogether. Most channels appeared to be ebb dominated; peak speeds of 60–70 cm/s that were vertically homogeneous throughout the core of the water column were capable of producing intense shears near the bed and suspended-sediment concentrations on the order of 3000 mg/l. During ebbing tide, transmissometer profiles showed step structures, reverse gradients, and variability in the frequency range 0.03–0.008 Hz (∼ 30 s–2 min). Although most of the energy was concentrated in spectral peaks at 125, 95, 66, and 50 s, approximately 40% of the records contained variance at frequencies too low to be resolved. Surges in transparency and associated sediment billows 2–3 h before low tide were a product of internal flow instabilities. A record of the first summer (monsoonal) storm showed enhanced ebb flows and transparency values near zero between peak ebb and peak flood. A 5–10 cm thick layer of soft, fluid-like mud was eroded from the tidal flat surface during the storm and probably explains the low transparency values. The combined erosion by storms and daily dispersal by channelized ebb currents requires a substantial return of sediments in order for the tidal flats to maintain their elevation. The apparent lack of bathymetric relief in the subtidal region indicates that either sediments carried seaward in ebb channels are removed by shelf processes (and thus returned at some later time) or are simply reintroduced to the intertidal region by flood currents.