Sedimentary processes of fine-grained material and the effect of seawall construction in the Daeho macrotidal flat-nearshore area, northern west coast of Korea

Abstract

Sedimentary processes on the lower tidal flat of the reclaimed Seosan Bay (Daeho tidal flat), at the west coast of Korea, have undergone significant changes since 1984 when seawalls over 8 km long were constructed at the bay mouth. Comparison of pre- and post-construction tidal flow models indicates that in general tidal currents have increased considerably in front of the Daeho tidal flat, with tidal flow directions having changed from shore-normal to shore-parallel. The 12-h measurements of currents and suspended sediment concentrations at four anchored hydrographic stations show nearshore suspended sediments drifting southward throughout the year, bypassing the Daeho tidal flat. Cyclic seasonal variations occur prominently in the grain texture of the tidal-flat sediments and associated sedimentation rates. In summer, fine-grained sediments deposit at a rate of 2-6 mm/month, particularly in sheltered places. During the winter monsoon, erosional processes are,greatly enhanced by the interplay of tidal currents and waves (up to -30 mm/month). This sedimentary seasonality is reflected in the variations in the concentration of nearshore suspended matter. The concentration of suspended matter is much higher in winter (60-80 mg/l), because of vigorous resuspension activities, compared with the summer concentration (10-30 mg/l). The direct effect of seawall construction, coupled with strong natural seasonality, can be seen from the continuous monitoring of sedimentation rates on the tidal flat. The 2-year (1993-1995) measurements indicate that the entire Daeho tidal flat was eroded during that period at the abnormally high average rate of -10 cm/year. The causal blocking of bayward sediment and water movements by any artificial structure might lead to similar erosional conditions to many other bays facing the Yellow Sea, on the both Chinese and Korean coastlines subjected to energetic winter-wave attack. (C) 1999 Elsevier Science B.V. All rights reserved.