Smooth Bed Research Articles

Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained both in its bulk properties, for example geology, the presence of sediment, and the presence of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of 2 decades of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability in along-track topography – enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow and, second, from bed-echo scattering – enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth beds, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard- or soft-bed boundary constraint for ice-sheet models.

Dye-tracer technique for rill flows by velocity profile measurements

Water flow on hillslope soil surface supplies energy which is required to detach soil particles, to transport and deposit sediments, therefore flow velocity is a key variable related to hillslope hydrodinamics of soil erosion processes. Among the different methods available for measuring velocity of shallow interrill and rill flow, the trace technique is widely used. Trace technique is applied by adding a material (salt, magnetic material, water isotope, floating object) and then measuring the speed of the material to travel a known distance from the injection point.When flow velocity is measured using a dye-tracing method, the mean velocity is calculated by multiplying the measured surface velocity of the leading edge of the tracer plume by a correction factor which was generally empirically deduced. The main uncertainty of the dye-tracing technique stands in the estimate of this correction factor, which is the ratio between the mean flow velocity and the surface velocity. The main goal of this paper is establishing a theoretical relationship for calculating the correction factor by using a power velocity profile. At first, the developed analysis demonstrated that the correction factor only depends on the exponent of the power velocity distribution (Eq. (5)). Then, this theoretical expression of the correction factor was applied using the velocity profiles measured by Baiamonte and Ferro for the condition of sediment-free flow in motion on a rough bed and by Coleman for a sediment-laden flow moving on a smooth flume.Using the correction factor values calculated by the velocity measurements carried out for the sediment-free flow in motion on a rough bed, the relationship between the correction factor and roughness height was established. In agreement with a previous study by Ali et al., for a sediment-free flow the correction factor increases with roughness height.Finally, the velocity profile measurements carried out for a sediment-laden flow in motion on a smooth bed were used to state the effect of sediment load on the correction factor. This last analysis allowed to conclude, in agreement with Li and Abrahams and Zhang et al., that the correction factor decreases when the sediment load increases.

Hydrological Characteristics of Estuary in Wulan Delta in Demak Regency, Indonesia

Understanding estuarine characteristics is crucial for managing aquatic ecosystem and aquaculture. A delta is a dynamic area that influence by both sea and river. Research on a micro-tidal estuary remains limited, particularly in Indonesia. To understand the estuarine management in the delta, this research provides an overview about the hydrological characteristics in Wulan Delta Estuary. This research is important to maintain agriculture, aquaculture, and mangrove ecosystem. Morphological data and hydrological were collected in wet season and dry season. The tidal data were collected from 2007–2016. As the results, the river is predominant in the mixing process in the wet season and the tides in the dry season. A low mixing energy causes stratification in the water column, particularly in a depth water depth. In a shallow water depth, salinity circulation is homogenous. In dry season, the sea water can penetrate into the river up to more than 5 km, due to smooth river bed that may harm the agricultural area in surrounding.

Evolution of irregular wave shape over a fringing reef flat

The main objective of this paper was to examine the influence of forereef bottom slope and the effects of coral degradation on the transformation of wave shapes over a fringing reef flat. Based on verification from a physical experiment, the numerical model FUNWAVE 2.0, based on the fully nonlinear Boussinesq equations, was employed to simulate irregular waves propagating over fringing reefs of different topographies. Empirical formulae relating wave skewness and asymmetry to local Ursell numbers were extended to three dimensions, including the effect of forereef slope and roughness of reef surface. Results demonstrated that the gentle forereef slope will lead to a larger wave asymmetry on a reef flat. Otherwise, wave asymmetry on a smooth reef bed was slightly larger than that on a rough bed, indicating that coral degradation may increase wave asymmetry over the reef flat. It was found that the different evolution of wave asymmetry on the reef flat could be primarily attributed to the different infragravity wave motions, which were induced by different forereef bottom slopes. Moreover, coral degradation would increase the asymmetry of infragravity waves, leading to some differences in wave asymmetry on the reef flat. However, wave skewness from different types of fringing reefs was almost identical, implying that the aforementioned factors have a negligible effect on wave skewness.

