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Study of Impact from Water Resouces Development on the Stability of Upper NAN and Its Tributaries
The Upper Nan above Sirikit reservoir is composed of 7 main tributaries in which tributary longitudinal section near the outlet is steeper than 1:700 (except Namwa). Therefore water can carries large size of sediment. Above Nan city, longitudinal slope of Nan river is 1:2,500 and becomes 1:3,000 before flows to Sirikit reservoir. Height of Nan city is 190 m.MSL. The sediment deposited almost up to the weir crest constructed on the tributary usually takes two to three years. Near the confluence of Nan river, some tributaries, the cross section are composed of main channel with width of 20-40 m. and the depth varying from 3 to 5 m. and shallow flood plain of 200-300 m. up to almost 1 km. wide. When water inundates on flood plain, it usually flows through the damage of weir side wall constructed on the main channel. Flood damaged on Nam Meamang, a sub-tributary of Namwa is discussed. The sediment usually fills up the tributary excavation within few years and often follows up by river bank erosion downstream. Many river bank erosions take place both on tributaries and Nan river. It is recommended that study of alternative river bank protection methods instead of reinforced concrete structure at present should be carried on.