제주 우도 홍조단괴해빈과 연안의 퇴적특성 및 시·공간적 변화

Abstract

Under the goal of examining the characteristics and changing factors of sedimentary environment in the rhodoliths beach and shore on Udo, this study traced changes to the particle structure along the shore of the subject area and carried out sounding, seafloor imaging analysis, and shallow stratum survey. The study also examined long-term changes to the beach area along with beach survey to figure out the deposition and sedimentation trends of the rhodoliths beach on Udo. The characteristics of sedimentary environment on the sea floor of the subject area were categorized into four sections according to the depth of water and local features: the section of 10 m or less in the depth of water, those of 10∼20 m in the depth of water in north and south, and that of 20 m or more in the depth of water. The section of 10 m or less in the depth of water was characterized by the development of bedrock on the seabed and vegetation of seaweeds. As the seabed was a combination of rudaceous and sandy sediments, it was impossible to penetrate into the bottom of sediment layer due to its strong acoustic reflection. The section of 10∼20 m in the depth of water in north featured a gentle slope and coverage of rudaceous rhodoliths all over the shore. The section of 10∼20 m in the depth of water in south showed big changes to the water level in a sharp slope. The seabed was widely covered by sandy sediments and ripple marks all over the shore. The shallow stratum survey results reveal that the depth of sediment layer was in the range of 1∼4 m. Finally, the section of 20 m or more in the depth of water was characterized by rapidly increasing water level and inclination and development of bedrocks on the seabed. The shallow stratum survey results show no continuous sediment layers. As for the composition changes of surface sediments, the sandy content increased around the shore from spring to autumn with the progress of granulation. The rhodoliths beach tended to be high in northwest and low in southeast with the inclination rising along the coastline and shore protection roads. The changes to the beach volume were examined according to the seasons in total five measurements. The findings show that there was maximum erosion and sedimentation in north and south, respectively, in spring and maximum sedimentation and erosion in north and south, respectively, in summer. In autumn, the entire beach reached the highest level of volume. There was a continuing trend of erosion for the last three years. The changes to the beach area for the last 30 years indicate a trend of continuous erosion since 2008. The study looked into the composition characteristics of surface sediments and found the prevalent distribution of rudaceous sediments all over the beach and the distribution of sandy sediments in the upper parts. The study also examined the distribution of average grain size and found that there were coarse grains with a constant average grain size in south and relatively fine grains in north with the ongoing progress of coarsening from spring to autumn.