Sheet Flow Sediment Transport Research Articles

Net sheet-flow sediment transport rate: Additivity of wave propagation and nonlinear waveshape effects

Net wave-induced sheet-flow sediment transport rate, Qnet, can be produced by two co-existing effects: nonlinear waveshape and wave propagation, which alter the intra-wave boundary layer flow and sediment concentration. A intriguing question is whether the two effects can be simply added (the additivity hypothesis). This paper addresses this question based on simulations of a one-phase numerical model for sheet-flow sediment transport and examinations of the governing equations for boundary layer flow and sediment concentration. The additivity for the mean velocity (or streaming) is first demonstrated under typical field conditions. The main reason is that the two driving forces, i.e., momentum transfer produced by wave propagation and intra-wave asymmetry of flow turbulence produced by nonlinear waveshape, do not affect each other. The net sediment flux is separated into wave- and current-related components. Since the mean concentration profile is determined by the first-order process, the ‘current’-related flux produced by the two effects, following the mean velocity, can be superimposed. After examining the leading Fourier components, the wave-related flux is also found to follow the additivity hypothesis. The findings are confirmed by numerical experiments. The implication of this work is that a practical formula for Qnet can be designed with two separate components for nonlinear-waveshape and wave-propagation effects.

A numerical study of sheet flow driven by velocity and acceleration skewed near-breaking waves on a sandbar using SedWaveFoam

A new methodology capable of concurrently resolving free surface wave field, bottom boundary layer, and sediment transport processes throughout the entire water column was recently developed in the OpenFOAM framework, called SedWaveFoam. In this study, SedWaveFoam is validated with large wave flume data for sheet flow driven by near-breaking waves. Good agreements are obtained for free surface elevation, flow velocity, turbulence kinetic energy, sediment concentration, and sheet flow sediment fluxes. Model results are used to investigate the joint effects of velocity skewness, acceleration skewness, and progressive wave streaming on sheet flow sediment transport. SedWaveFoam results are contrasted with rigid-lid one-dimensional-vertical model results to isolate the effect of the free surface. Onshore directed near-bed flow velocity and sediment flux are enhanced due to the presence of the free surface via progressive wave streaming. However, the enhancement of net onshore sediment transport for the near-breaking condition with both high velocity and acceleration skewness is several factors greater than that found in the nonbreaking condition with only high velocity skewness. Model results suggest that the large horizontal pressure gradient, which has a Sleath parameter exceeding 0.2, may play a key role. Momentary bed failure is identified via near-bed instability of the sheet flow layer, associated with a large bed shear stress and horizontal pressure gradient. Instantaneous near-bed vortices due to the near-bed instability correspond to the increase of horizontal pore pressure gradient during the wave crest, consistent with measured data. Model inter-comparison suggests that a two-dimensional model is crucial to capture the effect of momentary bed failure that increases sediment suspension during wave crest passage and net onshore sediment transport.

Experimental study of sheet-flow sediment transport under nonlinear oscillatory flow over a sloping bed

The seabed is sloped in shallow coastal regions, where strong nonlinear shoaling waves can generate sheet-flow sediment transport. Both wave nonlinearity and bottom slope can lead to a wave-averaged (net) sediment transport rate, but the bottom slope effect is often overlooked in existing sediment transport models. A full-scale experimental study of sheet-flow sediment transport under the oscillatory flows with nonlinear features (skewness or asymmetry) was conducted in an inclinable oscillatory water tunnel, so the bottom-slope-induced net transport rate in the context of nonlinear waves was experimentally investigated. The tests cover a variety of nonlinear wave shape, bottom slope (0 to 2.5∘) and two sediment diameters (0.24 and 0.51 mm). Since the downslope direction is the “offshore” direction in our tests, it is found that bottom slope reduces the net “onshore” transport rate due to wave nonlinearity. The slope contribution to the net transport rate is evaluated by taking the difference between the measured net transport rate from a sloping-bed experiment and the measured net transport rate from the corresponding horizontal-bed experiment. The results suggest that slope-induced net transport rate is insensitive to wave nonlinearity, and its variation with bottom slope can be considered linear in the context of predicting net transport rate. Comparisons between this study and a preceding sinusoidal-flow study of Yuan et al. (2017b) suggest that nonlinear waves can be approximated as sinusoidal flows for estimating the slope contribution to the net transport rate. Subsequently, an empirical model for predicting net sheet-flow transport rate due to bottom slope is established based on dimensional analysis. This model can be simply added to existing sheet-flow sediment transport models to improve their applicability in coastal regions.

