Abstract
The surface roughness of river beds affects flow resistance and sediment transport. In rough‐bed rivers (RBRs), where flow is shallow relative to roughness height, the surface roughness is difficult to define due to complex multi‐scale roughness elements (bedrock, boulders, and sediment patches). Here, neither the sediment grain size distribution percentiles (e.g., ) nor the bed elevation standard deviation fully captures the surface roughness. This paper uses high‐resolution digital elevation models of 20 RBR reaches to evaluate their channel morphology and surface roughness. A set of 29 different multi‐scale elevation, gradient‐based, and area‐based, roughness metrics are assessed. Correlation analysis and robust feature selection identified interchangeable metrics, revealing which roughness metrics provided independent information on channel characteristics. Principal component analysis and hierarchical clustering analysis showed that a comprehensive description of RBR topography requires the concurrent use of multiple metrics encompassing (a) a vertical or horizontal scale‐based roughness metric, (b) a slope‐ or area‐based metric, and (c) surface elevation skewness or kurtosis. Slope‐ and area‐based metrics can include roughness directionality relative to the bulk flow. We demonstrate how surface roughness metrics, specifically the use of multiple metrics in unison, are suitably capable of representing and distinguishing between RBRs with differing characteristics. In some cases, rivers with different morphology types (e.g., boulder bed or bedrock) are found to have greater similarity in their surface roughness metrics than rivers classified as morphologically similar. We then discuss RBR morphological and roughness characteristics in the context of flow resistance and sediment transport processes.
River beds can be made of solid rock (bedrock) with differing amounts of sediment on the bed (from none to fully covered), including boulders (sediment grains larger than 0.265 m wide). We explore how these differing features affect how uneven the shape of the bed surface is; we call this variation in the bed the “surface roughness”. It is important to accurately measure how “smooth” or “rough” a river beds is, as this controls how fast water flows and how deep the flow is during flood events. We calculate the surface roughness for 20 river beds using 29 different surface roughness measurements. We show that in order to distinguish between these river beds using their measured surface roughness, we need to use a combination of multiple different measurement types based on: (a) the vertical or horizontal size of the river, (b) the local slope or area of the bed surface, and (c) how much difference there is in height between different areas of the bed (skewness or kurtosis). In the future, this advanced ability to describe the river bed surface roughness will help us to improve our ability to manage rivers, for example, by predicting how deep the river flow will be during floods.
Using high‐resolution river bed topography, surface roughness metrics are quantified across various spatial scales A comprehensive description river bed topographic variability requires the concurrent use of multiple roughness metrics These data are interpreted in terms of flow resistance and sediment transport