River dams provide many benefits, including flood control. However, due to constantly evolving ch... more River dams provide many benefits, including flood control. However, due to constantly evolving channel morphology, downstream conveyance of floodwaters following dam closure is difficult to predict. Here, we test the hypothesis that the incised, enlarged channel downstream of dams provides enhanced water conveyance, using a case study from the lower Yellow River, China. We find that, although flood stage is lowered for small floods, counterintuitively, flood stage downstream of a dam can be amplified for moderate and large floods. This arises because bed incision is accompanied by sediment coarsening, which facilitates development of large dunes that increase flow resistance and reduce velocity relative to pre-dam conditions. Our findings indicate the underlying mechanism for such flood amplification may occur in >80% of fine-grained rivers, and suggest the need to reconsider flood control strategies in such rivers worldwide.
Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments,... more Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments, and may occur either in isolation or in groups. This study uses high-resolution particle-image velocimetry (PIV) experiments using fixed-bed models to examine the effects of barchan dune interaction upon the flow field structure. The barchan dune models were based upon an idealized contour map, the shape and dimensions of which were based upon previous empirical studies of dune morphology. The experimental setup comprised two, co-axially aligned barchan dune models that were spaced at different distances apart. In this paper, two volumetric ratios (Vr, upstream barchan dune: downstream barchan dune) of 1.0 and 0.175 were examined. Models were placed in an Eiffel-type, open-circuit wind tunnel and flow quantification was achieved using PIV at 0.5Hz. PIV measurements of the mean and turbulent flow field were made in the streamwise-wallnormal plane, along the centerline of the barchans(s), at an average Reynolds number of 59,000. The presence of an upstream barchan dune of equal volume to the downstream barchan dune (Vr = 1) induces a 'sheltering effect' on the flow, manifested by a significantly shorter separation bubble and both reduced streamwise velocity and turbulence intensity in the downstream barchan dune leeside, as compared to an isolated barchan. The volumetric ratio Vr = 0.175 shows enhanced turbulence production over the downstream barchan dune leeside, that is proposed to be caused by interacting shear layers from the up-and downstream dunes. The influence of the upstream dune is greater for a larger volumetric ratio due to the sheltering effect of the upstream bedform. Proper orthogonal decomposition (POD) analysis shows that the distribution of turbulent kinetic energy is shifted to higher modes (i.e. smaller spatial scales) over interacting barchan dunes, which also reflects the role of the leeside free shear layer in dominating the flow field by generation, or redistribution, of TKE to higher modes.
Earth Surface Processes and Landforms, Apr 19, 2022
Columbia River, as well as Hans Moritz from the Portland District office of the US Army Corp of E... more Columbia River, as well as Hans Moritz from the Portland District office of the US Army Corp of Engineers (USACE) for providing information regarding the timing, boundaries, and effects, of federal navigation channel dredging operations from 2005-2010. We also thank Pat Killion, captain of the Tansey Point, whose years of experience on the lower Columbia River were paramount towards enabling this research. Lastly, we especially thank Bob Dalrymple for his extremely insightful and stimulating review, as well as the thorough reviews from one anonymous referee and editor that greatly improved the paper. AUTHOR CONTRIBUTIONS E.W. Prokocki: conceptualisation (co-lead); investigation (lead); writingoriginal draft (lead); formal analysis (lead); writingreview and editing (lead); investigation (lead); methodology (co-lead); software (co-lead); project administration (co-lead); visualisation (co-lead). J.L. Best: conceptualisation (co-lead); funding acquisition (co-lead); project administration (co-lead); resources (co-lead); writingoriginal draft (supporting; equal); writingreview and editing (supporting; equal). M.M. Perillo: conceptualisation (co-lead); investigation (supporting); formal analysis (supporting); methodology (co-lead); software (co-lead); writingoriginal draft (supporting). P.J.
Earth Surface Processes and Landforms, Jan 31, 2017
The erosion of rock-bedded channels generally is considered a slow process caused mainly by abras... more The erosion of rock-bedded channels generally is considered a slow process caused mainly by abrasion due to bedload or suspended sediments, but the mechanisms of rapid erosion remain unclear. Cavitation is a clear-fluid erosive process, well known for its effect on engineering structures, when water vapour bubbles collapse and the resultant pressure shocks erode the boundary. However, although the occurrence of cavitation erosion in natural watercourses has long been a matter of debate, as yet there are no incontrovertible examples of cavitation damage to natural river beds. Using flume experiments, we show for the first time that only weakly-cavitating clearwater flows can occur for the range of flow velocities observed in rivers, and these do not erode medium-hardness rocks after 68 hours. During this time period, only a very soft rock featured erosional marks due to dissolution. Thus, our results cast significant doubt on the likelihood of identifying cavitation damage in most rivers, and provide pointers to those river systems that might be investigated further to identify cavitation erosion.
