| The records of daily water and sediment discharge are basic elements to show characteristics of water and sediment discharge of the Huanghe (Yellow River), which could reflect the variation of sediment load and water discharge rapidly and sensitively, especially some extreme-event variation, for example catastrophic flood peaks, huge sediment peaks, dry-off in lower reaches, high concentration flows etc., could be reflected from daily water and sediment process records, it's meaningful to recognize the water-sediment delivering characteristics and guide disaster prevention activities. On the other hands, peak restraint and water regulation caused by reservoirs in main streams have great impacts on water-sediment peaks and flow distribution in a year, which could be also reflected in daily water-sediment records clearly, it can show the details of reservoir regulation impacts on flood and disaster prevention, water consumption variation.Based on the daily data of the water and sediment discharge recorded at Huayuankou and Lijin hydrographic stations in the Huanghe lower reaches in 55 years (1950-2004), characteristics of the daily water and sediment discharge and its variation at the two stations for 55 years were analyzed and compared through 5 phases in corresponding to the periods of operation of 4 large reservoirs in the Huanghe main stream. Characteristics of daily water and sediment discharge of two unique years (1973, 1977) with high sediment peak of extreme-high concentrations and its link with occurrence of the sediment gravity flow in the Huanghe estuary were discussed.The result shows that the characteristics of the daily water and sediment discharge has changed significantly in 55 years in a stepwise way in coincidence with the time of operation of the 4 large reservoirs on the main river stream. The natural character of the daily water-sediment process before the reservoir construction has been changed totally to be a human-control one after the operation of Xiaolangdi reservoir. The major change after that was as follows: water flood peak and sediment peak disappeared; difference in terms of water and sediment discharge between flood season and non-flood season became ambiguous; sediment load during the water and sediment regulation period accounts for above 2/3 of its annual amount, and the river bed of the lower reaches turn to be eroded rather than accumulated.Comparison of the daily records between the two stations illustrates that the lower reaches plays a notable role in flood peak-shaping, flood restraint and sediment trapping effects in the water and sediment transport process from Huayuankou to Lijin. The difference in daily water-sediment discharge between the 2 stations from March to June increased rapidly in the last 30 years and reached as high as to 20 times, indicating that the water consumption in the lower reaches is concentrated in this spring irrigation period and is a major cause for water reduction in the lower reaches.The daily hydrographs of two years with high sediment peaks of extreme-high concentration indicate that more than 2/3 of yearly sediment load was delivered during the short period of the sediment peaks, as events of sediment transport. And huge amount of sediment was accumulated in the short peak period. More than 86% of the yearly sediment discharge with high concentration for formation of sediment gravity flow would reach the estuary in the two years. In recent years, even the runoff and sediment load of the Huanghe decreases sharply, however, the sediment concentration at Lijin during the water_sediment regulation period after flood season still reaches high enough to form sediment gravity flow in the estuary.Variation of the daily water-sediment process in the Huanghe lower reaches in the past 55 years shows human activity has become the major controlling factor of the daily water-sediment process in the Huanghe lower reaches.
|
PDF Full Text Request