Date of Award

2017

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Civil Engineering (MS)

Administrative Home Department

Department of Civil and Environmental Engineering

Advisor 1

Brian Barkdoll

Committee Member 1

Zhen Liu

Committee Member 2

Casey Huckins

Abstract

Local scour is a major cause of concern for the stability of bridge piers and the safety of the public using the bridges. Structural countermeasures and flow-altering devices have been developed to deal with local scour. Many armoring structural countermeasures are very effective and efficient in clear water conditions but are susceptible to moving bed forms and leaching. The flow-altering devices are effective in both clear water conditions and moving bed forms, but are prone to clogging, less efficient in changing flow direction, etc. This study aims to reduce the local scour by injecting air in order to reduce the effect of the scour-inducing downward roller flow pattern by harnessing the buoyancy of the injected air bubbles.

A clear-water scour experimental study is conducted in a rectangular channel with a pier embedded in a sand bed. Air is injected through a horizontal diffuser pipe wound around the upstream half of the cylindrical bridge pier. Measurements were taken for maximum local scour depth, water surface profile, rate of scour, centerline scour elevation and channel bed profile at the equilibrium condition. The ratio of velocity of air to the velocity of water (Va/Vw) was chosen as the non-dimensional variable for this study and was considered the most suitable representation and for ease of scalability. The air injection reduced the local scour at bridge piers in equilibrium conditions by nearly 35%. This scour reduction is caused by a change in flow patterns around the bridge piers with the injection of air bubbles. A dye test was performed for the base case (without air injection) and the optimal case (Va/Vw = 57.1) to observe the change in flow behavior around the pier.