Spatial and temporal variations in basal melting at Nivlisen ice shelf, East Antarctica, derived from phase-sensitive radars

Abstract. Thinning rates of ice shelves vary widely around Antarctica, and basal melting is a major component of ice shelf mass loss. In this study, we present records of basal melting at a unique spatial and temporal resolution for East Antarctica, derived from autonomous phase-sensitive radars. These records show spatial and temporal variations of basal melting in 2017 and 2018 at Nivlisen, an ice shelf in central Dronning Maud Land. The annually averaged basal melt rates are in general moderate (∼0.8 m yr−1). Radar profiling of the ice shelf shows variable ice thickness from smooth beds to basal crevasses and channels. The highest basal melt rates (3.9 m yr−1) were observed close to a grounded feature near the ice shelf front. Daily time-varying measurements reveal a seasonal melt signal 4 km from the ice shelf front, at an ice draft of 130 m, where the highest daily basal melt rates occurred in summer (up to 5.6 m yr−1). In comparison with wind, air temperatures, and sea ice cover from reanalysis and satellite data, the seasonality in basal melt rates indicates that summer-warmed ocean surface water was pushed by wind beneath the ice shelf front. We observed a different melt regime 35 km into the ice shelf cavity, at an ice draft of 280 m, with considerably lower basal melt rates (annual average of 0.4 m yr−1) and no seasonality. We conclude that warm deep-ocean water at present has a limited effect on the basal melting of Nivlisen. On the other hand, a warming in surface waters, as a result of diminishing sea ice cover, has the potential to increase basal melting near the ice shelf front. Continuous in situ monitoring of Antarctic ice shelves is needed to understand the complex mechanisms involved in ice shelf–ocean interactions.

Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

AbstractGravel riverbeds naturally present a range of sediment size that can span several orders of magnitude. However, fine sediment (i.e., size <2 mm) additions affect sediment transport and are potentially harmful to the river ecology. Limited research has been done to understand the effects of sand addition on gravel bed topography. For this study, we conducted flume experiments testing the effectiveness of topography remote sensing to measure the response of a water‐worked gravel bed to varying sand additions and bed flushing. Repeated measurements of the bed topography with through‐water photogrammetry show that sand deposits primarily in low‐lying areas of the bed, where it is sheltered. Observed changes in bed elevation standard deviation, skewness, and inclination index indicate a smoother bed, as sand fills the depressions on the surface and conceals gravel imbrication. The overall bed morphology (e.g., small bedforms and coarse grain arrangement), however, is preserved. Increasing the sand supply resulted in a gravel bed buried by sand and the formation of sand dunes. Bed flushing with a flow sufficient to move sand allowed a return of the bed roughness to the prepulse conditions, while flushing the bed with a discharge equivalent to the armoring discharge reworked the normally stable surface layer without removing infiltrated sand from the subsurface. This new data set contributes to providing missing information on the small‐scale topographic effects of sand addition upon gravel beds. It might be used with measurements of near‐bed flow hydraulics obtained in other studies to explain variations of sediment transport.

Multi-phase-flow modeling of underwater landslides on an inclined plane and consequently generated waves

An underwater landslide is a complex multiphase flow phenomenon with potential to cause large waves and destruction. Particle size is an important parameter that affects flow behaviors of multiphase flows. This numerical study examines underwater granular landslides on an inclined plane and their resultant waves with the purpose of exploring how the particle size affects landslide kinematics and wave amplitudes. A rheology-based multi-phase flow model is employed, newly incorporating the Navier-slip condition. Smooth and rough beds that reflect ideal and practical conditions, respectively, are simulated. Five types of particle are used; they are very fine sand, medium sand, coarse sand, granules, and pebbles. The granular masses are initially in the loose-packing condition. Our numerical results demonstrate that a smooth bed results in the faster landslides and the larger impulsive waves than a rough bed. The effects of particle size differ between the two bed conditions. For the rough bed, the relationship between the landslide speed and the particle size is negative, and so is the relationship between the wave amplitude and the particle size. For the smooth bed, the two relationships become concave and non-monotonic.