EarthArXiv (California Digital Library), Jan 15, 2018
New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These... more New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These diagrams can be used to distinguish between turbulent Newtonian, transitional, and laminar non-Newtonian flow behavior, on the basis of the balance between turbulent forces (approximated by the horizontal components of flow velocity and turbulence intensity) and cohesive forces (approximated by the suspended clay concentration and rheology). Stability regimes for five different flow types are defined using a comprehensive series of laboratory flume experiments at depth-averaged flow velocities ranging from 0.13 m s-1 to 1.47 m s-1 , and at volumetric kaolinite clay concentrations ranging from 0.03% (= 0.8 g L-1) to 16.7% (= 434 g L-1). As clay concentration increases, five flow types can be distinguished: turbulent flow, turbulenceenhanced transitional flow, lower and upper transitional plug flow, and quasi-laminar plug flow. The turbulent properties of transitional flows are shown to be considerably more complex than the common notion of gradual turbulence damping. Turbulence-enhanced transitional flows display higher turbulence intensity than turbulent flows of similar velocity, with such enhancement originating from development of a highly turbulent basal internal shear layer within ~ 0.01 m of the bed. In lower transitional plug flows, the basal internal shear layer separates a lower region of high vertical gradient in horizontal velocity and strong turbulence from an upper region of plug flow with a much gentler velocity gradient and lower turbulence intensity. Kelvin-Helmholtz shear instabilities within the highly turbulent shear layer are expressed as distinct second-scale oscillations in the time series of downstream velocity. Turbulence damping dominates upper transitional plug flows, because strong cohesive forces, inferred to be caused by gelling of the highconcentration clay suspension, start to outbalance turbulent forces. In quasi-laminar plug flows, gelling is pervasive and turbulence is fully suppressed, apart from some minor residual turbulence near the base of these flows. With very few exceptions, all flows pass through the same development stages as clay concentration increases, regardless of their velocity, but the threshold concentrations for turbulence enhancement, gelling, and development of internal shear layers and plug flows are proportional to flow velocity. At flow velocities
This paper details the dynamics of coherent flow structures generated in shallow flows around imp... more This paper details the dynamics of coherent flow structures generated in shallow flows around impermeable and permeable 2-dimensional bedforms overlaying a highly-permeable idealised bed. Particle imaging velocimetry (PIV) measurements were conducted in order to characterise flow both over and underneath idealised 2dimensional dunes overlaying a packed bed of uniform size spheres. Experiments were conducted in free surface flow conditions (Froude number = 0.1; Reynolds number = 25,000) for one bedform height: flow depth ratio (0.31). The flow above the dune was measured using a standard PIV technique while a novel endoscopic PIV (EPIV) system allowed collection of flow data within the pore spaces beneath the dune. These results show that the permeability of the bed has a critical impact on flow around the bedform, inducing a significant interaction between the freeflow and subsurface flow. The interaction between the free-flow and hyporheic flow is significant; in the leeside, recirculation in the separation zone is replaced by a mechanism of asymmetric alternate vortex shedding. The paper will discuss the implications of these results for the morphodynamics of coarse-sediment bedforms.
14th International Symposium on Particle Image Velocimetry, 2021
Barchan dunes are crescent-shaped bedforms that form in aeolian (i.e., wind-driven) environments ... more Barchan dunes are crescent-shaped bedforms that form in aeolian (i.e., wind-driven) environments (including both Earth and other planets, such as Mars) as well as subaqueous environments. Under the forcing of the aloft turbulent boundary layer, they migrate downstream at a rate inversely proportional to their size, which results in complex interactions between neighboring dunes of disparate scales. In particular, it has been observed that dunes will interact at a distance, causing changes in morphology without contacting each other, which is thought to be driven by the way dunes modify the local flow field Bristow et al. (2018); Assis and Franklin (2020). In this study, the coherent structures formed in the wakes of barchan dunes are investigated using measurements of the flow over fixed-bed (i.e., solid) barchan models, both in the wake of an isolated barchan and the interdune region between interacting barchans (Fig. 1(a)). Furthermore, the interactions between the flow structures...