Dam-break flow experiments over mobile bed: velocity profile

ABSTRACTKnowledge of the velocity distribution is key for understanding and modelling any type of flow. For steady flows over fixed beds, theoretical and experimental approaches evidenced a logarithmic law in the inner region, in the vicinity of the bed. For flows over mobile beds or for unsteady flows, the velocity profile progressively adapts to the changing flow conditions. In the present paper, new experiments of dam-break flows over a bed made of coarse sand are presented. Detailed velocity measurements both in the clear water layer and in the moving sediment layer were obtained. The measurements show that a logarithmic velocity profile develops not only in the clear-water layer but also in the upper part of the moving sediment layer, in a similar way to previous observations in steady flow with sheet-flow sediment transport.

Modeling net sheet-flow sediment transport rate under skewed and asymmetric oscillatory flows over a sloping bed

A one-dimensional-vertical model is developed for net sheet-flow sediment transport rate under oscillatory boundary layer flows. This model couples the boundary-layer model proposed by Yuan and Madsen (2015) with new predictors for bedload and suspended-load transport rates. Sediment concentration is modeled by solving the advection-diffusion equation with a parameterized turbulence diffusivity and a bottom pick-up rate. The slope effect is included in the predictions of instantaneous bedload transport rate and sediment pick-up rate, so the model can rigorously account for wave nonlinearity (velocity skewness and asymmetry) together with a mild bottom slope. The model performance on net sediment transport rate is excellent for large grain sizes, except that the velocity-asymmetry effect seems to be moderately underestimated. For small grain sizes, the model performance deteriorates, but is still acceptable for many cases. This is possibly because the model does not consider turbulence damping due to stratification and underestimates the phase-lag effect. Using the validated model, it is found that a boundary layer streaming due to velocity skewness and/or asymmetry can be a major contributor to net transport rate, when significant sediment suspension occurs. A simple case study also suggests that a mild bottom slope can be equally important as the wave nonlinearity for producing net sediment transport rate. Model predictions show that wave-nonlinear and bottom-slope effects can be linearly superimposed, which provides a simple way for incorporating bottom slope into existing transport-rate formulas.

Observations of wave‐induced pore pressure gradients and bed level response on a surf zone sandbar

AbstractHorizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep, near‐breaking waves in the surf zone. A barred beach was constructed in a large‐scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar. Horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed. Colocated observations of erosion depth were made during asymmetric wave trials with wave heights between 0.10 and 0.98 m, consistently resulting in onshore sheet flow sediment transport. The pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle, directed predominantly upward under the trough and then rapidly rotating onshore and downward as the wavefront passed. The magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth. Momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore‐directed pore pressure gradients, but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds. An expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at nonnegligible values of both forces.

Soft shore protection methods: The use of advanced numerical models in the evaluation of beach nourishment

Beach nourishment is one of the worldwide most common soft shore protection methods. However, the design of these projects is usually based on empirical equations and rules, leaving large margins of error regarding their expected efficiency. In the present work, an advanced wave and sediment transport numerical model is developed and tested in the evaluation of beach nourishment. Non-linear wave transformation in the surf and swash zone is computed by a non-linear breaking wave model based on the higher order Boussinesq equations, for breaking and non-breaking waves. The new Camenen and Larson (2007), transport rate formula for non-cohesive sediments (involving unsteady aspects of the sand transport phenomenon) is adopted for estimating the sheet flow sediment transport rates, as well as the bed load and suspended load over ripples. Suspended sediment transport rate is incorporated by solving the 2DH depth-integrated transport equation. Model results are compared with experimental data of both profile (cross-shore) and planform morphology evolution; the agreement between the two is considered to be quite satisfactory.

