Assessing intra-bar variations in grain roughness using close-range photogrammetry

Abstract

Evidence of downstream fining in sediment size along the length of a gravel bar has frequently been observed. However, there is limited quantitative information on the variation of other roughness statistics. Developments in the acquisition of high-resolution topographic data provide the opportunity for assessing roughness variations along and across a gravel bar, to quantify existing theoretical observations of bar sorting. Here, close-range photogrammetry is used for the first time to assess intra-bar variations in roughness at 14 locations on a single gravel bar in the Whakatiwai River, New Zealand. An extensive range of roughness parameters are used, including the standard deviation of elevations, skewness, inclination index, and horizontal roughness lengths from second-order structure functions. A reduction with distance down bar was found in all roughness parameters, except skewness, along with a decrease in the variability of the data at the bar tail for all parameters. Lateral variation in roughness parameters was also assessed, showing evidence of an increase in roughness parameters with distance from the water edge. These findings can be used to validate and calibrate existing flow-resistance equations and morphodynamic models. General trends in roughness statistics indicate coarser sediment at the bar head and near the river bank. These trends reflect the formative flows and are used to infer sedimentation patterns, which suggest that the gravel bar undergoes development through lateral accretion. Although complexities in the sedimentation patterns are evident, due to multiple cycles of erosion and deposition, a greater understanding of these patterns is needed for the implementation of successful river management for this river, and others.