Effect of tailwater depth and apron length on scour downstream of a weir

• Main Author: Ei Thandar Aung
• Other Authors: Lim Siow Yong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/168079
• Institution: Nanyang Technological University

The study is carried out to research on the scouring downstream of an overflowing weir. The scouring is caused by the movement of sediments due to the impinging jet. The focus of the study is the equilibrium maximum scour depth as the depth of scouring could undermine the stability of hydraulic structures. To investigate the flow variables that affect the equilibrium scour condition, literature on the key concepts and previous studies investigating scouring phenomenon was reviewed. This is followed by running 25 experiments on a flume with the dimensions of 8.0m by 0.3m by 0.6m. The experiments were done in the laboratory and included a filming set-up to record the run across the full duration. Since the process of scouring was filmed, the relationship between scour depth and time was briefly discussed using the scour depth data from the video. The experiments were designed to investigate the effect of varying tail water depths and apron lengths on scouring across different flowrates, with all other variables kept the same. The results of the experiments reveal that the equilibrium scour condition is based on those three variables. The scour profiles of suitable runs are examined to compare between variations made in the flow conditions. It was found that the following flow conditions result in an increase in equilibrium maximum scour depth: increase in flowrate, decrease in tailwater depth and decrease in apron length. For runs without apron, an equation was established to predict the equilibrium maximum scour depth given the flow conditions. The prediction from the equation has an error range of ±30% , which is considered sufficiently reliable for future usage. For runs with apron, the correlation between apron length and scour depth was explored. The investigation established that the apron installed must be of a minimum length such that the apron can act as protection for the erodible bed. Below the minimum length, the impinging jet strikes the erodible bed instead of the apron, making the apron redundant. Additionally, the scour depth is lowered by 90% when the ratio of apron length to weir height tends to 3.5.