Channel Adjustment to River Transformation from Permanent to Seasonal and Effects on Flood Control ——The Case of the Yellow River and the Hutuo River

The Hutuo River has been strongly regulated after two large reservoirs were built on the river in 1959. The annual runoff and mean sediment concentration in the lower reaches of the river in the period from 1969 to 1988 were decreased by 13.7 and 9.2 times respectively than those in the period before 1960. In the recent over two decades, there were only small water flows down the reservoirs in flood season, and the river kept dry in the rest time of the year. The discharge of the lower Yellow River decreased by over 1/3 in the period 1986-1996 comparing with that in the 1950's and the 1960's, but the mean sediment concentration changed inconsiderably. The phenomenon of flow breaking occurred as early as 1972 in the river, and the time and coverage of flow breaking had increased steadily thenceforth. Therefore, two of them have become essentially seasonal rivers. The paper discusses the adaptive adjustment of channels of the Yellow River and the Hutuo River to the change from a permanent to a seasonal river. The analyses show that the channel was eroded upstream and silted downstream along the lower reaches of the Hutuo River and was cut in the lower part and uplifted in the upper part cross sectionally. Since the river has been nearly cut off the flow and sediment load, the natural heavy sediment accumulation before 1959 has disappeared, and the changes in channel geometry and sinuosity were not significant. However, the hydraulic coarseness of the channel has been nearly doubled as a result of the invasion of vegetation into the channel and other reasons, resulting in the frequent occurrence of large disasters caused by small discharges which were harmless before. In the case of the Yellow River, the concentrated sediment accumulation in main channel has taken place in recent years since the discharge of the river was lowered conspicuously. Although the channel was deepened in the braided reach, it was seriously uplifted and narrowed, with obvious decrease in capacity, in the overall lower reaches. The result of the channel adjustment was the obvious rising of water level of a certain discharge and the huge disasters occasioned by small floods in recent years. With the enhancement of regulation, the possibility of increase in hydraulic roughness of the channel of the Yellow River due to the invasion of vegetation as the case of the Hutuo River should be paid more attention.