Introduction:
Riverbank and canal erosion significantly disrupt the morphological and ecological equilibrium of river systems, posing serious threats to aquatic habitats, agricultural lands, infrastructure, and human settlements in riparian zones. This degradation not only contributes to the loss and reduction of adjacent land areas but also diminishes water quality by increasing sediment load and turbidity. Among various hydraulic countermeasures, breakwaters have proven to be one of the most effective and practical solutions for controlling and mitigating erosion along riverbanks and canal shores. These structures are primarily employed to deflect and redirect the flow of water away from vulnerable banks towards the center of the channel, thereby reducing the shear stress acting on the banks and preventing further erosion. The widespread use of breakwaters in river engineering is largely attributed to their cost-effectiveness, ease of installation, and adaptability to various environmental conditions. Breakwaters are classified according to several criteria. Based on construction material, they may be composed of wood, concrete, stone, or composite materials. In terms of permeability, breakwaters are categorized as permeable (which allow partial flow through the structure, promoting energy dissipation and sediment deposition) and impermeable (which fully obstruct the flow and divert it more sharply). With respect to submergence, they can be either submerged or non-submerged, influencing how they interact with surface flows. Functionally, breakwaters are designed to serve distinct roles: energy-absorbing (to reduce flow momentum), energy-dissipating (to break turbulence and eddies), or water-diverting (to change the direction of the main current). While impermeable breakwaters create strong deflection zones, permeable ones reduce the velocity of flow gradually, thus offering a more natural transition and enhanced sediment stabilization. Understanding these classifications and their hydraulic impacts is crucial for selecting appropriate breakwater configurations in river restoration and erosion control projects.
Methods:
To analyze the hydraulic behavior of flow and turbulence structures around spur dikes, a comprehensive series of controlled laboratory experiments was conducted. The accuracy and reliability of the results were ensured by an optimized experimental design, precise and calibrated measurement instruments, and well-defined geometric configurations of the experimental setup. All experiments were performed in a glass flume, located at the Hydraulic Laboratory of Isfahan University of Technology, which was a specialized experimental channel with a length of 15 meters, a width of 0.9 meters, and a height of 0.6 meters. The flume was equipped with a high-precision feeding pump, a digital flowmeter to accurately measure discharge, and an energy dissipator installed at the inlet to stabilize flow. Water was supplied continuously from two interconnected reservoirs to ensure steady flow conditions throughout the experiments. Water depth measurements were conducted using a rail depth gauge with an accuracy of ±1 mm. A tailgate located at the downstream end of the flume allowed precise control of flow depth and submergence conditions. Flow visualization techniques were employed using a dye injection system with potassium permanganate, enabling clear identification of recirculation zones and stagnation points within the flow field. The flow entered the flume through a calming tank and a stilling basin, ensuring fully developed flow profiles over an approach length of 10 meters before reaching the spur dikes. Two types of spur dikes were examined: impermeable structures made of stone and permeable structures constructed from gabions. The impermeable dikes had widths of 20, 30, and 50 cm, whereas the permeable dikes maintained a constant width of 30 cm with a porosity of approximately 41%. The first spur dike was positioned 7.5 meters from the inlet. Various submergence levels, including non-submerged, semi-submerged, and fully submerged conditions, were tested, defined by the submergence ratio Sr = h/H. In total, 39 tests were conducted under a wide range of flow rates and structural configurations. The detailed results and analyses are provided in the following sections.
Results:
This section presents the analysis of results obtained from 39 experimental tests, focusing on the characterization of recirculation zones downstream of permeable and impermeable spur dikes. The influence of hydraulic and structural parameters such as discharge rate, spur dikes width, submergence ratio, and porosity on the length of the recirculation zone was systematically evaluated. For impermeable spur dikes, tests were conducted with widths of 20, 30, and 50 cm under three flow discharges of 29, 49, and 65 L/s. Results showed that for narrower spur dikes, increasing submergence led to a shorter recirculation zone. At a width of 30 cm and a discharge of 29 L/s, increasing submergence ratio from 0.37 to 0.47 reduced the recirculation zone length from 105 cm to 90 cm. In contrast, wider spur dikes (50 cm) exhibited longer recirculation zones at higher discharges, reaching up to 205 cm at 65 L/s, likely due to greater flow deflection and enhanced vortex stability. In tests with permeable spur dikes (41% porosity, 30 cm width), the recirculation zones were consistently longer, reaching 208 cm at 65 L/s. This was attributed to smoother internal flow through the structure, leading to momentum diffusion and broader recirculation. To study the effect of spur dike spacing, a second dike was positioned at varying distances relative to the first. When placed precisely at the end of the first recirculation zone (160 cm), the second zone extended to 118 cm. However, placing the second spur dike at upstream (120 cm) or downstream (199 cm) reduced the length to 100 cm and 80 cm, respectively. These findings highlighted the importance of optimal spur dike spacing for maximizing flow control efficiency. In summary, spur dike width, porosity, and placement significantly affected flow structures. Wider and permeable spur dikes were more effective in forming extended recirculation zones, while optimal spacing between multiple spur dikes enhanced their combined performance.
Conclusion:
Based on the findings of this study, the hydraulic behavior of the stagnant zone formed downstream of scour protections was thoroughly investigated through a series of physical experiments under varying conditions, including flow discharge, spur dike width, porosity, and submergence levels. The results demonstrated that the spur dike width and flow discharge had a significant impact on the length of the stagnant zone. Increasing the spur dike width and discharge led to a noticeable extension of the stagnant zone, as these factors enhance the energy redistribution and recirculation of flow. In contrast, higher submergence ratios, especially in cases of deep submergence, reduced the stagnant zone length due to the diminished influence of the spur dike structure on the flow. Moreover, permeable spur dikes were more effective in forming a larger stagnant zone than impermeable ones, aligning with previous studies. Additionally, the positioning of a second spur dike had a notable effect, with its placement at the end of the stagnant zone providing the best performance in terms of reducing lateral erosion. Overall, the optimal design of spur dike configurations requires a careful balance of hydraulic and geometric factors to enhance hydraulic stability and mitigate erosion in waterway structures.