稳定封河期冰塞发展对水流运动结构及河床演变影响机制研究

Ice jam is one of the most complicated hydraulic phenomenon, which can block the flow of water area, cause location of the maximum velocity moving down to readjust the velocity distribution and turbulent flow shear stress, and changes the starting velocity of sediment imbalance, resulting in sediment of imbalance and riverbed severe adjustment, and even the river migration. In view of, nowadays, the ice jam development effect on flow structure and bed evolution study is less, and the lack of the support of the measured data. Therefore, this project intends to adopt the field prototype test method, on the section of the Yellow River in Inner Mongolia local ice plug for field test as to collect ice-cover and ice jam development, velocity distribution under the ice, sediment concentration, water depth, bed material gradation, section topography changing process. Based on the measured data and considering each other relations on atmosphere, the rules of ice jam evolution, ice jam hydraulic resistance changing, velocity distribution, turbulent shear stress change are studied. Considering the relationship water-sediment-river bed, sediment starting velocity, river bed hydraulic resistance, the rules of sediment transport and bed evolution are researched. The ice jam development that effect on flow structure and river bed evolution is analysed. Research results can deepen understanding of ice jam evolution process, also for the study of sediment under ice, freeze-up period riverbed evolution mechanism to provide the reference and further enrich the basic theory on dynamics of river ice, ice jam disaster prevention for alpine areas to provide technical support, has important academic significance and application value.

河流冰塞是最复杂的水力现象之一。冰塞形成后，阻塞水流的过水面积，下移最大流速点位置，重新调整流速分布和紊动切应力，改变泥沙起动流速，造成输沙的不平衡及河床剧烈调整，甚至是河道的偏移。目前有关冰塞影响下的泥沙输移规律研究大多基于理论分析，缺少实测资料的支撑。为此，本项目拟对稳定封河期黄河内蒙古河段发生的冰塞开展连续测验，观测气象、冰盖及冰塞发展、泥沙和断面地形变化过程等资料。基于实测资料，考虑气-冰-水之间的关系，研究冰塞演变，冰塞水力阻力变化，流速分布、紊动切应力变化等规律；考虑水-沙-河床之间的相互影响关系，研究泥沙起动流速，河床水力阻力，泥沙输移与河床演变等规律；从能量平衡的角度分析稳定封河期冰塞发展对水流运动结构及河床演变影响机理。项目研究成果能深化对冰塞发展过程的认识，丰富冰下输沙、封河期河床演变研究成果，为高寒地区预防冰塞灾害提供技术支撑，具有重要的学术意义和应用价值。

冰花堆积形成的冰塞是凌汛期冰凌灾害产生的主要原因之一，冰塞形成受水力、热力、河道边界条件等多种因素影响，形成后对断面流速分布、泥沙输移、河床等影响较大。受现场恶劣测验环境影响，目前的测验资料及研究成果不深入。本项目委托具有水文测验资质的黄河水利委员会宁蒙水资源局，在2019/2020年凌汛期开展了黄河头道拐水文断面及上下游共三个断面的加密测验，开展了三个方面研究：①冰塞的发展过程及物理性质研究②冰塞水力阻力变化过程和水流运动结构研究③冰塞存在与河床演变相互影响关系研究。研究取得5个方面的重要结果：（1）冰塞体是由3cm以下形状不规则的片状薄冰组成，孔隙率介于0.4～0.5，初始封河期、开河期孔隙率稍大。冰塞截面多为倒三角形态，最大厚度一般为2.5m-3.0m，主体多位于主槽横向比降较小一侧，占冰期过水面积的20%～60%。（2）实测初始封河期冰盖糙率为0.050～0.070，稳定封河期糙率为0.013～0.017；河床糙率从封河前的0.0125左右增大至0.0145左右，封河后河道水位壅高0.5m～2.0m，初封期壅高大于稳定封河期。（3）封河期流速中间大，表层与底层较小，深泓线附近最大流速靠近河床，最大1m/s左右，冰塞下表面与河床间水深越大流速也越大，实测三个断面最大流速在0.8m/s～1.0m/s。在冰塞、河床附近形成了涡旋，u流速（0.6 m/s左右）>v流速（0.2 m/s左右）>w流速（0.02m/s左右），是造成河床冲刷的主要原因。（4）畅流期及开河期流量越大含沙量也越大，平均含沙量在2.0kg/m3以上，封河期含沙量基本在0.2kg/m3以下。悬沙D50从封河前的0.02mm细化至0.005mm。（5）封河后水体粘度增大约1 倍，泥沙启动流速增大，泥沙沉速减小。研究揭示了冰塞发展对水流运动结构及河床演变影响机制，丰富了河冰动力学和河床演变学的研究内容。对开展寒区水资源开发利用及河道整治工程建设，防止或减少冰凌灾害生成具有重要的现实意义。