Preliminary Study On The Similarity Of Model Sand Wave

Sand wave, which often exists in most natural rivers, is the coupling effects of the water flow, sediment and riverbed and also the primary form of the bed-load sediment transport. The similarity of sand wave in physical river model is the vital preliminary of not only the interaction similarity of the river flow, sediment and riverbed but also the riverbed evolution similarity, which is especially important for the physical river model in erosion state. Sand wave similarity includes similarities of geometry, kinematics and sand wave compositions. However, researches on sand wave similarity are still not systematical and comprehensive at present. Therefore, the author studies sand wave similarity under different conditions and the influence of particle diameter scales of the model sand, weight ration, and model distortion on sand wave similarity, which is of great theoretical and practical significance for a better understanding of sand wave transport characteristics and the improvements of moveable-bed river model experiment theories and methods.In this thesis, the author does systematical research on sand wave geometry similarity by using field data, theoretical analysis and flume experiment ways. The diameter variations of sand wave sediment particles in natural rivers are analyzed. The differences of sediment diameter scales, sediment-moving incipient velocity and sediment fall velocity caused by suspension similarity, incipient motion similarity or other similarity laws under different conditions are also quantitatively studied. Experiments of different sand choices, geometry distortion, weight rations of model sand and prototype sand are did to study their effects on sand wave similarity. What is more, based on experiment results, the author establishes the formulations of sand wave height and length. The main conclusions are as follows:(1)The minimum particle diameter of the bed material load, which is the threshold particle diameter of bed material load and wash load and also the main composition of sand wave, shows a complicated spatiotemporal pattern. On the one hand, erosion in the downstream of the dams increases the threshold particle diameter which shows fluctuation by time. On the other hand, threshold particle diameter decreases in the downstream of dams, but not reduces unanimously. In some reaches, threshold particle diameter changes throughout the year. Threshold particle diameter in the non-flood season can be even larger than that in the flood season. Therefore, in order to simulate bed material load accurately, it is imperative to take a full consideration of the spatiotemporal changes of the threshold particle diameter.(2)There exists deviation in particle diameter, threshold and settling velocity of sediment when prior consideration is given to such different similarity laws as suspension, incipient motion and threshold-suspension similarity. The deviation varies according to the geometric scale, effective unit weight scale and distortion. In normal models, the scale can be unified with different similarity laws when using light materials as model sediment, in which case both suspension and incipient motion similarities are satisfied. When using heavy model, it's impossible to get the same scale when different similarity laws are taken into consideration such as suspension, incipient motion and threshold-suspension similarity.(3)In a normal model with the same shape and density between model sand and natural sand, sand wave geometry is similar when particles are conformed to incipient motion similarity law. The average deviation of the height, length and width of wave is within 6%. When particles are conformed to suspension similarity law, model waves are greater as a whole which reflects dissimilarity. When both incipient motion similarity and suspension similarity are taken into consideration, the sand wave similarity still needs improvement with medium deviation. Therefore, when it aims to reflect the fluvial process dominated by erosion, incipient motion similarity should be the primary governing criterion in choosing the appropriate model sand.(4) In a normal model designed with different geometric scales, if model sand subject to incipient motion similarity shares the same unit weight and geometry with natural sediment, sand waves agree with the similarity law in height, length and width compared with natural sand waves. The geometric scale distortion and unit weight have little impact on the similarity of sand waves in terms of the height other than the length. In an abnormal model whose model material is subject to suspension similarity, it's better to take heavy sands as model particles to reduce the deviation of sand wave height. The deviation of the sand wave geometry will increase with the light sand.(5)After analyzing the key elements on sand waves, particle dimensionless parameter D., relative smoothness h/D and relative sediment transport capacity Tt are introduced to derive the non-dimensional equations about sand wave height and length. According to the adequate test data, empirical equation of wave height and length is established. After being verified with tests conducted by other institutions, the equation with the parameters D,, h/D and T, is proved to have better precision.