Bed Topography Research Articles

Experimental and numerical modelling of scour at vertical wall abutment under combined wave-current flow in low KC regime

The present study investigates the scour at vertical-wall abutments under a strong current-dominated combined wave-current and current-only environment using experiments and computational fluid dynamics (CFD) modeling. The CFD modelling is performed using an open-source three-phase numerical model (REEF3D), which solves Reynolds-Averaged Navier Stokes (RANS) equations with k-ω turbulence closure. The CFD model is first validated using the experiments and then used for further investigation. The Exner equation was used to compute alterations in bed elevation, while the Level-Set method recorded the free surface and bed topography in a realistic manner. The experimental findings demonstrate that the initial phase of scour development is more rapid under combined wave-current flow, but the equilibrium scour depth is higher in current-only conditions. The increase in Keulegan Carpenter (KC) number increases the scour depth at vertical-wall abutments for a specific relative flow velocity (Ucw). In contrast, a mild increase in vertical-wall abutment scour depth is observed with an increase in Ucw. To the best of the authors’ knowledge, the present work is the first of a kind of experimental and three-dimensional CFD study that estimates abutment scour under strong current-dominated combined wave-current flow.

Plastic pollution in riverbeds fundamentally affects natural sand transport processes

Over the past 50 years, rivers have become increasingly important vectors for plastic pollution. Lowland riverbeds exhibit coherent morphological features including ripple and dune bedforms, which transport sediment downstream via well-understood processes, yet the impact of plastic on sediment transport mechanics is largely unknown. Here we use flume tank experiments to show that when plastic particles are introduced to sandy riverbeds, even at relatively low concentrations, novel bedform morphologies and altered processes emerge, including irregular bedform stoss erosion and dune “washout”, causing topographic bedform amplitudes to decline. We detail (i) new mechanisms of plastic incorporation and transport in riverbed dunes, and (ii) how sedimentary processes are fundamentally influenced. Our laboratory flume tank experiments suggest that plastic is not a passive component of river systems but directly affects bed topography and locally increases the proportion of sand suspended in the water column, which at larger scales, has the potential to impact river ecosystems and wider landscapes. The resulting plastic distribution in the sediment is heterogeneous, highlighting the challenge of representatively sampling plastic concentrations in river sediments. Our insights are part of an ongoing suite of efforts contributing to the establishment of a new branch of process sedimentology: plastic – riverbed sand interactions.

Hydro-morphodynamic responses of rivers to the construction of hydropower dams: a case study – the Kor River, Iran

Rivers are one of the main components of the ecosystem that has changed because of more pressing human activities, such as the building of small and large dams along rivers. The impact of dams on fluvial hydro-morphology in arid and semi-arid regions, such as the Middle East, has received little attention, even if such fluvial corridors represent major sources of livelihood for the local population. In this study, the influence of the Mollasadra Dam, on the hydro-morphological conditions of the Iranian Kor River is numerically investigated using iRIC MFlow_02, by considering different flooding conditions. Simulating different scenarios with and without the dam, and looking at key parameters like flow rate and velocity, sediment transport rate, and bed topography, it is evident that the dam significantly reduces the water discharge and the relative flow velocities, while it has a relatively reduced impact of the bed morphology and sediment transport. This can be ascribed to the fact that the Kor River is a capacity-limited watercourse, characterized by rather coarse material and reduced sediment transport.

