Jim Best

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Papers by Jim Best

Fluvial form in modern continental sedimentary basins: Distributive fluvial systems: COMMENT

Geology, Dec 1, 2010

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Sedimentology and kinematics of a large, retrogressive growth-fault system in Upper Carboniferous deltaic sediments, western Ireland

Sedimentology, Dec 1, 2004

. ( ) * /0 ∃1∋ 022+02/3 .

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$Research paper thumbnail of Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbulent flow structure over 3D bedforms$

Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbulent flow structure over 3D bedforms

Bulletin of the American Physical Society, Nov 20, 2017

Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbu... more Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbulent flow structure over 3D bedforms 1 NATHANIEL BRISTOW, GIANLUCA BLOIS, Univ of Notre Dame, JAMES BEST, University of Illinois at Urbana-Champaign, KENNETH CHRIS-TENSEN, Univ of Notre Dame -Barchan dunes are three-dimensional, crescentshaped bedforms found in regions of unidirectional flow and limited sediment supply, and while most commonly associated with aeolian environments, recent observations have shown them to exist in river beds, along continental shelves, and on the surfaces of Mars and Titan. As barchans migrate in the direction of the flow, they interact with their neighbors, typically by way of a collision. The morphodynamics of such collision processes are complex, where the role of the turbulent flow structure is strongly coupled to that of the sediment transport and morphological change. Here we study the flow structure in a decoupled manner through measurements of the turbulent flow over fixed-bed models of barchan dunes in various configurations involved in a barchan collision process. Particle image velocimetry is used to measure the flow in a refractive-index matched flume environment that enables uninhibited access to the whole flow field around these geometrically complex bedforms. Presented herein are results from stereo PIV measurements showing, for the first time experimentally, the turbulent flow structure in the cross-plane behind the horns of a barchan undergoing a collision with a larger, downstream barchan.

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Beyond just floodwater

Nature sustainability, Jul 14, 2022

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Effects of low clay concentrations on nearly isotropic turbulence

Physical Review Fluids

We conducted laboratory experiments to quantify the effect of low concentrations of clay, C ∈ [0,... more We conducted laboratory experiments to quantify the effect of low concentrations of clay, C ∈ [0, 2]%, on turbulence in water-clay flows under nearly isotropic conditions using a novel approach. Full optical access was possible using Laponite RD TM , a synthetic clay capable of producing clear suspensions of clay particles. Turbulence was generated in a customized cubic mixing box using flow actuators placed symmetrically at each of the eight corners. High-frame-rate particle image velocimetry was used to quantify the spatiotemporal dynamics of turbulence around the center of the mixing box. The results showed that the presence of clay particles strongly modulated the structure of turbulence. Monotonic reduction of the turbulent kinetic energy (TKE) and mean dissipation rate and increase of the Taylor microscale λ was induced by increasing the clay concentration. In particular, the measurements indicated a relation between clay concentration and the p factor, the known Kolmogorov power-law correction -5/3 + p(Re λ ) of the inertial subrange, where Re λ is the Taylor microscale Reynolds number. The p factor decreased approximately linearly within 1% C 2%. This parameter and the third-order structure function S 3 supported the scale changes obtained from direct estimation with particle image velocimetry (PIV) data. These results demonstrated the significant impact of clay on the multiscale dynamics of turbulence even at low clay concentrations.

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On the turbulence dynamics induced by a surrogate seagrass canopy

Journal of Fluid Mechanics, 2022

The distinct turbulence dynamics and transport modulated by a common seagrass species were invest... more The distinct turbulence dynamics and transport modulated by a common seagrass species were investigated experimentally using a flexible surrogate canopy in a refractive-index-matching environment that enabled full optical access. The surrogate seagrass replicated the dynamic behaviour and morphological properties of its natural counterpart. The flows studied were subcritical with Froude numbers $Fr<0.26$ and concerned five Reynolds numbers $Re\in [3.4\times 10^{4}, 1.1\times 10^{5}]$ and Cauchy numbers $Ca\in [120, 1200]$ . Complementary rigid canopy experiments were also included to aid comparative insight. The flow was quantified in wall-normal planes in a developed region using high-frame-rate particle image velocimetry. Results show that the deflection and coordinated waving motion of the blades redistributed the Reynolds stresses above and below the canopy top. Critically, in-canopy turbulence associated with the seagrass lacked periodic stem wake vortex shedding present in ...

