Near-bed flows in rough-bed open-channels are of relevance to river engineering, freshwater ecology and hydraulic design. Detailed measurements of near-bed flow velocity and turbulence distributions are rarely available. This paper aims to improve our understanding of near-bed flow characteristics. We performed experiments of turbulent open-channel flow over roughness elements, and made fine-scale measurements of three-dimensional velocity and turbulence quantities, using an acoustic Doppler velocimeter. The experiments allowed for the d-type, the intermediate-type and the k-type of surface roughness. The fine scale measurements covered the entire flow region in the bed vicinity, including cavities below the crests of roughness elements. Below the crests, the longitudinal velocity field displays one weak eddy in the cavity between adjacent elements for the d-type of roughness, two eddies of different strengths for the intermediate-type of roughness, and boundary-layer velocity structures for the k-type of roughness. Above the crests, the main (or longitudinal) flow velocity shows a variety of vertical structures for the three types of surface roughness. At the elevation equal to one half the roughness height above the crests, there is a lack of longitudinal variations in the main flow velocity over the length of element—cavity sequence. In other words, main-flow velocity fluctuations are small; the velocity distributions have a standard deviation equal to 5%, 6% and 7% of their mean values for the d-type, the intermediate-type, and the k-type of roughness, respectively. Practically the influence of roughness surface may be considered as capped at that elevation. The secondary flow has the highest intensity nearest the bed. Turbulence shear has the highest average value at the crest level of roughness elements.

中文翻译：

---

表面粗糙度下明渠近床速度和切应力

粗糙床明渠中的近床水流与河流工程，淡水生态学和水力设计有关。几乎无法获得近床流速和湍流分布的详细测量结果。本文旨在增进我们对近层流特征的理解。我们进行了在粗糙元件上的湍流明渠流动实验，并使用声学多普勒测速仪对三维速度和湍流量进行了精细测量。实验允许表面粗糙度为d型，中间型和k型。细刻度测量覆盖了床附近的整个流动区域，包括粗糙度元素波峰下方的空腔。在波峰下方 纵向速度场在d型粗糙度的相邻元素之间的腔体中显示出一个弱涡流，对于中间类型的粗糙度显示出强度不同的两个涡流，对于k型粗糙度显示边界层速度结构。在波峰上方，对于三种类型的表面粗糙度，主（或纵向）流速显示出各种垂直结构。在高程等于峰顶上方粗糙度高度的一半时，在元素-腔序列的整个长度上主流流速缺乏纵向变化。换句话说，主流速度波动很小；对于d型，中间型和k型粗糙度，速度分布的标准偏差分别等于其平均值的5％，6％和7％。实际上，粗糙表面的影响可以认为是在那个高度上的上限。二次流在离床最近的位置具有最高强度。湍流剪切在粗糙度元素的波峰水平具有最高的平均值。