Effect of tailwater depth and critical apron length on jet-flipping scour downstream of a submerged weir

• Main Author: Wong, Weiyu
• Other Authors: Lim Siow Yong
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Civil engineering
• Online Access: http://hdl.handle.net/10356/76401
• Institution: Nanyang Technological University

Weirs are low-head hydraulic structures that allow water to flow freely above the weir crest before plunging to the river bed which result in a local scouring phenomenon. Local scour downstream of a weir may cause damage or affect the stability of the weir resulting in failure of the structure. As such, many studies are being done to study the effects of different hydraulic parameters on scour downstream of a submerged weir. Man-made structure, such as an apron, is also introduced as an alternative solution to solve the problem of local scouring but some studies show that scour depth increases with increasing apron length. Therefore, the main objective of this Final Year Project is to 1) identify the critical apron length where jet flipping phenomenon will occur, and there is a reduction in the scour depth downstream of the submerged weir. 2) To investigate the effects of apron length and tailwater depth on scour development and the occurrence of jet flipping phenomenon as they are the more sensitive parameters that affect the flow regimes. The study was based on 15 experiment runs. The runs were conducted in a rectangular flume. A solid metal platform will be used to simulate a rigid apron, and an acrylic plate will be glued to the platform as the weir. The flow rate of 3L/s and weir height of 100mm were kept constant while the apron length and different tailwater depth of 105mm, 110mm and 115mm were adjusted. The apron was adjusted by removing the weir and glueing it at the location of interest along the apron. The experiments results show that the higher the tailwater depth, the smaller the scour depth and lower occurrence of jet-flipping. As the tailwater depth gets higher, the flow changes from bed jet to jet-flipping and then to surface jet. During the transition, the duration of bed jet will become shorter while the duration of surface jet become longer. With the introduced of an apron, the scour depth will increase as the apron length increases till it reaches a critical apron length. After the critical length, the scour depth will start to decrease. Before the apron length reaches its critical length, the effect of tailwater on the scour depth remains the same where the higher the tailwater depth, the smaller the scour depth. However, after the apron length exceeds its critical length, the higher the tailwater depth will have larger scour depth than those of lower tailwater depth. Lastly, a dimensionless analysis was done to generalised different parameters trends into one unified graph. This better represents the relationship between the different parameters and have general applicability.