Wall-Roughness Effects on Flow and Scouring in Curved Channels with Gravel Beds
Wall-Roughness Effects on Flow and Scouring in Curved Channels with Gravel Beds
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DOI：
10.1061/(asce)hy.1943-7900.0001039
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发表时间：
2016
影响因子：
2.4
通讯作者：
D. Hersberger;M. Franca;A. Schleiss
中科院分区：
文献类型：
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作者：
D. Hersberger;M. Franca;A. Schleiss
壁面粗糙度对卵石床弯道水流冲刷的影响在高寒地区狭窄的河谷中，河流经常流经建设区，流经村庄和城市。由于空间有限，防止洪水泛滥往往需要用防护墙来保证。在洪水期间，这些防护墙可能会受到冲刷现象的威胁，特别是如果它们位于弯道上。在过去，充分的基础深度可以减少冲刷下建筑物的潜在危险。通过在位于河弯外侧的墙的表面设置垂直肋条等粗糙度元素，可以显著减少沿墙脚的局部侵蚀，从而降低墙基础的成本。这些意见是在对瑞士的几个防洪工程进行水力学模型试验审查和优化时提出的。一项文献回顾表明，直到今天，还没有对这些肋骨对弯道冲刷和水流的影响进行系统研究(§2)。这项研究项目通过研究冲刷的发展作为主要参数的函数来弥补这一差距。该研究基于半径为6m的90°弯道的试验研究，包括流速、水位、河床地形、泥沙特性、装甲层的粒度分布、流量和推移质(§4和5)。这一大组试验涵盖了广泛的流量、床面坡度和相当高的弗劳德数，但在亚临界区域，有许多肋骨间距和深度。观察到了两个冲刷坑的形成。在没有宏观粗糙度的情况下，在90°弯道的末端，进口河段内侧壁在外壁延伸处出现了第一个冲刷洞，出现了第二个冲刷洞。对试验结果的分析得出以下结论：现有的大多数冲刷公式大大低估了粗卵石河床山区河流的冲刷深度(§3)。观测到自由水面(驻波)和冲刷深度都有明显的波动，特别是第二个冲刷坑。这两个冲刷坑有不同的原因。第一个主要是由于主流方向的改变(对壁面的冲击)和诱导的二次流，第二个主要是由于在渠道内侧形成点坝后流速波动增加(§6)。在弯道的平均流场中，最高的主速度从靠近表面的中心线向外壁移动，然后向底部移动(§6)。在第一次冲刷时，在地面附近发现了最高的主速度。由于粗碎石混合物的使用，在横断面上观察到了显著的颗粒分选过程，导致外壁堆积粗泥沙，内岸沉积细颗粒。通过在外侧壁上施加垂直肋条，可以观察到对冲刷过程和流场的重要影响(§6)。宏观粗糙度对冲刷深度有以下影响：沿外侧冲刷深度明显减小，突出的冲刷几乎消失。随着壁面粗糙度的增加，第一个冲刷坑向下游方向移动，而第二个冲刷坑的位置保持不变。在没有宏观粗糙度的情况下，水面和冲刷深度的显著波动减少了约50%，冲刷以一种更平滑的方式发展。流场发生了明显的变化：最高流速远离外墙，减少了外墙基础底部的冲刷。沿着自由面的外壁，可以观察到在外岸有一个二次流单元。这一单元的重要性与银行保护效果显示出重要的相关性。最佳肋骨间距是至关重要的，因为不适当的间距可能会导致冲刷深度的显著增加。降低了弯道的运力。在天然河流中，这一现象可以通过增加河床坡度来弥补。弯道的上游和下游两端受肋骨的影响：弯道上游，由于弯道水头损失，水深增加；下游，在渠道中心发现一些额外的冲刷。除粗泥沙带面积增大外，肋骨的存在对颗粒分选过程影响不大。此外，本报告还提出了一个新的经验公式，用于估算宽粒径分布的山区河流的最大冲刷深度(§7)，该公式是基于物理参数建立的，这些参数是平均水深与河道宽度的比率，平均流速与水力半径的无因次比率，最后是底质的摩擦角。该公式还可用于计算最大冲刷断面的横向床面剖面。外墙存在宏观粗糙度时的最大冲刷深度可以用一个公式得到，该公式取决于肋骨间距、水力半径、弗劳德数和无因次剪应力与临界护盾参数之差(§7)。最后，给出了对水利工程师的建议(§8.3)，以便于在外滩上应用垂直肋条作为宏观粗糙度。
Wall roughness effects on flow and scouring in curved channels with gravel bed In the narrow valleys in Alpine regions, rivers frequently flow across constructed zones, passing through villages and cities. Due to limited space, the protection from high floods often needs to be ensured by protection walls. During floods, these protection walls may be endangered by scour phenomena, especially if they are located in bends. In the past, the potential danger of underscoured structures was reduced by sufficient foundation depth. By providing roughness elements such as vertical ribs at the surface of walls located at the outer side of river bends, the local erosion along the foot of the wall can be considerably decreased, reducing the cost of wall foundations. Such observations were made during the review and optimization of several flood protection projects with hydraulic model tests in Switzerland. A literature review showed, that no systematic study of the influence of these ribs on scour and flow in bends was performed up to this day (§ 2). This research project covers this gap by investigating the development of the scour as a function of main parameters. The study is based on an experimental investigation in a 90° bend with a radius of 6 m including measurements of the velocities, the water level, the bed topography, the sediment characteristics, the grain size distribution of the armoring layer, the discharge and the bed load (§ 4 and 5). The large set of tests covers a wide range of discharges, bed slopes, rather high Froude numbers, but in subcritical regimes, many rib spacings and depths. The formation of two scour holes was observed. Without macro roughness, a first scour hole occurs in the prolongation of the inner sidewall of the entry reach at the outer wall and a second scour hole appears at the end of the 