The flow field over dunes has been extensively measured in laboratory conditions and there isnow ... more The flow field over dunes has been extensively measured in laboratory conditions and there isnow a general understanding of the nature of the flow over dunes formed under equilibrium flow conditions.However, fluvial systems typically experience unsteady flow and therefore the sediment-water interface is constantly responding and reorganizing to this unsteadiness, over a range of both spatial and temporal scales. This is primarily through adjustment of bed forms (including ripples, dunes and bar-forms) which then subse-quently alter the flow field. This paper investigates, through the application of a Large Eddy Simulation (LES) model, the influence of these roughness elements on the overall flow and the variation in flow re- sistance during a change in flow conditions. To provide boundary conditions and a validation dataset for theLES model, a series of physical experiments were undertaken in a flume, 16m long and 2m wide, where fine sand (D 50 of 239µm) was water worked under a ran...
Barforms of mesotidal to macrotidal fluvial–tidal transitions, regardless of fluvial‐discharge, a... more Barforms of mesotidal to macrotidal fluvial–tidal transitions, regardless of fluvial‐discharge, are currently thought to display a sedimentary architecture dominated by tidal signatures. Due to the scarcity of observations from modern mesotidal fluvial–tidal transitions, especially those of multi‐channelled large‐rivers (mean annual discharge ≥7000 m3 s−1 and peak discharges ≥15 000 m3 s−1) with mid‐channel bars, this concept remains unproven. The present study analyses data produced by a combination of high‐resolution ground penetrating radar and coupled shallow vibracores (<5 m depth), collected from modern fluvial–tidal mid‐channel bars of the mesotidal multi‐channelled Lower Columbia River, Washington/Oregon, USA, which can experience peak discharges ≥18 000 m3 s−1. These data were used alongside time‐sequenced aerial imagery to characterize the spatio‐temporal sedimentological evolution of these barforms in singular flows or combined flows consisting of river, tidal and/or w...
Most past experimental investigations of flow over river dunes have focused on conditions that ma... more Most past experimental investigations of flow over river dunes have focused on conditions that match semiempirical flow‐depth scaling laws, yet such equilibrium conditions are of limited value because they rarely occur in natural channels. This paper quantifies the structure of mean and turbulent flow over fixed 2‐D laboratory dunes across a range of nonequilibrium conditions within the dune flow regime. The flow field was quantified using 2‐D particle imaging velocimetry for 12 conditions, including flows that are too deep, too shallow, too fast, or too slow for the size of the fixed dunes. The results demonstrate major departures in the patterns of the mean flow and structure of turbulence when compared to dunes formed under equilibrium flow conditions. The length of flow reattachment scales linearly with the ratio of mean depth‐averaged streamwise velocity to shear velocity at the dune crest ( ), which provides a new predictive measure for flow reattachment length. Depth‐averaged...
Images from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanne... more Images from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure‐from‐Motion (SfM) techniques and application of a depth‐brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near‐bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth‐brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low‐turbidity and shallow (< 2 m deep) water.The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18 km reach and three ~1 km long reaches respectively. Dune and unit bar migration rates...
Although permeable sediments dominate the majority of natural environments past work concerning b... more Although permeable sediments dominate the majority of natural environments past work concerning bed form dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modeled flow which allow the effects of bed permeability on bed form dynamics to be assessed. Simulation of an isolated impermeable bed form over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe‐shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in ser...
To date, published studies of alluvial bar architecture in large rivers have been restricted most... more To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometre‐scale bars vary within a multi‐kilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometre‐scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the Río Paraná – Río Paraguay confluence, where a significant volume of fine‐grained suspended sediment is introduced into the network. Bar‐scale cross‐stratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of bar‐scale sets are found on top of finer...
The flow field over dunes has been extensively measured in laboratory conditions and there isnow ... more The flow field over dunes has been extensively measured in laboratory conditions and there isnow a general understanding of the nature of the flow over dunes formed under equilibrium flow conditions.However, fluvial systems typically experience unsteady flow and therefore the sediment-water interface is constantly responding and reorganizing to this unsteadiness, over a range of both spatial and temporal scales. This is primarily through adjustment of bed forms (including ripples, dunes and bar-forms) which then subse-quently alter the flow field. This paper investigates, through the application of a Large Eddy Simulation (LES) model, the influence of these roughness elements on the overall flow and the variation in flow re- sistance during a change in flow conditions. To provide boundary conditions and a validation dataset for theLES model, a series of physical experiments were undertaken in a flume, 16m long and 2m wide, where fine sand (D 50 of 239µm) was water worked under a ran...
Barforms of mesotidal to macrotidal fluvial–tidal transitions, regardless of fluvial‐discharge, a... more Barforms of mesotidal to macrotidal fluvial–tidal transitions, regardless of fluvial‐discharge, are currently thought to display a sedimentary architecture dominated by tidal signatures. Due to the scarcity of observations from modern mesotidal fluvial–tidal transitions, especially those of multi‐channelled large‐rivers (mean annual discharge ≥7000 m3 s−1 and peak discharges ≥15 000 m3 s−1) with mid‐channel bars, this concept remains unproven. The present study analyses data produced by a combination of high‐resolution ground penetrating radar and coupled shallow vibracores (<5 m depth), collected from modern fluvial–tidal mid‐channel bars of the mesotidal multi‐channelled Lower Columbia River, Washington/Oregon, USA, which can experience peak discharges ≥18 000 m3 s−1. These data were used alongside time‐sequenced aerial imagery to characterize the spatio‐temporal sedimentological evolution of these barforms in singular flows or combined flows consisting of river, tidal and/or w...
Most past experimental investigations of flow over river dunes have focused on conditions that ma... more Most past experimental investigations of flow over river dunes have focused on conditions that match semiempirical flow‐depth scaling laws, yet such equilibrium conditions are of limited value because they rarely occur in natural channels. This paper quantifies the structure of mean and turbulent flow over fixed 2‐D laboratory dunes across a range of nonequilibrium conditions within the dune flow regime. The flow field was quantified using 2‐D particle imaging velocimetry for 12 conditions, including flows that are too deep, too shallow, too fast, or too slow for the size of the fixed dunes. The results demonstrate major departures in the patterns of the mean flow and structure of turbulence when compared to dunes formed under equilibrium flow conditions. The length of flow reattachment scales linearly with the ratio of mean depth‐averaged streamwise velocity to shear velocity at the dune crest ( ), which provides a new predictive measure for flow reattachment length. Depth‐averaged...
Images from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanne... more Images from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure‐from‐Motion (SfM) techniques and application of a depth‐brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near‐bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth‐brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low‐turbidity and shallow (< 2 m deep) water.The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18 km reach and three ~1 km long reaches respectively. Dune and unit bar migration rates...
Although permeable sediments dominate the majority of natural environments past work concerning b... more Although permeable sediments dominate the majority of natural environments past work concerning bed form dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modeled flow which allow the effects of bed permeability on bed form dynamics to be assessed. Simulation of an isolated impermeable bed form over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe‐shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in ser...
To date, published studies of alluvial bar architecture in large rivers have been restricted most... more To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometre‐scale bars vary within a multi‐kilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometre‐scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the Río Paraná – Río Paraguay confluence, where a significant volume of fine‐grained suspended sediment is introduced into the network. Bar‐scale cross‐stratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of bar‐scale sets are found on top of finer...
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Papers by Jim Best