Characterization of Porous In-Stream Structures to Assess Their Implications on Flow Dynamics and Sediment Transport

In aquatic environments, the presence of porous obstacles induces intricate flow dynamics as the flow passes through and around them. These flows exhibit large local vertical and lateral gradients, influencing the evolution of downstream flow structures across various scales. In this study, we investigated flow around five idealized porous obstacles with varying porosity and pore arrangements using Particle Image Velocimetry (PIV). By introducing a two-layer model and computing turbulent kinetic energy budgets, we quantified jet velocity and length to predict the development of downstream flow structures. Recirculation zones were observed downstream of obstacles with small pore sizes, while forward flow motions prevailed downstream with larger pore sizes due to increased jet velocity and length. To study the effect of multiple porous obstacles, we installed a second obstacle at various downstream distances, which showed minimal influence on jet length and velocity once the distance between obstacles exceeded the jet length determined from single obstacle analysis, particularly with obstacles featuring large pore sizes. Our study identifies the need to properly characterize in-stream obstacles based on both their porosity and their representative pore sizes, as the jets created through the obstacles significantly alter the expected flow structures from solid-obstacle predictions. Based on the insights from the hydrodynamic study and using the balance between resistance and driving force of sediment motions, we discuss ecological and geomorphic applications in the vicinity of porous obstacles, highlighting the potential locations for sediment erosion and deposition.