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Hydraulic roughness is a fundamental property in river research, as it directly affects water levels, flow strength and the associated sediment transport rates. 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). Roughness inferred from dune geometry explains at best 31% of the variance. 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 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. Plain Language Summary: Hydraulic roughness is the resistance that a river flow experiences from the bed and banks of the channel. Studying hydraulic roughness aids in the prediction of flooding, as increased resistance causes the water level to rise. Previous research on hydraulic roughness has mainly focused on the effect of the shape of river dunes at the river bed. Dunes cause resistance, which has been captured in many equations predicting roughness. In this study, we tested several of those equations capturing dune roughness, and examined alternative sources of flow resistance. A 78 km‐long segment of the river Rhine and river Waal in the Netherlands was used as a case study. Hydraulic roughness was inferred from longitudinal water surface level profiles measured with a survey vessel, and also from river dune dimensions. Both methodologies resulted in similar values and the same general trend of roughness, however, river dune dimensions only explained about one third of the total observed variation. We found that, contrary to expectations, multiple‐kilometer long fluctuations of the river bed elevation influence roughness as well. As the river deepens, the flow weakens, increasing roughness in that region. This is an important finding, since many rivers feature such multi‐kilometer depth variation. Key Points: Roughness predictions based on bedform geometry explain about 1/3th of the variance in friction factors inferred from water surface profilesMetrics capturing dune leeside angle statistics do not outperform classical hydraulic roughness predictors in explaining friction variationBed‐level gradients oscillate in counter phase with the friction factor, indicating the importance of multi‐kilometer depth variations [ABSTRACT FROM AUTHOR]

Abstract: Particulars of 6km long HTS AC power cable for Amsterdam project are: a cable has to fit in an annulus of 160mm, with only two cooling stations at the cable ends . Application of existing solutions for HTS cables would result in excessively high coolant pressure drop in the cable, possibly affecting public acceptance of the project. In order to solve this problem, a model cryostat was developed consisting of alternating rigid and flexible sections and hydraulic tests were conducted using sub-cooled liquid nitrogen. In the 47 m-long cryostat, containing a full-size HTS cable model, measured pressure drop amounts 11 mbar at the mass flow rate of 0.3kg/s and temperature 65K. For a 6 km-long HTS cable this gives a pressure drop below 2bar, which is acceptable. In order to achieve this result, the cryostat was manufactured from alternating straight rigid sections and corrugated flexible sections. A flexible dummy HTS cable was inserted into this cryostat and sub-cooled liquid nitrogen was circulated in the annulus between the dummy cable surface and the inner cryostat surface. In the paper details are presented of the cryostat, of the measurement setup, of the experiment and of the results. [Copyright &y& Elsevier]