Bed morphology of rivers covering secondary bedform and roughness contribution

Bedform morphologies are intricately linked to fluvial sediment transport, hydrodynamics, and resistance balance. Many studies have concentrated on the morphological parameters and drag effects of the primary bedforms, while ignoring the secondary bedforms located above them. This paper quantified and analyzed the geometric features of primary and secondary bedforms using high-precision riverbed measurements, revealing significant differences in relative height, steepness, and leeside angle. The secondary bedform was significantly steeper than the host bedform, and the height of the secondary bedform received the influence of the host bedform and water depth, and its abundance and relative height showed a negative correlation. Further, based on the mean dimensional measurements of bed morphology collected from 296 groups of rivers and flumes, it was found that secondary bedform roughness height could serve as a better descriptor of bedform resistance in large and deep rivers with water depths greater than 2.5 m, because the impeding effect of the primary bedform height in deep rivers may be offset by the slow leeside slope gradient due to the low steepness and low leeside angle of the primary bedform in these rivers. Based on this understanding, the secondary bedform resistance of the study reaches was quantified and analyzed for its contribution to the total resistance along the reach and found to explain only up to 19.1% of the variation in river resistance, implying that although the secondary bedform resistance can be valuably close to the bedform resistance, the variability of the bedform resistance is difficult to be explained by a single factor, and some potential influencing factors such as terrain variation and 3D index are discussed and further study is needed.