Local scour at bridge piers in overtopping flows

• Main Author: Ng, Xin Yi
• Other Authors: Chiew Yee Meng
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Civil engineering::Water resources
• Online Access: http://hdl.handle.net/10356/77892
• Institution: Nanyang Technological University

Numerous studies have been conducted to study the effects of scour due to the everchanging impacts of global warming, which result in rising sea levels. It is crucial for bridge engineers to estimate the scour protection measures as well as understand the scouring action under bridge inundation, which may result in the failure of bridge structures. Extensive research has been conducted in the past on pier scours in open-channel flow. However, there are comparatively fewer works dedicated to study overtopping flows as well as the orifice conditions. The objective of this experimental study is to determine the relationship between the local scour depth and water height relative to the pier in the orifice as well as overtopping conditions. The experiment was conducted in the hydraulics modelling laboratory, where various experimental runs were conducted to study the maximum scour depth at the equilibrium condition. The model was placed at various depths in the sediment recess while maintaining the approach water depth and flow rate in the experiment. Three different kinds of flow are identified in the study, namely open-channel flow, orifice flow and overtopping flow. Open-channel flow is the free-flow condition in which the water depth is below the bridge deck, as such, the flow does not encounter any interference from the bridge deck. Orifice flow is the condition in which the water depth is at the bridge deck level, hence inducing a pressurised flow. Overtopping flow is the condition in which the water depth is higher than the bridge deck, hence inundating the bridge deck. This results in splitting the appearance flow, one flowing below and the other above, the bridge deck. The experimental data collected over the course of this study shows that while both the orifice and overtopping flow conditions will result in a deeper scour depth as compared to the open-channel flow, it is obvious that the overtopping flow condition has the most detrimental impacts to scour depth when compared to the other two flow conditions. In overtopping flow conditions, it is observed that both the leading jet, the upstream flow upstream of the pier at the front region of the bridge deck, and trailing jet, the downstream flow after the pier in the back region of the bridge deck, contribute to an increase in pier scour depth. However, in the case of the orifice flow, the downflow caused by the pressurised leading jet is the main contributor. In summary, this experimental study shows that varying the height of the model (bridge deck and pier) with respect to the water depth can affect the maximum equilibrium scour depth. The leading and trailing width of the bridge deck was also found to contribute significantly to the equilibrium scour depth. Hence, it is recommended that the length of the bridge deck be varied for a more comprehensive study of the local scour in orifice and overtopping flows in the future.