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The role of receiving basin substrate cohesivity in delta morphodynamics

Deltas are home to hundreds of millions of people worldwide and form a key part of many coastal e... more Deltas are home to hundreds of millions of people worldwide and form a key part of many coastal environments. Due to their low elevation, many deltas are threatened by sea level rise as well as direct human influences on flow, sediment delivery and subsidence. The calibre and cohesivity of river sediment have both been shown to be important factors in determining the erosion, deposition and stability regimes within a delta system. However, the influence of the receiving basin substrate characteristics is poorly constrained. It has been shown that basin substrates composed of finer sediment lead to more incisive channel. However, the exact controls of substrate characteristics on delta morphology and morphodynamics has not been quantified.

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Effects of biofilm on flow over and through a permeable bed

Bulletin of the American Physical Society, 2016

constitute an important form of bacterial life in aquatic environments and are present at the int... more constitute an important form of bacterial life in aquatic environments and are present at the interface of fluids and solids, such as riverbeds. Biofilms are permeable, heterogeneous, and deformable structures that can influence the flow and mass/momentum transport, yet their interaction with flow is not fully understood in part due to technical obstacles impeding quantitative experimental investigations. The porosity of river beds results in the generation of a diverse mosaic of 'suction' and 'ejection' events that are far removed from typical assumptions of turbulent flow structure over an impermeable bed. In this work, the effect of biofilm on bed permeability is studied. Experiments are conducted in a closed water channel equipped with 4-cm-deep permeable bed models consisting of horizontal cylinders normal to the bulk flow direction, forming an idealized two-dimensional permeable bed. Prior to conducting flow experiments, the models are placed within an independent biofilm reactor to initiate and control the biofilm growth. Once a targeted biofilm growth stage is achieved, the models are transferred to the water channel and subjected to transitional and turbulent flows. Long-distance microscopic particle image velocimetry measurements are performed to quantify the effect of biofilm on the turbulence structure of the free flow as well as the freestream-subsurface flow interaction.

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Drainage and erosion of Cambodia’s great lake in the middle-late Holocene: The combined role of climatic drying, base-level fall and river capture

Quaternary Science Reviews, 2020

We provide evidence for a large-scale geomorphic event in Cambodia's great lake, the Tonlé Sap, d... more We provide evidence for a large-scale geomorphic event in Cambodia's great lake, the Tonlé Sap, during the middle Holocene. The present-day hydrology of the basin is dominated by an annual flood pulse where water from the Mekong River raises the lake level by c. 8m during the monsoon season. We present new subsurface geophysical data, allied to new and past core studies, which unequivocally show a period of major mid-Holocene erosion across the entire Tonlé Sap basin that is coincident with establishment of the lake's flood pulse. We argue that this widespread erosion, which removed at least 1.2 m of sediment across the lake's extent, was triggered by up to three, likely interacting, processes: (1) base-level lowering due to mid-Holocene sea-level fall, leading to (2) capture of the Tonlé Sap drainage by the Mekong River, and (3) a drying climate that also reduced lake level. Longer-term landscape evolution was thus punctuated by a rapid, river capture-and base-level fallinduced, lake drainage that established the ecosystem that flourishes today. The scale of change induced by this mid-Holocene river capture event demonstrates the susceptibility of the Tonlé Sap to ongoing changes in local base-level and hydrology induced by anthropogenic activity, such as damming and sand mining, within the Mekong River Basin.

