Study On Turbulence Characteristics In Dredge-out And Stability Of Dredging Of Straight Channel In Branching Reach With Bend

There are many pebble shoals of branching reach with bend in mountainous rivers, which are characteristic of bend, shallow and rapid. The pebble shoal of branching reach with bend is one of the most complicated shoal and rapid in channel regulation. Because of the limited research on this kind of shoal and rapid, many regulation projects couldn't solve the problem of obstructing-navigation to the core. So it is important to research obstructing-navigation mechanism of pebble shoals and regulation methods of branching reaches with bend. The flow and sediment features are analyzed of pebble shoals of branching reach with bend. Turbulence characteristics in dredge-out and stability of dredging of straight channel in branching reach with bend are studied by means of experiments and theoretics.The obstructing-navigation characteristic and regulation practice are studied and analyzed of pebble shoals of branching reach with bend in the reach between Chongqing and Yibin in Yangtze River. Based on the field data and experimental results of physical model of typical pebble shoals, obstructing-navigation mechanism, the features of flow and sediment movement, the rules of riverbed evolution are summarized. It is shown that the curved branch channel is too bend and shallow, and its flow condition for navigation is too poor to improve in branching reach with bend. In order to achieve permanent improvement of navigation condition, it is suggested to excavate straight channel at central bar for navigation.Turbulence characteristics in dredge-out are researched systemically by means of laboratory experiments. Turbulence characteristics of flow in different dredge-cut are measured by a MicroADV system. The distributions of time-averaged velocities, turbulent intensities along depth are further analyzed and calculated. In addition, the distribution characters of longitudinal, lateral and vertical turbulent intensities are compared. Furthermore, the turbulence characteristics are analyzed of dredge-cut combined with spur dikes.The experimental results show that in certain value scope the bottom stress and turbulence of narrow-deep pattern of dredging are larger than broad-shallow pattern, so it is better than the broad-shallow one in aspect of reducing sediment back-silting.Stability of newly excavated channel in branching river with bend is studied. It is shown that flow velocity, which meets the demands of dredge-cut stability at regulation discharge, is advisable to be 1.1～1.3Vc. The sediment transport intensity achieves maximum at width-depth ratio (B/H) being equal to 17.8. In order to improve navigation condition thoroughly, it is suggested to excavate straight channel at central bar for navigation. As to newly excavated channel, the low-water regulation methods of narrow-deep type of dredging coupled with diking are recommended.