Laboratory Investigation of the Effect of Air Injection and Trapezoidal Collar on Reducing Local Scouring around Bridge Pier

Abstract

Air jet system (AJS) models are assessed on the upstream face of a cylindrical pier at six depths. The system can reduce scouring by reducing the upstream vertical velocity of the pier and the downflow strength. The efficiency of the AJS against local scouring depends on the air outlet nozzle placement depth; the greatest scour reduction of 29% occurs when the air outlet nozzle is installed at the lowest depth (exactly on the bed). Various trapezoidal collar (TC) shapes and sizes are assessed. The collar with a length three times the pier diameter and a width twice the pier diameter allows a 59% reduction in scouring. There is no delay in onset of pier scouring for the collar models with the same width as the pier diameter. The combined air jet system–trapezoidal collar (AJS-TC) model reduces the local scouring rate by 82%. The primary capabilities of AJS-TC models are energizing the boundary layer, encountering the downflow, and direct bed coverage. In assessing the turbulence intensity profiles, it is revealed that: (1) the coherent air jet reduces the turbulence intensity; and (2) the discrete air bubbles increase the turbulence intensity rate.