A Conductivity Concentration Profiler for Sheet Flow Sediment Transport

A new sensor for measuring sediment concentration under sheet flow conditions is presented. Electrical conductivity, measured using a four-electrode method, is used as a proxy for sediment concentration. The relationship between conductivity and sediment concentration was calibrated using known masses of neutrally suspended sediment in a heavy liquid and agrees well with existing linear and power-law relationships ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> >; 0.98). A 29-point conductivity profile at 1-mm resolution is generated by multiplexing through a vertical array of 32 plate electrodes. Numerical simulations of the voltage field around the sensor indicate that the horizontal extent of measurement volume is 1.5 times the sensor width. The finite extent of the measurement volume leads to smoothing of the vertical concentration profile. The sensor resolves sheet flow layers with a thickness greater than 3.5 mm, and a correction formula is introduced to correct the measured sheet thickness for the smoothing effect. Initial field results in the swash zone of a natural beach quantify sheet flow processes with unprecedented detail. Short-lived sheet flow with a maximum thickness of 19 mm was observed during the uprush, and a longer duration sheet flow with a maximum thickness of 8 mm was observed during the backwash.

PREDICTION OF SEDIMENT TRANSPORT DUE TO IRREGULAR WAVE MOTION

&lt;p&gt;In general, waves in coastal environments are irregular and have a random shape with a height and period that was not constant. The accuracy of sediment transport rate prediction is the most important stages in the study of morphology and coastal marine environments. In addition, the predictive model of coastal morphology is more efficient to use the bottom shear stress calculation approach for practical purposes rather than a more complex approach to the modeling of two phases. In this paper, the calculation of sediment transport was based on the bottom shear stress modelling purposed with data validation from the experimental results in the turbulent bottom boundary layer over rough bed under irregular waves. The new approach to estimate the bottom shear stress was based on combining velocity and acceleration terms. Furthermore, a new approach of the bottom shear stress was applied to formulate the sheet flow sediment transport rate for irregular waves by using the experimental data from Dibadjnia and Watanabe (1998) and the empirical formula was found.&lt;/p&gt; &lt;p&gt;Keywords: sediment transport, bottom shear stress, irregular waves&lt;/p&gt;

PREDICTION OF SEDIMENT TRANSPORT DUE TO IRREGULAR WAVE MOTION

In general, waves in coastal environments are irregular and have a random shape with a height and period that was not constant. The accuracy of sediment transport rate prediction is the most important stages in the study of morphology and coastal marine environments. In addition, the predictive model of coastal morphology is more efficient to use the bottom shear stress calculation approach for practical purposes rather than a more complex approach to the modeling of two phases. In this paper, the calculation of sediment transport was based on the bottom shear stress modelling purposed with data validation from the experimental results in the turbulent bottom boundary layer over rough bed under irregular waves. The new approach to estimate the bottom shear stress was based on combining velocity and acceleration terms. Furthermore, a new approach of the bottom shear stress was applied to formulate the sheet flow sediment transport rate for irregular waves by using the experimental data from Dibadjnia and Watanabe (1998) and the empirical formula was found. Keywords: sediment transport, bottom shear stress, irregular waves

RANS-based simulation of turbulent wave boundary layer and sheet-flow sediment transport processes

A numerical model coupling the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations with two-equation k−ω turbulence closure is presented and used to simulate a variety of turbulent wave boundary layer processes. The hydrodynamic model is additionally coupled with bed and suspended load descriptions, the latter based on an unsteady turbulent-diffusion equation, for simulation of sheet-flow sediment transport processes. In addition to standard features common within such RANS-based approaches, the present model includes: (1) hindered settling velocities at high suspended sediment concentrations, (2) turbulence suppression due to density gradients in the water–sand mixture, (3) boundary layer streaming due to convective terms, and (4) converging–diverging effects due to a sloping bed. The present model therefore provides a framework for simultaneous inclusion of a number of local factors important within cross-shore wave boundary layer and sediment transport dynamics. The hydrodynamic model is validated for both hydraulically smooth and rough conditions, based on wave friction factor diagrams and boundary layer streaming profiles, with the results in excellent agreement with experimental and/or previous numerical work. The sediment transport model is likewise validated against oscillatory tunnel experiments involving both velocity-skewed and acceleration-skewed flows, as well as against measurements beneath real progressive waves. Model capabilities are exploited to investigate the importance of boundary layer streaming effects on sediment transport in selected velocity-skewed conditions. For the medium sand grain conditions considered, the model results suggest that streaming effects can enhance onshore sediment transport rates by as much as a factor of two. Moreover, for fine sand conditions streaming (and related convective) effects are demonstrated to potentially reverse the direction of net transport (i.e. from offshore to onshore) relative that predicted in oscillatory tunnel conditions. The developed model is implemented within the popular Matlab environment, and hence may be attractive for both research and educational purposes.

