3D confined jet scour at a wall

• Main Author: Chee, Wei Hong
• Other Authors: Lim Siow Yong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/163594
• Institution: Nanyang Technological University

Scouring is a natural phenomenon of sediment transportation caused by the lifting and reposition of sediments by strong current or rapid flowing water. At berthing ports for large ships, the soil sediments located at the seabed near the toe of the port walls contribute towards the resisting force and moment to prevent geotechnical failure of these retaining walls, such as overturning or sliding failures. The exposed sediment bed is highly vulnerable to the thrusting force from ship’s propeller, which causes scouring at the toe of the wall. If left without scour control measures, the erosion may reach a point where it reduces the Factor of Safety (Over Design Factor) to below unity, hence compromising the stability. Thus, it is of paramount importance to gather constructive data regarding the various scour profiles generated by ships’ jet thrusters, to determine if a seabed scour protection is required to prevent failure of the retaining walls. The current study utilized a confined jet scour experimental setup. The effects of varying sediment sizes and jet velocities (to simulate jet thrusters from berthing ships) against an obstructive wall were investigated. On the other hand, jet diameter, offset height and lateral distance of water jet, and duration of water jet acting against the wall were kept at the critical value determined by Chow (2020). The existence of the wall caused the jet to deflect backward to the jet outlet. Beyond the jet outlet location, the scoured sediment bed was observed to be uneven likely due to the recirculating flow affected by the ridges formed. The wall also served as an obstruction for water to reflect off and into the sediment bed, thus generating further scour. The results showed that the spatial properties of the scour hole are larger with an increase in jet efflux velocity and a decrease in sediment size. The data collected from the current study were analysed and compared with the predictive formulas from Chow (2020) and against the unobstructed jet scour equations proposed by Tan (2018). The comparison showed that most of the current data were being overestimated by the equations by about 2 to 4 times. The present data and relevant data from previous studies were then compiled and analysed using dimensional analysis and multiple regression analysis and new empirical equations were proposed with a reasonably high correlation coefficients ranging from 0.91 to 0.97. These equations can be used to estimate the maximum scour hole dimensions caused by 3-D jets at a wall. However, more data need to be collected to test the general applicability of the proposed equations.