Experimental investigations on developments of velocity field near above a sandy bed during regular wave-induced fluidized responses

Regular waves were applied in a laboratory flume to investigate the evolutions of the velocity fields near above a fine sandy bed (d₅₀=0.073 mm) during fluidized responses. Measurements of 2D velocity components and suspended sediment concentration (SSC) at 1 cm above the bed in addition to water surface displacements and sub-soil pore pressures were carried out with an acoustic Doppler velocimeter and an optical probe. The results have shown similar three typical soil responses including one unfluidized and two fluidized responses to previous report in other fine-grained soil beds. In the post- and pre-fluidized stages of a resonantly fluidized response, amplitudes of horizontal velocity component can be decreased by a maxima value of 50% while vertical components can be amplified up to 5 times larger. The developments of near-bed velocity field become less significant in consecutive non-resonantly fluidized responses. Particularly, the evolutions of the velocity field are closely dependent on the deepening of fluidized surface soil layers d_f and the characteristics of soil fluidization responses. The amplified vertical velocity components are clearly contradictory to the dissipated overloading waves near above a fluidized bed but are critical to much drastic sediment suspensions by interactions between overloading waves and fluidized bed soils.     

Unfluidized soil responses of a silty seabed to monochromatic waves

A flume experimental study on unfluidized responses of a silty bed (d₅₀=0.05 mm) to monochromatic water waves had shown that pore pressure variations were generally poro-elastic in the bulk body and displayed two other characteristic features not found in previous laboratory sand tests. They were an immediately fluidized thin surface layer induced by wave stresses inside the seabed's boundary layer and a porous skeleton with internally suspended sediments due to channeled flow motions. The analyses verified that on soils beneath the measurement points, both features resulted in relatively small-step pore pressure build-ups, while the former played a primary role. Besides, laboratory observations confirmed that there were some near-bed sediment suspensions during wave actions resulting in a flat bed form over a silty bed compared to small-scaled ripples over a sandy bed with no clearly identified suspended sediments. These characteristic silt responses suggest that sediment transport is critically associated with the internal soil responses and some field-observed sediment suspensions near above sandy beaches can further be approached in the laboratory by utilizing fine-grained soils.     

Laboratory flume studies on monochromatic wave-fine sandy bed interactions: Part 1. Soil fluidization

Tests on two fine sandy soils (d₅₀ = 0.134 mm and 0.092 mm) under monochromatic wave actions were conducted in a wave flume of 37 m (L) by 1.2 m (H) by 1 m (W) to investigate characteristics of fluidized responses. The pore pressure measurements demonstrate only an unfluidized response in the coarser sandy bed, while in the finer one, two more feature fluidized responses. Fluidized responses are similarly classified into resonantly and non-resonantly fluidized according to Foda and Tzang [Foda, M.A., Tzang, S.-Y., 1994. Resonant fluidization of silty soil by water waves. J. Geophys. Res., 99-C10: 20463–20475.]. At a given depth, they are in principle defined by magnitude of fluidization ratio between excess pore pressure and static soil stresses and by the occurrence of a resonance event in the same test series. Inside the sandy bed, the excess pore pressures of a fluidized response are almost initiated simultaneously. Their magnitudes are essentially in static balance to the integrated weight of overlaying fluidized soil layers. Comparisons with previously reported data from a silty bed (d₅₀ = 0.05 mm) by Foda and Tzang have immediately indicated the importance of grain fraction. With less fine constituents, surface layers of the two sandy soils are less susceptible to fluidization. Resonance mechanism is evidently diminishing in a resonantly fluidized response, and re-fluidization becomes less potential in the subsequent tests. In a resonantly fluidized response, pore pressures at a given depth would start to resonantly grow from a fluidization ratio of 7–14%. In a few wave cycles, resonant growth subsides at a fluidization ratio of greater than 50%, which value increases with depth. The analyses clearly illustrate that fluidization tends to be initiated in surface layers and fast spreads into lower layers. Fluidization is dependent on finer constituting grains, smaller shear modulus G and permeability k and thinner boundary layers in bed soils. Measurements of previous silt tests are analyzed to show that lower limits of wave steepness on resonantly fluidizing a soil bed increase linearly with relative water depth ranging from 0.13 to 0.23. Data of present fine sand tests have preliminarily confirmed the linear trend. Over a fluidized sandy bed, similar vivid sediment suspensions were observed during wave generations as had been reported in silt tests.     