Despite an increasingly large body of work advancing our understanding of flow interactions occur... more Despite an increasingly large body of work advancing our understanding of flow interactions occurring at the interface of a turbulent flow overlying a permeable bed, little is known concerning how such flow may be affected by the presence of biofilms, which exist in nearly all aquatic environments. This study quantifies the effects on flow exerted by biofilms grown over experimental laboratory permeable beds until biofilm detachment, and then compares this to the residual effects after its detachment. The investigation is conducted in a flow channel by immersing two‐dimensional permeable beds with idealized geometry and different porosities in order to explore different bed permeabilities. Sequences of increasingly higher flow velocity conditions, followed by lower flow, were considered to explore the effect of detachment. Measurements were performed using particle image velocimetry. The total wall shear stress and friction velocity were found to increase in the presence of pregrown...
Channel confluences are key nodes within large river networks, and yet surprisingly little is kno... more Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to th...
The recognition of large fluvial channels in the geological record is of great importance for reg... more The recognition of large fluvial channels in the geological record is of great importance for regional palaeohydraulic and palaeogeographical reconstructions, inputs to reservoir modelling, and estimating the input of sediment to sedimentary basins, with consequent larger-scale implications for modelling basin fill. However, available criteria for the interpretation of the scale of ancient fluvial systems are still poorly tested, particularly the widelyadopted assumption that the abundance of large-scale dunes in some deep channels implies that abundant large-scale cross-strata sets will be preserved in similar palaeochannels. To test this hypothesis, high-resolution multibeam echo-sounding imaging of two reaches in the Amazon River where large dunes are common were investigated, yielding an extensive dataset concerning dune geometry, position within the channel and, most importantly, the presence and distribution of smaller superimposed dunes on their lee sides. These results show that despite 90% of the bedforms at water depths >20 m being constituted by up to 12Á2 m high compound dunes, 94% of the lee sides of these dunes are covered by smaller superimposed dunes. These results suggest that steep avalanche foresets that are several metres in height may be rare in the preserved stratigraphic record of these large channels, which are instead more commonly represented by decimetrescale cross-stratified cosets formed by superimposed dunes migrating down the lee side of the large-scale host bedforms. This observation thus suggests that the recognition of compound dune cosets is key to the interpretation of river-channel scale, since compound dunes are the principal bedform in most large river channels. Consequently, successions dominated by decimetre-scale thick cross-strata sets, but that show rarer preservation of outsized metre-scale avalanche foresets, and abundant similar-sized cosets near the base of fining-upward cycles are probably the most common bedform record of large-river channels.
The overall objectives of this project are as follows: Application of the Wavelet Transform (WT) ... more The overall objectives of this project are as follows: Application of the Wavelet Transform (WT) technique in the spatial domain to characterize the spatial distribution of co-existing bedforms of multiple dimensions generated under different flow field scenarios (waves, unidirectional flows and combined flows).
The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , with a ... more The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , with a significant fraction being sequestered in large deltas, home to over 500 million people. Most (>70%) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a sustainable supply of fluvial sediment is therefore critical in preventing deltas being 'drowned' by rising relative sea levels 2,3,4. Here, we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones (TCs) in transmitting suspended sediment to one of the world's great deltas. We demonstrate that spatial variations in the Mekong's suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations in TC climatology, and that a significant portion (32%) of the suspended sediment load reaching the delta is delivered by runoff generated by TC-associated rainfall. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 Mt over recent years (1981-2005), of which 33.0 ± 7.1 Mt is due to a shift in TC climatology. Consequently TCs play a significant role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast.
This is a repository copy of Bedform genesis in bedrock substrates: Insights into formative proce... more This is a repository copy of Bedform genesis in bedrock substrates: Insights into formative processes from a new experimental approach and the importance of suspension-dominated abrasion.