SHEETFLOW SEDIMENT TRANSPORT UNDER SKEWED - ASYMMETRIC WAVES AND CURRENTS

Prototype scale laboratory experiments have been conducted to investigate the sheetflow sediment transport of uniform sands under different skewed-asymmetric oscillatory flows. Experimental results reveal that in most of the case with fine sand, the "cancelling effect†, which balances the on-/off-shore net transport under pure asymmetric/skewed flows and results a moderate net transport, was developed for combined skewed-asymmetric flow. However, under some certain conditions (T &gt; 5s) with coarse sands, the onshore sediment transport was enhanced by 50% under combined skewed-asymmetric flows. Sand transport mechanism under oscillatory sheetflow conditions is also studied by comparing the maximum bed shear stress and the phase lag parameter at each half cycle. A comparison of measurements including the new experimental data with a number of practical sand transport formulations shows that the Dong et al. (2013) formulation performs the best in predicting the measured net transport rates over a wide range of experimental conditions

A sheetflow sediment transport model for skewed-asymmetric waves combined with strong opposite currents

Prototype scale physical model tests were conducted to investigate the sheetflow sediment transport of uniform sand under different skewed-asymmetric oscillatory flows with and without the presence of relatively strong currents in the opposite direction against wave propagation. Experiments show that in most cases with fine sands, the “cancelling effect” which balances the on-/off-shore net transport under pure asymmetric/skewed oscillatory flows and results a moderate net transport was developed for combined skewed-asymmetric shaped oscillations. However, under certain conditions (T>5s) with coarse sands, the onshore sediment transport was enhanced for combined skewed-asymmetric flows. Additionally, the new experimental data under collinear oscillatory flows and strong currents show that offshore net transport rates increase with decreasing velocity skewness and acceleration skewness. Sediment movement behaviors were investigated through analysis of experimental data obtained from the image analysis technique and attempts were made to estimate and formulate the sheetflow layer thickness. Accordingly, sediment transport under oscillatory sheetflow conditions was studied and successfully explained by comparing the bed shear stress and the phase lag parameter at each half cycle. Consequently, these parameters were incorporated in an improved Dibajinia and Watanabe's type sediment transport model. The formula is calibrated against a comprehensive experimental data (331 in total). Good agreement obtained between predictions and measurements shows that the new formula is fulfilled for practical purposes.

Assessing sediment stress closures in two-phase sheet flow models

A crucial component of models using a two-phase flow approach to simulate sediment transport is the closure employed to describe the sediment phase stresses. We assess and compare several formulations which have been successfully implemented in simulations of sheet flow sediment transport and in multidimensional studies. To that end, we use numerical results of a two-phase flow model which uses a one-equation granular flow sediment stress closure and which has been extensively validated. We first assess existing empirical parametric sediment stress formulations under steady and oscillatory sheet flow conditions. The numerical results only partially support existing mathematical expressions, and lead to a ratio between normal and shear stresses which is not a constant. We also directly compare the numerical results obtained using three different sediment stress closures against intrawave sediment flux data.

