Time-Averaged Turbulent Velocity Flow Field through the Various Bridge Contractions during Large Flooding

Kwang Yoon,Seung Hong,Seung Lee

doi:10.3390/w11010143

Abstract

Extreme rainfall events, larger than 500-year floods, have produced a large number of flooding events in the land and also close to the shore, and have resulted in massive destruction of hydraulic infrastructures because of scour. In light of climate change, this trend is likely to continue in the future and thus, resilience, security and sustainability of hydraulic infrastructures has become an interesting topic for hydraulic engineering stakeholders. In this study, a physical model experiment with a geometric similarity of the bridge embankments, abutments, and bridge deck as well as river bathymetry was conducted in a laboratory flume. Flow conditions were utilized to get submerged orifice flow and overtopping flow in the bridge section in order to simulate extreme hydrologic flow conditions. Point velocities of the bridge section were measured in sufficient details and the time-averaged velocity flow field were plotted to obtain better understandings of scour and sediment transport under high flow conditions. The laboratory study concluded that existing lateral flow contraction as well as vertical flow contraction resulted in a unique flow field through the bridge and the shape of velocity profile being “fuller”, thereby increasing the velocity gradients close to the bed and subsequently resulting in a higher rate of bed sediment transport. The relationships between the velocity gradients measured close to the bed and the degree of flow contraction through the bridge are suggested. Furthermore, based on the location of maximum scour corresponding to the measured velocity flow field, the classification of scour conditions, long setback abutment scour and short setback abutment scour, are also suggested.

Highlights

Bridge foundation failure occurs due to the processes of (1) contraction scour-scouring across the entire channel due to the flow contraction caused by the bridge opening and deflection of floodplain flow into the main channel and (2) local scour at the base of piers and abutments caused by local flow contraction, down flow, and formation of a horseshoe vortex that wraps around the obstructions
Initial hydraulic parameters measured in fixed bed have been summarized in Table 1: Q is the total discharge; Vf1 /Vfc1 and Vm1 /Vmc1 is approach flow intensity in the floodplain and in the main channel, respectively, where Vf1 and Vm1 is approach flow velocity and Vfc1 and Vmc1 is approach flow critical velocity calculated by Keulegan’s equation; yf1 and ym1 is the water depth of floodplain and main channel in the approach section, respectively; qf2 /qf1 and qm2 /qm1 is unit discharge contraction ratio in the floodplain and main channel, respectively; W is the setback distance; Lm is the traverse distance from the toe of the abutment to the maximum scour hole depth
Recent extreme rainfall events associated with climate change can often result in submerged orifice flow with or without overtopping flow, in which the flow field through the bridge is more complex because of the simultaneous existence of local turbulence around the base of the structure as well as vertical flow contraction in addition to the lateral flow contraction

Summary

Introduction

Bridge foundation failure occurs due to the processes of (1) contraction scour-scouring across the entire channel due to the flow contraction caused by the bridge opening and deflection of floodplain flow into the main channel and (2) local scour at the base of piers and abutments caused by local flow contraction, down flow, and formation of a horseshoe vortex that wraps around the obstructions. The contraction scour and the local scour (pier scour and abutment scour) have been considered as two separate types of scour caused by different processes. The higher velocity resulted in increased erosive force, so more bed material is removed from the contracted section. In addition to the higher velocity due to the flow contraction, local flow structures associated with the base of the abutment result in additional erosion around the abutment

Methods

Results

Conclusion