Modelling open channel flow for the features of a flexible groyne

Abstract

Large-scale measures within the river programmes such as Room for the River, Natura2000 and Water Framework Directive have increased the biodiversity in Dutch rivers and have achieved a more natural landscape. However, recent river programmes have shown conflicts between safety against flooding and riverine nature rehabilitation with maintaining navigational water depths. Therefore, an integrated approach is called out upon to improve the river management within these programmes. Conventional groynes are typically used to improve functions within the river programmes, whose primary function is to maintain navigational water depth. Groynes are transverse structures in which large turbulent structures are observed around the groynes. Consequently, significant bed shear stresses are developed, leading to significant local scour. Investigations for improving alternative groyne configurations are often studied for optimizing the groyne structure. The flexible groyne is considered an optimization of the conventional groyne structure. The flexible groyne consists of steep slopes and has a permeable characteristic. Although a fair amount of research has already been carried out on various groyne configurations, detailed flow characteristics around and through permeable, sloped groynes are limited. Hence, this research aims to numerically quantify the effects of permeability and head steepness of groynes. A literature study is executed to investigate the critical processes required for capturing within the numerical model to answer the research question. From the literature study, it appears that the most important processes are the non-hydrostatic effects, adapting large turbulent structures and implementing the porous zone using a non-linear function. Multiple software packages have been analyzed. Fluent is chosen, which is analyzed to capture the required processes adequately.
A new numerical model has been set up for investigating the research question by simulating flow around groynes. The model is validated against three experimental studies for various characteristics. The important mean flow characteristics have been validated within an acceptable range. Still, it appears that the numerical model tends to underestimate the mean streamwise flow velocities, overestimate the Reynolds shear stresses and shift the peak values of the Reynolds shear stresses downstream. Four configurations are identified for the simulations of varying head steepness and porosity. It appears that the increase of the porosity reduces the large turbulent structures and bed shear stresses close to the groyne and shifts the peak values of the Reynolds shear stresses, and bed shear stresses further downstream. The porosity reduces the maximum Reynolds shear stresses and bed shear stresses compared with the Reynolds shear stresses, and bed shear stresses for an impermeable groyne. These reductions are because of the momentum exchange between the free flow region and the flow through the porous structure, which reduces the mean flow velocity in the free flow region. For decreasing steepness, the large turbulent structures and bed shear stresses are observed close to the groyne due to increasing deflection. The flow appears to follow the geometry of the sloped groyne more smoothly. This research is seen as a first approach for modelling a porous, sloped groyne. Further improving and analyzing numerical modelling for porous, head sloped groynes are advised to increase the model's accuracy. Furthermore, more simulations for varying the head steepness and porosity is suggested to improve the relations between the increase of porosity and head steepness for the flow characteristics. In addition, including sediment transport models within the model is expected to increase the understanding of the hydrodynamics and morphology around these specific groynes.