An experimental study of seabed responses around a marine pipeline under wave and current conditions

Experiments on three types of soil (d₅₀=0.287, 0.057 and 0.034 mm) with pipeline(D=4 cm) either half buried or resting on the seabed under regular wave or combined with current actions were conducted in a large wave flume to investigate characteristics of soil responses. The pore pressures were measured through the soil depth and across the pipeline. When pipeline is present the measured pore pressures in sandy soil nearby the pipeline deviate considerably from that predicted by the poro-elasticity theory. The buried pipeline seems to provide a degree of resistance to soil liquefaction in the two finer soil seabeds. In the silt bed, a negative power relationship was found between maximum values of excess pore pressure p_max and test intervals under the same wave conditions due to soil densification and dissipation of the pore pressure. In the case of wave combined with current, pore pressures in sandy soil show slightly decrease with time, whereas in silt soil, the current causes an increase in the excess pore pressure build-up, especially at the deeper depth. Comparing liquefaction depth with scour depth underneath the pipeline indicates that the occurrence of liquefaction is accompanied with larger scour depth under the same pipeline-bed configuration.     

风暴过程中潮滩悬沙浓度和悬沙输运的变化及其动力机制——以长江三角洲南汇潮滩为例

悬沙浓度是淤泥质海岸重要的环境指标。为探讨潮滩悬沙浓度和悬沙输运对风暴事件的响应过程及其动力机制,于2014年9月"凤凰"台风过境前、中、后在长江三角洲南汇潮滩进行了现场观测,获得同步高分辨率的水深、波高、近底流速和浊度剖面时间序列(9个潮周期)。结果表明,风暴中平均和最大波高、波-流联合底床剪切应力、悬沙浓度和悬沙输运率可比平静天气高数倍;风暴期间高潮位低流速阶段悬沙沉降导致近底发育数十厘米厚的浮泥层(悬沙浓度大于10 g/L)。研究认为风暴事件中淤泥质海岸悬沙浓度和悬沙输运的剧烈变化其根本动力机制是风暴把巨大能量传递给近岸水体,进而显著增大波-流联合底床剪切应力,导致细颗粒泥沙再悬浮。     

Sheet flow and suspension of sand in oscillatory boundary layers

after revisionTime-dependent measurements of flow velocities and sediment concentrations were conducted in a large oscillating water tunnel. The measurements were aimed at the flow and sediment dynamics in and above an oscillatory boundary layer in plane bed and sheet-flow conditions. Two asymmetric waves and one sinusoidal wave were imposed using quartz sand with D₅₀ = 0.21 mm. A new electro-resistance probe with a large resolving power was developed for the measurement of the large sediment concentrations in the sheet-flow layer. The measurements revealed a three layer transport system consisting of a pick-up/deposition layer, an upper sheet flow layer and a suspension layer.In the asymmetric wave cases the total net transport was directed “onshore” and was mainly concentrated in the thin sheet flow layer (< 0.5 cm) at the bed. A small net sediment flux was directed “offhore” in the upper suspension layer. The measured flow velocities, sediment concentrations and sedimenl fluxes showed a good qualitative agreement with the results of a (numerical) 1DV boundary-layer flow and transport model. Although the model did not describe all the observed processes in the sheet-flow and suspension layer, the computational results showed a reasonable agreement with measured net transport rates in a wide range of asymmetric wave conditions.     

