Experimental Study Of Non-uniform Sediment Movement On Bed Surface

In recent years, with the development of digital image processing technology, using high speed camera to get a lot of particles movement trajectories, and on the basis of the detail particles movement characteristics to establish the transport model based on physical process have become one of the trends to solve bed load problems. In this paper, a large flume bed load transport movement measurement system has been developed, bed load particles movement flume experiment under the condition of non-uniform sediment bed surface has been carried out, the coordinates and trajectories of particles are obtained by digital image processing technology, the movement parameters such as particles velocity and movement/wating time are studied. The main conclusions have been made as follows:(1) A green background flat board is set up on the top of funnel for collecting sediment in downstream of the flume, a digital camera is set up on the side of the flume, and sediment particles entering through the flat board would be recorded by the camera. By this way, real-time bed load sediment transport rate would be measured by countering particles from the video information. This method not only can provide real-time measurement data of bed load sediment transport rate, but also can be employed for further analysis such as particle size and transverse distribution of the bed load flux.(2) The solution shooting particles movement from the top of a large flume is put forward, and the specific system is developed, which mainly includes:a mobile vehicle, a piece of glass which can be moved up and down and with upwards bend edge on both sides upstream and downstream, a illumination system, a bubble broken screen and a shot hole on the top of glass, and so on. The results show that this device can effectively solve the problem that one can only get blurred pictures when shooting on the top of an open channel due to the fluctuation of free surface. This system provides a new approach to get lots of clear particles movement information synchronously.(3) Sediment particles are recognized under the condition of pure color background by edge detection method, mathematical morphology method and boundary search method, the results show that:Mathematical morphology method and boundary search method have good effect for coarse particles recognition but not for fine particles, and edge detection method has a better recognition for coarse and fine particles at the same time. Then the classic operators on edge detection method (Sobel, Prewitt, Roberts, Laplacian, Log) are compared, the result shows that Sobel operator is more suitable for sediment particle identification under the condition of pure color background, and the threshold value selection and particle recognition effect are also analyzed. Finally, the accurate information of sediment particles under the condition of pure color background are obtained by this method.(4) The sediment particles movement on bed surface is shot by a digital camera with speed of 50fps, there are a total of 3000 frames pictures in 60s for identification, which include 268 non-uniform sediment particles, nearly 10000 the particle coordinates data are obtained. Bed load particle movement characteristics are further analyzed with these data.(5) Bed load particles motion state transform frequently, during the study period, more than 200 particles are with at least two state transitions, namely "rest-motion-rest-motion", the maximum state transition times is up to 9 times. Due to the particles motion are caused by the water flow action at the bottom, the frequent change of particles motion state indicates that turbulence structure changes dramatically along the track of the movement of particles.(6)Bed load particle trajectories are greatly influenced by large particles on bed surface, there is an obvious phenomenon that small particles move around large particles, there are some "passage ways" for small particles to move on bed surface. However, some small particles are captured by the wake flow of larger particles, and which are confined to the wake zone of large particles.(7) The movement distance of bed load which have the complete movement process including "rest-motion-rest" are analyzed. For the measured non-uniform particles, the minimum value of the single movement distance is less than 1d, the maximum value is up to 166d,91% particles movement distance is less than 20d. Compared with the existing uniform sediment results, the single movement distance of non-uniform bed load particles is smaller, but the probability density distribution is similar to uniform particles, which also accord with or is close to Gamma distribution. The further analysis shows that shields parameters and the single movement distance of particles has no obvious correlation.(8) The continuous movement of particles, namely go-stop time of moving particles that occur state transition frequently are analyzed, and the results show that the rest time of particles is larger than the motion time, which indicates when most of particles complete one movement, which need a certain waiting time and then begin to move again, and the waiting time is greater than its movement time.(9) Under the condition of non-uniform sediment bed surface, the probability density distribution of longitudinal velocity and transverse velocity with bed load follows exponential-like form of distribution, which agrees with the results of uniform sediment, whereas the longitudinal velocity is much smaller than uniform sediment of existing results, either taking grain Reynolds number or the shields parameters for comparison. The results further reveal that large particles at the bottom on bed surface have great influence to movement of water and sediment near the bed.(10) Under the condition of non-uniform sediment bed surface, the instantaneous transport rate of bed load shows strong fluctuation characteristics, the analysis results of the moving average sediment discharge show that, when the time scale is more than 10 seconds, bed load sediment transport rate tends to be unchanged during the study period. Ballio’s instantaneous bed load sediment transport rate method is expanded to the non-uniform sediment, then compared and analyzed by the experimental data. The analysis area will be further divided, and the different regional transport rate is analyzed, the results show that, with the effect of coarse particles at the bottom, sediment transport on bed surface under the condition of non-uniform sediment shows obvious spatial non-uniformity, sediment flux in different areas is significantly different.