The Influence Of A Dredged Hole Evolution In Riverbed On The Scour Under Bridge

In recent years, a large amount of sand in rivers is excavated for the industrial purpose, therefore a dredged hole or holes are formed on the river bed. The hole usually evolves due to the flow dynamic role, which may do great damage to the bridges near the dredged site. As a result, it is significant to predict and assess the effect of the dredged hole on the scour under bridge to bridge safety. In presented paper, A one-dimensional flow dynamic model, coupled with the Exner equation, is adopted to describe the variation of the flow characteristics and the dredged hole evolution in riverbed. The model is verified by comparing with measured data and it is shown that the results are in good agreement. Modify the relevant conditions so as that the model is capable of simulating compression scour caused by the bridge piers. Combine with the characteristics of the above two aspects to research the adverse effects on the scour under bridge of evolution of the dredge hole. The following conclusions can be obtained.(1)The hole results in the temporary loss of the equilibrium among the flow, sediment and riverbed. And the erosion occurs in the upper reaches of the hole due to the increase of hydraulic gradient, deposition occurs in the hole and scour occurs due to the decrease in sediment concentration in the lower reaches of the hole. (2)Gradual decrease in the depth of upstream erosion is along with the increase in the distance from upstream of the hole until the depth of upstream scour becomes negligibly small at a certain location upstream from the hole. With the gradual silting of the hole, the hole is tend to migrate downstream. When the hole migrated to the mouth of the lower edge of the hole, the trend of downstream bed is converted into deposit.(3)The dredge hole makes a very substantial influence on the hydraulics and morphodynamics, but the influence is bound to decay and the ultimate state is naturally equilibrium. However, the bed elevation and the water surface elevation inevitably lower when the hole reaches the state of equilibrium.(4)The processes with different conditions of water discharge, size of the hole, sediment concentration and time effect are analyzed. It is shown that the variations of scour depth and deposition depth increase with the water discharge, size of the hole and the time.(5)When there are many holes in the riverbed, the bed deformation tends to be more intense than that under the condition of single hole. Deformation in bed between holes is the most intense so that the holes connect each other to form a bigger hole.(6)The compression scour of the bridge piers is related to the boundary setting of compression and compression ratio. Different boundaries produce different calculated results. As the compression ratio becomes smaller, which means the degree of compression is greater, the erosion is tend to be more intense.(7) If the dredge hole is located at downstream of bridge, the influence of a dredged hole evolution in riverbed on the scour under bridge is mainly caused by river incision caused by upstream erosion; If the dredge hole is located at upstream of bridge, the influence of a dredged hole evolution in riverbed on the scour under bridge is mainly caused by the migration downstream of the hole. The influences of the two conditions are both related to the distance between the hole and the bridge and scale of the hole. The degree of adverse effects becomes greater as the distance becomes smaller or the scale becomes greater.