Coastal Bottom Boundary Layer Research Articles

Sensitivities of Bottom Stress Estimation to Sediment Stratification in a Tidal Coastal Bottom Boundary Layer

The bottom friction velocity (U*), which controls seabed erosion and deposition, plays a critical role in sediment transport in tidal coastal bottom boundary layers. Approaches have been proposed to calculate U*, including the log profile (LP) estimation, the direct covariance (COV) measurement, and the turbulent kinetic energy (TKE) method. However, the LP method assumes homogeneous flow and the effects of stratification need to be taken into account. Here, field investigations of hydrodynamics and sediment dynamics were carried out on the Jiangsu Coast, China. Two acoustic Doppler velocimeters (ADV) velocity measurements at 0.2 and 1 m above the seabed have been used to estimate U*, based on the aforementioned three methods. The COV and TKE methods provided reasonable estimations of U*, while a pronounced overestimation was identified when using the LP method. This overestimation can be attributed to the stratification effects associated with the vertical suspended sediment concentration (SSC) gradient near the bottom. Then, three models were utilized to correct the overestimation, in which the gradient/flux Richardson number was modified with empirical constants α, β, and A to parameterize the stratification effects in the logarithmic velocity distribution. The values of α, β, and A derived from the observation are smaller than the results from previous investigations. These modified logarithmic velocity distribution models can be applied in numerical simulations when sediment stratification is important.

Sediment Suspension by Straining‐Induced Convection at the Head of Salinity Intrusion

AbstractThe tidal straining can generate convective motions and exert a periodic modification of turbulence and sediment transport in estuarine and coastal bottom boundary layers. However, the evidence and physics of convection and sediment suspension induced by tidal straining have not been straightforward. To examine these questions, mooring and transect surveys have been conducted in September 2015 in the region of the Yangtze River plume influence. Field observations and scaling analyses indicate an occurrence of convective motions at the head of saline wedge. Theoretical analyses of stratification evolution in the saline wedge show that unstable stratification and resultant convection are induced by tidal straining. Vertical turbulent velocity and eddy viscosity at the head of saline wedge are both larger than their neutral counterparts in the main body, largely enhancing sediment suspension at the head of saline wedge. Moreover, sediment suspension in both neutral and convection‐affected flows is supported by the variance of vertical turbulent velocity, rather than the shearing stress. Finally, the stability correction functions in the Monin‐Obukhov similarity theory can be simply derived from the local turbulent kinetic energy balance to successfully describe the effects of tidal straining on turbulent length scale, eddy viscosity, and sediment diffusivity in the convection‐affected flow. These recognitions may provide novel understanding of estuarine turbidity maxima, and the dynamical structure and processes for coastal hypoxia.

Observations of the sensitivity of beam attenuation to particle size in a coastal bottom boundary layer

[1] The goal of this study was to test the hypothesis that the aggregated state of natural marine particles constrains the sensitivity of optical beam attenuation to particle size. An instrumented bottom tripod was deployed at the 12-m node of the Martha's Vineyard Coastal Observatory to monitor particle size distributions, particle size-versus-settling-velocity relationships, and the beam attenuation coefficient (cp) in the bottom boundary layer in September 2007. An automated in situ filtration system on the tripod collected 24 direct estimates of suspended particulate mass (SPM) during each of five deployments. On a sampling interval of 5 min, data from a Sequoia Scientific LISST 100x Type B were merged with data from a digital floc camera to generate suspended particle volume size distributions spanning diameters from approximately 2 μm to 4 cm. Diameter-dependent densities were calculated from size-versus-settling-velocity data, allowing conversion of the volume size distributions to mass distributions, which were used to estimate SPM every 5 min. Estimated SPM and measured cp from the LISST 100x were linearly correlated throughout the experiment, despite wide variations in particle size. The slope of the line, which is the ratio of cp to SPM, was 0.22 g m−2. Individual estimates of cp:SPM were between 0.2 and 0.4 g m−2 for volumetric median particle diameters ranging from 10 to 150 μm. The wide range of values in cp:SPM in the literature likely results from three factors capable of producing factor-of-two variability in the ratio: particle size, particle composition, and the finite acceptance angle of commercial beam-transmissometers.

