The Flow Resistance Of Suspended Flexible Curtains And Its Effectiveness Of Sediment Accretion

Scour and deposition of sediments plays a significant role in rational exploitation and utilization of water resource.In recent decades,with the social development and economic take-off,human activities have played a strong impact on river sediment discharge and coastline shifting,so sediment accretion becomes more and more important to water conservancy and ocean engineering projects.The suspended flexible accretion curtain is a new type of sedimentation-promoting and scour-prevention device to harvest sediments.The bottom of the curtain is fastened by mooring cables to the river bed and keeps a certain distance away from the bed while the body suspends in the water freely and when water runs,the curtain inclines downstream wards with a certain angle,such that a low-velocity horizontal vortex(the recirculation zone)is formed in the leeside of the curtain.The incoming sediments would therefore be harvested in this low velocity zone,and gradually a sediment hump would yield.The curtains not only increase energy consumption by surface friction,but also form a low-velocity zone which further increases energy consumption.It is seen that the suspended flexible curtains for sediment accretion have the advantages of effective sediment accretion,little resistance to flood detention,simple installation and removal.Through flume experiments with fixed bed and movable bed,an investigation on the flow resistance characteristics of suspended flexible curtains and the effectiveness of sediment accretion were carried out in this thesis,which further proved that the effectiveness of sediment accretion of this new device.The estimation methods for resistance coefficient f and roughness n as well as drag coefficient C_D of the curtains are proposed respectively,which may provide a reference for practical application of the curtains.92 sets of experiment for fixed bed were conducted.The water surface and velocity distribution in vertical direction were measured,respectively.It shows that the curtain height and the spacing of curtains are main factors influencing the water-surface profile;while the vertical distribution of flow velocity between the adjacent flexible curtains has a phenomenon of“inflection point”that decreases first and then increases within the influencing height of curtain.The hydraulic gradients were obtained through water-surface curves.A study on the drag coefficient C_D of the curtain,Manning roughness n and Darcy-Weisbach resistance factor f was studied in this thesis.It was showed that the resistance of the flow in open channel with suspended flexible curtains is mainly produced by the curtains,which is related to the curtain surfaces and the flow separation at the leeside of the curtains;there exists a critical spacing for the installation of curtains where the mutual interference among curtains almost disappears,the leeside eddy can develop fully in recirculation zone behind each curtain which creates the largest energy dissipation.On the basis of theoretical analysis combined with experimental results,it is found that the curtain drag coefficient C_D varies between 1.60 and 2.05in turbulent flow conditions;The Manning roughness n is mainly related to the relative height and arrangement density of SFCG,while the Darcy-Weisbach friction factor f also depends on the curtain Reynolds number.Formulae were proposed for the estimation of n and f,respectively.8 tests under movable bed conditions were conducted.The influence of discharge,water depth,curtain height and spacing on the effectiveness of sediment accretion enhancement of suspended flexible curtains were studied.Each test lasted 12 hours and the bed elevation was measured at center line every 2 hours.The process of morphological change as well as the differences of the amount of sedimentation between inside and outside curtains were measured.Formulae were proposed for the estimation of average and maximum value of the sedimentation quantity which were obtained by regression.It was showed that the suspended flexible curtains play an effective sediment accretion while the size and form of sedimentation body behind the curtain was related to the relative height and the spacing of curtains.In order to gain the proper benefit and effectiveness of sediment accretion,the suitable height and the spacing of curtains should be carefully determined before the construction.