Investigation of scour and turbulent flow characteristics around L Head Groynes in gravel bed

Abstract

Riverbank protection is a critical aspect of hydraulic engineering, particularly in gravel-bed rivers where erosion and sediment transport significantly impact channel stability. Groynes are widely employed as river training structures to regulate flow, protect banks from erosion, and enhance aquatic habitats. While conventional straight-type groynes have been extensively studied, groynes with varying head shapes exhibit distinct hydrodynamic behaviors that can offer improved performance in river management. Among these, L-head groynes (LHGs) are particularly effective due to their ability to dissipate flow energy, reduce local scour, and promote sediment deposition in their wake zones. Despite their potential advantages, limited research exists on the scour and turbulence characteristics around LHGs, particularly in gravelbed rivers. This study aims to bridge this gap by experimentally investigating the scour mechanism and turbulent flow field around single and multiple LHGs, and comparing their performance with other groyne configurations, namely I-head groynes (IHGs) and T-head groynes (THGs). This study presents an experimental and numerical investigation into the scour patterns, flow field characteristics, and turbulence structures around groynes, with a focus on L-head groynes (LHGs). Experiments were conducted in a flume and they covered a range of flow conditions characterized by Froude numbers varying from 0.5 to 0.61. Contour plots of equilibrium scour depth, mean flow velocity, and Reynolds stresses were generated to provide a comprehensive understanding of the underlying hydrodynamic processes. Velocity measurements and streamline analyses were performed to capture the complex flow structures, including flow separation zones, horseshoe vortex systems, and wake turbulence. Turbulence characteristics were quantified through Reynolds stresses, bed shear stress distributions, and turbulent kinetic energy to determine the underlying mechanisms responsible for sediment entrainment and deposition. A comparative assessment of the three groyne types revealed that LHG and T-head groynes (THGs) induced the deepest scour, while I-head groynes (IHGs) exhibited a relatively lower scour depth. The location of peak scour varies between configurations: for IHG, it occurs at the groyne tip due to localized flow separation, whereas LHG experiences maximum scour at the junction of its two perpendicular faces. In contrast, THG exhibits its deepest scour upstream due to pronounced downflow and horseshoe vortex generation at the groyne’s perpendicular face. iv The results indicate that IHG provides the highest bank protection efficiency, quantified through the normalized length of bank protection (Lp), reaching a maximum of 1.2L1, where L1 is the transverse length of the groyne. The study also investigated the effect of multiple LHGs in series, demonstrating that increasing groyne spacing from L1 to 2L1 results in a 12% increase in scour depth. Additionally, for three groynes arranged in series with L1 spacing, the maximum scour depth was found to be 25% greater than for two groynes under similar conditions, highlighting the cumulative influence of successive groynes on local flow dynamics and sediment transport. A cost-benefit analysis was also conducted to assess the economic viability of each groyne configuration, revealing that IHG is the most cost-effective option due to its lower scour depth and higher bank protection efficiency. These findings provide crucial insights into groyne selection for riverbank stabilization in gravel-bed streams, particularly in mountainous regions where streambed composition and hydrodynamic forces present unique engineering challenges.