Hydraulic Jump below Abrupt Asymmetric Expanding Stilling Basin on Rough Bed

The present research describes a laboratory study of hydraulic jump in the abrupt asymmetric expansion stilling basin as an energy dissipator by changing the geometry of walls and bed roughness elements. The experiments were carried out in a horizontal flume with 10 m length, 0.5 m width, and 0.5 m depth for a range of the upstream Froude numbers ( F r 1 ) from 5 to 11. Four physical models with expansion ratio of α = 0.33, 0.5, 0.67, and 1 and asymmetry ratio of Δ = 0.16 were installed in the flume and two different heights of roughness elements ( h = 1.4 and 2.8 cm) were also considered. The results indicated that the sequent depth and the jump length as well as the roller length below abrupt asymmetric expansion on the rough bed were decreased in comparison to the same parameters of the jump in a prismatic channel with smooth bed. It was revealed that the roughness elements have the effective role on stabilization of the hydraulic jump location. The analysis of energy dissipation efficiency confirmed that the spatial jump in the abruptly expanded basin with roughened bed was more efficient than classical jump. In order to estimate the hydraulic jump characteristics, empirical relationships associated with expansion ratio of basin walls, relative height of roughness elements and upstream Froude number were proposed based on the experimental data that resulted in preliminary design of an abrupt asymmetric enlarged basin.

Experimental study of the energy dissipation on rough ramps

ABSTRACT Water ramps are one of the most conveyance structures for surface water, irrigation and drainage networks, as well as wastewater collection and discharge systems. To reduce the size of the dissipaters or completely eliminate it, it is required to decrease flow energy over the water ramps. In this paper, results from an experimental investigation into ramps with different slopes and roughened beds are presented. The effects of hemispherical roughness on the kinetic energy dissipation have been studied. The experimental studies were conducted into energy dissipation a ramps with three different slopes (15, 22.5 and 30 degrees), three different roughness height (0.043, 2.2 and 3.2 cm), five roughness arrangements on the bed, and discharge rates of 5 to 30 lit/s. Results show that the energy dissipation in two models with a rough bed and different arrangements increased by 10–32% higher than the control model with a smooth bed. In addition, the energy dissipation of flow in all slopes increased with increasing roughness height. According to the results, a slope at a 30-degree angle had the maximum energy dissipation. Finally, an energy dissipation equation was developed to predict the energy dissipation in any rough model.

Volume fraction and velocity fields of nearly uniform granular flows in a narrow channel geometry with smooth bed

The combined knowledge of the velocity and volume fraction fields is crucial for investigating the dynamics of granular flows, especially in the dense-collisional regime where both frictional and collisional dissipation mechanisms are significant. A laboratory investigation on steady dry granular flows in a straight channel is reported, where slip conditions are allowed at the basal surface and side walls. The stochastic-optical method (SOM), proposed by Sarno et al. (2016) for estimating the volume fraction in granular mixtures, is applied for the first time to granular flows. The velocity at the free surface and at the flume sidewall is measured by using a multi-pass particle image velocimetry (PIV) approach. The measurements of the velocity and volume fraction reveal a superimposition of different dynamic structures, which can be distinguished by means of a volume fraction threshold. Additionally, the profiles of measured volume fraction are exploited to estimate the pressure distribution, so as to numerically describe the velocity profiles by using the μ(I) rheology. It is found that the employment of the experimental volume fraction is superior in describing the flow dynamics, especially near the free surface.

Effect of different silica particles on flowability of gypsum powder for 3D powder printing

3D powder printing (3DPP) attracts much attention in the field of construction and decoration. However, the strong hygroscopicity and high tendency to agglomerate of gypsum powder give rise to a poor flowability, which is crucial during large-size 3DPP procedure. This study aims to compare gliding effect of silica particles with different sizes, shapes and surface properties on poor-flow gypsum powder. Angle of repose (AOR), flow rate of the modified powders and surface morphology of the modified particles were evaluated. The results showed that silica particles of different hydrophobicity presented various extent of glidant effect. For hydrophobic nanosilica, an optimum content (1.0 wt% in this study) could best minimize AOR of the modified powder, leading to more smooth and voidless powder bed of gypsum powder than unmodified gypsum powder. The research found an optimum glidant which could improve the flowability of gypsum powder, resulting in good quality during 3DPP procedure.