Field measurements of turbulence at an unstable interface between current and wave bottom boundary layers

[1] In situ particle image velocimetry measurements, at a resolution of 3.5 Kolmogorov scales, have been performed in the inner part of the coastal bottom boundary layer. The spatial details enable us to directly determine the vertical distributions of mean velocity, Reynolds shear stress, shear production and dissipation rates, energy spectra, and abundance of eddies. Focusing on cases with wave velocity of similar magnitude as the mean current, velocity profiles have logarithmic distributions in the upper half of the sample area. Below the log layer, but well above the bottom ripples, an inflection point appears, indicating a region of flow instability. Based on data interpretation, which includes variations in wave phase with height, this inflection occurs near the interface between current and thinner wave boundary layer (WBL) below it. Scaling of mean velocity profiles with shear velocity and characteristic roughness is effective only above the inflection point, while turbulence parameters scale reasonably well at all elevations. Instabilities associated with the inflection are manifested by a peak in turbulent shear production rate and a rapid increase in small-scale turbulence, as is evident from trends of the dissipation rate, energy spectra, and distribution of eddies with elevation. Therefore, the presence of a WBL generates a shear production peak and rapid increase in the dissipation rate at higher elevations than those found in rough-wall steady boundary layers. Transition between current and wave boundary layers is also characterized by broad Reynolds stress peaks and shear production exceeding the dissipation rate.

IMPROVEMENT OF BOTTOM BOUNDARY LAYERS MODELING UNDER INTERACTIONS OF WAVE AND WAVE-INDUCED CURRENT

The bottom boundary layer characteristics beneath waves transforming on a natural beach are specifically affected both by wave and wave-induced current. This study presents an improved approach for coastal bottom boundary layers modeling under interactions of wave and wave-induced current. The improvement is achieved by formulating the mean horizontal pressure gradient term in the boundary layer equation with wave parameters and mean water level. This formulation represents the balance between the wave excess momentum flux gradient and the hydrostatic pressure gradient in a spatially transforming wave field, accounting for the effect of the wave-induced cross-shore current. Model is validated with experimental data for normally incident shoaling wave over a sloping bed. Calculated results agree well with data for instantaneous velocity profiles, wave oscillating amplitudes and mean velocity profiles. In particular, model reasonably reproduces the observed local onshore mean flow near the bottom beneath shoaling wave. It is revealed that the proposed formulation of the mean horizontal pressure gradient plays an important role in bottom boundary layer modeling under wave transforming over an variable near-shore bathymetry, and that the present model can be conveniently and reliably coupled with a sediment transport model to study coastal processes in engineering applications.

Implications of intermittent turbulent bursts for sediment resuspension in a coastal bottom boundary layer: A field study in the western Yellow Sea, China

Previous experiments carried out in the laboratory flumes and open channels have shown that close to the wall sediment resuspension is largely dominated by a sequence of intermittent, powerful and large-scale motions called coherent structures (or burst cycle) which mainly account for Reynolds stress generation. However, the difficulties in simultaneously measuring the fluctuations of velocities and sediment concentration close to the seabed during the field observations inhibit the further understanding of this laboratory discovery in the marine regime. In the present paper, field data sets collected within a coastal bottom boundary layer (BBL) in the western Yellow Sea are examined comprehensively to reveal the relationships between coherent structures and sediment resuspension. The following results are obtained: (1) the bulk of turbulent sediment flux ( c ' w ' ― ) is accomplished by ‘ejection’ (56%) and ‘sweep’ (42%) events, while contributions coming from outward and inward interactions are negligible; (2) wall turbulence containing coherent structures vertically transport momentum more efficiently than suspended sediments; (3) spectral scaling of sediment concentration reveals that two types of suspended sediment clouds with different sizes were existent during the peak flood. The large-size particle clouds contain coarse sediments locally resuspended from seabed, whereas the smaller particle clouds consist of silts and clays, and act as “washload” advected from Jiaozhou Bay and Yellow Sea by tidal flows; (4) wavelet transform can provide a more intuitive way to study and visualize the relationship between the burst cycle and sediment suspension, and the results clearly show that the intermittent Reynolds stress generation is accompanied by intense sediment resuspension.

The Coastal Bottom Boundary Layer: A Note on the Model of Chapman and Lentz

Abstract The bottom boundary layer of a stratified flow on a coastal continental shelf is examined using the model of Chapman and Lentz. The flow is driven by a surface stress, uniform in the alongshore coordinate, in a downwelling-favorable direction. The stress diminishes in the offshore direction and produces an Ekman pumping, as well as an onshore Ekman flux. The model yields an interior flow, sandwiched between an upper Ekman layer and a bottom boundary layer. The interior has a horizontal density gradient produced by a balance between horizontal diffusion of density and vertical advection of a background vertical density gradient. The interior flow is vertically sheared and in thermal wind balance. Whereas the original model of Chapman and Lentz considered an alongshore flow that is freely evolving, the present note focuses on the equilibrium structure of a flow driven by stress and discusses the vertical and lateral structure of the flow and, in particular, the boundary layer thickness. The vertical diffusivity of density in the bottom boundary layer is considered so strong, locally, as to render the bottom boundary layer’s density a function of only offshore position. Boundary layer budgets of mass, momentum, and buoyancy determine the barotropic component of the interior flow as well as the boundary layer thickness, which is a function of the offshore coordinate. The alongshore flow has enhanced vertical shear in the boundary layer that reduces the alongshore flow in the boundary layer; however, the velocity at the bottom is generally not zero but produces a stress that locally balances the applied surface stress. The offshore transport in the bottom boundary layer therefore balances the onshore surface Ekman flux. The model predicts the thickness of the bottom boundary layer, which is a complicated function of several parameters, including the strength of the forcing stress, the vertical and horizontal diffusion coefficients in the interior, and the horizontal diffusion in the boundary layer. The model yields a boundary layer over only a finite portion of the bottom slope if the interior diffusion coefficients are too large; otherwise, the layer extends over the full lateral extent of the domain.

