Shallow water flow around an elongated bridge pier

Abstract

This work describes the free surface flow around an elongated vertical pier, i.e. rectangular pier with round noses. Measurements were made using two-component Laser Doppler Anemometry, with the main objective of studying the turbulent flow field around the pier. The flow was studied for a water depth equal to 5 cm, corresponding to a shallow water flow, and a ratio of 0.6 between the mean approach velocity (0.17 ms-1) and the estimated critical velocity of sand with a mean diameter of 0.376 mm. It was observed that the flow is completely developed as it reaches the pier. Measurements of horizontal and vertical velocities in the vicinity of the pier show the strong interaction between the structure and the flow. The flow decelerates as it approaches the pier, and is deflected towards the bottom. In free surface flows Reynolds shear stresses are usually negative, but in the present case as the flow approaches the pier Reynolds shear stresses become positive and increase. The wake of the pier is characterized by high turbulence intensities. Reverse horizontal velocities were observed downstream of the pier, until a distance equal to one pier transversal dimension.