Experimental Study On The Failures Of Spur Dike Under Unsteady Flow

The spur dike is commonly used as one of the engineering measures of channel regulation, but it is often destroyed by flow in the flood season when running, so regular maintenance is required. In order to reduce the amount of post-maintenance it is urgently needed to carry out research of the failure of spur dike under unsteady flow conditions. Based on the analysis of flow process feature upper reaches of the Yangtze River and using fixed bed and movable bed test of generalized sink, the change process of the hydraulic elements, the spur dike destroyed, the reason of groin destroyed and the development of the scour hole around the spur dike are researched under seven unsteady flow processes. And carry out the comparative tests between the constant flow with the unsteady flow. The achievements provide reference for optimization designing groins. The main results obtained in the paper like the following:(1) The natural annual daily-average discharge hydrograph upper reaches of the Yangtze River is a non-symmetric and unbalanced random process with a single peak or bimodal or multimodal Morphology. Generally the peak number is much fewer and the maximum value of the peak is much greater. The different frequency flood process is established according to one-dimensional first-order probabilistic model.(2) According to the characteristics of the prototype river and the spur dike, the sink and groin for test is generalized. Using the controlling and inspection system of unsteady flow, the average daily unsteady flow process controlling and water level, flow rate of the process measurement is achieved.(3)When it is dry season, the spur dike usually is in non-submerged state, the scour hole is smaller and the spur dike is perfect.With the flow rate increasing, the flow submerges over the top of the dam. There is a large difference between the downstream and the upstream of the spur dike and waterfall appearing the frontier of the groin at the same time. The hydraulic inclination upstream and downstream and the frontier of the spur dike is larger but the flow rate is smaller on the top of the groin, and the more vulnerable parts is the head of the spur dike.; When it is flood season, the hydraulic inclination and flow rate of the frontier of the spur dike are always larger but the flow rate and hydraulic inclination of the top of the spur dike are often larger than those of the head of the spur dike and the mainstream, so the more vulnerable parts is mainly the body of the spur dike. Thus the frontier of the spur dike is sustained the high-speed flow for a long time and the top of the spur dike is suffered the larger flow force, the frontier and body of the spur dike are all the vulnerable parts. Which are the parts that should be enhanced whin designning structure of spur dike.(4) Under unsteady flow conditions, the maximum of hydraulic inclination around the spur dike appears earlier than that of the discharge, and the hydraulic inclination of the rising limb is larger than that of the drop. Ultimate scour depth and range of the frontier of the spur dike and the degree of spur dike destroyed are much greater than that of the corresponding constant flow.(5) When the flow increasing, flow velocity and scour hole of the groin head are rising and gradually developing individually. The rock block of the spur dike will begin slipping instability due to losing the backing of the riverbed. When the first flood peak arriving, the flow rate increases sharply, the scour hole develops rapidly, the downstream slope and the frontier and the top of the spur dike will suddenly collapse by flow attacking; With the subsequent flow up and down, the scour hole develops relatively slowly.(6) The spur dike is destroyed which response to the riverbed deformation and flow increasing during flood season; The combined effects of the spiral flow of the groin head and mainstream flow and the running-over-groin flow make the flow rate and hydraulic inclination of the top of the spur dike increase rapidly, the vortex follows the diving flow and arrives the head and surface of the spur dike, and forms a lot of energy when crushing, impacting and separating. The spur dike suddenly collapses because the loose stone was dragged away by flow directly due to the large fluctuating velocity.