387 / 2024-03-08 16:54:29

Numerical Analysis of Flow Structure and Suspended Matter Behavior around Hybrid Groyne in the Aizuma River

hybrid groyne,Numerical analysis,Suspended solids,IRIC

Subtheme 1: Water-related Hazard and Risk Reduction > Topic 6: Safety Risk Prevention and Control of Water Engineering

The Aizuma River, which flows through Aichi Prefecture, has three right-angled groyne on its right bank approximately 300 m downstream from the confluence to prevent riverbank erosion during floods. This hybrid groyne combines the impermeable characteristics of a masonry structure and the permeable characteristics of a piled-out groyne.

Although the authors have conducted field investigations, there has been no comparison of the flow structure around groyne and impermeable groyne during outflows. In addition, the relationship between the flow field and suspended sediment behavior in the vicinity of groyne has not been fully investigated.

The objective of this study is to obtain new knowledge on river flow with hybrid groyne by numerical analysis of the flow field and suspended sediment behavior around the groyne.

This study is analyzed using the NaysCUBE solver of iRIC software. Suspended solids are represented as a sphere swarm model, and the behavior of the spheres constituting the suspended solids is tracked independently using the Lagrangian equations.

The analysis pattern is to compare the differences in river flow and suspended sediment behavior when no groyne, impermeable groyne only, and hybrid groyne are installed for three different boundary conditions. Since the Aizuma River is located in the middle and lower reaches of the Sakai River system, small-scale suspended solids such as grasses and river litter were assumed for the suspended solids.

The analysis results show that in the case of impermeable groyne, the transverse flow velocity was large at the tip of the groyne located at the most upstream end. In addition, the impermeable groyne had a pronounced control effect of concentrating the flow direction in the center of the main flow section, resulting in the generation of large circulating flows inside the groyne. Such large circulation flows and strong hydrodynamic forces exerted on the groyne tip are considered to be the cause of local scour around the impermeable groyne.

In the analysis of suspended solids behavior, in the case of the impermeable groyne, the suspended solids flowing downstream from the upstream were trapped along the flow toward the inside of the groyne and stayed there, because the circulation flow around the groyne was remarkable. On the other hand, in the case of the hybrid groyne, there was almost no circulation flow, and the suspended solids flowed downstream along the flow. The suspended solids under the conditions of this analysis were captured more effectively by the impermeable groyne than by the hybrid groyne.