Temporal variation of local scour at complex rectangular pier

Due to geotechnical and financial reasons, actual piers are built on foundation, while there is limited number of study available about effect of the foundation on the local scour. Overestimation of the scour depth results in designing a deep foundation level and thus leads to an uneconomical design of bridges. However, the underestimation of scour depth would result in shallow foundation and providing a chance to expose the foundation to the flow. It is definitely dangerous for bridge safety. Previously, extensive research has been carried out about local scour around uniform piers. However, only some studies are available in literature to predict the time variation of local scour at complex piers.
In this study, time variation of local scour have been experimentally investigated around uniform and non-uniform rectangular piers under clear water conditions. The local scour is investigated with variation of foundation level (Z), pier dimension and foundation dimension. The results are compared at different foundation level.

A flume with rectangular cross section and the dimension of 12.0 m long, 0.4 m wide, and 0.6 m deep was chosen for all experiments. Three uniform and non-uniform rectangular piers were chosen for tests (Table 1). Uniform sediment with d50 = 0.70 mm maintain the clear water conditions, the flow velocity was set close to the critical velocity of sediment (U/Uc between 0.94 and 0.99).

The trend of local scour at non-uniform pier is time dependent. To investigate the effects of foundation level (Z) on temporal variation of local scour around pier, four levels of 0.0, 1.0, 2.0 .3.0 cm were chosen for Z. The experimental data and flow condition are given in Table 1. In this table, P and FP were selected for uniform and non-uniform piers, respectively. The scour depth develops to top of foundation quickly, and then the foundation postpones the scour development for a certain time (lag –time). It was observed that during of lag-time, the scour hole is slightly extended in parallel to and in front of the pier. In addition, development of local scour in parallel to pier (in the flow direction) is faster than those in front of abutment (upstream). The scour hole in the foundation nose is enlarged in the area, and it is somewhat deeper than other parts at the upstream side. The deepest depth at the upstream of non-uniform pier gradually develops around the sides of the foundation to create a shallow groove parallel to the foundation. Subsequently, the depth of the scour ahead of the foundation is more increased due to the formation of a vortex at upstream of the foundation. This vortex enlarges and deepens the scour at the corner of the foundation, and the sediments besides of the foundation are carried out to downstream of the abutment. Although, the foundation postpones development of scour depth firstly, but if the foundation exposes to the scour hole, the vortex in front of exposed foundation increases the scour depth. The lag-time (latency) directly depends on pier width (L), foundation width (Lf) and level of foundation under the sediment bed (Z). The rate of sediment transport decreases with increasing the scour hole dimension, and it will be stopped approximately close to the equilibrium scour depth. The reduction of scour depth due to lag –time is very useful to prevent the failure of bridge especially in the flood events that bridges are the main structures in transportation. Generally, the peak of flood may not be long-lasting to develop equilibrium scour depth and the flood may be stopped within lag-time. Therefore, the lag-time postpones the maximum scour depth and provides an opportunity to repair the bridge foundation after the flood events. Generally, For 0<Z/L1.2,the local scour increasing with increasing foundation level (Z) and for Z≥2.4L the local scour at compound pier is similar to uniform pier.

In this study, temporal variation of local scour at non-uniform piers is investigated experimentally under the clear water conditions. The non-uniform piers were included a rectangular pier founded on a larger rectangular pier. In all experiments, the scour depth is developed to top of foundation quickly, and then the foundation postpones the scour development for a certain time (lag–time). Duration of lag-time is depended on the dimension of pier, foundation size and the foundation level. A comparison between the uniform and non-uniform piers indicated that the trend of scour depth at non-uniform and uniform piers is similar to each other. This study highlights that a proper design of foundation level decreases the scour depth and increases the duration of scouring. Furthermore, the lag-time provides an opportunity to repair the bridge foundation after the flood events.