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Proto-dune formation under a bimodal wind regime

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Observations and scaling of tidal mass transport across the lower Ganges-Brahmaputra delta plain: implications for delta management and sustainability

Earth Surface Dynamics Discussions, 2018

The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associ... more The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges and Brahmaputra Rivers, is now maintained almost exclusively by tidal processes as the fluvial system has migrated to the east through the Holocene. In natural areas such as the Sundarbans National Forest, year-round spring-tide inundation delivers sufficient sediment for vertical accretion to keep pace with relative sealevel rise. However, recent human modification of the landscape in the form of embankment construction has terminated this pathway of sediment delivery for much of the region, resulting in a startling elevation imbalance, with inhabited areas often sitting >1 m below mean high water. Restoring this landscape, or preventing land loss in the natural system, requires an understanding of how rates of water and sediment flux vary across time scales ranging from hours to months. In this study, we combine time-series observations of water level, salinity, and suspended sediment concentration, with shipbased measurements of large tidal channel hydrodynamics and sediment transport. To capture the greatest possible range of variability, cross-channel transects designed to encompass a 12.4-h tidal cycle were performed in both dry and wet seasons, during spring and neap tides. Regional suspended sediment concentration begins to increase in August, coincident with a decrease in local salinity, indicating the arrival of the sediment-laden, freshwater plume of the combined Ganges-Brahmaputra-Meghna rivers. We observe profound seasonality in sediment transport, despite somewhat modest seasonal variability in the magnitude of water discharge, indicating the importance of this seasonal sediment delivery. On tidal time-scales, spring tides transport an order of magnitude more sediment than neap tides in both the wet and dry seasons. In aggregate, sediment transport is flood-oriented, likely a result of tidal pumping. Finally, we note that rates of sediment and water discharge in the tidal channels are of the same scale as the annually averaged values for the Ganges or Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as the quintessential tidal delta in the classification scheme of Galloway. These data also inform critical questions regarding the timing and magnitude of sediment delivery to the region, which are especially important in predicting, and preparing for, future change under changing environmental conditions.

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Fluvio-deltaic avulsions during relative sea-level fall

Geology, 2015

Understanding river response to changes in relative sea level (RSL) is essential for predicting f... more Understanding river response to changes in relative sea level (RSL) is essential for predicting fluvial stratigraphy and source-to-sink dynamics. Recent theoretical work has suggested that rivers can remain aggradational during RSL fall, but field data are needed to verify this response and investigate sediment deposition processes. We show with field work and modeling that fluvio-deltaic systems can remain aggradational or at grade during RSL fall, leading to superelevation and continuation of delta lobe avulsions. The field site is the Goose River, Newfoundland-Labrador, Canada, which has experienced steady RSL fall of around 3-4 mm yr -1 in the past 5 k.y. from post-glacial isostatic rebound. Elevation analysis and optically stimulated luminescence dating suggest that the Goose River avulsed and deposited three delta lobes during RSL fall. Simulation results from Delft3D software show that if the characteristic fluvial response time is longer than the duration of RSL fall, then fluvial systems remain aggradational or at grade, and continue to avulse during RSL fall due to superelevation. Intriguingly, we find that avulsions become more frequent at faster rates of RSL fall, provided the system response time remains longer than the duration of RSL fall. This work suggests that RSL fall rate may influence the architecture of falling-stage or forced regression deposits by controlling the number of deposited delta lobes.

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Three-dimensional numerical modeling of flows over a natural dune field

Proceedings of the International Conference on Fluvial Hydraulics, Lisbon, Portugal, 6-8 September 2006, 2006

The presence of vegetation plays a significant role in the sustainable development of aquatic sys... more The presence of vegetation plays a significant role in the sustainable development of aquatic systems in steams and rivers, and it affects water hydrodynamics and scalar mixing process. This paper developed a three dimensional large eddy simulation model for flows and scalar transport in an open channel with vegetation. An Eulerian and Lagrangian model was applied to simulate the scalar transport in an open channel with vegetation, respectively. A random walk model was used to describe the mechanical dispersion coefficient in the Eulerian and Lagrangian model. The model was validated by three tests, i.e., the flows in an open channel with submerged vegetation, the flows in an open channel with partly submerged vegetation and the scalar transport in a vegetated channel. The simulation results are in good agreement with the measurement data. As a result, the model has a potential application in simulating the flow and/or scalar transport in an open channel with vegetation.