Effect of bed roughness on tsunami-like waves and induced loads on buildings

Tsunami, impulse-waves and dam-break waves afflict humanity with casualties and damages. An insight into the flow mechanisms of these waves is important to provide safety and reduce reconstruction costs. This experimental study focuses on the effect of bed roughness on the main hydrodynamic properties of surges propagating on dry bed. In addition, the resulting wave impact forces on buildings with and without openings are studied. Results pointed out that dry bed surges on a rough bed had a lower front celerity and a higher flow depth, resulting into inundation depths during the impact around 20% higher as compared to the smooth bed. Furthermore, a rough bed induced a lower momentum flux during wave propagation, resulting into lower impact forces exerted on the building. The rough bed configuration also caused shorter impact durations, leading to lower impulse values transferred to the building. Results pointed out that even on rough bed, openings within the buildings linearly reduced impact forces, thus providing some helpful information for the design of safer coastal structures.

Analysis of sediment transport in sewer pipes using a coupled CFD-DEM model and experimental work

ABSTRACTThe present work aims to experimentally and numerically study the self-cleansing velocity for non-cohesive sediments in urban sewers as well as the influence of the particle size and bed roughness on the sediment transport. Experiments were conducted in a circular laboratory pipe using sand and gravel over smooth and rough beds. The results were used to validate three-dimensional simulations performed with a coupled computational fluid dynamics (CFD) model and discrete element method (DEM). The CFD part of the simulation was carried out in ANSYS Fluent, which is two-way coupled to the DEM software EDEM. Results have shown that sediment size has a greater effect on critical velocity than bed roughness, while sediment velocity is strongly dependant on the bed roughness. Good correspondence between numerical and experimental results indicated that a CFD-DEM model could be a good tool for an analysis of sediment transport.

MECHANICAL PROPERTIES OF CARBON NANOTUBES-MODIFIED EPOXY GROUT FOR PIPELINE REPAIR SYSTEM

Epoxy grout properties are theoretically important in predicting the behaviour of the composite pipeline repair system. Usually, it is used as an infill material to fill the gap or irregularity on the surface caused by pipe corrosion and ensures a smooth bed before fibre wrapper can be applied to recover the pipeline strength. In this research, the existing commercially available epoxy resin grout has been strengthened by using Carbon Nanotubes (CNTs) at the amount 0.1% of weight fractional to evaluate their apropos behaviour to the neat epoxy grout. The various mechanical tests were performed on this modified grout to identify its compression, tensile, flexural and lap shear strength. In addition, the dispersion process of CNTs was carried out by using ultrasonication and three-roll mill technique to ensure an optimum enhancement in the properties of the polymer matrix. By comparing the strength, 0.1% of CNTs filler has significantly improved the strength of grout in flexural, tensile and shear bonding but not in compression. In addition, the results also indicate that CNTs filler has increased the modulus of elasticity of the infill material. Therefore, it demonstrates the intrinsic potential of the CNTs in modifying the properties of the epoxy grout.

Research on dam-break flow induced front wave impacting a vertical wall based on the CLSVOF and level set methods

The problem of a dam-break flow induced front wave impacting a vertical wall placed at the downstream end was numerically investigated by using the Level Set (LS) method and Coupled Level Set/Volume Of Fluid (CLSVOF) method. For the LS approach, the advection equation of the level set function to capture the interface is discretized by the fifth-order, weighted, essentially non-oscillatory (WENO5) scheme. For the CLSVOF approach, the Tangent of Hyperbola for Interface Capturing/Weighed Line Interface Calculation (THINC/WLIC) scheme is adopted to capture the interface and improve mass conservation. The interface normal vector in the WLIC reconstruction procedure can be estimated from the LS function instead of VOF function. The Navier–Stokes solver which uses an explicit Adams–Bashforth algorithm is performed on a staggered Eulerian grid. The numerical experiments are carried out in a rectangular flume with a smooth horizontal wet bed with four different tailwater levels. This study investigates the formation and propagation of the negative wave resulting from the reflection of flood waves against the downstream end wall. The numerical results are compared with laboratory experiment results studied in Ref (Kocaman and Ozmen-Cagatay, 2015). and a reasonable agreement is achieved. Numerical results show that the CLSVOF solutions are better than LS solutions in terms of mass conservation.

Utjecaj hrapavosti na spregnutu dubinu kod vodnih skokova na neravnoj podlozi

Hydraulic jump is an important phenomenon in open channel flows such as rivers and spillways. Hydraulic jump is mainly used for kinetic energy dissipation at the downstream side of a spillway with the assist of baffle blocks. It has been demonstrated that corrugated or rough beds show considerably more energy dissipation than smooth beds. The experimental research evaluating the effect of crushed stones on the hydraulic jump is presented in this paper. Five different-size sets of crushed stones were used. Results show that the effect of rough bed does not increase after a certain height of crushed stone is reached.

