Modulation of the flow structure by progressive bedforms in the Kinoshita meandering channel

Abstract

An in-house fully three-dimensional general-purpose finite element model is applied to solve the hydrodynamic structure in a periodic Kinoshita-generated meandering channel. The numerical model solves the incompressible Reynolds-averaged Navier-Stokes equations for mass and momentum, while solving the k À ε equations for turbulence. The free surface is described by the rigid-lid approximation (using measured water surface data) for flat (smooth-bed) and self-formed (rough-bed) conditions. The model results are compared against experimental measurements in the 'Kinoshita channel', where three-dimensional flow velocities and turbulence parameters were measured. This validation was carried out for the upstream-valley meander bend orientation under smooth (flat bed) conditions. After validation, several simulations were carried out to predict the hydrodynamics in conditions where either it was not possible to perform measurements (e.g. applicability of the laboratory acoustic instruments) and to extrapolate the model to other planform configurations. For the flat smooth-bed case, a symmetric (no skewness) planform configuration was modeled and compared to the upstream-skewed case. For the self-formed rough-bed case, prediction of the hydrodynamics during the progression of bedforms was performed. It appears that the presence of bedforms on a bend has the following effects: (i) the natural secondary flow of the bend is disrupted by the presence of the bedforms, thus depending on the location of the dune, secondary flows might differ completely from the traditional orientation; (ii) an increment on both the bed and bank shear stresses is induced, having as much as 50% more fluvial erosion, and thus a potential increment on the migration rate of the bend. Implications on sediment transport and bend morphodynamics are also discussed in the paper.

Numerical evaluation of the effects of planform geometry and inflow conditions on flow, turbulence structure, and bed shear velocity at a stream confluence with a concordant bed

“…Besides the usual coherent structures present in open channels [e.g., see Hardy et al ., , ; Frias and Abad , ; Abad et al ., ], the confluence hydrodynamic zone is characterized by large‐scale turbulent coherent structures generated by differential velocities and momentum along the mixing interface (MI) between the confluent flows. The momentum ratio between the two incoming streams is defined as: where ρ i , Q i , and U i 0 are density, discharge, and bulk velocity in the lateral [ i = 2] and main [ i = 1] tributaries.…”

Modulation of outer bank erosion by slump blocks: Disentangling the protective and destructive role of failed material on the three‐dimensional flow structure

“…Such effects also find parallels in the recent work of Abad et al . [] who document the influence of migrating bedforms on bank shear stresses. A fruitful avenue for future studies will be to better quantify the turbulent fluxes and Reyonlds stresses associated with these slump block effects.…”

“…Res. Lett., 42, 10,663-10,670, doi:10.1002 physical experiments [Mizumura and Yamasaka, 2002;McCoy et al, 2007McCoy et al, , 2008Yossef and de Vriend, 2011] and numerical models [Mcbride et al, 2007;Blanckaert et al, 2010Blanckaert et al, , 2012Blanckaert et al, , 2013Abad et al, 2013], or studied the effects of artificial bend-way weirs, wing-dikes, and groins [Abad et al, 2008;Jamieson et al, 2011]. The current absence of detailed 3-D measurements documenting the effects of bank roughness on near-bank flow in natural channels is partly due to the complexity and spatial scale of the processes involved and remains a significant research gap [Motta et al, 2014].…”

“…Although past studies have investigated the effect of bank roughness on near‐bank flows, these have either been of insufficient spatial resolution to resolve fully the three‐dimensional flow near the bank [ Jin et al ., ; Thorne and Furbish , ], or they have documented flow associated with large‐scale roughness elements in physical experiments [ Mizumura and Yamasaka , ; McCoy et al ., , ; Yossef and de Vriend , ] and numerical models [ Mcbride et al ., ; Blanckaert et al ., , , ; Abad et al ., ], or studied the effects of artificial bend‐way weirs, wing‐dikes, and groins [ Abad et al ., ; Jamieson et al ., ]. The current absence of detailed 3‐D measurements documenting the effects of bank roughness on near‐bank flow in natural channels is partly due to the complexity and spatial scale of the processes involved and remains a significant research gap [ Motta et al ., ].…”

Turbulence in Rivers

“…They may be conditioned or even generated by turbulent instability at smaller scales and, in turn, may be responsible for changes in the overall turbulence of the flow, and include secondary currents in river bends (Blanckaert and De Vriend 2005;Termini and Piraino 2011;Nikora and Roy 2012;Abad et al 2013), horizontal coherent vortices at the boundary between the main channel and floodplains (Knight and Shiono 1990;Shiono and Muto 1998;van Prooijen et al 2005;Proust et al 2013), and lowfrequency large-scale flow structures traveling in low-submergence flows (Kirkbride and Ferguson 1995;Roy et al 2004;.…”