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Sediment Transport, Bed Morphology and the Sedimentology of River Channel Confluences

River Confluences, Tributaries and the Fluvial Network, 2008

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Modelling flow through a permeable bed: a combined physical-numerical approach

In this paper, a CFD code developed to model turbulent flow across the surface-subsurface interfa... more In this paper, a CFD code developed to model turbulent flow across the surface-subsurface interface of a high-conductivity permeable bed is validated by using experimental data collected at the scale of the individual pore space. High-resolution endoscopic PIV data are compared with numerical results for the simple model of a permeable bed comprising uniform size spheres (D = 4 cm) packed in a cubic arrangement. We present here details of both the numerical and experimental techniques combined for investigating such turbulent interstitial flow, and highlight the major issues involved in both the numerical and physical modelling.

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Shear-Layer Interactions Between Surface-Mounted Obstacles at Varying Streamwise Spacings

The American Physical Society Shear-Layer Interactions Between Surface-Mounted Obstacles at Varyi... more The American Physical Society Shear-Layer Interactions Between Surface-Mounted Obstacles at Varying Streamwise Spacings T. KIM, J.L. BEST, K.T. CHRISTENSEN, University of Illinois at Urbana-Champaign -Surface obstacles occur in a variety of flows, such as roughness elements in engineering flows and barchan dunes in natural eolian environments on both the Earth and Mars. Depending upon the arrangement and spacing between such obstacles, the flow over one obstacle can significantly alter the flow over those positioned downstream. Such flow interactions occur in fields of barchan dunes that are closely spaced and aligned in the flow direction, and where flow sheltering may play a significant role. To better understand these flow interactions, experiments were conducted for a pair of identical, upright cylinders extending into the log layer and aligned at various spacings in the streamwise direction of a turbulent channel flow at Re τ ∼ 1200. Particle-image velocimetry measurements of the flow around the cylinders reveal strong interactions between the shear layers generated downstream of the cylinders, and particularly a weakening of the downstream-most shear layer for small cylinder spacings (< 4 -6D). Modifications of the vortex-shedding processes at the downstream cylinder are under investigation, as these interactions are thought to play a critical role in the formation and evolution of surface obstacles when the surface is cohesionless and mobile.

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$Research paper thumbnail of Flow over interacting barchan dunes studied in a refractive-index-matched environment$

Flow over interacting barchan dunes studied in a refractive-index-matched environment

Flow over interacting barchan dunes studied in a refractive-indexmatched environment Z. TANG, N. ... more Flow over interacting barchan dunes studied in a refractive-indexmatched environment Z. TANG, N. JIANG, Tianjin University, G. BLOIS, J.M. BARROS, J.L. BEST, K.T. CHRISTENSEN, Univ. of Illinois -Barchan dunes are three-dimensional topographic features characterized by a crescentic shape. Very common on Earth's surface, barchans are produced by unidirectional flows in regions of sediment starvation and are characterized by migration rates that are a function of their volume. This results in complex dune-to-dune interaction mechanisms that are poorly understood. In order to quantify the flow structure produced by interacting barchans, PIV measurements were made wherein the dune models were immersed in a flowing fluid that was refractive-index-matched to the dune material. Doing so provided full optical access to the obstructed regions of flow and eliminated reflections from the liquid-solid boundaries, allowing near-wall data to be collected. Clear barchan models with different volumetric ratios were arranged in tandem, and flow-field measurements were made in multiple streamwise-wall-normal and streamwise-spanwise planes. Ensemble-averaged flow fields and Reynolds stresses were obtained for different barchan spacings and compared to the reference case of an isolated barchan. Proper orthogonal decomposition analysis was employed to study the spatial characteristics of the energy distribution both between and downstream of the aligned dunes.

