Scale‐Dependent Evanescence of River Dunes During Discharge Extremes

Abstract

During high river discharge extremes, the growth of dunes can reach a maximum beyond which a transition to upper stage plane bed may occur, enhancing the river's conveyance capacity and reducing flood risk. Our predictive ability of this bedform regime shift in rivers is exclusively built upon high Froude number flows dominated by asymmetric dunes with steep downstream‐facing slipfaces that are rare in natural rivers. By using light‐weight polystyrene particles as a substrate in an experimental flume setting, we present striking dune morphodynamic similarity between shallow laboratory flow conditions and deep rivers, preconditioned that both flow and sediment transport parameters are accurately scaled. Our experimental results reveal the first observation of upper stage plane bed in a shallow laboratory flume that is reached for a Froude number well below unity. This work highlights the need to rethink widely used dune scaling relationships, bedform stability diagrams, predictions of flow resistance, and flood risk.

Rapidly Migrating Secondary Bedforms Can Persist on the Lee of Slowly Migrating Primary River Dunes

“…Dunes in flumes, formed in limited water depth and high Froude number flows, are typically asymmetrical and have steep lee side angles. In natural rivers, so‐called low‐angle river dunes are often formed (Bradley & Venditti, 2017; Naqshband & Hoitink, 2020; Naqshband et al., 2014a; Van der Mark et al., 2008). Lee side angles observed in natural rivers can be even smaller than 10° (Cisneros et al., 2020; Hendershot et al., 2016; Lefebvre et al., 2016; Ten Brinke et al., 1999).…”

The Influence of Slipface Angle on Fluvial Dune Growth

“…Dune slipface angles—determinative for flow separation and turbulence production—are often assumed to instantaneously reach high angles sloping at the angle of repose (∼30°). Our experimental study with lightweight polystyrene grains allow for dune morphodynamic similarity between shallow laboratory flow conditions and rivers that are an order of magnitude deeper (Naqshband & Hoitink, 2020). We show that dune slipface angles adjust to the imposed flow at time scales similar to the evolution of dune height and length (Figure 3d).…”

“…To investigate dune growth and adaptation, a 15-cm thick layer of uniformly distributed, lightweight polystyrene particles was installed as a surrogate for sand after Naqshband and Hoitink (2020). This allowed dynamic similarity of both flow characteristics (Froude number) and sediment transport conditions (Shields number) between our shallow laboratory flows and rivers that are an order of magnitude deeper.…”

“…Our experimental design, procedure, and instrumentation are briefly outlined below. For more specific details of our methodology and experimental techniques see Naqshband et al (2017) and Naqshband and Hoitink (2020).…”

“…For more specific details of our methodology and experimental techniques see Naqshband et al. (2017) and Naqshband and Hoitink (2020).…”