Cover stones on liquefiable soil bed under waves

The paper describes the results of an experimental study on the behavior of cover stones on a liquefiable soil bed exposed to a progressive wave. The soil was silt with d₅₀ = 0.098 mm. Stones, the size of 4 cm, were used as cover material. The effect of packing density of stones, and that of number of stone layers (including the effect of an intermediate filter layer) were investigated. Pore pressure was measured across the soil depth. The experiments show that the soil liquefaction depended mainly on two parameters: the packing density of stones, and the number of stone layers. When the liquefaction occurs, stones sink in the soil. Mechanisms of liquefaction and sinking are described, and practical guidelines are recommended.     

Phase-lag effects in sheet flow transport

The inception of the sheet flow regime as well as the effects of the phase lag when the sheet flow regime is established were investigated for oscillatory flows and combined steady and oscillatory flows. A new criterion for the inception of sheet flow is proposed based on around 300 oscillatory flow cases from experiments. This criterion was introduced in the Camenen and Larson [Camenen, B., Larson, M., 2005. A bedload sediment transport formula for the nearshore. Estuarine, Coastal and Shelf Science 63, 249–260.] bed load formula in order to take into account phase-lag effects in the sheet flow regime. The modification of the Camenen and Larson formula significantly improves the overall agreement with data and yields a correct behavior in relation to some of the main governing parameters, which are the median grain size d₅₀, the orbital wave velocity U_w, and the wave period T_w. The calibration of the new formula was based on more than 200 experimental data values on the net sediment transport rate for a full wave cycle. A conceptual model was also proposed to estimate the ratio between sediment transport rate with and without phase lag, (r_pl = q_s,net / q_s,net,ϕ=0). This simple model provides accurate results and may be used together with any quasi-steady model for bed load transport.     

Prediction of berm geometry using a set of laboratory tests combined with teaching–learning-based optimization and artificial bee colony algorithms

Understanding sediment movement in coastal areas is crucial in planning the stability of coastal structures, the recovery of coastal areas, and the formation of new coast. Accretion or erosion profiles form as a result of sediment movement. The characteristics of these profiles depend on the bed slope, wave conditions, and sediment properties. Here, experimental studies were performed in a wave flume with regular waves, considering different values for the wave height (H₀), wave period (T), bed slope (m), and mean sediment diameter (d₅₀). Accretion profiles developed in these experiments, and the geometric parameters of the resulting berms were determined. Teaching–learning-based optimization (TLBO) and artificial bee colony (ABC) algorithms were applied to regression functions of the data from the physical model. Dimensional and dimensionless equations were found for each parameter. These equations were compared to data from the physical model, to determine the best equation for each parameter and to evaluate the performances of the TLBO and ABC algorithms in the estimation of the berm parameters. Compared to the ABC algorithm, the TLBO algorithm provided better accuracy in estimating the berm parameters. Overall, the equations successfully determined the berm parameters.     

Suspended-sediment reference concentration under waves: field observations and critical analysis of two predictive models

