若尔盖泥炭型弯曲河流的崩岸机理研究

Bank Failure Mechanism of Peatland Meandering Rivers in the Zoige Basin

弯曲河流凹岸在近岸水流淘刷作用下发生崩岸是河湾横向迁移的直接动力。2011—2016年野外调查表明,黄河源的若尔盖盆地发育众多的弯曲河流,其河岸上部为泥炭层,下部为粉沙层,可称为泥炭型弯曲河流。通过分析泥炭型弯曲河流的河岸粉沙层淘刷过程,以及崩岸临界力矩平衡条件,从机理上研究泥炭型弯曲河流的横向淘刷速率与崩岸机制。泥炭型弯道的崩岸分为粉沙层淘刷与泥炭层崩塌这两个过程,并将河岸粉沙层的淘刷简化为3个阶段:1)凹岸的弯顶下游处最先受到水流的淘刷;2)开始阶段以横向淘刷直至最大距离;3)逐渐转变以纵向淘刷为主。进而,建立近岸流速与淘刷距离的关系式,近岸流速随淘刷距离增加而减少,推导出粉沙层横向淘刷速率的表达式,提出确定淘刷系数的方法,推导出泥炭层拉张临界破坏的力矩平衡方程,获得了泥炭块的临界崩塌宽度表达式,其计算值与实测值较接近。

Under the action of lateral scour by near-bank flow,bank failure of concave bank of meandering river is the direct drive of bend lateral migration.Field surveys in 2011—2016 suggested that numerous meandering rivers,called peatland meandering river,had developed in the peatland swamp of the Zoige basin in the Yellow River source,where the upper layer of river bank was peat and the bottom layer is silt.Based on field investigation and lateral scour process,the process of lateral scour and critical torque balance of bank failure were analyzed to reveal the bank failure mechanism.Bank failure of peatland meandering river consisted of lateral scour in the silt layer and bank collapse in the peat layer.Three stages of lateral scour in the silt-layer were simplified as firstly flow scour of downstream bend apex in concave bank,lateral scour up to the maximum distance,and finally longitudinal scour along the bank.Furthermore,the relation between near-bank velocity and lateral scour distance was obtained,i.e.near-bank reducing with increasing lateral distance.The formula of lateral scour rate was deduced that a simple method was proposed to estimate the lateral scour coefficient.The prediction value of critical width of peat slump blocks was in agreement with the measurements.