Numerical investigation of tsunami bore effects on structures, part II: effects of bed condition on loading onto circular structures

In this paper, a multiphase three-dimensional numerical reproduction of a large-scale laboratory experiment of tsunami-like bore interaction with a surface-piercing circular column is presented. The numerical simulation is conducted using OpenFOAM. A dam-break mechanism is implemented in order to generate tsunami-like bores. The numerical model is validated using the results of experiments performed at the Canadian Hydraulics Center of the National Research Council in Ottawa, Canada. Unsteady Reynolds-Averaged Navier–Stokes equations are used in order to treat the turbulence effects. The Shear Stress Transport k–ω turbulence model showed a high level of accuracy in replicating the bore–structure interactions. A scaled-up domain is used to investigate the influence of bed condition in terms of various downstream depths and roughnesses. Finally, a broad investigation on bore propagation characteristics is performed. The stream-wise forces exerted on the structural column as well as the bore velocity are compared and analyzed for smooth, rough, dry and wet beds with varying depths.

Characteristics of Positive Surges in a Rectangular Channel

A positive surge is an unsteady open channel flow motion characterized by an increase of flow depth. In previous experimental studies, a positive surge was typically induced by either a sudden increase of discharge in a channel or by the rapid closure of a downstream sluice gate, thus leading to a steep initial profile. However, in many instances, the evolution of a positive surge is of a progressive manner (e.g., in the downstream navigation canal during the emptying operation of lock chambers). In the present work, the inception and development of a positive surge induced by a progressive increase of discharge was investigated in a rectangular channel with a smooth bed. Both undular and breaking surges were studied. The results demonstrate that the maximum wave height at the first wave crest of an undular surge is in very close agreement with the McCowan theory. Additionally, the wave amplitude essentially shows a linearly increasing trend with an increasing surge Froude number up to Fr0 = 1.26 to 1.28, whereas it tends to suggest a power law reduction for larger surge Froude numbers. Moreover, the dispersion of undular surges is consistent with the linear wave theory only for surge Froude numbers close to unity. Overall, the present study demonstrates the unique features of positive surges induced by a progressive increase of discharge.

Predicting the relative energy dissipation of hydraulic jump in rough and smooth bed compound channels using SVM

Abstract Channels with different shapes and bed conditions are used as useful appurtenances to dissipate the extra energy of a hydraulic jump. Accurate prediction of hydraulic jump energy dissipation is important in design of hydraulic structures. In the current study, hydraulic jump energy dissipation was assessed in channels with different shapes and bed conditions (i.e. smooth and rough beds) using the support vector machine (SVM) as an intelligence approach. Five series of experimental datasets were applied to develop the models. The results showed that the SVM model is successful in estimating the relative energy dissipation. For the smooth bed, it was observed that the sloping channel models with steps performed more successfully than rectangular and trapezoidal channels and the step height is an effective variable in the estimation process. For the rough bed, the trapezoidal channel models were more accurate than the rectangular channel. It was found that rough element geometry is effective in estimation of the energy dissipation. The result showed that the models of rough channels led to better predictions. The sensitivity analysis results revealed that Froude number had the more dominant role in the modeling. Comparison among SVM and two other intelligence approaches showed that SVM is more successful in the prediction process.

The Effect of Infill Steel Plate Thickness on the Cycle Behavior of Steel Plate Shear Walls

The effect of changing in the bed slope of stilling basins produces changing in characteristics of the hydraulic jump such as sequent depth ratio, length of jump ratio, length of the roller and energy dissipation ratio, consequently the dimensions of stilling basin changed. In this study hydraulic jump investigated on smooth bed (without any appurtenances) for three adverse slopes (- 0.03, - 0.045, - 0.06) in addition to horizontal bed slope, the experiments were applied for the range of Froude number (Fr1) between 3.99 and 7.48. The results showed a reduction about10 % in sequent depth ratio, 22.1 % in length of jump ratio, 20.51 % in length of roller ratio and 13.87% in the energy dissipation ratio when the adverse slope (- 0.06) used instead of horizontal bed for the same Froude numbers. Empirical equations for the sequent depth ratio, length of roller ratio and the energy dissipation ratio were obtained from the experimental data