Study on the morphological equilibrium condition on river mouths and tidal inlets

沙质海滩的河口和潮汐入海口通过捕获沿岸漂流来形成沙子。航道缩窄往往造成行洪或通航受阻，航道的合理治理是海岸管理的重要课题之一。本研究的目的是找出河道形态平衡的普遍条件，为河道管理服务。通过沿岸漂移的泥沙补给和河流、潮流的排沙，通过泥沙平衡来量化河道的形态发育。从理论上推导出平衡条件为供给量与去除量的平衡，并与形态的稳定条件等价。将平衡状态下的无因次河道宽度表示为两个无因次参数Φ_R和Φ_T的函数，这两个参数分别与河流输沙量和潮流输沙量对输沙量的相对重要性成正比。印度尼西亚的河流OPAC、Serayu、Wawar、Tipar和Bogowanot都在贾瓦岛南海岸面向印度洋，在高海浪的旱季容易关闭。日本的六条主要河流也被检查了。美国太平洋、大西洋和墨西哥湾沿岸的潮汐入口处的情况也被检查了。文中推导出的平衡条件具有普适性，并且统一了河口和潮汐入海口。

River mouths and tidal inlets in sandy beaches develop sand spits by trapping littoral drift. Since narrowing of the channels often causes obstruction of flood passage or navigation, appropriate control of the channels is one of the important subjects in coastal management. The aim of this study is to find the universal condition of the morphology equilibrium for the channels in order to serve the management.Morphological development is quantified by the sediment budget with the sediment supply from littoral drift and sediment removal by river and tidal flows. The equilibrium condition is theoretically derived as the balance of the supply and removal and is equivalent with the stationary condition of the morphology. The dimensionless channel width in equilibrium is expressed as a function of two dimensionless parameters, Φ_R and Φ_T which proportional to the relative importance of the sediment removals by the fiver flows and tidal flows to the sediment supply respectively.This equilibrium condition on the river mouths is verified by the field data. The Indonesian rivers, Opac, Serayu, Wawar, Tipar, and Bogowanot, which all face the Indian Ocean in the south coast of Jawa Island and prone to close in the dry season for the high wave, were examined. Six major rivers in Japan were also examined.The condition was also examined for the tidal inlets in the Pacific, Atlantic and Gulf coasts in USA. The equilibrium condition derived here is shown to be universal and to unify the river mouths and tidal inlets.