Experimental study on entrainment of large wood in lowland rivers

Wood transport in rivers, typically occurring during flood events, represents a relevant hazard for its potential to create obstruction at bridges and narrow cross-sections. Therefore, the understanding and prediction of entrainment and transport dynamics of woody material of different shapes, density and dimensions is of great interest for river managers. The paper presents results from laboratory experiments carried out to assess the entrainment conditions of large wood in lowland rivers, i.e. with negligible longitudinal slopes, relatively smooth bed and low flow Froude numbers. The tests were performed in a straight flume with fixed bed and smooth side walls for several flow conditions. Entrainment was studied for circular and square logs having different initial orientation. Integrating the results with other data available in the literature allowed to derive a semi-empirical entrainment threshold based on a force balance. This threshold is based on simple parameters, such as normal flow characteristics, log size and density as well as median diameter of bed sediments.河流中的木材输移（Wood transport）通常发生于洪水事件期间，因其可能在桥梁与狭窄河道断面处形成阻塞，故而构成显著安全隐患。因此，针对不同形状、密度与尺寸的木质物料的起动（entrainment）及输移动力学开展理解与预测，对河道管理人员而言具有重要的实践价值。该论文呈现了针对低地河流中大木材（large wood）起动条件开展的室内试验结果，此类河流具备纵向坡度可忽略、河床相对平整且水流弗劳德数（Froude number）较低的特征。试验在固定床、侧壁光滑的直水槽（flume）中开展，涵盖多种水流工况。研究针对不同初始朝向的圆形与方形原木，分析了其起动特性。将本试验结果与已发表文献中的其他数据相结合，可推导出基于力平衡的半经验起动阈值。该阈值仅依赖于常规水流特征参数、原木尺寸与密度以及河床沉积物中值粒径等简单参数。