14:30 〜 14:45
[HCG28-04] メコンデルタ海岸線変化の長期的観点
キーワード：人間活動、海岸、東南アジア
The Mekong delta, southern Vietnam, has suffered serious coastal erosion, following catchment disturbances, such as hydropower dams and river sand mining, over the last decades. Indeed, the delta area has shrunk since 2005 in contrast to the overwhelming expansion during Holocene. Here, long-term perspectives on coastal changes of the Mekong delta are presented by taking into account temporal and spatial variability based on sedimentological, geomorphological, and chronological characterization. The Mekong delta represents a typical mixed-energy delta, in which relative contributions of fluvial, wave, and tidal processes are spatially variable; tide-modulated wave processes dominate the open coast, whereas distributary channels are characterized by an upstream to downstream transition from fluvial to tide-dominated estuarine processes. All regions are influenced by strong contrasts between the winter and summer monsoons, which define an annual sedimentary cycle; the weaker southwesterly summer monsoon causes river flooding and sediment supply, whereas the northeasterly winter monsoon is dry and stronger, associated with limited fluvial sediment supply and asymmetric longshore sediment drift toward the southwest. The shrinkage of the Mekong delta has not happened evenly over the coast, but reflected a decline in accretion rate in the sandy river mouth area and significant erosion of the downdrift muddy coast. Behaviors of the sandy coast are characterized by 200–600 year cycles in which an emergence of river mouth bar causes a jump of elongate barrier shoreline several kilometers offshore, followed by a filling of back barrier with mud. This sporadic nature introduces uncertainties in estimate of sediment budget from short-term coastal mapping just over several decades. The serious erosion of the downdrift coast has continued for > 100 years and thus is not attributable to river dams and sand mining in the last decades. It may have reflected land-use changes in the catchment, construction of canal networks on the delta plain, climate changes, and possibly autoretreat. These spatially- and temporally-variable behaviors of the Mekong delta coast questions a view to simply link human disturbances with the delta shrinkage and motivates further sedimentological and geomorphological investigations.