Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

Gravel riverbeds naturally present a range of sediment size that can span several orders of magnitude. However, fine sediment (i.e., size <2 mm) additions affect sediment transport and are potentially harmful to the river ecology. Limited research has been done to understand the effects of sand addition on gravel bed topography. For this study, we conducted flume experiments testing the effectiveness of topography remote sensing to measure the response of a water-worked gravel bed to varying sand additions and bed flushing. Repeated measurements of the bed topography with through-water photogrammetry show that sand deposits primarily in low-lying areas of the bed, where it is sheltered. Observed changes in bed elevation standard deviation, skewness, and inclination index indicate a smoother bed, as sand fills the depressions on the surface and conceals gravel imbrication. The overall bed morphology (e.g., small bedforms and coarse grain arrangement), however, is preserved. Increasing the sand supply resulted in a gravel bed buried by sand and the formation of sand dunes. Bed flushing with a flow sufficient to move sand allowed a return of the bed roughness to the prepulse conditions, while flushing the bed with a discharge equivalent to the armoring discharge reworked the normally stable surface layer without removing infiltrated sand from the subsurface. This new data set contributes to providing missing information on the small-scale topographic effects of sand addition upon gravel beds. It might be used with measurements of near-bed flow hydraulics obtained in other studies to explain variations of sediment transport.