Experimental investigation of sediment transport around an elongated pier

Abstract

This work describes the experimental investigation of sediment transport, originated by the interaction between the flow around an elongated pier and an erodible bed of sediments. Sediment transport is strongly influenced by the horseshoe vortex that develops immediately upstream of the pier. The horseshoe vortex excavates the sediment bed originating a scour hole. As the sediments are suspended in the flow, they travel downstream and eventually sediment in regions of lower turbulence.

Experiments were conducted in a 0.4 m wide and 16.7 m long flume. The test section consisted of a vertical rectangular pier with round nose shapes, 4 cm wide and 8 cm long. The pier was mounted vertically in a box, 10 cm deep and 1.8 m long, filled in with uniform sand with a mean diameter of 0.376Â mm. For comparison purposes, the flow around the pier in case of a rigid flat bed with the same roughness was also studied. The flow rate was controlled by a gate valve and measured by an electromagnetic flow meter (ABB, model IDE41F). Water depth was controlled by a sluice gate at the downstream extremity of the flume.

Measurements were made using two-component Laser Doppler Anemometry (LDA). The light source was an Argon-ion Laser (Spectra-Physics, Model 177-G0232) and the optical system was a Dantec 60X41 FiberFlow, combined with an 85 mm probe together with a beam expander and 500 mm front lens, used in forward scatter mode. A colour separator (Dantec 55X35) split the collected light into 2 wavelengths before reaching the corresponding photomultipliers (Dantec 57X18). LDA data acquisition and processing were performed using BSA F60 Flow Analyser (Dantec). Simultaneous measurements of longitudinal (horizontal) and vertical velocities were made using the overlapped coincidence method.

The flow above the scour hole was studied for water depths of 5 and 15 cm and mean horizontal approach velocities between 0.17 and 0.30 ms-1. Results include horizontal and vertical velocity fields, velocities probability density functions and Reynolds stresses.