Abstract:
The reach between the Three Gorges Project and the Gezhouba Project, is important for navigation in the Yantze River. The reach between the two dams has double flow characteristics of reservoir area and natural river. And river regime and riverbed morphology cause the special flow condition with deep water, steep slope of water level and high velocity under large discharge which is larger than 25000m3/s.
There have some famous navigation-obstructing shoals with “four shoals and one bend”. Four shoals are respectively Liantuo, Xitan, Dashaba, Piannao, and one bend is Shipai. Liantuo is the most typical and dangerous shoal of them for ship navigation. Under large discharge, the velocity is over 3.0m/s and the max is close to 5.0m/s, local slope of water level is over 7.5‰. There exists return flow, boil-vortex flow, and scissors-like flow. And boil-vortex flow is variable randomly in intensity and location. Return flow reduces the efficient navigation width. Boil-vortex flow causes ships worse steerage. Scissors-like flow makes main flow concentrate. These navigation-obstructing characteristics are much more obvious with larger discharge. So it is very necessary to channel regulation to improve navigation condition of Liantuo under large discharge.
The physical model is built and validated to test the flow condition, including water level, velocity, and flow pattern. The two remote-control self-propelled ship models are built and validated to test ship navigating condition, including the minimum ship speed with opposite bank, steering angle, drift angle, etc. The physical model is normal and the scale is 1:100. The ship models are respectively 1500T dry bulker carrier and 300TEU container ship. The scales of ship models are also 1:100.
It analyzes the change of flow characteristics and ship navigation parameters from the discharge of 25000m3/s to 45000m3/s. And 25000m3/s is the initial of large discharge, while 45000m3/s is prohibited to ship navigation for safety. In present situation, the test results show that navigable condition of Liantuo is out of order. The Shuitianjiao, at the entrance of the shoal, has steep slope of water level and high velocities. In the middle of the shoal, return flow with wide range and high intensity exists on both sides of the main flow belt, which are face-to-face. The width of them accounts for 66% of the total. And the main flow belt is narrow and the velocities increase significantly. And navigation-obstructing flow belts formed by clustered boil-vortex flow exist between return flow and main flow. So the navigable range is narrow and difficult for self-propulsion of ships to go through the shoal. When the discharge is over 30000m3/s, the test ship can’t safety navigate in Liantuo.
By analyzing the reach characterizes and flow variation, the navigation-obstructing reasons are obtained. The width of river sudden-expansion and sudden-contract causes scissors-like flow and return flow. The rises and falls of riverbed change the flow between kinetic energy and potential energy which leads to boil-vortex flow. The cross section most presents like “U” or “V” shape. Increasing rate of discharge sectional area is less than that of the discharges. So the velocity is higher with larger discharge. Then principles for channel regulation are proposed. They are expanding the upper part, lifting the lower part, and making overall plan, which can expand discharge sectional area, adjust velocity distribution and banish most factors that cause flow energy changing.
Based on analysis, the scheme of single engineering measure is tested such as dredging or dumping fill. The best dredging range and elevation is obtained. And the best dumping elevation is also obtained. On the basis, and the test of comprehensive improvement scheme is proposed, tested, and optimized both dredging and dumping fill. The results show that, the navigation flow condition has been improved obviously, and the capacity has been enhanced of ships going through the shoal. Thus, regulation goal of increasing limited discharge of navigation has been achieved to 35000m3/s. So regulation idea and principles are effective for the shoals with deep water and rapids.
Keywords: Shoal with deep water and rapids; navigation-obstructing characteristics; navigation regulation; physical model, remote-control self-propelled ship model