Quantifying Hydraulic Roughness From Field Data: Can Dune Morphology Tell the Whole Story?

Abstract

Hydraulic roughness, which quantifies the resistance to flow by objects protruding into the water column (Chow, 1959), is a fundamental property in hydraulics. Due to its influence on water levels, flow structure and the associated sediment transport, understanding roughness is crucial to comprehend river dynamics. Quantification of hydraulic roughness is challenging, since it is not directly measurable in the field.Hydraulic roughness at the bed of a main channel with abundant bedforms, consists of grain-friction drag and form drag. In lowland rivers, bedforms (bars, dunes, ripples) are typically the major cause of form roughness (Gates & Al-Zahrani, 1996;Julien et al., 2002). Form drag induced by dunes is estimated with predictors based on dune height and length (

Dune Geometry and the Associated Hydraulic Roughness in the Fluvial to Tidal Transition Zone of the Fraser River at Low River Flow

“…A similar relation was observed in the Rhine and Waal rivers in the Netherlands by de Lange et al. (2021), who hypothesized that multi‐kilometer depth oscillations induce flow divergence associated with depth increase, which in turn causes energy loss. This is reflected in an elevated hydraulic roughness.…”

“…This in turn suggests that variable dune roughness might not be needed to simulate large‐scale (tidal) flow. Similar conclusions were drawn from a fluvial system where dune roughness calculated from dune geometry explained only 31% of the variance of the roughness inferred from the water surface slope (de Lange et al., 2021) and the remaining variance could not be explained by leeside angle statistics.…”

“…These changes in dune morphology do not affect reach‐scale hydraulic roughness, because the calibrated model roughness is similar to the dune‐derived roughness based on dune height and dune length. As a result, hydraulic model performance using a calibrated, constant, roughness is not improved by implementing dune‐derived bed roughness. In the Fraser River, large‐scale variations river geometry are more important than dune morphology in controlling variations in reach‐scale roughness, a finding which is not unique to the Fraser River (de Lange et al., 2021). In the Fraser river, oscillations in river depth elevate hydraulic roughness in the tidal region, but this does not occur in the fluvial‐tidal regime because changes in depth are compensated for by changes in width, keeping the cross‐sectional area of the channel relatively constant.…”

Coexistence of two dune scales in a lowland river

“…At the highest discharge levels, this correlation is unclear. In the Waal river, at higher discharges, part of the water is discharged through the floodplains, and the shear stress does not necessarily increase with discharge (Wilbers and Ten Brinke, 2003;De Lange et al, 2021). The observation that dune height scales with discharge and thus water depth and shear stress is in line with existing literature (Yalin, 1964;Julien and Klaassen, 1995;Bradley and Venditti, 2017), although at high transport stages dunes are expected to decrease in height, because they wash out to upper stage plane bed (Allen, 1978;Van Rijn, 1984c).…”

“…Dunes are the key components of sand-bedded rivers and have been extensively researched to better understand and manage fluvial systems (Wilbers, 2004;Best, 2005;Parsons et al, 2005;Nittrouer et al, 2008;Cisneros et al, 2020;Naqshband and Hoitink, 2020;De Lange et al, 2021;Zomer et al, 2021). Dunes control local flow dynamics.…”