Jet scour on gravel bed

• Main Author: Chiang, Jia Jun
• Other Authors: Lim Siow Yong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/136737
• Institution: Nanyang Technological University

Local scouring is a natural phenomenon by the action of excessive excavation of sands and gravels from the bed by the flowing water. Scouring contributes to most of the failure of offshore structures in the world and are responsible for structure instability of the man-made hydraulic structures. Hence, it is important to provide appropriate safety factor for the stability of the hydraulic structure. This study investigated the effect of tailwater depth on local scour on gravel bed with a median sediment size of 5.27 mm caused by horizontal circular wall jet under submerged condition by means of laboratory experiments. The study confirmed that the tailwater water depths (ranges from 190 mm to 70 mm or with tailwater depth ratio, H/do, from 7.6 to 2.8) showed some effects on the resulting scour depths. The experimental results were compared to the equation proposed by Tan (2018) based on a modified densimetric Froude number, Fo,d84. The results show that Tan’s proposed equation (dse/Rh = 1.22 Fo,d84) agrees reasonably well with the current gravel bed data. Based on Zhao et. al’s (2019) viewpoint on the importance of the sediment size parameter d50/Rh on jet scour, and the fact that the tailwater may have a certain influence on the scour, the author attempts to include the tailwater parameter, H/Rh and the sediment size parameter d50/Rh to Tan’s (2018) equation. To this end, the current experimental data, together with 404 past jet scour data compiled by Tan (2018) were used. The regression analysis with Fo,d84 as the dominant parameter and the additional d50/Rh and H/Rh produces an equation with a correlation coefficient of R2 of 0.861 which reflects that the effects of tailwater and the sediment size parameters are also factors to be considered in the prediction of the scour depth caused by 3-D jets.