HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY Department of Civil & Structural Engineering CIVL 252-Hydraulics TA: Wong Minghoi Laboratory A-Flow and Scour around a Bridge Pier Spring of 2002 Laboratory A: Flow and Scour around a Bridge Pier 1. Introduction Local scour is a site specific phenomenon that a loose bed channel drops in bed level in the region where hydraulic structures exist. The aim of this experiment is to study erosion of bed material around a circular bridge pier. Formation of turbulence and high bed shear stresses around the bridge pier is illustrated in the experiment. The pattern and extent of bed erosion, i.e. maximum depth of scour around the pier, can be investigated. These details are of great importance for the safety of the structure supported by the pier. 2. Background The term “scour” implies enlargement of a flow section by removal of material comprising the boundary through the action of the fluid in motion (Laursen 1952). Scour occurs when the rate of sediment supply is less than the rate of sediment transport. For a given sediment, increase in shear stress results in increase of the rate of sediment transport. The presence of a bridge pier in a flowing fluid generates secondary circulation which is associated with the bending of streamlines. Flow pressure on the upstream side of the pier is high causes a horse-shoe vortex surrounding the pier. The horse-shoe vortex generates a strong shear stress which causes scour around boundary of the pier. At the levels not influenced by the scour effects, lines of “vortex street” are generated behind the pier. The well-known “Karmon Vortex” can be observed at low Reynolds numbers ranging from 40 to 200. The local scour is a complex problem, there are numbers of formulae developed based on laboratory data for prediction of local scour around bridge piers. Laursen (1962) developed a formula in which the local scour depth is an implicit variable. The diameters of the sediment used in the study were in the range between 0.46 and 2.2mm and the formula takes the form: 1 ) 1 5 . 111 ( 5 . 5 7 . 1 0 0 0 − + = DD DD Db s s (1) where b is the pier width; 0 D is the mean depth of flow upstream of pier; and s D is the depth of scour below mean bed elevation. Shen et al. (1969) proposed that the scour depth is function of Froude number. The suggested formula for prediction of scour depth is: (medium diameter of sediment: 0.16-0.68mm) 3 /1 0 3 /2 0 ) ( 4 . 3 b D F b Ds = (2) where 2 /1 0 0 0 ) /(gD U F = is the Froude number; g is the gravitational acceleration; and 0 U is the mean velocity upstream of pier. In 1975, the Colorado State University developed a formula for prediction of scour depth around circular piers. Similar to equation 2, the scour depth is function of Froude number: 43 . 0 0 65 . 0 0 0 ) 5 . 111 ( * 0 . 2 F Db DDs = (3) Figure 1 The Colorado State University formula for scour depths of circular piers. 3. Apparatus The apparatus for the experiment of scour around bridge pier consists of, 1)an inlet tank; 2) a long tilting flume bed with sediment transport channel; 3) an over-shot weir at discharge end; 4) a manual jacking system; 5) a recirculating pump; 6) a collecting/settling tank; 7) a fabricated frame: 8) a drain tap; 9) an earth leakage breaker; and 10) a vertical bridge pier. Details are shown in the following figure: Figure 2 The apparatus for scour around bridge pier 4. Procedure 1. place sand (grade size: 0.1 to 0.3mm) in the flume to a uniform depth, level with the top of the overspill weir. 2. Tamp down the sand surface so that it is uniformly packed, lightly compacted and as flat as possible. 3. Fill the catch tank with clean water up to the “water level” mark. 4. Place the cylinder to be used to simulate a bridge pier in the working reach of the flume. The cylinder must be inserted vertically with the bottom well buried in the sand, as a deep scour will form in this region. 5. Set the slope to a zero and switch on the pump on setting 1. 6. Increase the slope in very small increments until a scour hole begins to develop around the bridge pier but there is no initiation of bedload movement on the upstream side of the channel. Note the geometry of the scour hole. In particular, observe the formation of a “horse-shoe vortex” and associated scour trench on the upstream side of the pier. Lines of “vortex street” are also generated behind the pier. The organised structure of the vortices are more easy to be detected when the flow velocity is slow. The flow pattern can be visualized by the use of a coloured tracer introduced at various locations using a syringe or an eye-dropper. 7. Record the vortex systems formed at different locations. 8. Measure the dimensions of the scour hole when it becomes stable together with the dimensions of the channel and bridge pier, discharge flowrate, slope and mean approach flow depth. 9. Repeat (6) and (7) by increasing the slope and/or the discharge. 5. Results and Discussions Discuss the formation of the horse-shoe vortex and vortex street Draw and discuss the geometry of the scour hole in association with the vortex systems, e.g. describe the movement of the sediment eroded from the scour hole around the pier, etc. Compare the observed scour depth with the predictions from equations (1) to (3) and discuss (a) the discrepancies between the results; and (b) the formulation of the equations Discuss how the scour intensity will be different in case that (a) the slope or the discharge is increased; (b) sediment transport at the channel bed is initiated Discuss the changes of the geometry of the scour hole if the bed material is a cohesive sediment Discuss how the shape of the pier wil affect the scour Comment on any possible experimental errors