ANALYSIS OF FLOW AROUND GROYNES WITH DIFFERENT HEAD SHAPES

Abstract

Groynes are essential river training structures widely employed to protect riverbanks, facilitate navigation, and enhance ecological diversity. While extensive research exists on conventional straight-type I-head groynes (IHGs), T-head groynes (THGs) have drawn attention for their ability to modify flow patterns and improve hydraulic performance. Despite this potential, comprehensive studies on THGs, particularly in series configurations, remain limited. This study investigates the scouring and deposition patterns as well as the flow field characteristics around single and multiple THGs under clear-water, unsubmerged conditions in a mobile bed channel. A total of twelve experiments were conducted, maintaining uniform bed material and consistent flow conditions to enable direct comparisons between configurations. Key design parameters, including groyne dimensions, constriction ratios, and spacing in series configurations, were systematically studied to understand their influence on hydraulic performance. Comparative analyses with corresponding IHG configurations were performed to evaluate the impact of groyne head shape on flow and scour dynamics. The results indicate that THGs outperform IHGs by effectively redistributing stress zones away from groyne structures and enhancing sediment deposition within embayments through improved ponding effects. While THG exhibited higher equilibrium scour depths near their tips when installed in isolation, they demonstrated superior bank protection by shifting high shear zones along the detached shear layer further from the bank, safeguarding approximately 1.5 times more bank length compared to IHGs. Turbulent flow dynamics, including the interaction of the horseshoe vortex system, detached shear layer, and downflow at the groyne face, were identified as primary drivers of scouring near the groyne tips. Based on these findings, a framework for cost-benefit analysis was developed to assess the costeffectiveness of different groyne head shapes. A comparative analysis revealed that while THGs offer superior hydraulic performance and sediment management, IHGs remain more costeffective for achieving adequate bank protection under comparable hydraulic conditions. In series configurations, both groyne types exhibited similar maximum scour depths at the upstream groyne. However, THG series demonstrated stronger horizontal vortex systems and greater flow separation at the tip of the first groyne, contributing to more complex flow dynamics. Both THG and IHG series effectively reduced streamwise velocities within the groyne fields, ii supporting bank protection. Maximum scour depths occurred near upstream groynes, while downstream groynes experienced minimal scour, emphasizing the need for reinforced foundation protection for upstream structures. The spacing and constriction ratios significantly influenced flow patterns, sediment dynamics, and groyne field stability. Increased constriction ratios generally resulted in greater scour depths, while reduced spacing between groynes stabilized ponding zones but intensified turbulent kinetic energy and bed shear stresses along the groyne faces. The interaction between detached shear layers and downstream horseshoe vortices was identified as a critical factor influencing scour depths at upstream groyne tips. Secondary flows with positive transverse velocities were more prominent in IHG series, while THG series exhibited smaller positive velocity pockets, which diminished with increased spacing, indicating stronger flow exchange in IHGs and more intricate flow dynamics in THGs. These findings provide valuable insights into the hydraulic behavior, scour mechanisms, and economic feasibility of THGs, contributing to the understanding required for informed design considerations in riverbank protection. Furthermore, the experimental dataset presented here not only contributes to the understanding of flow dynamics and sediment transport processes around group of groynes but also serves as a benchmark for numerical simulations. While the findings may not directly predict full-scale river flow, they provide valuable insights for the development and validation of computational models involving groynes in series and assist in decision-making processes for selecting groyne head shapes based on specific field requirements. These insights will contribute to optimizing river system management concerning navigation, bank protection, and ecology.