山区河流浅水河床漂石对局部水流结构影响试验研究

Experimental Study on Influences of Boulder on Flow Structure in Mountain River with Shallow Depth Conditions

西南山区河道普遍分布着大量的卵砾石、漂石等大粒径床沙,漂石颗粒急剧调整局部水流结构与泥沙输移,从而制约着河床响应过程。以漂石河床水流结构为研究对象,结合野外典型河段调查和概化试验,利用3维多普勒流速仪(ADV)详细测量漂石区域的3维流速,探讨浅水条件下漂石颗粒对局部水流运动的影响。分析表明:漂石对水流结构的影响集中在其下游中心区域,尤其是远部尾流区内,其纵向范围主要集中在约–1<Δx/D<2区域,其中漂石附近的近底流速变化明显,远部尾流区边界高度也从受漂石影响较小的上游0.3~0.4倍水深发展到受漂石影响较大的下游0.5~0.7倍水深处;中心纵剖面紊动能和雷诺应力数值显著大于左右两侧,且其数值在漂石下游激增至1~10倍和1~16倍,两者出现峰值的位置都集中在漂石远部尾流区末端。研究还发现,漂石相对淹没度(H/D,平均水深与漂石有效直径之比)对漂石附近的水流结构有较大影响,随着相对淹没深度(H/D)的增加,其对流速、紊动能和雷诺应力的影响逐渐增强,在处于临界淹没度H/D=1.2时最为强烈,之后强度递减。不同相对淹没度(H/D)下,流速、紊动能和雷诺应力恢复区长度相近,且与远部尾流区尺度变化基本一致。总体而言,漂石通过改变局部沿程流速、紊动参数和远部尾流区分布,从而显著影响浅水河床局部水流结构,其具体影响因子和响应规律有待进一步深入研究。

There were big amount of large-size sediments such as pebbles and boulders on the bed of mountain rivers in Southwest China,which adjusting the surrounding water structure and sediment transportation,and thus greatly affected the process of bed transformation.ln this study, the influence of boulders on flow movement with shallow depth conditions was analyzed and discussed.The research included both the field investigation of typical riverbed and open channel experiments,in which the Acoustic Doppler Velocimeter （ADV） was used to measure the three-dimensional velocities in vicinity of the boulder to understand the flow mechanism better.The analysis showed that the effect of boulder on longitudinal velocity concentrated in the central plain at the downstream of the boulder, especially in the far-wake region,and the longitudinal impact of boulder was concentrated at the range of- 1 〈△x/D〈2,the near-bottom velocity changed significantly near the boulder, and the boundary height of far-wake region raised from about （0.3-0.4）D in the less influenced upstream region to about （0.5-0.7）D at the more influenced downstream region;the values of turbulence kinetic energy and Reynolds stress in central plain were much higher than those measured at the left and right sides,both increased sharply by about （1 - 10）,（ 1 - 16） times compared with the initial value and both of their peak occurred at the end of the far-wake region;the flow structure also was found to be greatly affected by the boulder particle submergence degree （H/D,the ratio between mean water depth and boulder effective diameter）,the influence on the hydraulic parameters such as velocity,turbulence kinetic energy and Reynolds stress firstly increased with the increase of the boulder particle submergence degree （H/D）,reached at a maximum value at the critical-sub- mergence degree of H/D=- 1.2,and then decreased.With different particle submergence degree （H/D）,different hydraulic parameters＇ recovery length and the scale of far-wake region were comparable.In summary,the boulder has great impact on flow structure through changing surrounding longitudinal velocity,turbulent parameters and the distribution of far-wake region,the details of effective factors and the laws should be further analyzed.