Two commonly adopted but fundamentally different approaches for predicting time-averaged suspended-sediment reference concentration (C̄_REF) under waves are tested against field measurements and compared with each other. The first model relates C̄_REF to the cube of the non-dimensional skin friction, whereas the second model adopts a more complex function of excess skin friction incorporating the empirical constant γ₀. The dataset is from the zone of wave shoaling seaward of an open-coast surfzone and includes measurements of waves, currents, suspended sediment and bedforms. Estimates of C̄_REF are derived from acoustic backscatter data, and the seabed and suspension process are described from video footage. When waves were energetic, the bed was deformed into large hummocks; during less energetic conditions, the bed was rippled. The time-averaged concentration profiles over the ripples were consistent with settling flux balanced by pure gradient diffusion and a sediment diffusivity that is constant with elevation above the bed. C̄_REF in that case is shown to apply at z=0, where z is the elevation above the bed. Over the hummocks, there was a sheet flow at the base of the suspension and C̄_REF is shown to apply at z=1 cm. The concentration profiles over the hummocks implied sediment diffusivity that varied linearly with elevation within ∼10 cm of the bed and constant sediment diffusivity above that level. For both rippled and hummocky beds, γ₀ derived from the field data was found to be sensitive to the value assumed for critical stress for initiation of sediment motion, which could explain the range of values reported in the literature for γ₀. γ₀ was also found to vary in a complex way with skin friction, which suggests that the reference-concentration model based on excess skin friction is not correctly formulated. Nevertheless, two functions for γ₀ (one applying to rippled beds and the other to hummocky beds) were contrived to make the model fit the data. The model based on non-dimensional skin friction was found to be a good predictor of C̄_REF when a correction was made for flow contraction over ripples. The correction was not required for the hummocky bed, where sediment was being entrained in a thin sheet flow layer. The model based on non-dimensional skin friction correctly portrayed the relationship between flow and sediment response without contrivance and therefore should be the favoured approach in predicting reference concentration.     

Effect of bed roughness on turbulent boundary layer and net sediment transport under asymmetric waves

This paper presents an investigation of the roughness effects in the turbulent boundary layer for asymmetric waves by using the baseline (BSL) k–ω model. This model is validated by a set of the experimental data with different wave non-linearity index, N_i (namely, N_i = 0.67, N_i = 0.60 and N_i = 0.58). It is further used to simulate asymmetric wave velocity flows with several values of the roughness parameter (a_m/k_s) which increase gradually, namely from a_m/k_s = 35 to a_m/k_s = 963. The effect of the roughness tends to increase the turbulent kinetic energy and to decrease the mean velocity distribution in the inner boundary layer, whereas in the outer boundary layer, the roughness alters the turbulent kinetic energy and the mean velocity distribution is relatively unaffected. A new simple calculation method of bottom shear stress based on incorporating velocity and acceleration terms is proposed and applied into the calculation of the rate of bed-load transport induced by asymmetric waves. And further, the effect of bed roughness on the bottom shear stress and bed-load sediment transport under asymmetric waves is examined with the turbulent model, the newly proposed method, and the existing calculation method. It is found that the higher roughness elements increase the magnitude of bottom shear stress along a wave cycle and consequently, the potential net sediment transport rate. Moreover, the wave non-linearity also shows a big impact on the bottom shear stress and the net sediment transport.     

The inception of sheet flow in oscillatory flow

A simple model is developed to study the inception of sheet flow in oscillatory flow based on the available experimental data. The inception of sheet flow in oscillatory flow is well defined by the simple model: A/d=KA²ω/ν+B, where A is the semi-excursion of wave orbital motion near the bed, d is the grain size, ω is the angular frequency, ν is the kinematic viscosity of water, and K and B are the coefficients and dependent on sediment properties only. The inception velocity of sheet flow derived from the model is shown to be the function of grain size d, oscillatory period T and specific sediment density s. For a given sediment, the inception velocity is found to increase sharply initially with T and then approach a constant at T>6.0 s. The present model is quite simple and gives good agreement with the available experimental data.     

An experimental study on geometric characteristics of beach erosion profiles

The beach profile and sediment transport are very important factors in the design of coastal structures, and the beach profile is mainly affected by a number of parameters, such as wave height and period, beach slope, and the material properties of the bed. In this study, considering wave height (H₀=6.5, 11.5, 16, 20, 23, 26 and 30 cm), wave period (T=1.46 and 2.03 s), beach slope (m=1/10 and 1/15) and mean sediment diameter (d₅₀=0.18, 0.26, 0.33 and 0.40 mm), an experimental investigation of coastal erosion profile (storm profile) was carried out in a wave flume using regular waves, and geometric characteristics of erosion profile were determined by the resultant erosion profile. Dimensional and non-dimensional equations were obtained by using linear and non-linear regression methods through the experimental data and were compared with previously developed equations in the literature. The results have shown that the experimental data fitted well to the proposed equations with respect to the previously developed equations.     