Radar stratigraphy-a method for analysing 3D GPR data in sedimentary environments as exemplified ... more Radar stratigraphy-a method for analysing 3D GPR data in sedimentary environments as exemplified by fluvial sediments TOMASZ ŻUK
Flows with high suspended sediment concentrations are common in many sedimentary environments, an... more Flows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi-laminar plug flows. The characteristics of these transitional flows are a function of both clay concentration and type as well as the applied fluid stress. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds, which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow (TF), turbulenceenhanced transitional flow (TETF), lower transitional plug flow (LTPF), upper transitional plug flow (UTPF) and a quasi-laminar plug flow (QLPF). Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in TF, TETF and LTPF. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under TETF and LTPF regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater leeside erosion. As clay concentration increases further from a LTPF, ripples cease to exist under the UTPF and QLPF conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross-stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not to form in cohesive flows with a Reynolds number smaller than ~12,000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain-size segregation and longitudinal flow transformation.
In order to experimentally investigate the flow dynamics around a 3D barchan dune, herein we adop... more In order to experimentally investigate the flow dynamics around a 3D barchan dune, herein we adopt a new experimental approach that enables us to quantify flow around complex morphologies using a refractive index matching (RIM) flume. This technique allows the model barchan to be rendered invisible, thus permitting use of standard optical techniques for flow-field quantification. Here, we present full details of this unique new RIM facility and detail its use to investigate flow around a model barchan dune. These results are compared to past experimental and numerical models of barchan dune flow dynamics.
This paper details the dynamics of coherent flow structures generated in shallow flows around imp... more This paper details the dynamics of coherent flow structures generated in shallow flows around impermeable and permeable 2-dimensional bedforms overlaying a highly-permeable idealised bed. Particle imaging velocimetry (PIV) measurements were conducted in order to characterise flow both over and underneath idealised 2dimensional dunes overlaying a packed bed of uniform size spheres. Experiments were conducted in free surface flow conditions (Froude number = 0.1; Reynolds number = 25,000) for one bedform height: flow depth ratio (0.31). The flow above the dune was measured using a standard PIV technique while a novel endoscopic PIV (EPIV) system allowed collection of flow data within the pore spaces beneath the dune. These results show that the permeability of the bed has a critical impact on flow around the bedform, inducing a significant interaction between the freeflow and subsurface flow. The interaction between the free-flow and hyporheic flow is significant; in the leeside, recirculation in the separation zone is replaced by a mechanism of asymmetric alternate vortex shedding. The paper will discuss the implications of these results for the morphodynamics of coarse-sediment bedforms.
The flow within the pore spaces of a permeable bed beneath a turbulent free-flow is investigated ... more The flow within the pore spaces of a permeable bed beneath a turbulent free-flow is investigated through a specially-developed endoscopic PIV technique. Specifically, we present results conducted at high-Reynolds number (21,000) in the presence of a 2D-bedform. The interstitial flow induced by the bedform is investigated at different pore space locations in order to address the impact of the bedform on the pore flow structure. The interstitial flow is characterized by jets, of varying orientation and magnitude, driven by a local pressure gradient. Measurements show intense turbulence in the pore space, resulting in intense velocity fluctuations and the formation of interstitial large vortical structures. Analysis of the instantaneous data allows the mechanism of formation and advection of these structures to be established. The precise nature of the pore flow is also shown to be strongly dependent on the location of the pore with respect to the overlying bedform. For example, the structure of the flow in the region directly beneath the bedform is highly complex and characterized by the interaction of multiple jets. Conversely, the region of the bed located beneath the wake of the bedform is characterized by diagonally converging flow that then moves vertically upward. The implications of these results for the interaction of the pore flow with the free flow are discussed.
Flow processes measured in the laboratory over fixed, 2D or 3D bedforms have mostly been conducte... more Flow processes measured in the laboratory over fixed, 2D or 3D bedforms have mostly been conducted at one flow depth and with bedform dimensions set by scaling laws based upon "equilibrium" flow conditions. These results thus have limited applicability to many natural situations where bedforms and flow fields are co-evolving at different rates in response to transient conditions, such as changes in flow depth and flow discharge associated with a flood. The research presented herein investigates flow processes over 2D fixed bedforms under a range of non-equilibrium, transient, states in order to quantify the spatio-temporal changes in turbulence associated with steady conditions that are set at non equilibrium depths and velocities. Flow field information was obtained at steady states for a range of flow depths and mean flow velocities, mimicking conditions during the transient evolution of flow and bedforms during a flood wave. This allowed quantification of flow fields over bedforms under transient boundary conditions, including shear stress profiles and the spatial variation in the dynamics of the separation zone. These findings provide data for a preliminary assessment of the link between sediment transport lag and transient flow dynamics, and facilitate an analysis of the implications of variable dune height: flow depth for flood wave propagation and bedform response.
