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.