黄河下游的河性

THE CHARACTERISTICS OF THE HUANGHE RIVER (YELLOW RIVER) AT LOWER REACHES

本文探讨了黄河一般特性的综合关系,讨论了河道演变中均衡与非均衡的问题、滩槽冲淤的利与害和主与从的问题,同时讨论了河道冲淤演变特性、与水沙条件的关系、河槽淤积率和淤积比等。

To understand the river characteristics, the cause of formation and the possible measures to reform them is best way to achieve great success in river training. Effects of scouring and silt depositing on the flood plain and in the channel are not equivalent in the view of fluvial processes. Either silt depositing on the flood plain or scouring in the channel will make the channel deeper, narrower, stabler, and will enlarge the capacities of water and silt transportation. On the contrary, scouring on the flood plain and silt depositing in the channel would make the conditions of the river worse. If the silt load is heavier in low water period and relatively lighter in flood period, the river channel would get shallower and broader, In the other words, a small value of m in the water-silt discharge relationQs=kQm would cause a shallow-broad channel while large value of m would cause a deep-narrow one. In a river which has a successive accumulation process, such as the Huanghe River, the rising of the channel bed will take the leading role, while that on the flood plain is the follower. The sums of water discharges and silt discharges of the four rivers upstream the Dongguan narrow path (namely, the Northern Main Stream of the Huanghe River, the tributaries of Weihe River, Beilo River and Fenhe River) have a relation-ship of Q, ?KQ' with a value of m = 2.8, which represents the characteristics of a meander river. But, the detention effect of the Dongguan narrow path transfers a. large amount of silt in the flood period into the low water peiod, and decreases the value of m to about 2. This unsatisfactory relationship eventually imposes on the Huanghe River at the lower reaches and is the cause for the river to form its bad behavior. Actually, the large amount of silt transferred into the low water period makes the channel of the Lower Huanghe River to be silted seriously, and only recovered somewhat by scouring in flood period. This makes the channel very shallow and broad. The depth-width ratio at Huayuankou reach is only 1/30 of that at Wuhan in the Changjiang River. It is well known that a shallow-broad cross section has much small capacities of water and silt transportation than a deep-narrow one. The symbolic water discharge symbolic silt discharge and symbolic silt concentration of Huay-uankou are only 1/110,1/1600 and 1/16 of that of Wuhan correspondingly. The small capacity of silt transportation, combining with the large amount of incoming silt, makes the accumulation in the river very serious.Heavy accumulation makes steep slopes; and steeper slope makes the river bed more changeable. The movability factor at Huayuankou reach is 200 times of that at Wuhan reach of the Changjiang River. All of these eventually compose the main characteristics of the Huanghe River: messily branchy, unstable, intensively shifting. Since 1972, the operation of the Sanmenxia reservoir has been changed to reta-in silt but release fresh water in non-flood season, and to remain as a detention reservoir in flood season. Then, the channel of the Henan reach is scoured in non-flood season but built up by the silts sluiced from the reservoir in low water period (? < 1800m3/s) of the flood season. Owing to the latter, the main charac-teristics of the Huanghe River remain nearly unchanged. According to the water-silt discharge relations of cross sections, the depositing rate A?, and depositing ratio may be expressed as follows: Thus, the paper worked out a formular for calculating the silt amount and deposition process in the Shandong reach, and found the maximum silting rate will occur when the discharge is about 1000m3/s. When Q > 1800m3/s, the channel will be scoured.