河流极限流速及其与河床演变的关系

High flow velocity will cause a lot of problems, such as flood through time shortened, riverbed scoured, the survival of riparian life threatened etc. The limit velocity law studies the range of flow velocity, which can provide the basis for high flow velocity control in integrated river management. In alluvial rivers, in general, the average velocity increases with an increase in discharge when the discharge is small. As the discharge exceeds the bank-full discharge, any further increase in discharge does not result in an increase in velocity. The average velocity approaches a limit, which is the so-called limit velocity. However, up to now, there is no systematic research result of limit velocity for semi-alluvial river and non-alluvial river. In this project, the study of flow velocity for semi-alluvial river and non-alluvial river is presented by field investigation and analyzing hydrological data including daily average discharges, sediment concentrations, stages, and cross sections from the Yangtze, Yellow, Songhua, Yarlung Zangbo Rivers etc. The cross-sectional average velocities will be calculated from the discharge and the cross-sectional area. According to the analysis of alluvial river, semi-alluvial river, and non-alluvial river,summarize the limit velocity law. Study the change of limit velocity with stage, discharge, bed material, and the cross-sectional shape and size. Reveal the relationship between limit velocity and sediment movement and fluvial processes. Analyze impacts of human activities on limit velocity. Propose a river management program which is based on limit velocity and will meet the sustainable development of people and water. The limit velocity law can not only improve the theory of flow velocity, but also provide necessary scientific support for integrated river management.

河流流速较高将导致行洪时间缩短、河道冲刷、滨河生物生存受到威胁。极限流速定律研究流速的变动范围与极限，为河流综合管理控制高流速提供依据。冲积河流的研究表明，当流量较小时，流速随流量的增加而增大，达到一定流量后，流速趋近一个极限，即极限流速。但是，对于半冲积、非冲积河流，目前尚无极限流速的系统研究成果。本课题拟对长江、黄河、松花江、雅鲁藏布江的典型半冲积、非冲积河段进行水文资料收集和野外调查测量，利用水位、流量、实测大断面资料根据连续性方程计算流速，或采用多普勒流速测量仪实测流速、流量、河床断面。根据冲积、半冲积、非冲积河流的分析结果总结极限流速定律。研究极限流速随水位、流量、河床性质的变化，揭示极限流速与泥沙运动及河床演变的关系。分析人类活动对极限流速的影响，提出以极限流速为基础的符合人水和谐的河流治理方案。极限流速定律将不仅丰富流速理论，还可为河流综合管理提供必要的科学支撑。

通过对长江、黄河、松花江典型冲积、半冲积、非冲积河段进行野外调查、水文资料分析和流速计算，发现冲积河流存在极限流速，当流量较小时，流速随流量的增加而增大，达到一定流量后，流速趋近一个极限；半冲积河流符合极限流速定律，但极限流速比冲积河流大；非冲积河流不符合极限流速定律。从河床形态与演变特性、剪力、扬动流速、河流输沙四个方面分析了极限流速的产生原因：1）冲积河流一般具有复式河床断面形态，流量较小时，水流在主槽内流动，当流量增大到某一数值时，水流漫滩，断面面积增大，使得流量的进一步增加不会引起流速的显著变化，流速会保持在极限流速附近；非冲积河流通常两岸受到山体约束，参与造床的泥沙不多，河流不能自动调整，没有极限流速；半冲积河流通常一侧河床是有一定厚度的二元结构，一侧受山体约束，由于大量泥沙参与造床，使得半冲积河流也具有极限流速，但比冲积河流的极限流速高。2）基于床面泥沙运动颗粒的垂向位置和运动特征以及泥沙起动、止动和扬动的临界剪力分析，发现冲积河流存在极限流速，且极限流速出现在水流剪力与扬动剪力相等的条件下。3）冲积河流在主槽深度条件下，极限流速接近悬移质泥沙最粗材料的扬动流速；分析推移质泥沙的扬动条件，发现冲积河流的极限流速不能超过泥沙的扬动流速，且小于3m/s。4）在动平衡条件下，造床流量时，河道平均流速的最大值应控制在扬动流速之内，如果超过扬动流速，水流对粗颗粒泥沙的输移量将增加，引起河床冲刷，同时推移质起悬变为悬移质，扩大河槽面积降低流速，使得极限流速最终仍回到扬动流速附近。河流极限流速具有地貌学意义和生态学意义，值得开展深入的研究。人类活动通常会影响河流的极限流速，使得断面平均流速接近或超过极限流速。河流出现较高流速后，水流将侵蚀岸壁和河床，增加河床失稳可能性，同时会破坏生态系统平衡。河流综合管理必须模拟河流的自然演变过程并满足极限流速定律，这样才能维持生态和谐，地貌稳定。