90° bend. The analysis of the performed tests results in the following conclusions: Most existing scour formulae considerably underestimate the scour depth in mountain rivers with coarse gravel bed (§ 3). Significant oscillations, both of the free water surface (stationary waves) and of the scour depth were observed, especially for the second scour hole. The two scour holes have different reasons. The first one is essentially due to the change of the main flow direction (impact on the wall) and the induced secondary current, whereas the second one is mainly due to increased velocity fluctuations after the point bar formed at the inner side of the channel (§ 6). In the average flow field in a bend, the highest main velocities are shifted from the centerline close to the surface toward the outer wall and then towards the bottom (§ 6). At the first scour, the highest main velocities are found next to the ground. A significant grain sorting process is observed over the cross-section due to the use of coarse gravel mixture, resulting in the accumulation of coarse sediments at the outer wall and depositions of fine material on the inner bank. By applying vertical ribs on the outer sidewall, an important impact on the scour process and on the flow field can be observed (§ 6). The macro-roughness has the following effects: The scour depth along the outer sidewall is significantly reduced and the prominent scours almost disappear. With increasing wall roughness, the first scour hole shifts in the downstream direction whereas the second one remains at about the same position. Significant oscillations of the water surface and the scour depth, observed without macroroughness, are reduced by about 50% and the scour develops in a "smoother" way. The flow field undergoes a pronounced modification: the highest velocities are kept away from the outer sidewall, reducing the scour at the bottom of the outer wall foundations. Along the outer wall at the free surface, an secondary flow cell at the outer bank can be observed. The importance of this cell shows an important correlation with the bank protection effect. Optimum rib spacing is essential since an inappropriate spacing may lead to an important increase in scour depth. The transport capacity in the bend is reduced. In natural rivers, this phenomenon is compensated by a steepening of the bed slope. The upstream and downstream extremities of the bend are influenced by the ribs: upstream of the bend, the water depth increases due to the head losses in the bend, and downstream, some additional erosion is found in the center of the channel. The grain sorting process is not significantly influenced by the presence of the ribs beside an increase of the area of the coarse sediment zone. This report furthermore presents a new empirical scour formula for the estimation of the maximum scour depth for mountain rivers with wide grain size distribution (§ 7), established with physically based parameters, which are the ratio mean water depth to channel width, a dimensionless ratio combining the mean velocity with the hydraulic radius and finally the friction angle of the bed material. The lateral bed profile in the maximum scour cross section can also be computed with this equation. An estimation of the maximum scour depth in the presence of macro-roughness on the outer wall can be obtained with a formula depending on the rib spacing, the hydraulic radius, the Froude number and the difference between the dimensionless shear stress and the critical Shields parameter (§ 7). Finally recommendations for hydraulic engineers (§ 8.3) are given to facilitate the application of vertical ribs on outer banks serving as macro-roughness.