Time-averaged near-bed suspended sediment concentrations under waves and currents: Comparison of measured and model estimates

Profiles of suspended sediment concentration and velocity were measured over a 15-day period at a near-shore site off Santa Cruz, CA in Monterey Bay. The concentration and velocity data were collected with an Acoustic Backscattering System (ABS) and Acoustic Current Profiler (ACP) that were mounted on a bottom tripod. High-resolution bottom scanning sonar was also attached to the tripod to provide images of bed features during the experiment. Hourly time-averaged near-bed concentrations of suspended sediment were calculated from three models and compared with the measurements. Surface waves and currents that were generated by a storm of moderate intensity caused bed stresses that exceeded threshold stress for D 50=0.02 cm, the median size of the moderately well-sorted bottom sediment, over a period of about 7 days. Estimates of the concentration at 1 cm above the bottom, C a1, were obtained using the ABS measurements. These observations have been compared with predictions for the concentration at 1 cm above the bottom, C 1. Nielsen's models for reference concentration C o [Nielsen, P., 1986. Suspended sediment concentrations under waves. Coastal Engineering 10, 32–31; Nielsen, P., 1992. Coastal Bottom Boundary Layers and Sediment Transport, Advanced Series on Ocean Engineering. World Scientific, Hackensack, NJ.] are purely wave-based and do not include effects of bottom currents on bed stress and bedform scales. C 1 calculated from this model compared well with measured C a1 when currents were weak and small oscillatory ripples were observed in the sonar images. However, during the 3-day period of highest bottom stresses modeled C 1 did not compare well to C a1. The other two models for C 1, Glenn and Grant [Glenn, S.M., Grant, W.D., 1987. A suspended sediment stratification correction for combined wave and current flows. Journal of Geophysical Research 92(C8), 8244–8264.] and van Rijn and Walstra [Van Rijn, L.C., Walstra, D.J.R., 2004. Description of TRANSPOR2004 and implementation in Delft3D-ONLINE. Interim Report prepared for DG Rijkswaterstaat, Rijksinstituut voor Kust en Zee. Delft Hydraulics Institute, The Netherlands.], accounted for combined wave-current stresses and included different formulations for predicting bedform scales. C 1 calculated from these models compared well with measurements throughout the early and middle period of the storm, but underpredicted measured values during the latter part of the experiment. An alternative method based on dimensional analysis provided a straightforward way to relate C 1 to bed stress and sediment parameters. This new relationship based on dimensional analysis explained about 77% of the variability in C a1 derived from the ABS data.

Settling velocity, effective density, and mass composition of suspended sediment in a coastal bottom boundary layer, Gulf of Lions, France

Particle size distribution and size-specific settling velocity are critical parameters for understanding the transport of fine sediment on continental margins. In this study, observed floc size versus settling velocity, volume distributions of particles 2 μm–1 cm in diameter, and calculated effective densities for all particle sizes provided estimates of the mass distribution in suspension, which is used to apportion mass among component particles, microflocs, and macroflocs. Measurements were made during relatively quiescent environmental conditions. Observations of size distributions based on mass demonstrate an increase in the component particle fraction through time. The increase in the percentage of component particles in suspension had implications on water column properties, as small changes in the component particle fraction affected water column optical transmission in a way that was not as easily detected by changes in the volume concentration distribution or total mass concentration. Flocs larger than 133 μm in diameter only comprised one quarter to one third of the mass in suspension. This finding may explain why suspension bulk clearance rates are often an order of magnitude lower than those predicted by other methods.

