LANDWARD MIGRATION OF ISOLATED SHELLY SAND RIDGE (CHENIER) ON THE MACROTIDAL FLAT OF GOMSO BAY, WEST-COAST OF KOREA - CONTROLS OF STORMS AND TYPHOON

A shelly sand ridge, 860 m long, 30-60 m wide, and np to 1.6 m high, p arallels the shoreline on a high tidal mudflat in Gomso Bay, which ope ns to the eastern Yellow Sea. The ridge is composed entirely of fine t o medium sand and shell material, with some subangular gravel and over lies tidal mad discordantly. The lithostratigraphy of the bay deposits and C-14 dates of shells from vibracores suggest that the shelly sand ridge, together with mid to lower sand hats, has accumulated on the t idal mud since 1800 yr B.P. This geologically recent development sugge sts that ridge evolution has been independent of regional sea-level ch ange. River influence also seems to be negligible because of the appar ently meager riverine sediment input into the bay. Judging from a seri es of aerial photographs spanning the past two decades (1967-1989), br oad sand shoals on the mid to lower flat have moved landward and have become the shelly sand ridge. Vertical sections from a trench cut acro ss the ridge also attest to its landward migration, with gently landwa rd-dipping interbeds of sand and shell. Repeated measurements of the m orphological change of the ridge over a two-year period (1990-1992) in dicate that net transport was landward, on the order of 8 m/yr, except for during the typhoon season. The migration rate in the stormy winte r monsoon season was generally two or three times greater than that of the remaining season. However, in the summer of 1992, a typhoon enter ing the Yellow Sea vigorously shifted the ridge landward, causing a di splacement of up to 11 m in a few days. This suggests that strong wave s associated with storms or typhoons have predominantly driven the she lly sand ridge and offshore sands onto the tidal flat.