Experimental study on influence of different patterns of patch on the flow field and scour/deposition process in the wake region

Flume experiments were conducted to comprehend the impact of different patterns of a patch on the flow field and the scour process in natural rivers. Velocity measurements, flow visualization, and scour tests were undertaken around different patch patterns, which were simulated inspiring from expansion process of a typical instream vegetation. The patches were idealized in which a circular patch of rigid emergent stems becomes elongated due to positive and negative feedback. Three characteristically different patches with a gradual decrease in density towards the downstream side were examined. This variation in density and length was assumed to occur due to sediment/seedling deposition, and the establishment of new plants. The expansion of patch was considered to occur in three stages: (a) stage 1, (b) stage 2, and (c) stage 3. These stages were replicated individually by only increasing in density and elongating the patch. In this way, two processes (i.e., elongation and increase in permeability), which usually have hydrodynamically opposite effects on flow fields, were simulated at the same obstruction. Despite generally elongated obstacles being streamlined bodies, the morphometric analysis of obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and enhances localization of the local scour hole. This situation implies as the patch expands, in the wake region, the steady-wake region becomes shorter, turbulence diminishes, lateral shear stress enhances, and deposition cannot occur far from the patch. Consequently, as the patch expands, the hydrodynamic consequences may restrict further patch expansion after a certain length/density.为探究天然河道中不同斑块格局对水流场及冲刷过程的影响，本研究开展了水槽试验（flume experiments）。试验以典型河道植被的扩张过程为原型，针对不同斑块格局开展了流速测量（velocity measurements）、水流可视化（flow visualization）与冲刷试验（scour tests）。本次研究将斑块理想化：由刚性挺水茎秆（rigid emergent stems）组成的圆形斑块会因正负反馈（positive and negative feedback）发生伸长变形。本研究选取了三类特征各异的斑块，其密度沿下游侧（downstream side）逐渐降低。这种密度与长度的变化被认为由泥沙/幼苗沉积（sediment/seedling deposition）以及新植株定植所导致。斑块的扩张被划分为三个阶段：(a) 阶段1、(b) 阶段2 与 (c) 阶段3。仅通过提升斑块密度与延长斑块尺寸，即可分别复现这三个阶段。通过该方法，本研究可在同一阻水结构上同时模拟通常对水流场具有水动力相反效应（hydrodynamically opposite effects）的两个过程，即斑块伸长与渗透性提升。尽管常规伸长型障碍物属于流线型物体（streamlined bodies），但激光扫描仪（laser scanner）获取的形态学分析结果显示，透水斑块的流线型伸长会加剧整体冲刷（global scour），并强化局部冲刷坑（local scour hole）的局域集中分布。该现象表明，随着斑块扩张，尾流区内的稳定尾流区（steady-wake region）长度缩短，湍流衰减（turbulence diminishes），横向剪切应力（lateral shear stress）增强，且沉积物无法在距离斑块较远的位置沉积。因此，随着斑块扩张，其水动力效应可能会在斑块达到一定长度或密度后，限制其进一步扩张。