The Research Of Pier Scour Under Tidal Flow

Data indicated that most of the collapse of bridge should owe to severe floodscour or insufficient burial depth. With the development of economy and populationin east coast regions, highway extends to tidal waterways and offshore harbors. At thesame time, flood and tide always endanger the estuary region, so the practical projectsmake a higher expectation of the research about pier scour in tidal flow.The progress of pier scour can be divided into three parts: scour of natureevolution, general scour and local scour. Local scour is dependent on water flow, pierdesign, sand and riverbed. Most of existing bridges appear to turn to physical modelto predict the scour depth, as a result, it leaves much to be desired about the generalformula of scour depth under tidal flow. It is widely believed that the scour depthunder tidal flow can take example by that of steady flow, and research for mechanismabout tidal scour is under further study.HMMC calculating measure and control system of hydraulic model is used in thisphysical model test. By referring silt parameter and tidal parameter of Pearl RiverValley and Yangtze River Valley, three kinds of sediment grain size and three kindsof water flow process are chosen. Four kinds of pier types are chosen, which iscylinder, rectangle, streamline and clump of piles. The scour pit and changes of depthare recorded by photographing and measuring, which arrive at a conclusion of the pierscour process in tidal flow. By comparing the scour depth in tidal current and that inconstant flow, the reduction factors of scour under different conditions were obtained,which vary from0.75to0.92. The maximum velocity of rising tide and tide ebb issuggested to replace constant flow velocity, which used to be a major factor informula. Because of the decrease of effective acting velocity and effective acting time,effective sediment decreases, thus extending the total scour process in tidal flow. It issuggested that when the particle size of sand is smaller than0.1millimeter, thecoefficient of riverbed grain the result of scour depth is less than4, as a result,deviation occurs between the result of formula calculation and that of the test. Thescour depth largen as soon as the sediment median size decrease, which contradictfrom theoretical analysis and model test, which recommend improvement for theformula in standard. The simulation computational software FLUENT is used to verify the result ofphysical model experiment. It is the standard k-ε turbulent model that simulates flowfields around cylinder pier, which obtains the shear stress of flow fields aroundcylinder pier. A UDF mode is developed to calculate the current changes of riverbedelevation. The comparison result indicates that the model experiment result arereasonable.