Analysis of impacts of sedimentation on flooding disasters in China

Abstract

The flooding and inundated area in China has been increased since the end of the 1960's. The area of farmland subjected to flooding or waterlogging increased at a rate of 4.69 million hm 2 annually in 1965~1978, and rose to 16.74 million hm 2 in 1991~1997. Climate condition is the number one important factor influencing the flooding disasters. However, changes in precipitation in China in the past decades did not support the considerable increase in flooded areas. Also, there was a decrease in the area damaged by flooding calculated in light of the disaster prevention level of the waterlogging prone farmland since lots of flood control and waterlogging mitigation works have been constructed in the same period. On the other hand, about 1.2 billion tons of sediment were deposited annually in river channels and lakes in the plains, reservoirs, irrigation areas, and flood detention areas in China. Supposing the annual increase in the volume of water flooding the farmland is equal to the annual volume of the 1.2 billion tons of deposits, there would be a good agreement between the calculated changes in flooded area and the actual increase in the flooded or damaged area after the year 1965. It suggests that the sediment accumulation in the rivers, lakes, reservoirs, etc. in China may be considered as one of the main causes for aggravating flooding disasters. The patterns of impacts of sedimentation on flooding disaster are generalized as deposition in lakes and reservoirs, deposition in river channels, deposition in river mouth, and high sediment concentration in flows. Sediment accumulation results in the decrease in the flood-regulating capacity of the lakes and reservoirs. It lowers the drainage capacity of the channels and canals and also raises the riverbed to form a perching river channel. The extension of river mouth due to sediment accumulation will lower the slope of channel, enhancing sediment deposition upstreams and deteriorating the flood prone situation of delta areas. The large amount of sediment carried by water flows can obviously enlarge the discharge of peak flood, and the hyper-concentrated flows show an unstable behavior, leading to abrupt rise and fall of peak flood, an unfavorable situation to flood control. To alleviate the impacts of sedimentation on flooding disasters, soil erosion control, enlargement of sediment transport capacity of the river channels, decrease of the height difference between the river channels and surrounding plains, and reduce of deposition-induced extension of river mouth should be the measures. However, the feasibility of these measures in the senses of economy and technology is not so clear now and they may have negative effects on economic development, environment and society. Thus, the feasibility as well as the positive and negative effects of these measures should be studied and compared comprehensively in order to achieve the best results in reducing the impacts of sedimentation on flooding disasters in the future.

綜合分析近幾十年中國洪澇災害的增加與泥沙之間的關系 ,劃分出 4種泥沙影響水災的形式 ,闡明泥沙加重洪澇災害的機理。基于對泥沙與洪澇災害關系的認識 ,提出我國洪澇災害治理的可能途徑和存在的問題。