Abstract
Unprecedented flooding due to heavy rainfall occurred in southern Thailand, in November 1988. Damage caused by rain and flooding accompanying the north-easterly monsoon extended over the whole Tapi River basin. We have conducted the research on the state of flooding in Surat Thani, the lower reach of the Tapi River and the Chawang River Basin, which is one of tributaries of the Tapi River.
The lower reach of the Tapi River is divided into two parts : a natural levee zone and a delta. The former is noted for floods of the “bank-collapse” and “concentrated” flooding type and the latter for floods of the “overflow” type. In the case of the Tapi River, the backswamp and former river course in the natural levee zone showed deep and long stagnation in this flooding, because this long, narrow zone is situated directly above a constriction at Phunphin. In the delta, below Phunphin, the coastline advanced because of the great quantity of sand and gravel transported down by the river.
Ban Na San lies at the foot of a mountain of granit and granodiolite, which have a tendency to collapse, and consequently many landslides occurred along the Chawang and Lam Phun Rivers, which are tributaries of the Tapi. After three consecutive nights of rain, sand and gravel together with logs poured down on the residential areas of Ban Na San.
Two factors account for the heavy flood damage : one is the topographical and geological situation, the other is socio-economic development, such as, reckless deforestation, exploiting natural resources, and bringing the land at the foot of mountain under cultivation.
