ベトナム北部ハイハウ海岸の発達と侵食過程に関する研究 [in Japanese] STUDY ON LONG-TREM EROSION OF HAI HAU COAST, NORTHERN VIETNAM [in Japanese]
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ベトナム北部ナムディン県にあるハイハウ海岸とその背後地は地域の漁業，農業等を支える重要な沿岸域である．海岸の主要部（延長約30 km）では100年来に及ぶ数百メートル規模の広域的，長期的な侵食が進行している．一方，この海岸の北部と南部の河口域では堆積が今も進んでおり，紅河とその支流からは一定の土砂供給があると推定される．本研究では，現在見られている侵食のメカニズムを説明することを目的として，過去数百年の海岸の発達過程の分析とこれが長期間に亘る侵食に転じた原因を，文献調査，採取資料の年代測定，One-line modelによる数値的検討を通じて調べた．その結果，かつては同海岸の中央部に紅河の流路がありここを中心にデルタが発達してきたが，これが1787年の洪水による紅河の流路変更によりハイハウ海岸のある区間が長期的な侵食へと転じたことを明らかにした．
Hai Hau Coast, Nam Dinh Province, northern Vietnam is an approximately 30 km long sandy beach, which is suffering long-term erosion in the last hundred years. The amount of erosion exceeds 1, 000 m, and several villages in the hinterland disappeared during this recession. On the other hand, at the river estuaries, north and south of the main part of Hai Hau Coast, satellite images capture formations of newly deposited areas, which are indicating that the rivers are still putting a huge amount of sand to the coast. This study tires to understand the formation process of Hai Hau Coast, and why it turned to erosive circumstance of nowadays, from literature survey, dating of sampled shells from the hinterland, and by a numerical computation of shoreline changes with one-line model. From literature survey, the area was so-called river dominated delta, and was continuously accumulating by the sediment input of old Red River. A large flood diverted the old Red River to the north and south, the present river estuaries, and the coast turned to so-called wave dominated delta, and the long-term erosion has started after this event. From the results of carbon dating of the shells, speed and amount of accumulation before the flood event were estimated. After the diversion, one-line model was applied to discuss the shoreline changes, which well reproduced the on-going erosion of Hai Hau Coast. The numerical result suggests that reclamation of the deposited sediments at the estuaries is one of the possible reasons of long-term erosion, which prevents feeding sand from the river to the coast.
- Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering)
Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering) 73(2), I_781-I_786, 2017
Japan Society of Civil Engineers