River dams provide many benefits, including flood control. However, due to constantly evolving ch... more River dams provide many benefits, including flood control. However, due to constantly evolving channel morphology, downstream conveyance of floodwaters following dam closure is difficult to predict. Here, we test the hypothesis that the incised, enlarged channel downstream of dams provides enhanced water conveyance, using a case study from the lower Yellow River, China. We find that, although flood stage is lowered for small floods, counterintuitively, flood stage downstream of a dam can be amplified for moderate and large floods. This arises because bed incision is accompanied by sediment coarsening, which facilitates development of large dunes that increase flow resistance and reduce velocity relative to pre-dam conditions. Our findings indicate the underlying mechanism for such flood amplification may occur in >80% of fine-grained rivers, and suggest the need to reconsider flood control strategies in such rivers worldwide.
Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments,... more Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments, and may occur either in isolation or in groups. This study uses high-resolution particle-image velocimetry (PIV) experiments using fixed-bed models to examine the effects of barchan dune interaction upon the flow field structure. The barchan dune models were based upon an idealized contour map, the shape and dimensions of which were based upon previous empirical studies of dune morphology. The experimental setup comprised two, co-axially aligned barchan dune models that were spaced at different distances apart. In this paper, two volumetric ratios (Vr, upstream barchan dune: downstream barchan dune) of 1.0 and 0.175 were examined. Models were placed in an Eiffel-type, open-circuit wind tunnel and flow quantification was achieved using PIV at 0.5Hz. PIV measurements of the mean and turbulent flow field were made in the streamwise-wallnormal plane, along the centerline of the barchans(s), at an average Reynolds number of 59,000. The presence of an upstream barchan dune of equal volume to the downstream barchan dune (Vr = 1) induces a 'sheltering effect' on the flow, manifested by a significantly shorter separation bubble and both reduced streamwise velocity and turbulence intensity in the downstream barchan dune leeside, as compared to an isolated barchan. The volumetric ratio Vr = 0.175 shows enhanced turbulence production over the downstream barchan dune leeside, that is proposed to be caused by interacting shear layers from the up-and downstream dunes. The influence of the upstream dune is greater for a larger volumetric ratio due to the sheltering effect of the upstream bedform. Proper orthogonal decomposition (POD) analysis shows that the distribution of turbulent kinetic energy is shifted to higher modes (i.e. smaller spatial scales) over interacting barchan dunes, which also reflects the role of the leeside free shear layer in dominating the flow field by generation, or redistribution, of TKE to higher modes.
Earth Surface Processes and Landforms, Apr 19, 2022
Columbia River, as well as Hans Moritz from the Portland District office of the US Army Corp of E... more Columbia River, as well as Hans Moritz from the Portland District office of the US Army Corp of Engineers (USACE) for providing information regarding the timing, boundaries, and effects, of federal navigation channel dredging operations from 2005-2010. We also thank Pat Killion, captain of the Tansey Point, whose years of experience on the lower Columbia River were paramount towards enabling this research. Lastly, we especially thank Bob Dalrymple for his extremely insightful and stimulating review, as well as the thorough reviews from one anonymous referee and editor that greatly improved the paper. AUTHOR CONTRIBUTIONS E.W. Prokocki: conceptualisation (co-lead); investigation (lead); writingoriginal draft (lead); formal analysis (lead); writingreview and editing (lead); investigation (lead); methodology (co-lead); software (co-lead); project administration (co-lead); visualisation (co-lead). J.L. Best: conceptualisation (co-lead); funding acquisition (co-lead); project administration (co-lead); resources (co-lead); writingoriginal draft (supporting; equal); writingreview and editing (supporting; equal). M.M. Perillo: conceptualisation (co-lead); investigation (supporting); formal analysis (supporting); methodology (co-lead); software (co-lead); writingoriginal draft (supporting). P.J.
Earth Surface Processes and Landforms, Jan 31, 2017
The erosion of rock-bedded channels generally is considered a slow process caused mainly by abras... more The erosion of rock-bedded channels generally is considered a slow process caused mainly by abrasion due to bedload or suspended sediments, but the mechanisms of rapid erosion remain unclear. Cavitation is a clear-fluid erosive process, well known for its effect on engineering structures, when water vapour bubbles collapse and the resultant pressure shocks erode the boundary. However, although the occurrence of cavitation erosion in natural watercourses has long been a matter of debate, as yet there are no incontrovertible examples of cavitation damage to natural river beds. Using flume experiments, we show for the first time that only weakly-cavitating clearwater flows can occur for the range of flow velocities observed in rivers, and these do not erode medium-hardness rocks after 68 hours. During this time period, only a very soft rock featured erosional marks due to dissolution. Thus, our results cast significant doubt on the likelihood of identifying cavitation damage in most rivers, and provide pointers to those river systems that might be investigated further to identify cavitation erosion.
