Quantifying hydraulic roughness from field data: can dune morphology tell the whole story?

Hydraulic roughness is a fundamental property in river research, as it directly affects water levels, flow strength and the associated sediment transport rates. It is a key property in hydrodynamic and sediment transport models used to address societal risks such as flooding and stranding of ships during low water conditions. However quantification of roughness is challenging, as it is not directly measurable in the field. In lowland rivers, bedforms are a major source of hydraulic roughness. Decades of research have focused on dunes to allow parameterization of roughness, with relatively little focus on field verification. This study aims to establish the predictive capacity of current roughness predictors, and to identify reasons for the unexplained part of the variance in roughness. We quantified hydraulic roughness based on the Darcy-Weisbach friction factor (f) calculated from hydraulic field data of a 78 km long trajectory of the river Rhine and river Waal in the Netherlands. This is compared to predicted roughness values based on dune geometry, and to the spatial trends in the local topographic leeside angle, both inferred from bathymetric field data. Results from both approaches show the same general trend and magnitude of roughness values (0.019 < f < 0.069, mean 0.035). Roughness inferred from dune geometry explains 31% of the variance, for the best performing predictor. Efforts to explain the remaining variance from statistics of the local topographic leeside angles, which supposedly control flow separation, were unsuccessful. Unexpectedly, multi-kilometer depth oscillations explain 34% of the total roughness variations. We suggest that, similar to what is well-known at the scale of an individual dune, flow divergence associated with depth increase causes energy loss, which is reflected in an elevated hydraulic roughness. Multi-kilometer depth variations occur in many rivers worldwide, which implies a cause of flow resistance that needs further study.