Wave interaction with a submarine pipeline in clayey soil due to random waves

The wave pressure and uplift force due to random waves on a submarine pipeline (resting on bed, partially buried and fully buried) in clayey soil are measured. The influence of various parameters viz., wave period, wave height, water depth, burial depth and consistency index of the soil on wave pressures around and uplift force on the submarine pipeline was investigated. The wave pressures were measured at three locations around the submarine pipeline (each at 120° to the adjacent one). It is found that the wave pressure and uplift force spectrum at high consistency index of the soil is smaller compared to that of low consistency index. Just burying the pipeline (e/D=1.0) in clayey soil reduces the uplift force to less than 60% of the force experienced by a pipeline resting on the seabed (e/D=0.0) for I_c=0.33.     

波流作用下海底边界层沉积物再悬浮与影响因素研究

海底沉积物再悬浮及其分布取决于海洋水动力、沉积物类型与床面形态之间复杂的相互作用,准确地理解和确定沉积物再悬浮过程对于沉积物输运的研究具有重要的意义。本文在祥云湾海洋牧场典型海域开展现场原位观测,获取研究区波浪、海流及悬浮沉积物浓度数据;分析了波、流作用下海底边界层悬浮沉积物垂向分布特征,并探究了海洋水动力和床面形态对悬浮沉积物垂向分布的影响。结果表明,研究区波流之间的相互作用不显著,沉积物再悬浮受控于风暴浪作用,风暴浪作用下底床切应力可以达到沉积物临界切应力的10～15倍,沉积物的再悬浮滞后于风暴浪作用2～3 h。在波浪荷载微小的情况下,悬浮沉积物垂向分布呈现"I"型,波浪荷载下,悬浮沉积物垂向分布呈现幂指函数分布,表现为"L"型;床面形态随波、流作用而演化,影响沉积物的再悬浮过程,u_(?w)/u_(?c)=1.00可作为波浪和海流起主导控制作用的床面形态的判别依据,纯波浪荷载作用下的u_(?w)/u_(?c)显著高于波浪主控作用下,但二者之间的界线随着波浪荷载的增加而升高。     

Measurement and modelling of the influence of grain size and pressure gradient on swash uprush sediment transport

The paper examines the dependency between total sediment transport, q, and grain size, D (i.e. q ∝ D^p) under dam break generated swash flows. Experiments were performed in a dam break flume over a sloping mobile sand bed with median grain sizes ranging from 0.22 mm to 2.65 mm. The total sediment transport was measured by truncating the flume bed and collecting the sediment transported over the edge. The experiments were designed to exclude pre-generated turbulence and pre-suspended sediment so as to focus solely on the swash flow. The magnitude and nature of the grain size dependency (i.e. p value) were inferred for different flow parameters; the initial dam depth, d_o, the integrated depth averaged velocity cubed, ∫ u³dt, and against the predicted transport potential, q_p, using the Meyer-Peter Muller (MPM) transport model and variations of that model. The data show that negative dependencies (p < 0) are obtained for d_o and q_p, whilst positive dependencies (p > 0) are obtained for ∫ u³dt. This indicates that a given d_o and q_p transport less sediment as grain size increases, whereas transport increases with grain size for a given ∫ u³dt. The p value is found to be narrowly ranged, 0.5 ≤ p ≤ − 0.5. On average, the incorporation of a pressure gradient term via the piezometric head into the MPM formulation reduces q_p by 4% (fine sand) to 18% (coarse sand). The measured total transport for fine and coarse sands is best predicted using MPM and MPM + dp*/dx respectively. However, the inferred optimum transport coefficient in the MPM formulation is about 30, much higher than the standard coefficient in a steady flow and this is not due to the presence of the pre-suspended sediment. The optimum transport coefficient indicates some sensitivity to grain size, suggesting that some transport processes remain unaccounted for in the model.