Study On The Hydraulic Characteristics Of End-dump Closure With The Assistance Of Auxiliary Closure Structure

River closure is a key step in water dam construction, and the end-dump closure is a general way to cut off the river flow. For the end-dump closure with single dike in the closure project with large closure discharge and high closure drop, the flow in the closure gap is of large hydraulic gradient, energy concentration, and high velocity, which would results in high closure difficulty. The flow hydraulic conditions at the closure gap of end-dump closure with single dike can be improved to some extent by closure with wide dike or double dikes, but the construction organization is complex, the construction tensity and the requirements of construction space and the traffic condition are high, and it's not convenient to control the hydraulic condition at closure gap. Thus based on the study that the impact of water level at the downstream of closure dike on the hydraulic condition at closure gap, this paper proposes the concept of auxiliary closure structure, which efforts to reduce the flow velocity and improve the hydraulic condition at the closure gap, and explores new closure methods to reduce the difficulty of end-dump closure.The main contents are as follows:(1) A 3D numerical model is established to simulate the flow pattern at the closure gap of end-dump closure, and the impact of the water level at downstream of closure dike on the flow pattern at the closure gap is analyzed. First, according to the flow characteristics of end-dump closure, a 3D turbulence numerical model based on time-averaged N-S equation and RNG k-? model, which are verified by the results of physical model tests, is established to simulate the flow of end-dump closure. Then, by using the established numerical model, the impact of downstream water level of the closure dike on the flow pattern at the closure gap of end-dump closure is analysed. At last, the approaches to raise the water level at the downstream toe of closure gap are summarized.(2) The method of reducing the closure difficulty by using auxiliary closure structure is proposed. For the river closure project with large closure discharge and high closure drop, it's necessary to take measures to decrease the flow kinetic energy at the closure gap, and weaken the erosion of riverbed and closure material. The auxiliary closre structure is defined as the engineering measure which decreases the flow kinetic energy and flow velocity, and reduces the closure difficulty by increasing the energy dissipation or optimizing the energy conversion process of flow at the closure gap. The auxiliary closure structure is divided into energy dissipation closure structure and energy storage closure structure according to the definition of auxiliary closure structure. Based on the summarizing and analyzing of the existing auxiliary closure structure, two new auxiliary closure structures are presented, including the backwater-sill and the wedge dike head, and also the combined auxiliary closure structure by combining the two.(3) The hydraulic characteristics and the main hydraulic parameters calculation method in the closure gap of end-dump closure with backwater-sill are studied. The construction method of end-dump closure with backwater-sill is introduced. The impact of backwater-sill on the flow pattern in the closure gap and the feasibility of reducing the closure difficulty of end-dump closure is investgated. The backwater-sill significantly raises the water level at the downstream toe of closure gap, decreases the flow hydraulic gradient and flow velocity, and reduces the closure difficulty. The control variable method is adopted to analyse the impact of closure dike and backwater-sill geometries on the hydraulic condition at the closure gap of end-dump closure. The results show that the height of backwater-sill is the key factor affecting hydraulic conditions, while the influence of other factors are relatively small and can be neglected. The higher the backwater-sill, the larger the flow depth, the smaller the closure drop and the flow velocity, and the better for closure. The total flow energy equation and continuity equation are applied to derive the calculation method of the main hydraulic parameters at the closure gap, non-liner regressions are performed for the relevant coefficients, and the main hydraulic parameters calculation method of end-dump closure with backwater-sill is obtained.(4) The hydraulic characteristics and the main hydraulic parameters calculation method in the closure gap of end-dump closure with wedge dike head are studied. The hydraulic characteristics of different closure gap shape is analyzed, which indicates that the closure gap with contraction shape is of low hydraulic gradient and flow velocity, and is the most favorable for river closure, and illustrates that the closure difficulty of end-dump closure can be reduced effectively by wedge dike head. The control variable method is adopted to analyse the impact of closure dike and wedge dike head geometries on the hydraulic condition at the closure gap of end-dump closure and to investigate the impact mechanism. The results show that the inlet width and outlet width of the closure gap are the main factors affecting hydraulic conditions, while the influence of other factors are relatively small and can be neglected. The larger the width of inlet and the smaller width of outlet, the smaller the closure drop and the flow velocity, and the better for closure. Based on the classic hydraulics, the calculation method of the main hydraulic parameters at the closure gap is derived, non-liner regressions are performed for the relevant coefficients, and the main hydraulic parameters calculation method of end-dump closure with wedge dike head is obtained.(5) The hydraulic characteristics and the main hydraulic parameters calculation method at the closure gap of end-dump closure with combined closure structure is studied. Based on the analysis of the limitations of backwater-sill and wedge dike head, the combination of the two auxiliary closure structures to form a combined closure structure is proposed. The comparative analysis of the hydraulic characteristics of end-dump closure with single dike, end-dump closure with backwater-sill, end-dump closure with wedge dike head, and end-dump closure with combined closure structure shows that the combined closure structure integrates the advantages of backwater-sill and wedge dike head, which further raises the downstream toe water level, decreases the hydraulic gradient and flow velocity, and reduces the closure difficulty. The control variable method is adopted to analyse the impact of closure dike, wedge dike head and backwater-sill geometries on the hydraulic condition at the closure gap of end-dump closure and to investigate the impact mechanism. The results show that the closure gap inlet width and outlet width and the height of backwater-sill are the main factors affecting hydraulic conditions, while the influence of other factors are relatively small and can be ignored. The larger the width of inlet, the smaller the width of outlet and the higher the backwater-sill, the more favorable for closure. Based on the classic hydraulics and non-liner regressions for the relevant coefficients, and the main hydraulic parameters calculation method of end-dump closure with combined closure structure is obtained.With the consideration of reducing the flow kinetic energy and flow velocity, the concept of auxiliary closure structure is proposed, which provides important theoretical guiding significance for river closure design and construction. Backwater-sill, wedge dike head and combined closure structure all improve the hydraulic characteristics at the closure gap and bring convience to construction organization, and significantly reduce the difficulty of end-dump closure. End-dump closure with backwater-sill has been applied in the diversion channel closure of the Lower Sesan 2 hydro-power project, which has achieved great success.