EarthArXiv (California Digital Library), Jan 15, 2018
New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These... more New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These diagrams can be used to distinguish between turbulent Newtonian, transitional, and laminar non-Newtonian flow behavior, on the basis of the balance between turbulent forces (approximated by the horizontal components of flow velocity and turbulence intensity) and cohesive forces (approximated by the suspended clay concentration and rheology). Stability regimes for five different flow types are defined using a comprehensive series of laboratory flume experiments at depth-averaged flow velocities ranging from 0.13 m s-1 to 1.47 m s-1 , and at volumetric kaolinite clay concentrations ranging from 0.03% (= 0.8 g L-1) to 16.7% (= 434 g L-1). As clay concentration increases, five flow types can be distinguished: turbulent flow, turbulenceenhanced transitional flow, lower and upper transitional plug flow, and quasi-laminar plug flow. The turbulent properties of transitional flows are shown to be considerably more complex than the common notion of gradual turbulence damping. Turbulence-enhanced transitional flows display higher turbulence intensity than turbulent flows of similar velocity, with such enhancement originating from development of a highly turbulent basal internal shear layer within ~ 0.01 m of the bed. In lower transitional plug flows, the basal internal shear layer separates a lower region of high vertical gradient in horizontal velocity and strong turbulence from an upper region of plug flow with a much gentler velocity gradient and lower turbulence intensity. Kelvin-Helmholtz shear instabilities within the highly turbulent shear layer are expressed as distinct second-scale oscillations in the time series of downstream velocity. Turbulence damping dominates upper transitional plug flows, because strong cohesive forces, inferred to be caused by gelling of the highconcentration clay suspension, start to outbalance turbulent forces. In quasi-laminar plug flows, gelling is pervasive and turbulence is fully suppressed, apart from some minor residual turbulence near the base of these flows. With very few exceptions, all flows pass through the same development stages as clay concentration increases, regardless of their velocity, but the threshold concentrations for turbulence enhancement, gelling, and development of internal shear layers and plug flows are proportional to flow velocity. At flow velocities
This paper details the dynamics of coherent flow structures generated in shallow flows around imp... more This paper details the dynamics of coherent flow structures generated in shallow flows around impermeable and permeable 2-dimensional bedforms overlaying a highly-permeable idealised bed. Particle imaging velocimetry (PIV) measurements were conducted in order to characterise flow both over and underneath idealised 2dimensional dunes overlaying a packed bed of uniform size spheres. Experiments were conducted in free surface flow conditions (Froude number = 0.1; Reynolds number = 25,000) for one bedform height: flow depth ratio (0.31). The flow above the dune was measured using a standard PIV technique while a novel endoscopic PIV (EPIV) system allowed collection of flow data within the pore spaces beneath the dune. These results show that the permeability of the bed has a critical impact on flow around the bedform, inducing a significant interaction between the freeflow and subsurface flow. The interaction between the free-flow and hyporheic flow is significant; in the leeside, recirculation in the separation zone is replaced by a mechanism of asymmetric alternate vortex shedding. The paper will discuss the implications of these results for the morphodynamics of coarse-sediment bedforms.
14th International Symposium on Particle Image Velocimetry, 2021
Barchan dunes are crescent-shaped bedforms that form in aeolian (i.e., wind-driven) environments ... more Barchan dunes are crescent-shaped bedforms that form in aeolian (i.e., wind-driven) environments (including both Earth and other planets, such as Mars) as well as subaqueous environments. Under the forcing of the aloft turbulent boundary layer, they migrate downstream at a rate inversely proportional to their size, which results in complex interactions between neighboring dunes of disparate scales. In particular, it has been observed that dunes will interact at a distance, causing changes in morphology without contacting each other, which is thought to be driven by the way dunes modify the local flow field Bristow et al. (2018); Assis and Franklin (2020). In this study, the coherent structures formed in the wakes of barchan dunes are investigated using measurements of the flow over fixed-bed (i.e., solid) barchan models, both in the wake of an isolated barchan and the interdune region between interacting barchans (Fig. 1(a)). Furthermore, the interactions between the flow structures...
