Spatial variations of velocity and pressure fields induced by large-scale (single stalk) and small-scale (sediment) roughness elements

Abstract

Characterizing velocity and pressure fields in aquatic systems is crucial for understanding fundamental processes such as sediment transport, hyporheic flow, air-water exchange, and habitat quality. While large obstacles like vegetation stalks are known to create significant localized pressure gradients, the role of small-scale bed roughness in generating local pressure gradients remains poorly understood. Here, we explore flow dynamics around a vertical cylinder (simulating vegetation) over a coarse granular bed using stereo particle image velocimetry (SPIV) with refractive index-matched (RIM) fluid, integrated with Large Eddy Simulations (LES). Our combined approach reveals an important phenomenon: while large obstacles like vegetation stalks create localized pressure gradients, bed roughness elements generate frequent pressure fluctuations across the entire streambed. Although previous studies have focused primarily on large-obstacle effects, our findings show that grain-scale pressure variations generate stronger local gradients (up to ±250 mmH₂O/m) than those from large obstacles (±50 mmH₂O/m), and their widespread occurrence throughout the bed surface may collectively have substantial effects on hyporheic exchange. By quantifying pressure fields at both large and small scales, we demonstrate that bed roughness elements create persistent pressure gradients that, due to their widespread occurrence, may significantly influence surface-subsurface water interactions. Our results highlight the importance of considering grain-scale roughness effects when studying hyporheic processes in natural streams.