Machine learning enhanced high dynamic range fringe projection profilometry for in-situ layer-wise surface topography measurement during LPBF additive manufacturing

Fringe Projection Profilometry (FPP) is a cost-effective and non-destructive method, typically used for measuring finer features and reconstructing 3D topography of objects. However, to use the FPP method for measuring the dynamic topography of powder bed and printed layers during Laser Powder Bed Fusion (LPBF) based additive manufacturing (AM) process, unique challenges exist due to the varying material properties and ambient conditions in the build chamber. In this work, we aim to enhance the discernibility, accuracy, and resolution of FPP in the specific application scenario of measuring layer-wise surface topography during LPBF AM by integrating our recently developed LPBF-specific FPP sensing model that features localized sensor calibration and Fourier filter-aided unwrapping with an equipment-based High dynamic range (HDR) method and machine learning (ML) aided FPP data analysis. First, a projector based HDR method is applied to mitigate the shadowing and intensity saturation problems by projecting sinusoidal fringe patterns of varying intensities. Secondly, a ML framework is developed to improve the surface topography measurement accuracy (RMSE from 10.57 μm to 7.49 μm or even 4.35 μm for directly measurable points) and enhance resolution that is currently subjected to hardware limitations (from 38 μm to 5 μm laterally and from 10 μm to 1 μm vertically). Several different types of candidate neural networks (NNs) are trained and tested using the in-situ FPP measurement data and ex-situ standard optical microscopy characterization data. Multiple NN-based models are resulted and compared in terms of their ability to enhance the accuracy and resolution of FPP's end-result (height measurement). By selecting the best-performance NN enabled image super resolution model, the proposed ML integrated HDR FPP method is expected to measure the surface topography of printed layers during LPBF-AM more capably and efficiently, thus advancing the existing state-of-the-art methods towards the desired online inspection of LPBF print defects.

Subglacial hydrology modeling predicts high winter water pressure and spatially variable transmissivity at Helheim Glacier, Greenland

AbstractWater pressure beneath glaciers influences ice velocity. Subglacial hydrology models are helpful for gaining insight into basal conditions, but models depend on unconstrained parameters, and a current challenge is reproducing elevated water pressures in winter. We eliminate terms related to englacial storage, opening by sliding, and melt due to changes in the pressure-melting-point temperature, to create a minimalist version of the Subglacial Hydrology And Kinetic, Transient Interactions (SHAKTI) model, and apply this model to Helheim Glacier in east Greenland to explore the winter base state of the subglacial drainage system. Our results suggest that meltwater produced at the bed alone supports active winter drainage with large areas of elevated water pressure and preferential drainage pathways, using a continuum approach that allows for transitions between flow regimes. Transmissivity varies spatially over several orders of magnitude from 10−4to 103m2s−1, with regions of weak transmissivity representing poorly connected regions of the system. Bed topography controls the location of primary drainage pathways, and high basal melt rates occur along the steep valley walls. Frictional heat from sliding is a dominant source of basal melt; different approaches for calculating basal shear stress produce significantly different basal melt rates and subglacial discharge.

Artificial Roughness Dimensions and their Influence on Bed Topography Variations Downstream of a Culvert: An Experimental Study

Artificial Roughness Dimensions and their Influence on Bed Topography Variations Downstream of a Culvert: An Experimental Study

Characteristics of seismically-induced resonant sloshing waves and the effects of bed topography

The seismically-induced resonant sloshing wave (SIRSW) is a crucial concern in the design of storage tanks and reservoirs. To uncover the generative mechanism and characteristics of SIRSW and the effects of bed topography, a numerical study using a Navier-Stokes solver with the Virtual-Boundary-Force (VBF) approach for modelling structures is conducted. Various seismic excitations with low to high frequency contents are used to excite nonlinear sloshing. Simplified bed topographies, i.e. the uniformly bottom-mounted rectangular barriers, are considered. Special attention is paid to the correlation between the sloshing response and the seismic characteristics through two groups of parametric studies: 1) changing the peak ground velocity (PGV) under fixed peak ground acceleration (PGA); 2) changing the frequency band under fixed PGV. Results indicate that the seismic PGV, frequency content and frequency band are three significant parameters dominating the sloshing wave; the bed topography can enhance or suppress SIRSW depending on the predominant frequencies of the seismic excitation and their durations.