Despite an increasingly large body of work advancing our understanding of flow interactions occur... more Despite an increasingly large body of work advancing our understanding of flow interactions occurring at the interface of a turbulent flow overlying a permeable bed, little is known concerning how such flow may be affected by the presence of biofilms, which exist in nearly all aquatic environments. This study quantifies the effects on flow exerted by biofilms grown over experimental laboratory permeable beds until biofilm detachment, and then compares this to the residual effects after its detachment. The investigation is conducted in a flow channel by immersing two‐dimensional permeable beds with idealized geometry and different porosities in order to explore different bed permeabilities. Sequences of increasingly higher flow velocity conditions, followed by lower flow, were considered to explore the effect of detachment. Measurements were performed using particle image velocimetry. The total wall shear stress and friction velocity were found to increase in the presence of pregrown...
Channel confluences are key nodes within large river networks, and yet surprisingly little is kno... more Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to th...
The recognition of large fluvial channels in the geological record is of great importance for reg... more The recognition of large fluvial channels in the geological record is of great importance for regional palaeohydraulic and palaeogeographical reconstructions, inputs to reservoir modelling, and estimating the input of sediment to sedimentary basins, with consequent larger-scale implications for modelling basin fill. However, available criteria for the interpretation of the scale of ancient fluvial systems are still poorly tested, particularly the widelyadopted assumption that the abundance of large-scale dunes in some deep channels implies that abundant large-scale cross-strata sets will be preserved in similar palaeochannels. To test this hypothesis, high-resolution multibeam echo-sounding imaging of two reaches in the Amazon River where large dunes are common were investigated, yielding an extensive dataset concerning dune geometry, position within the channel and, most importantly, the presence and distribution of smaller superimposed dunes on their lee sides. These results show that despite 90% of the bedforms at water depths >20 m being constituted by up to 12Á2 m high compound dunes, 94% of the lee sides of these dunes are covered by smaller superimposed dunes. These results suggest that steep avalanche foresets that are several metres in height may be rare in the preserved stratigraphic record of these large channels, which are instead more commonly represented by decimetrescale cross-stratified cosets formed by superimposed dunes migrating down the lee side of the large-scale host bedforms. This observation thus suggests that the recognition of compound dune cosets is key to the interpretation of river-channel scale, since compound dunes are the principal bedform in most large river channels. Consequently, successions dominated by decimetre-scale thick cross-strata sets, but that show rarer preservation of outsized metre-scale avalanche foresets, and abundant similar-sized cosets near the base of fining-upward cycles are probably the most common bedform record of large-river channels.
The overall objectives of this project are as follows: Application of the Wavelet Transform (WT) ... more The overall objectives of this project are as follows: Application of the Wavelet Transform (WT) technique in the spatial domain to characterize the spatial distribution of co-existing bedforms of multiple dimensions generated under different flow field scenarios (waves, unidirectional flows and combined flows).
The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , with a ... more The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , with a significant fraction being sequestered in large deltas, home to over 500 million people. Most (>70%) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a sustainable supply of fluvial sediment is therefore critical in preventing deltas being 'drowned' by rising relative sea levels 2,3,4. Here, we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones (TCs) in transmitting suspended sediment to one of the world's great deltas. We demonstrate that spatial variations in the Mekong's suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations in TC climatology, and that a significant portion (32%) of the suspended sediment load reaching the delta is delivered by runoff generated by TC-associated rainfall. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 Mt over recent years (1981-2005), of which 33.0 ± 7.1 Mt is due to a shift in TC climatology. Consequently TCs play a significant role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast.
This is a repository copy of Bedform genesis in bedrock substrates: Insights into formative proce... more This is a repository copy of Bedform genesis in bedrock substrates: Insights into formative processes from a new experimental approach and the importance of suspension-dominated abrasion.
Radar stratigraphy-a method for analysing 3D GPR data in sedimentary environments as exemplified ... more Radar stratigraphy-a method for analysing 3D GPR data in sedimentary environments as exemplified by fluvial sediments TOMASZ ŻUK
Flows with high suspended sediment concentrations are common in many sedimentary environments, an... more Flows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi-laminar plug flows. The characteristics of these transitional flows are a function of both clay concentration and type as well as the applied fluid stress. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds, which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow (TF), turbulenceenhanced transitional flow (TETF), lower transitional plug flow (LTPF), upper transitional plug flow (UTPF) and a quasi-laminar plug flow (QLPF). Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in TF, TETF and LTPF. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under TETF and LTPF regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater leeside erosion. As clay concentration increases further from a LTPF, ripples cease to exist under the UTPF and QLPF conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross-stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not to form in cohesive flows with a Reynolds number smaller than ~12,000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain-size segregation and longitudinal flow transformation.
