Time Scale for Local Scour at Bridge Piers

Abstract

The temporal development of clear-water local scour depth at cylindrical bridge piers in uniform sand beds is considered. New data are presented and used to quantify the influence of flow duration on the depth of local scour. An equilibrium time scale (t*) is defined. The data show that both t* and the equilibrium scour depth (dse) are subject to similar influences of flow and sediment parameters, as might be expected because they are inherently interdependent. A method is given for determination of the time for development of dse for a given pier, sediment, and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole. The results show that the scour depth after 10% of the time to equilibrium is between about 50% and 80% of the equilibrium scour depth, depending on the approach flow velocity. of local scour d se is rapidly attained in live-bed conditions, but rather more slowly in clear-water conditions (Fig. 1). Clear- water scour occurs for mean flow velocities up to the threshold velocity for bed sediment entrainment, i.e., V # Vc, while live- bed scour occurs for V > Vc. The maximum equilibrium scour depth dse)max occurs at V = Vc. In armored cobble or cohesive sediment bed streams, multiple flood events may be required before the maximum clear-water scour is reached. This may take many years. The equilibrium scour depth in live-bed con- ditions fluctuates due to the effects of bed form migration. The dashed lines in Fig. 1 represent the temporal average scour depth under live-bed conditions. The diagram also shows the time taken, te, for the equilibrium scour depth to develop. The equilibrium time, te, is the focus of this paper. It increases rapidly with flow velocity under clear-water conditions, but then decreases rapidly for live-bed scour. Existing equations for depth of local scour at bridge piers give the equilibrium depth and are therefore conservative re- garding temporal effects. For the live-bed conditions that typ- ically pertain in floods, equilibrium scour depths are appro- priate. However, where clear-water scour conditions exist, the equilibrium depth of scour may be overly conservative. Peak flood flows may last only a number of hours or a few days in the field, and short floods have insufficient time to generate equilibrium depths. For example, bridge piers situated on the floodplain may be wet for periods of less than one day during a flood; typically, clear-water conditions pertain at such sites. The actual scour may be only a small fraction of the equilib- rium scour depth, which could take weeks to fully develop. Johnson and McCuen (1991) developed an analytical model to simulate the temporal process of local scour at piers. The model was applied to a hypothetical bridge pier using a gen- erated sequence of flood flows over a 75 year period, the in- dividual storms being of 24 h duration. At the end of the 75 year period, the scour depth was still increasing.