SHEETFLOW SEDIMENT TRANSPORT UNDER ASYMMETRIC WAVES AND STRONG CURRENTS

Experiments have been conducted to investigate the sheetflow sediment transport of uniform sand under asymmetric oscillatory flows in combination with relatively strong opposite currents. Two kinds of nearshore waves were performed, namely, velocity asymmetric waves and acceleration asymmetric waves. Image analysis technique is utilized to study major influences of wave shapes and current through observing the instantaneous sheetflow layer thickness. Maximum sheetflow layer thickness was formulated and incorporated to an enhanced Watanabe and Sato's formulation. The new conceptual model is examined its validity for a wide range of experimental conditions

Two-dimensional time dependent hurricane overwash and erosion modeling at Santa Rosa Island

A 2DH numerical, model which is capable of computing nearshore circulation and morphodynamics, including dune erosion, breaching and overwash, is used to simulate overwash caused by Hurricane Ivan (2004) on a barrier island. The model is forced using parametric wave and surge time series based on field data and large-scale numerical model results. The model predicted beach face and dune erosion reasonably well as well as the development of washover fans. Furthermore, the model demonstrated considerable quantitative skill (upwards of 66% of variance explained, maximum bias − 0.21 m) in hindcasting the post-storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied. The prediction skill ranged between 0.66 and 0.77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied. The sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay. The model sensitivity to the sheet flow sediment transport limiter, which served as a proxy for unknown factors controlling the resistance to erosion, was significantly greater than the sensitivity to the hydraulic forcing parameters. If no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response.

Modelling and observation of oscillatory sheet-flow sediment transport

Sand transport beneath large waves occurs in the plane-bed, sheet-flow regime. Comparisons between two sediment transport models and oscillatory sheet-flow experiments conducted in a flow tunnel are presented here. The experiments represent field-scale asymmetric (velocity-skewed) wave conditions over fine, medium and coarse sands, with median grain diameters of 0.13, 0.27 and 0.46 mm, respectively. The two numerical models used in the study are a two-phase flow model and a simpler two-layer, turbulence-closure model, both of which are one-dimensional vertical (1DV). The two-phase model takes account of the complete fluid–particle interactions, and the two-layer model uses an empirical description of the processes within the sheet-flow layer. The measured and predicted time-varying velocity, concentration and flux profiles, as well as the erosion depth and the net transport rates are compared and analysed. Overall, the predictions of both models are shown to be in good agreement with the measurements. The models predict the changing characteristics of the sheet-flow layer with grain size including the increasing importance of phase-lag effects for finer sands and the change in the net transport rate direction from onshore for coarse and medium sands to offshore for fine sands, with important implications for sediment sorting in the nearshore zone.

Characteristics of turbulent boundary layers over a rough bed under saw-tooth waves and its application to sediment transport

A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, φ and the acceleration coefficient, a c expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k– ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by Watanabe and Sato [Watanabe, A. and Sato, S., 2004. A sheet-flow transport rate formula for asymmetric, forward-leaning waves and currents. Proc. of 29th ICCE, ASCE, pp. 1703–1714.]. Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found.

Numerical Simulation of Vertical Sorting in Sheetflow Sediment Transport by Two-Phase Turbulent Flow Model

Simulations of the vertical sorting under the oscillatory flow condition have been conducted by using one-way model without considering solid-liquid interaction. In this paper, simulation by two-way model to treat the solid-liquid and solid-solid interactions was performed by using the distinct element method for tracking particle motion. The influence on the flow field by the difference between one-way and two-way models on the vertical sorting process was investigated.

Sheet flow sediment transport under waves with acceleration skewness and boundary layer streaming

The effect of acceleration skewness on sheet flow sediment transport rates q s ― is analysed using new data which have acceleration skewness and superimposed currents but no boundary layer streaming. Sediment mobilizing forces due to drag and to acceleration (∼ pressure gradients) are weighted by cosine and sine, respectively, of the angle φ τ · φ τ = 0 thus corresponds to drag dominated sediment transport, q s ― ∼ | u ∞ | u ∞ , while φ τ = 90° corresponds to total domination by the pressure gradients, q s ― ∼ d u ∞ / d ⁢ t . Using the optimal angle, φ τ = 51° based on that data, good agreement is subsequently found with data that have strong influence from boundary layer streaming. Good agreement is also maintained with the large body of U-tube data simulating sine waves with superimposed currents and second-order Stokes waves, all of which have zero acceleration skewness. The recommended model can be applied to irregular waves with arbitrary shape as long as the assumption negligible time lag between forcing and sediment transport rate is valid. With respect to irregular waves, the model is much easier to apply than the competing wave-by-wave models. Issues for further model developments are identified through a comprehensive data review.