In order to experimentally investigate the flow dynamics around a 3D barchan dune, herein we adop... more In order to experimentally investigate the flow dynamics around a 3D barchan dune, herein we adopt a new experimental approach that enables us to quantify flow around complex morphologies using a refractive index matching (RIM) flume. This technique allows the model barchan to be rendered invisible, thus permitting use of standard optical techniques for flow-field quantification. Here, we present full details of this unique new RIM facility and detail its use to investigate flow around a model barchan dune. These results are compared to past experimental and numerical models of barchan dune flow dynamics.
This paper details the dynamics of coherent flow structures generated in shallow flows around imp... more This paper details the dynamics of coherent flow structures generated in shallow flows around impermeable and permeable 2-dimensional bedforms overlaying a highly-permeable idealised bed. Particle imaging velocimetry (PIV) measurements were conducted in order to characterise flow both over and underneath idealised 2dimensional dunes overlaying a packed bed of uniform size spheres. Experiments were conducted in free surface flow conditions (Froude number = 0.1; Reynolds number = 25,000) for one bedform height: flow depth ratio (0.31). The flow above the dune was measured using a standard PIV technique while a novel endoscopic PIV (EPIV) system allowed collection of flow data within the pore spaces beneath the dune. These results show that the permeability of the bed has a critical impact on flow around the bedform, inducing a significant interaction between the freeflow and subsurface flow. The interaction between the free-flow and hyporheic flow is significant; in the leeside, recirculation in the separation zone is replaced by a mechanism of asymmetric alternate vortex shedding. The paper will discuss the implications of these results for the morphodynamics of coarse-sediment bedforms.
The flow within the pore spaces of a permeable bed beneath a turbulent free-flow is investigated ... more The flow within the pore spaces of a permeable bed beneath a turbulent free-flow is investigated through a specially-developed endoscopic PIV technique. Specifically, we present results conducted at high-Reynolds number (21,000) in the presence of a 2D-bedform. The interstitial flow induced by the bedform is investigated at different pore space locations in order to address the impact of the bedform on the pore flow structure. The interstitial flow is characterized by jets, of varying orientation and magnitude, driven by a local pressure gradient. Measurements show intense turbulence in the pore space, resulting in intense velocity fluctuations and the formation of interstitial large vortical structures. Analysis of the instantaneous data allows the mechanism of formation and advection of these structures to be established. The precise nature of the pore flow is also shown to be strongly dependent on the location of the pore with respect to the overlying bedform. For example, the structure of the flow in the region directly beneath the bedform is highly complex and characterized by the interaction of multiple jets. Conversely, the region of the bed located beneath the wake of the bedform is characterized by diagonally converging flow that then moves vertically upward. The implications of these results for the interaction of the pore flow with the free flow are discussed.
Flow processes measured in the laboratory over fixed, 2D or 3D bedforms have mostly been conducte... more Flow processes measured in the laboratory over fixed, 2D or 3D bedforms have mostly been conducted at one flow depth and with bedform dimensions set by scaling laws based upon "equilibrium" flow conditions. These results thus have limited applicability to many natural situations where bedforms and flow fields are co-evolving at different rates in response to transient conditions, such as changes in flow depth and flow discharge associated with a flood. The research presented herein investigates flow processes over 2D fixed bedforms under a range of non-equilibrium, transient, states in order to quantify the spatio-temporal changes in turbulence associated with steady conditions that are set at non equilibrium depths and velocities. Flow field information was obtained at steady states for a range of flow depths and mean flow velocities, mimicking conditions during the transient evolution of flow and bedforms during a flood wave. This allowed quantification of flow fields over bedforms under transient boundary conditions, including shear stress profiles and the spatial variation in the dynamics of the separation zone. These findings provide data for a preliminary assessment of the link between sediment transport lag and transient flow dynamics, and facilitate an analysis of the implications of variable dune height: flow depth for flood wave propagation and bedform response.
Uploads
Papers by Jim Best