Review of Mechanics of Coastal Sediment Transport by Jorgen Fredsoe and Rolf Deigaard

Sedimentation processes in coastal areas are fascinating. The analysis of sediment transport under steady flow seems complex enough that one can only welcome this analytical treatment of sediment transport under wind and tidal waves in coastal areas. The book is the third volume of a set of four titles published by World Scientific in the broad area of Advanced Series on Ocean Engineering. The first volume is entitled The Applied Dynamics of Ocean Surface Waves by C.C. Mei. The second volume is Water Wave Mechanics for Engineers and Scientists by R.G. Dean and R.A. Dalrymple. The fourth volume is entitled Coastal Bottom Boundary Layers by P. Nielsen. This third volume is divided into 12 chapters: (1) “Basic concepts and potential wave theory”; (2) “Wave boundary layers”; (3) “Bed friction and turbulence in wave-current motion”; (4) “Waves in the surf zone”; (5) “Wave-driven currents”; (6) “Current velocity distribution in the surf zone”; (7) “Basic concepts of sediment transport”; (8) “Vertical distribution of suspended sediment waves and current over a plane bed”; (9) “Current-generated bed waves”; (10) “Wave-generated bed forms”; (11) “Cross-shore sediment transport and coastal profile development”; and (12) “Long-shore sediment transport and coastline development”. The title of this text is very appropriate. The book is intended for graduate students and describes the processes of sediment transport and how to represent them in mathematical models. The book is divided into two main parts. The first relates to hydrodynamic theory and the second to sediment transport and morphological development. The hydrodynamic part first reviews the elementary theory of water waves, followed with chapters on the turbulent wave boundary layer and turbulent interaction between waves and currents, and finally covers surf zone hydrodynamics and wave-driven currents. The part on sediment transport introduces the basic concepts (critical bed shear stress, bed load, suspended load and sheet layer, near-bed concentration and sloping bed); it treats suspended sediment in waves and current and in the

Eddy viscosity and Eulerian drift over rippled beds in waves

This paper reports the results of an experimental study of the Eulerian drift induced by weakly asymmetrical progressive waves in the bottom boundary layer over rippled beds in the transitional flow regime. Fourteen tests were carried out in a wave flume and one test in a steady flow flume for the estimation of the Nikuradse roughness length k s of the artificial rippled bed which was used. The fluid velocities were measured with a two-component laser-Doppler anemometer. The edge of the bottom wave boundary layer is estimated to be located at one Nikuradse roughness length above the level midway between crest and trough of the ripples. Momentum transfer in this boundary layer is dominated by organized vortices. An analytical model based on a time-varying eddy viscosity was developed by Davies and Villaret [J. Geophys. Res. 104 (C1) (1999) 1465] for the estimation of the wave-induced Eulerian drift above rippled and very rough beds in the turbulent flow regime. Present data show that the applicability of this model can be extended to the “lower” part of the transitional flow regime in the parameter ranges 1000≤ R≤6500 and 1≤ a/ k s≤3, where R is the flow Reynolds number and a is the orbital amplitude of fluid, if adjusted values of the model coefficients which represent the variation amplitude of the symmetrical and asymmetrical time-varying components of the eddy viscosity are used. The drift at the edge of the boundary layer is oriented in the direction of wave propagation in the transitional flow regime, while this drift is oriented in the opposite direction in the turbulent flow regime. The vertical profiles of Eulerian drift, horizontally averaged over a ripple length, are also compared with Longuet-Higgins' [Philos. Trans. R. Soc. Lond., A 245(903) (1953) 535] solution for a laminar flow above a smooth bed and with Nielsen's [Coastal bottom boundary layers and sediment transport. Advanced Series on Ocean Engineering, Vol. 4, World Scientific, Singapore, Chap. 1, pp. 40–60, 1992] solution involving a time-invariant eddy viscosity for turbulent flows over fairly rough beds.

Optical and physical variability on timescales from minutes to the seasonal cycle on the New England shelf: July 1996 to June 1997

High‐resolution time series of physical and bio‐optical data were obtained using moored and bottom‐mounted instruments on the southern New England continental shelf during the Coastal Mixing and Optics experiment (CMO) from July 1996 through June 1997. The most prominent physical and bio‐optical signals observed during the experiment were associated with the seasonal variability. However, several important events interrupted the seasonal cycle. These episodic events appear to have had a great impact on biogenic and nonbiogenic matter. Hurricanes and storms passed over or near the CMO site, resulting in reduced stratification of the water column, particle redistribution, and sediment resuspension. Changing hydrographic conditions that resulted from the influence of several water mass intrusions greatly affected particle concentration on timescales of days to several weeks. The bottom boundary layer had an influence on particle movement in the water column and along the seafloor. The results suggest that there is likely considerable interannual variability in both the physics and bio‐optics because of active and diverse physical forcing. This experiment also sets the context for comparing our coastal ocean results with previous open ocean findings. Important differences arise because of coastal bottom boundary layer effects, large‐scale water mass intrusions, and the relatively greater role of tides on the shelf. Timescales of optical variability (e.g., changes in phytoplankton spectral shapes of absorption) are thus generally shorter for the coastal environment.