Riparian vegetation's effects on the evolution of an experimental meandering channel driven by an upper vortex flow

AbstractFew investigations of river evolution have incorporated the series of upper vortex flow disturbances induced by quasi‐knickpoints, and it is unclear whether the regulating ability of riparian vegetation in maintaining meandering channel patterns is valid. In this study, we therefore conducted meander evolution experiments to confirm the effects of vegetation species and vegetation strength on this complex system. Two nonleguminous plants with salinity and alkalinity resistance had non‐uniform degrees of normal seedling growth and different root network lengths. Our findings show that braided swales formed between the upstream developing zone of the vortex flow and the nearest bend and the riparian vegetation generally consolidated the single‐thread channel planform without branching outward under the conditions of flooding flow. Transverse point bars on inner banks were replaced by downstream scroll bars in the overall channel for long time, unless riparian vegetation and flood scour were coupled. Shallow‐rooted plants were inadequate to withstand the inner‐banks being cut by the upper vortex flow. Deep‐rooted plants can significantly stabilize bank lines and thalwegs but are vulnerable to locally low vegetation coverage. Using evolutionary spectral analysis based on thalwegs, we found that the streamwise high‐frequency distribution of bed topography was primarily concentrated downstream of the bifurcation interface when a flood was present in the unvegetated scenario, shrank to the isolated turning interface of the upper reach and the large‐scale spiral swale of the lower reach in the shallow‐rooted scenario and stood out along the area disturbed by bare roots in the deep‐rooted scenario. This experimental study broadens our understanding of vegetation effects in hydro‐bio‐geomorphological system engineering.

Bed topography and subglacial landforms in the onset region of the Northeast Greenland Ice Stream – CORRIGENDUM

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Experimental investigation of the effect of permeability and angle of gabion submerged vane on bed topography

Experimental investigation of the effect of permeability and angle of gabion submerged vane on bed topography

On the Morphodynamics of a Wide Class of Large‐Scale Meandering Rivers: Insights Gained by Coupling LES With Sediment‐Dynamics

AbstractIn meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large‐eddy simulation and bed morphodynamics simulations to create a unique database of hydro‐morphodynamic data sets for 42 meandering rivers with a variety of planform shapes and large‐scale geometrical features that mimic natural meanders. For each simulated river, the database includes (a) bed morphology, (b) three‐dimensional mean velocity field, and (c) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high‐fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large‐scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications.

Multi-Branch Deep Neural Network for Bed Topography of Antarctica Super-Resolution: Reasonable Integration of Multiple Remote Sensing Data

Bed topography and roughness play important roles in numerous ice-sheet analyses. Although the coverage of ice-penetrating radar measurements has vastly increased over recent decades, significant data gaps remain in certain areas of subglacial topography and need interpolation. However, the bed topography generated by interpolation such as kriging and mass conservation is generally smooth at small scales, lacking topographic features important for sub-kilometer roughness. DeepBedMap, a deep learning method combined with multiple surface observation inputs, can generate high-resolution (250 m) bed topography with realistic bed roughness but produces some unrealistic artifacts and higher bed elevation values in certain regions, which could bias ice-sheet models. To address these issues, we present MB_DeepBedMap, a multi-branch deep learning method to generate more realistic bed topography. The model improves upon DeepBedMap by separating inputs into two groups using a multi-branch network structure according to their characteristics, rather than fusing all inputs at an early stage, to reduce artifacts in the generated topography caused by earlier fusion of inputs. A direct upsampling branch preserves large-scale subglacial landforms while generating high-resolution bed topography. We use MB_DeepBedMap to generate a high-resolution (250 m) bed elevation grid product of Antarctica, MB_DeepBedMap_DEM, which can be used in high-resolution ice-sheet modeling studies. Moreover, we test the performance of MB_DeepBedMap model in Thwaites Glacier, Gamburtsev Subglacial Mountains, and several other regions, by comparing the qualitative topographic features and quantitative errors of MB_DeepBedMap, BEDMAP2, BedMachine Antarctica, and DeepBedMap. The results show that MB_DeepBedMap can provide more realistic small-scale topographic features and roughness compared to BEDMAP2, BedMachine Antarctica, and DeepBedMap.

