Research On Flow Characteristics In Compound Channels With Vegetated Floodplains

Natural rivers are usually characterized by compound channels with main channelsand the floodplains. Due to often no water flow on the floodplains, the floodplains arecommonly covered with vegetation such as trees, shrubs and grass. The vegetation hasthe important effect in the field of water and soil conservation and ecologicalrehabilitation. However, vegetation on the floodplain can significantly alter the structureof the flow. The hydraulic resistance due to vegetation leads to a reduction of capacityof flood, which in turn leads to complexity of the water flow in the river. In the thesis,the flow characteristics in open compound channels with emerged and submergedvegetated floodplains are studied, based on combination of research methods ofexperimental investigation, theory analysis and numerical simulation. The maincontents of the thesis are as follows:Based on large flume experiment in the Hydraulic Laboratory, the lateral andvertical distributions of the flow velocity in the open compound channels withnon-vegetated, emerged and submerged vegetated floodplains are studied.Lateral distributions of depth-averaged velocity in the open compound channelswith emerged and submerged vegetated floodplains are analyzed, based on an analyticalsolution to the depth-integrated Reynolds-Averaged Navier-Stokes equation with a termincluded to account for the effects of vegetation.The4calculation examples are given for verifying the proposed analytical modelwhich presents the analytical solution for lateral distributions of depth-averagedvelocity in open compound channels with emerged and submerged vegetatedfloodplains. The4calculation examples are: rectangular channel with submergedvegetated corner, compound channel with submerged vegetated floodplain, two-stagerectangular channel with emergent vegetated floodplain, and two-stage rectangularchannel with submerged vegetated floodplain. The analytical solutions of the three casesare compared with computational or experimental data. The corresponding analyticaldepth-averaged velocity distributions show good agreement with the experimental andcomputational data.Based on some phenomena in the experiments, the analysis for the vertical distribution of flow velocity is presented. Mainly due to the influence of the secondaryflow, the largest velocity value is not occued on the water surface, which is reffered asdip-phenomenon. In order to verify the reliability of the experimental data in presentobservations, the previous flume experiments by others for simple channel, compoundchannel and compound channel with vegetated floodplain are introduced to compare.The compared results are: under the same conditions, present experiment is similar tothe previous experiments by others, which have the same phenomena. Furthermore, thepresent flume experiment which is different from the previous experiments, presents thenew results. The results can provide the basis for the further theoretical analysis.According to the programming of Juhe River in Pinggu District of Beijing,different vegetations may be planted on the floodplain in order to protect slope andfloodplain, which lead to different roughness in the floodplain. The main velocity fieldin the River is simulated in order to unclose the characteristics of water flow under theinfluence of vegetation in meandering compound channel.