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.

中文翻译：

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大尺度（单茎）和小尺度（沉积物）粗糙度单元引起的速度和压力场的空间变化


表征水生系统中的速度和压力场对于了解沉积物运输、低流流、气水交换和栖息地质量等基本过程至关重要。虽然已知像植被茎这样的大型障碍物会产生显著的局部压力梯度，但小尺度河床粗糙度在产生局部压力梯度中的作用仍然知之甚少。在这里，我们使用立体粒子图像测速 （SPIV） 和折射率匹配 （RIM） 流体，并与大涡模拟 （LES） 集成，探索了粗颗粒床上垂直圆柱体（模拟植被）周围的流动动力学。我们的综合方法揭示了一个重要现象：虽然像植被茎这样的大型障碍物会产生局部压力梯度，但河床粗糙度元素会在整个河床上产生频繁的压力波动。尽管以前的研究主要集中在大障碍物效应上，但我们的研究结果表明，与大障碍物 （±50 mmH₂O/m） 相比，晶粒尺度压力变化会产生更强的局部梯度（高达 ±250 mmH₂O/m），并且它们在整个床面上的广泛出现可能共同对低流交换产生重大影响。通过量化大尺度和小尺度的压力场，我们证明了河床粗糙度元素会产生持续的压力梯度，由于它们的广泛存在，可能会显着影响地表-地下水的相互作用。我们的结果强调了在研究自然溪流中的低流过程时考虑颗粒尺度粗糙度效应的重要性。