Fast 3D ground penetrating radar simulations for glaciers

Modeling ground penetrating radar (GPR) reflection data on glaciers with methods that require the discretization of the full subsurface domain is extremely computationally expensive because of the combination of a large domain size and the comparatively short wavelength of the signal. To address this issue, we build on and extend a previously proposed approach based on the assumption of a homogeneous background medium (ice) in which various scattering objects (e.g., crevasses, channels, boulders) are embedded far from each other such that multiple scattering can be ignored. The glacier bed, below which no scatterers are assumed to exist, represents the lower limit of the modeling domain. With this method, the two-way propagation of the radar waves through the ice is simulated in a semi-analytical way, whereby scattering surfaces are represented with a set of planar elements of different electric and reflective properties, allowing a wide range of objects to be simulated. As we also take the antenna radiation pattern at the air-ice interface into account, this simple algorithm is able to produce realistic 3D GPR data in a fast and memory efficient way. In this study, we validate the presented algorithm with an analytical solution for a layered model, and we simulate radar data for a model of the Otemma glacier in Switzerland featuring a realistic topography of the glacier bed and a subglacial channel.

Experimental Analysis of 3D Flow Structures around a Floating Dike

Floating dikes have several advantages over spur dikes including less influence on riverine sediment transport, bed topography, and ecosystems, and a good adaptability to fluvial conditions. Despite these advantages, floating dikes have not been used in many river regulation schemes due to the limited understanding of the 3D flow structures around floating dikes. In this study, a series of experiments were conducted to investigate the 3D flow structures around floating dikes. Results show that, after installing a floating dike on one side of a flume, the surface water flow is deflected to the opposite side of the flume, and a backflow develops around the outer and downstream side of the dike, where both the vertical turbulent intensity and the absolute magnitude of the Reynolds stress are relatively large. Due to the blocking effect of the dike, the cross-sectional area decreases, causing an increase in velocities below and alongside the dike, as well as a decrease in velocities upstream of the dike. Increasing the submerged depth or length of the dike results in an increase in flow velocity adjacent to the dike, as well as an increase in the vertical or lateral scale of the backflow. On the contrary, increasing the dike thickness leads to a weakening or disappearance of the backflow, along with a decrease in the acceleration rate of flow adjacent to the dike.

An efficient semi-implicit friction source term treatment for modeling overland flow

In this study, a new numerical treatment of the friction source term is proposed for modeling shallow water flows over complex bed topography. We design a novel semi-implicit temporal discretization technique based on Taylor series expansion of friction source terms. A well-balanced positivity-preserving finite volume scheme on unstructured triangular grids is used to solve the shallow water model. We present a series of numerical tests to demonstrate the robustness and effectiveness of the proposed numerical model where we used rapidly varying flows over complex topography as well as flows involving small values of water depth and wet/dry zones. The simulation results of the numerical model based on the proposed approach for friction source terms are compared to experimental data and to those obtained by using other existing techniques. The proposed numerical model is stable and second-order accurate and our predictions are in good agreement with experimental observations.

Analysis of Bedforms in the Mississippi River at Vicksburg, Mississippi, for Select Flows 2011–2016

Quantification of sediment transport rates is necessary for river management but remains difficult due to large volumes of sediment in motion, spatiotemporal heterogeneity in discharge, depth, and morphology, the need for expensive equipment, and personnel costs. During efforts to quantify sediment bed load in the Mississippi (MS) River near Vicksburg, MS, US, the US Army Corps of Engineers-Engineer Research and Development Center (USACE-ERDC) collected acoustic multibeam topographic data during the 2011 MS River flood. Bed topography data were analyzed to quantify the effect of changing flow rates on bed topography. Bedforms were much higher and longer in the center section than near either bank, and superimposed bedforms were found during the falling limb of the 2011 flood hydrograph. Bedform amplitudes and lengths were positively correlated with flow depths, but amplitudes were typically higher and lengths shorter than predicted in previous research. Topography data from three lower flows are included to illustrate the morphometric variation induced by a wide range of discharges.

