黄河高含沙洪水"揭河底"机理探讨

“揭河底”发生前,河床上存在着不同时期形成的淤积物,随着淤积条件的变化,河床纵比降和横断面形态会进一步调整,当河床调整到一定程度时,若发生“晾河底”等现象,则为河床成层成块淤积物的形成以及块体边界剪应力及层间黏合力的消弱创造了条件,若遇大中尺度涡漩形成,垂向能量向底层传递,垂向紊动,脉动特性增强,在忽略层间黏合力和块体间的边界垂向剪应力条件下,水体可能掀动的河床淤积物块体厚度,取决于河床淤积物块体密度,浑水体密度以及作用在淤积物块体上下表面的脉动压力的最大可能振幅之和,脉动压强与时均压强的大小无关,大致与断面平均流速水头成正比,在合理确定和表达糙率系数及平均流速后,推导出了洪水可能掀起的淤积物块体最大厚度计算表达式,当河床淤积物块体厚度小于此计算值时,涡漩引起的垂向脉动特征增强可能在随机条件下促发“揭河底”发生。

Approach to "River Bottom Peeling Off" Mechanism of Hyper-Sediment Concentrated Floods of the Yellow River

Before the phenomenon of "river bottom peeling off" occurred, deposition formed at different periods and different layers exists on the riverbed. The vertical gradient and transverse profile morphology of the riverbed will make a further adjustment along with the changes of deposition conditions. When the riverbed is adjusted to a certain degree, if the phenomenon of "dried riverbed"occurred, it has created conditions to form hard deposits and weaken and vanish the shearing stress of piece boundaries and engaging force between layers. If a large or medium scale eddy current forms, under the conditions of binding power of the neglected layer and vertical boundary shearing stress of each piece, the body of water might lift deposited pieces on the riverbed due to vertical energy is transmitted towards bottom layer and the vertical turbulent fluctuation increased. The lifted thickness depends on the unit weight of deposited pieces on the riverbed and muddy water and the sum of the maximum probable amplitude of fluctuating pressure acted on the surface of deposited pieces. Due to the strength of fluctuating pressure intensity is in direct proportion with the mean water head velocity, the paper derives the maximum thickness calculation of deposited pieces that might be lifted by floods from reasonably decided roughness coefficient and mean velocity, i. e. when the thickness of deposited pieces on the riverbed is smaller than the value and vertical fluctuation pressure caused by eddy current increased, the phenomenon of "river bottom peeling off" might be occurred randomly.