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Flow fields over unsteady three dimensional dunes

The flow field over dunes has been extensively measured in laboratory conditions and there is gen... more The flow field over dunes has been extensively measured in laboratory conditions and there is general understanding on 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 these unsteady flows, 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 subsequently alter the flow field. This paper investigates, through the application of a numerical model, the influence of these roughness elements on the overall flow and the increase in flow resistance. A series of experiments were undertaken in a flume, 16m long and 2m wide, where a fine sand (D50 of 239µm) mobile bed was water worked under a range of unsteady hydraulic conditions to generate a series of quasi-equilibrium three dimensional bed forms. During the experiments flow was measured with acoustic Doppler velocimeters, (aDv's). On four occasions the flume was drained and the bed topography measured with terrestrial LiDAR to create digital elevation models. This data provide the necessary boundary conditions and validation data for a Large Eddy Simulation (LES) model, which provided a three dimensional time dependent prediction of flow over the four static beds. The numerical predicted flow is analyzed through a series of approaches, and included: i) standard Reynolds decomposition to the flow fields; ii) Eulerian coherent structure detection methods based on the invariants of the velocity gradient tensor; iii) Lagrangian coherent structure identification methods based upon direct Lyapunov exponents (DLE). The results show that superimposed bed forms can cause changes in the nature of the classical separated flow region in particularly the number of locations where vortices are shed and the point of flow reattachment, which may be important for sediment entrainment and sediment transport dynamics during bed form adjustment. Finally, the flow predictions enable a reassessment of the drag caused by the superimposed bed forms generated by unsteady flow.

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Sedimentology and kinematics of a large, retrogressive growth‐fault system in Upper Carboniferous deltaic sediments, western Ireland

Sedimentology, 2004

Growth faulting is a common feature of many deltaic environments and is vital in determining loca... more Growth faulting is a common feature of many deltaic environments and is vital in determining local sediment dispersal and accumulation, and hence in controlling the resultant sedimentary facies distribution and architecture. Growth faults occur on a range of scales, from a few centimetres to hundreds of metres, with the largest growth faults frequently being under‐represented in outcrops that are often smaller than the scale of feature under investigation. This paper presents data from the exceptionally large outcrops of the Cliffs of Moher, western Ireland, where a growth‐fault complex affects strata up to 60 m in thickness and extends laterally for ≈ 3 km. Study of this Namurian (Upper Carboniferous) growth‐fault system enables the relationship between growth faulting and sedimentation to be detailed and permits reconstruction of the kinematic history of faulting. Growth faulting was initiated with the onset of sandstone deposition on a succession of silty mudstones that overlie a...

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Electrical Resistance Tomography for Suspended Sediment Measurements in Open Channel Flows Using a Novel Sensor Design

Particle & Particle Systems Characterization, 2006

Understanding the transport and deposition of sediment (such as sand and clay) is essential to al... more Understanding the transport and deposition of sediment (such as sand and clay) is essential to allow the modelling and prediction of sediment dispersal, and the development of depositional morphology, within modern environments and ancient sedimentary basins. Electrical Resistance Tomography (ERT) has previously been used to provide a measure of particle concentration within moving suspensions. ERT measurements normally employ electrodes around a circular boundary of a container that allows measurement of multiple projections. However, measuring the concentration of a flow in a rectangular channel without disturbing the flow cannot use these well‐established sensor rings. This paper investigates the use of a linear sensor array in the channel bed, and a U‐shaped sensor array in the channel bed and walls, to measure spatial and temporal gradients in sediment concentration/conductivity of a channellised flow. To overcome the problems with the effect of sediment‐covered electrodes and ...

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Fluvial form in modern continental sedimentary basins: Distributive fluvial systems: COMMENT

Geology, Dec 1, 2010

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Sedimentology and kinematics of a large, retrogressive growth-fault system in Upper Carboniferous deltaic sediments, western Ireland

Sedimentology, Dec 1, 2004

. ( ) * /0 ∃1∋ 022+02/3 .

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$Research paper thumbnail of Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbulent flow structure over 3D bedforms$

Cross-plane Stereo-PIV measurements in a refractive index matched flume of to elucidate the turbulent flow structure over 3D bedforms

Bulletin of the American Physical Society, Nov 20, 2017

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Beyond just floodwater

Nature sustainability, Jul 14, 2022

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Effects of low clay concentrations on nearly isotropic turbulence

Physical Review Fluids

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On the turbulence dynamics induced by a surrogate seagrass canopy

Journal of Fluid Mechanics, 2022

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