Hydraulic Characteristics And Scour Mechanism Of Flexible Beach Protection Structure

Flexible mattress beach protection structure is one of the most widely used beach protection structures in the middle and lower reaches of the Yangtze River.It has the advantages of simple structure and construction technology and low cost.What’s more,the flexible mattress beach protection structure can adapt to certain riverbed deformation,and does not occupy water area.However,since the completion of the Three Gorges Project,due to the operation of the reservoir,the water and sediment conditions in the downstream of the dam have changed greatly,and the middle reaches of the Yangtze River are under the trend of long period of time and long distance of erosion.Many of the beach protection projects have been deformed or even damaged under scouring,which seriously affects the improvement effect of beach protection projects.In view of this serious problem,many anti-scouring measures have been proposed,among which the embedded treatment of the edge of the mattress is a simple and effective anti-scouring method.The edge of the mattress is embedded below the bed surface in advance to adapt to the riverbed after scouring and deformation and reduce the damage caused by the soft row in the scouring and deformation of the riverbed.Many problems are still lack of scientific explanation,such as how to determine the embedded depth,embedded row physical bear how much scour have no deformation in order to make sure the soft mattress,embedded software in what type water flow under the condition of losing impingement effect,loss of impingement effect after the deformation of soft mattress of how to develop,such as by further studies are needed to answer.Based on the analysis of the research progress,the hydraulic characteristics of the embedded soft drains under the Three Gorges Reservoir dispatching and the scouring mechanism at different stages before and after the deformation of the soft drains are studied in this paper supported by key research and development project.The main conclusions are as follows:（1）Based on the daily runoff data of Hankou Hydrological Station in the middle reaches of the Yangtze River from 1980 to 2015,the variation of hydrological characteristics before and after the operation of the Three Gorges Reservoir from two aspects of interannual variation and annual distribution was summarized,and the effect of the operation scheme of the Three Gorges Reservoir to the uniform distribution of runoff in the middle reaches of the Yangtze River was illustrated.A stochastic model of the average daily flow for small samples was established based on the support vector machine（SVM）classification method and the Copula function,and the stochastic simulation of the average daily flow process under insufficient hydrological samples was realized.The stochastic model was used to simulate the average daily flow process of Hankou Hydrological Station after the Three Gorges Reservoir was filled with 175m water lever.（2）The redistribution of velocity on the reverse slope was proved,a distribution model of velocity on the vertical line was established,and a formula of vertical average velocity was derived.The ratio y₀/h of the maximum velocity position from the river bottom to the water depth is distributed in the range of（0.7～1.0）due to the influence of the slope of the beach.The maximum velocity on the vertical line of the beach ridge is still located at the water surface.The maximum vertical velocity at the beach surface of gentle slope is about 0.7 times the water depth above the river bottom.The maximum vertical velocity at the junction of steep slope and beach trough is about 0.75～0.80times above the river bottom.Combined with the model test data,the fitting optimal entropy structure was determined,and the parameters of the model were defined.（3）Affected by the movement of sand waves on the bed surface,the scour pit moves downstream.The maximum scour depth of a single scour pit is positively proportional to the Froude number and relative velocity,which is similar to the response of sand wave height to hydraulic factors.One well line edge,U/U_C∈[3.1,4.6],Fr∈[0.21,0.29];in the water side,U/U_C∈[3.2,4.2],Fr∈[0.25,0.33].Based on the principle that the maximum scour depth under the general scour of the row is not more than the embedded depth,the embedded depth formula of the soft row is established.（4）The triggering mechanism of destructive scour was defined,and the critical conditions for triggering destructive scour were established respectively from the two aspects of flow Froude number and relative velocity.Using the Froude number as an indicator,2.4Fr（10）0.6h/D₅₀×10^-3（28）1 can be used as a critical condition for general scour and destructive scour.The top of the straight line indicates that the edge of the row is damaged by scouring,and the soft row will be deformed.The straight line below indicates that there is no destructive scour,and the soft plate will not be deformed.Based on the relative velocity index of edge row,U/U _c=4.7can be used as a trigger condition for destructive scour.Under destructive scour,the relationship between relative scour depth h_s/h and Froude number can be expressed by a power function,and the relationship between relative scour depth h_s/h and relative velocity U/U_C or effective velocity ratio（U-U_C）/ωcan also be expressed by a power function.A prediction formula for the scour depth of the lower edge of destructive scour was established.Under the condition of destructive scour,the slope of the stable scour pit is equal to the Angle of repose of sediment particles under water.Based on this,a method for predicting the width of the deformation sag of soft row has been established.