Monsoon-influenced deposition systems in a rhodolith beach on Udo Island, Korea

Abstract

The Seogwangri Beach of Udo Island, off Jeju Island, Korea, is formed almost entirely by long-term deposition of rhodoliths, which is an extremely rare sedimentological phenomenon. In 2004, this beach was designated a natural monument with significant geoheritage value. To date, no geological studies have been conducted to explore the sedimentary mechanisms and properties of beach, despite the potential for erosion due to recent development and climate change. Therefore, the objective of this study was to study beach evolution on Udo Island. We examined aerial photographs taken during the past several decades and conducted seasonal beach surveys to determine its sedimentation characteristics. Geomorphological characteristics of the beach were divided into to the northern (Reef area), with high altitudes and rocky coastlines, and to the southern (Sandy beach), with low altitudes and geomorphology controlled by waves. Gravel is distributed along the coastline, and particle size decreases toward the upper parts of the beach. Sediments in the north increased in size from spring to autumn, approaching the size of coarse sand. Sedimentary processes exhibited seasonal variation, dominated by erosion in the north and deposition in the south in spring. Also, beach received more sediment deposits in autumn than in any other season. Due to the influence of the East Asian monsoon climate, Udo Island is dominated by typhoons in summer and by tidal currents and winds from the north-northwest in autumn and winter. The beach area of Udo Island increased from 1985 to 2003, and continually decreased thereafter. And during the past three years, beach erosion has accelerated due to a decrease in the volume of sediments on the soutThe Seogwangri Beach of Udo Island, off Jeju Island, Korea, is formed almost entirely by long-term deposition of rhodoliths, which is an extremely rare sedimentological phenomenon. In 2004, this beach was designated a natural monument with significant geoheritage value. To date, no geological studies have been conducted to explore the sedimentary mechanisms and properties of beach, despite the potential for erosion due to recent development and climate change. Therefore, the objective of this study was to study beach evolution on Udo Island. We examined aerial photographs taken during the past several decades and conducted seasonal beach surveys to determine its sedimentation characteristics. Geomorphological characteristics of the beach were divided into to the northern (Reef area), with high altitudes and rocky coastlines, and to the southern (Sandy beach), with low altitudes and geomorphology controlled by waves. Gravel is distributed along the coastline, and particle size decreases toward the upper parts of the beach. Sediments in the north increased in size from spring to autumn, approaching the size of coarse sand. Sedimentary processes exhibited seasonal variation, dominated by erosion in the north and deposition in the south in spring. Also, beach received more sediment deposits in autumn than in any other season. Due to the influence of the East Asian monsoon climate, Udo Island is dominated by typhoons in summer and by tidal currents and winds from the north-northwest in autumn and winter. The beach area of Udo Island increased from 1985 to 2003, and continually decreased thereafter. And during the past three years, beach erosion has accelerated due to a decrease in the volume of sediments on the southern beaches. Topographical causes of sediment erosion are lack of berm area in the south. The results of this study will increase understanding of Rhodolith beach system elsewhere in the world.hern beaches. sediment erosion are lack of berm area in the south. The results of this study will increase understanding of Rhodolith beach system elsewhere in the world.