Exploiting high-slip flow regimes to improve inference of glacier bed topography

AbstractTheory and observation show that glacier-flow regimes characterized by high basal slip enhance the projection of topographic detail to the surface, motivating this investigation into the efficacy of using glacier surges to improve bed estimation. Here we adapt a Bayesian inversion scheme and apply it to real and synthetic data as a proof of concept. Synthetic tests show a reduction in mean RMSE between true and inferred beds by more than half, and an increase in the mean correlation coefficient of ~0.5, when data from slip- versus deformation-dominated regimes are used. Multi-epoch inversions, which partition slip- and deformation-dominated regimes, are shown to outperform inversions that average over these flow regimes thereby squandering information. Tests with real data from a surging glacier in Yukon, Canada, corroborate these results, while highlighting the challenges of limited or inconsistent data. With the growing torrent of satellite-based observations, fast-flow events such as glacier surges offer potential to improve bed estimation for some of the world's most dynamic glaciers.

Coupling between downstream variations of channel width and local pool–riffle bed topography

Abstract. A potential control of downstream channel width variations on the structure and planform of pool–riffle sequence local bed topography is a key to the dynamics of gravel bed rivers. How established pool–riffle sequences respond to time-varying changes in channel width at specific locations, however, is largely unexplored and challenging to address with field-based study. Here, we report results of a flume experiment aimed at building understanding of how statistically steady pool–riffle sequence profiles adjust to spatially prescribed channel width changes. We find that local bed slopes near steady-state conditions inversely correlate with local downstream width gradients when the upstream sediment supply approximates the estimated transport capacity. This result constrains conditions prior to and following the imposed local width changes. Furthermore, this relationship between local channel bed slope and downstream width gradient is consistent with expectations from scaling theory and a broad set of field-based, numerical, and experimental studies (n=88). However, upstream disruptions to coarse sediment supply through actions such as dam removal can result in a transient flipping of the expected inverse correlation between bed slope and width gradient, collectively highlighting that understanding local conditions is critical before typically implemented spatial averaging schemes can be reliably applied.

Flume Investigation Into Mechanisms Responsible for Particle Sorting in Gravel‐Bed Meandering Channels

AbstractMeandering gravel‐bed rivers tend to exhibit bed surface sorting patterns with coarse particles located in pools and fine particles on bar tops. The mechanism by which these patterns emerge has been explored in sand‐bed reaches; however, for gravel‐bed meandering channels it remains poorly understood. Here we present results from a flume experiment in which bed morphology, velocity, sediment sorting patterns, and bed load transport were intensively documented. The experimental channel is 1.35 m wide, 15.2 m long, and its centerline follows a sine‐generated curve with a crossing angle of 20°. Water and sediment input were held constant throughout the experiment and measurements were collected under quasi‐equilibrium conditions. Boundary shear stress calculated from near‐bed velocity measurements indicates that in a channel with mild sinuosity, deposition of fine particles on bars is a result of divergent shear stress at the inside bend of the channel, downstream of the apex. Boundary shear stress in the upstream half of the pool was below critical for coarse particles (>8 mm), leading to an armored pool. Inward directed selective transport was responsible for winnowing of fine particles in the pool. Fine and coarse sediment followed similar trajectories through the meander bend, which contrasts earlier studies of sand‐bedded meanders where the loci of fine and coarse particles cross paths. This suggests a different sorting mechanism for gravel bends. This experiment shows that a complex interaction of quasi‐equilibrium bed topography, selective sediment transport, and secondary currents are responsible for the sorting patterns seen in gravel‐bed, meandering channels.

Generative Elevation Inpainting: An Efficient Completion Method for Generating High-Resolution Antarctic Bed Topography

Generative Elevation Inpainting: An Efficient Completion Method for Generating High-Resolution Antarctic Bed Topography