Physical Model Study of the Fairfax Water Off-Shore Intake: New Modifications and the Study of the Flow Patterns around the Sand Barrier

Abstract

To withdraw better water quality from the Potomac River for a water treatment plant during flow conditions under 20,000 cfs, Fairfax Water in VA had an off-shore river intake built in 2004. The intake was comprised of a 36-ft diameter hexagon concrete structure sitting on the bed of the river with a sand barrier around it to minimize the sand withdrawal. The sand barrier is a nonagon concrete structure with approximately a 54 ft diameter. Since the intake started operating in late 2004, it has exhibited a significant amount of sand withdrawal. In 2007, a 1:10 scale model of the intake was built at the St. Anthony Falls Laboratory (SAFL) to study the causes of sand withdrawal, to modify the structure and to minimize the amount of sand withdrawal. The modifications conducted on the sand barrier resulted in a new geometry for the sand barrier, which was comprised of two wingwalls at the downstream end, a nose wall at the upstream end and a crown over the entire structure. The results of the tests conducted on this geometry showed that by building the new geometry for the sand barrier, the prototype bedload withdrawal should decrease by more than 60%. To further reduce the sediment withdrawal, a new study was required which is the subject of the current report. In this study, three more modifications were made to the sand barrier and tested. A total of 11 tests were conducted on the new geometries and the original geometry. In addition, the flow patterns around the original geometry of the sand barrier as well as the modified geometry were studied to determine if any further modification could decrease the bedload withdrawal. By studying the flow patterns and turbulence intensities around the structure, the final geometry was designed to include the crown, wingwalls and nose wall of the sixth geometry with a screen mounted on the half height of the walls around the structure. The screen was designed to prevent the eddies from dispersing the resuspended sediments in the water column along the height of the wall. The results of the tests conducted on this geometry suggest that the prototype bedload withdrawal should decrease by approximately 70%.