Temporal and spatial variation of surface sediment distribution in the open coast macrotidal flat, southern Ganghwa Island, Gyeonggi Bay, west coast of Korea

Abstract

Spatio-temporal variability of surface distribution was investigated in the Yeochari tidal flat, southern Ganghwa Island in Gyeonggi Bay, west coast of Korea. Surface sediments were collected at 135 locations encompassing an upper to lower intertidal zone, which was seasonally conducted 8 times from April in 2014 to January in 2016. Surface sediments are generally fining landward with notable exception around channel area. In the upper intertidal zone, coarse-grained sediments are locally dominant in the channel. In contrast, fine-grained sediments are locally present on the channel flank in the lower intertidal zone. From spring to summer in 2014, surface sediment became coarser in the upper intertidal zone and finer in the middle to lower intertidal zone. From summer to fall in 2014, overall intertidal zone showed a coarsening-trend except tidal creeks and shell fragment-covered upper intertidal flat. From fall in 2014 to winter in 2015, the western part of upper intertidal zone and the channel flank in the lower intertidal zone became coarser-grained, while southwestern part of the lower intertidal flat showed a fining trend. Spatio-temporal distribution of surface sediments in 2015 is generally similar to that in 2014. From summer to fall in 2015, most of locations show an overall fining trend except for tributary channels. Southern Ganghwa tidal flat is open-coast macrotidal flat, which is connected to the offshore in the southwest. North to northwest winds during winter expand sand flat in western part of Yeochari tidal flat where is closed to offshore as well as Sukmo channel. In contrast, south to southeast winds during summer broaden sand flat in eastern part of the tidal flat. Wind-induced wave energy and direction seems to be most important component to determine the surface sediment distribution. However, strong wave energy during winter generates resuspension of fine sediment and causes deposition of fine sediments near tributary channel when tidal current speed is close to zero during end of ebbing tide. Variable sedimentary facies observed in the middle to lower intertidal flat is due to its complex channel system. The expansion of the upper-intertidal mud flats during winter and spring seems to be associated with the genesis of biofilm as a result of the blooming of microphytobenthos, which is counterintuitive to overall coarsening in the lower intertidal zone because of enhanced wave activity. Large-scale dune distributed neighboring main channel tends to disappear during winter/spring with fining-grained trend and be developed during summer/fall with coarser grained. Present study revealed that tidal current and seasonally intensified waves exert a primary control on the surface sediment distribution. However, the spatio-temporal variation of the surface sediment complicated by biological process in the upper intertidal zone and autocyclic channel migration in the middle to lower intertidal zone.