Impact assessment of upstream large reserviors on sediment transport in the downstream of Red - Thai Binh river system of Hai Phong city

Abstract

     The Red-Thai Binh River System in Vietnam is the second largest river system in Vietnam (after Mekong River system) and plays an important role in socio-economic development of the country. Four large reservoirs were constructed in the upstream for flood control, hydropowe and water supply (agriculture, domestic, navygation and fisheries...). Besides these benefits, the reservoirs also created negative impacts such as changing sediment stransport in the downstream which causes erosion, sedimentation, salinization, and natural imbalance. In this paper, the module of sediment transport of MIKE 11 ST model was applied to assess changes in annual sediment transport in 5 major estuaries in the downstream of Red-Thai Binh River system of Hai Phong city before and after the reservoirs construction. Results showed that the amount of sediment in the downstream is significantly decreased after mainstreams locked by the large dams. Before 1970 when no reservoirs bulit, the total amount of sediment is 45.5 million tons/year; after Thac Ba and Hoa Binh reservoirs are in operation (1989-2006) the sediment load was 29.4 million tonnes/year (remains 65%); from 2007 till now when four large reservoirs are in operation, it was only 15.1 million tons/year (remains 33%). This presents the deficit of sediement in the river system and leads to unstability of river banks and river bed in the study area. Therefore, this work could provide valuable information for better management and planning of this river basin.