A Lagrangian drifter for surveys of water surface roughness in streams

Water surface roughness in rivers and streams is mainly driven by flow, turbulence and riverbed conditions. Conversely, water surface roughness gives easy access to information about hydrodynamics – provided that measurements are possible at sufficient spatial-temporal resolution and accuracy. We present a novel small-size (diameter 28 mm) drifter, which detects water surface roughness through acceleration measurements while floating on the water surface. Laboratory tests revealed a good agreement between drifter-based measurements and optical characterization of surface roughness. Only very short waves with periods <0.15 s remained undetected by the drifter. Results from laboratory and field experiments show strong correlations between the drifter-measured water surface roughness, flow and shear velocities, Froude and Reynolds numbers. The small-sized drifter is particularly appropriate for reach-scale measurements in streams, where the application of other devices or methods is too large or laborious.

河流与溪流的水面粗糙度主要受水流、湍流及河床条件驱动。反之，只要能够以足够的时空分辨率与精度开展测量，通过水面粗糙度便可便捷获取水动力学相关信息。本研究研发了一款新型小型漂流浮标（直径28毫米），该浮标漂浮于水面时可通过加速度测量来检测水面粗糙度。实验室测试结果表明，基于该漂流浮标的测量结果与水面粗糙度的光学表征结果具有良好的一致性，仅周期小于0.15秒的极短波长波浪无法被该浮标检测到。实验室与现场实验结果显示，该浮标测得的水面粗糙度与水流速度、剪切流速、弗劳德数（Froude number）及雷诺数（Reynolds number）之间存在显著相关性。这款小型漂流浮标尤其适用于溪流的河段尺度测量，其他设备或方法在此类场景中往往体积过大且操作繁琐。