Experimental Study On The Local Scour Process Of Bridge Piers Under Unsteady Flow

Excessive scour of the foundation under flooding is the main cause of bridge water damage.The floods occurring in natural rivers are all unsteady flow.Under unsteady flow conditions,the development speed of the local scour of the bridge foundation changes with the strength of the water flow,and even the phenomenon of scour and backfill occurs in the falling water stage.However,the existing design theory and maintenance methods of bridge foundation scour are based on the local scour research results under steady water flow.In this paper,the local scour process of bridge piers under unsteady flow conditions is studied.Based on the summary of existing scour monitoring technology,the local scour monitoring system based on ultrasonic technology is developed.In the local scour experiments,the theoretical analysis of the local scour process is carried out using the dimensional analysis method.The main results and conclusions obtained as flows:Under steady flow conditions,the local scour depth of the pier changes with time in accordance with the law of logarithm.The local scour process develops rapidly in the initial period.The local scour depth can reach more than 80%of the equilibrium scour depth in the first 10%of the time;The results of local scour experiments under the effect of steady flow show that the temporal scour depth evolution is basically not affected by the relative water depth（D/H）and the relative sediment particle size(D/d₅₀),but the influence of relative flow velocity（V_C/V）on the development progress of scour depth is more significant.The experimental verification of the calculation formula of the local equilibrium scour depth of the pier and the development model of the scour depth with time under steady flow conditions shows that the revised 65-1 equation and 65-2 equation underestimated the local equilibrium scour depth of piers,however,HEC-18 equation,Melville eauation and Sheppard/Melville eauation overestimate the local equilibrium scour depth of the pier;the local scour depth development models of Melville,Chang,Oliveto overestimated the local balanced scour depth of the pier,at the same time,the Melville model and the oliveto model overestimate the initial rate of scour development,however,the Chang model underestimates the initial rate of scour development.The main characteristics of unsteady flow have a significant impact on the local scour process.When the flooding rising time and the flood peak flow are the same,the final scour depth increases with the increase of the base flow;when the base flow and the flood peak flow are the same,the final scour depth increases with the the flooding rising time;whether the flooding stage affects the maximum scouring depth of the pier depends on whether the water flow work is effective during the flooding stage.The influence of the unsteady flow characteristics on the local scour depth can be reasonably explained by the local scour energy or local scour work.In order to predict the local scour depth development process under the unsteady flow conditions,the effective work work of water flow is introduced to comprehensively measure the accumulation capacity of any unsteady water flow process on the sediment around the bridge pier,based on the derivation of the dimensionless effective work function model,the impact of sediment scour resistance and the geometric characteristics of the pier on the local scour process are further considered,based on the measured data,the method of determining the key coefficients of the model is improved to significantly improve the applicability and accuracy of the effective flow work model.The research results of this paper are helpful to answer the real process of local scour of bridge piers during the operation of the bridge,and provide theoretical support for the economic construction and safety management of the actual bridge across the river.