Impact Scale Theory And Cumulative Effect Mechanism Of Spur Dike Group

Spur dike is one of the common river engineerings, which is widely used in river training projects, flood control projects, channel improvement projects and floodplain reclamation projects. As a human impact imposed on river system, spur dike would interfere river system to a certain extent. In order to better play the various functions of spur dike, spur dike often appears in the form of "group" in engineering practice. However, the current researches on spur dike, including flow characteristics or local scour mechanism, are more concentrated in partial impacts of single spur dike on the flow or riverbed and confined in the range of local reaches to the upstream or downstream spur dike. It is less involved in the combined effects and cumulative effects of spur dike group composed by a series of spur dikes on the flow or riverbed. In addition, the correlation between spur dikes is different under different dike spacing, so the corresponding cumulative effects are different. Consequently, it is necessary to carry out the research of impact scale of spur dikes. Currently, the cumulative effect studies of some river engineerings, such as reservoirs, hydropower stations, dams, artificial channels, etc., have been carried out and achieved some research achievements. However, as one of them, cumulative effect researches of spur dike are rarely involved. Therefore, based on the classification of impact scale of spur dikes, to carry out the research of cumulative effect mechanism of spur dike group with different impact scales has some academic value and important significance for understanding and evaluating the impact of river engineering on river system health.Firstly, flume experiments were conducted in Jiangong Testing Hall of Zhejiang University to obtain observed data of flow field of spur dike group. And then, three turbulence models based on different solutions of N-S equations and two kinds of treatment methods of free surface were selected to simulate the spur dike flow field. Under the comprehensive consideration of simulation accuracy and computation time, the suitable mathematical models of non-submerged and submerged spur dikes were obtained. On the basis of previous studies, numerical simulation was used to study on the classification criteria of impact scale of non-submerged and submerged spur dikes, and the advantages and disadvantages of each classification criterion were analysed and compared, meanwhile, the concept system of impact scale of spur dike group was built. Furthermore, the impact scale of non-submerged and submerged spur dikes was researched quantitatively and the cumulative effect mechanism of non-submerged and submerged spur dikes under different impact scales were carried out further. The major work and conclusions of the thesis are as follows:(1) Based on the observed data from flume experiments, three turbulence models--standard k-? model, Reynolds stress model (RSM) and large eddy simulation (LES)-were selected, and the free surface boundary respectively employed two methods--rigid-lid assumption and VOF method (volume of fluid) in each model. The six kinds of mathematical models were applied to the simulation of flow field of non-submerged and submerged spur dike. Considering the consistent degree between computed results and observed data as well as the model computation time, the appropriate turbulence model and free surface boundary for the simulation of flow field of non-submerged and submerged spur dike were obtained under different research purposes.(2) According to the conclusions of appropriate mathematical models of non-submerged and submerged spur dikes and the actual computation condition, mathematical model contained in standard k-? model and rigid-lid assumption was utilized to simulate the flow field of non-submerged spur dike, and mathematical model combined by standard k-? model and VOF model was used to simulate the flow field of submerged spur dike. Based on above mathematical models, the flow fields of non-submerged and submerged spur dikes with different spacing were simulated and analyzed. Three classification criteria of impact scale of non-submerged double spur dikes were proposed, which respectively based on non-overlapping of large-scale vortex, similarity of sectional velocity of double spur dikes and velocity recovery of downstream dike, and their advantages, disadvantages and application scopes were analyzed qualitatively. Meanwhile, a classification criterion of impact scale of submerged double spur dikes based on velocity recovery of downstream dike was proposed, and its advantages, disadvantages and application scopes were gotten by qualitative analysis.(3) The influence factors of spacing threshold of double non-submerged and submerged spur dikes were obtained by dimensional analysis and the corresponding simulation conditions were drawn up respectively. The quantitative research of impact scale of double non-submerged and submerged spur dikes were carried out by means of numerical simulation. Three empirical formulas of spacing threshold of double non-submerged spur dikes under three classification criteria were obtained. The comparion of calculation results from three formulas showed that the results based on non-overlapping of large-scale vortex were smallest, while the results from the other two were with little difference. Mechanism analysis showed that the latter two had the same essence but different applied scope, and the former criterion was the necessary but not sufficient condition for the judgment of impact scale of non-submerged spur dikes. Likewise, the empirical formula of spacing threshold of double submerged spur dikes under the classification criterion was obtained. And the quantitative comparison of spacing thresholds between non-submerged and submerged spur dikes was carried out further.(4) Finally, based on the three empirical formulas of spacing threshold of double non-submerged spur dikes, the cumulative effect mechanism of flow structure, velocity and turbulence intensity of non-submerged spur dike group under three kinds of impact scales were explored respectively. Comparison study showed that the cumulative effects of each flow component of non-submerged spur dike group were different under different impact scales. With the increase of spacing between adjacent dikes, the cumulative effects of flow structure were the first to disappear, followed by flow velocity and turbulence intensity. Similarly, based on the empirical formula of spacing threshold of double submerged spur dikes, the cumulative variation of water level, flow velocity and turbulence intensity of submerged spur dike group under different impact scales were discussed respectively. It was found that with the accumulation of the number of spur dike, the cumulative variation of water level and flow velocity were inverse and the cumulative effects almost disappeared simultaneously, while turbulence intensity was still the most difficult to recover among the flow factors.