A COMPARATIVE STUDY ON GEOMORPHOLOGY AND FLOODING AMONG THE PLAINS OF THE CHO SHUI CHI, CHAO PHYA, IRRAWADDY AND GANGES

Abstract

Distinct regional differences are to be found concerning geomorphology and flooding among the plains of the Cho Shui Chi (Taiwan), Chao Phya (Thailand), Irrawaddy (Burma) and Ganges (India and East Pakistan). (1) There is a large fan with a radius of 13 km along the Cho Shui Chi, but its natural F levees and delta are small. On the contrary, fans are scarcely developed in the Chao Phya and Irrawaddy plains, but with large natural levees and delta. (2) Almost all the soil textures of the plains of the Chao Phya, Irrawaddy and Ganges are of clay, but sandy in the Cho Shui Chi plain. (3) We can divide the plains of the Chao Phya, Irrawaddy and Ganges into three parts as follows: 1) natural levee and back-swamp area, 2) upper and 3) lower delta. The largest variation of the water level between dry and wet season is recognized in the natural levee and back-swamp area. State of flooding is moderate in the upper delta, and in the lower delta, flood from the sea (storm surge, etc.) predominates. (4) In the case of natural levee, term of inundation is short and depth of inundation is shallow. In the case of back-swamp, however term of inundation is long and depth of inundation is deep. Especially the term of inundation of the back-swamp of the Chao Phya, Irrawaddy and Ganges amounts to several months. Depth of inundation in back-swamps becomes deep in proportion to the relative height of natural levee. Thus the depth of inundation in back-swamps located in the upper reaches of plains is deeper than that of the lower reaches. The deepest part is seen in the plain of the Chao Phya, reaching down to the depth of about 4.5 meters. In such an area, cultivation of floating rice is a well developed agricultural practice. (5) Slopes of the deltas are very flat except the delta of the Cho Shui Chi. For example, that of the Chao Phya is only 0.02 to 1000. (6) The lower deltas along the Ganges and Irrawaddy are overed with thick forests of mangrove and others. Reclaimed lands are developed in the lower delta of the Cho Shui Chi. Velocity of seaward extension of the Cho Shui Chi delta is highest among these deltas. (7) Except the Cho Shui Chi, it takes roughly one to three months, until floods reach deltas from up-stream. Since the amount of rainfall in the plain is not enough for paddy, flood water coming from the upper reaches is quite important in these areas. (8) The flood of the Cho Shui Chi is very dangerous for the people. On the Contrary, flood water is beneficial to the people of the other river basins who practice irrigation. (9) Sea coast of the Cho Shui Chi delta is attacked by storm surges caused by occasional typhoons. But damage is not very large, because the ground is high. There are no storm surges along the sea coast of the Chao Phya delta. Coasts of the Ganges and Irrawaddy are attacked now and then by storm surges caused by cyclones.