Morphological responses of barrier islands to natural versus anthropogenic stress in the Nakdong River delta, the southeastern coast of Korea

Abstract

Morphological responses of barrier islands to natural versus anthropogenic stresses in the Nakdong River delta, the southeastern coast of Korea

Manimeldura Chathurika Lakshi Zoysa

Department of the Convergence Study on the Ocean Science and Technology Ocean Science and Technology School of Korea Maritime and Ocean University

Extended Abstract

Coastal barriers and barriers–island systems make up 15% of the world’s ocean shoreline. Its’ consisting mostly sand, with a minority of gravel dominated barriers in glaciated area. Due mainly to their offshore location, they play frequently significant role in protecting and mitigating the mainland by catastrophic events such as typhoons and storm surges. Morphology of the barrier system mediated to the natural and human alteration. In this context, barriers in the Nakdong River delta, Busan, Korea are a human natural system proper. Over the last half of century, dramatic changes in the barrier island system in the Nakdong River delta, Busan, South Korea were documented particularly by anthropogenic impacts. There were two estuary barrages constructed across the Nakdong river namely Noksan dam and Nakdong dam in year 1934 and 1983 respectively. In addition, there were several anthropogenic activities in this area. However, little known is the natural forcing to contribute to morphological changes under increasing storminess in that area since the late Holocene. The purpose of this study is therefore to preliminary document morphodynamic changes of barriers induced by natural and human interventions, and to evaluate how much the human forcing contribute to such a change regime shift from delta to wave dominated setting. Finally, we document modern typhoon signatures recorded in the barrier sediments and discuss how does barrier respond to extreme events such a summer typhoon in the area. In order to identify the time-series morphological changes, nautical maps published between years 1972 to 2016 were digitized by using an ArcGIS tool. Topographic changes during the typhoon were monitored using a VRS-GPS system at least five transect lines placed on the Doyodeung barrier island. Sediment samples were collected along the central transect line, these having been analyzed by dry sieving at 0.25 phi sieve intervals, and the mud fraction being analyzed by wet sieving method. Box-cores were collected additionally to document sedimentary facies with relief peels. Physical data such as tidal level, wave and wind directions were obtained from the Korean Hydrographic and Oceanographic Agency with reference to the two stations. Map analysis shows that the tidal prism has changed from 5.6x 109 m3 to 2.0x 109 m3 between year 1972 to 2016 respectively. In response to the reduction of the tidal prism, the east tidal inlet decreased in width from 1437 m to 419 m, i.e. 29% of its original size. The area of the barrier islands gradually increases with time. The new barrier, namely Doyodeung, formed in 2008. Its dramatic changes were notable among the other islands having migrated 0.17 km to the north. Sinjado island was formed in 1999 but it shrunk 3.5 km to 3.1 km. It migrated towards the west. The westernmost Jinwoodo was slightly expanded about 0.1 km between in 1972 to 2016, but it remains rather stable. Historical maps show barrier started to appear in the early 1900’s, the period that was not human-developed yet. The facts signify the regime shifts already in this area possibly from the deltaic to wave-dominated barrier system at least in the offshore. Human interferences may accelerate the formation in new barriers and fast growth of prograding barriers. However, the source of the barrier sands remain uncertain and further studies are required to clarify in the future. The topography survey reveals after the typhoon that the erosion and deposition along the transect lines. The mean grain size of the barrier sediments change very little with difference of 2.2 ɸ to 2.6 ɸ after the typhoon Kong-rey, 2.4 ɸ to 2.9 ɸ after the typhoon Tapha and 2.5 ɸ to 2.7 ɸ after the typhoon Mitag. However, modal changes were remarkable. Grain size frequency curves along the transect show bimodal distribution at beach and back barrier flat, but unimodal distribution at aeolian dune. After the typhoon invasions bimodal distribution expanded and aeolian dune field was occupied by beach sands. Box-core facies analysis shows that parallel-laminated sands with scattered shells and bubble sands with bioturbation were predominated. In coarse sands, fine sand layers and wave-ripple cross-laminations are noticed on the sedimentary facies. These signatures are suggestive of the washover depositions due to typhoon invasions.

KEY WORDS: Barrier island, Morphological change, Anthropogenic alteration, Typhoon impact, Regime shift, Nakdong River delta, Busan