Wave-dominated Flows Research Articles

Transport of heterometric sediments in wave-dominated flows – Tracer experiments

Sediment dynamics is a complex subject, playing an important role in coastal areas. The interaction between wave action and sediment particles is determinant to understand sediment transport processes. The knowledge of sediment transport comprising sand mixtures is relevant as coastal areas, usually, present heterogeneities of sediment particle sizes in the horizontal and vertical directions, denoting the existence of selective transport processes. This study investigates the effects of non-cohesive size-graded sands on sediment transport fluxes to understand selective transport mechanisms under wave-dominated conditions. A laboratory experiment was carried out at the large wave flume (Großer Wellenkanal, GWK) in Hannover, considering distinct sediment mixtures between two well-sorted sands with median grain sizes of 0.21 mm and 0.58 mm. The experiments were carried out for four sediment bed configurations with two different wave energy conditions. A fluorescent sand tracer technique was implemented to measure total and fractional net sediment transport rates and the outcomes are compared with the ones obtained with the mass conservation method. The experimental results show the occurrence of the interaction between different sand fractions: a decrease in the transport of fine sediments and an enhancement of the transport of the coarse sediments as the percentage of coarse sediments in the bed mixture increases. These effects are larger as the coarser fraction in the sand bed increases and they are noticeable for different sediment transport regimes (sheet flow and rippled beds).

Material Residence Time in Marine Canopies Under Wave-Driven Flows

Coastal canopies (e.g. seagrasses, coral reefs, and kelp forests) are vitally important ecosystems that provide a range of ecological services (e.g. oxygen production, sediment stabilization and trapping and recycling of nutrients). The long-term health, productivity and survival of these canopies rely heavily on the residence time of ecologically-significant materials in these environments. Recent studies have shown that the presence of submerged canopies induces strong mean current over the canopy top, even in purely wave-dominated environments. Thus, in addition to vertical mixing, the horizontal flushing of materials (resulting from these canopy-induced currents) will dictate rates of water renewal and, therefore, residence time in wave-dominated flows over submerged canopies. Building on this recently-improved understanding, this paper provides (for the first time) a framework for estimation of material residence time (Tres) and its variation with core system parameters, including both canopy and wave characteristics. This is done through consideration of a P'eclet number (Pe) which is the ratio of mixing to advective time scales. Prediction of residence time for a wide and realistic range of marine canopies (i.e. mixing dominated with Pe>1, and transitional (Pe ~ O (1)) reveals that while Tres decreases with wave height and increases with water depth, it has a complex relationship with canopy density and height. Importantly, residence time can vary from orders of seconds to hours, depending on wave and canopy properties. This has considerable ecological implications for marine canopies through the direct impact on a range of chemical and biogeochemical processes within the canopy. The framework presented here provides a critical step forward in being able to predict residence time in coastal canopies and test the interacting set of factors that influence the residence time in real, complex systems.

Characterization of Eddy Scales Over Tandem Obstacles for Surface Wave-Dominated Flow

Characterization of Eddy Scales Over Tandem Obstacles for Surface Wave-Dominated Flow

Geophysical turbulence dominated by inertia–gravity waves

Recent evidence from both oceanic observations and global-scale ocean model simulations indicate the existence of regions where low-mode internal tidal energy dominates over that of the geostrophic balanced flow. Inspired by these findings, we examine the effect of the first vertical mode inertia–gravity waves on the dynamics of balanced flow using an idealized model obtained by truncating the hydrostatic Boussinesq equations on to the barotropic and the first baroclinic mode. On investigating the wave–balance turbulence phenomenology using freely evolving numerical simulations, we find that the waves continuously transfer energy to the balanced flow in regimes where the balanced-to-wave energy ratio is small, thereby generating small-scale features in the balanced fields. We examine the detailed energy transfer pathways in wave-dominated flows and thereby develop a generalized small Rossby number geophysical turbulence phenomenology, with the two-mode (barotropic and one baroclinic mode) quasi-geostrophic turbulence phenomenology being a subset of it. The present work therefore shows that inertia–gravity waves would form an integral part of the geophysical turbulence phenomenology in regions where balanced flow is weaker than gravity waves.

The role of turbulent hydrodynamics and surface morphology on heat and mass transfer in corals.

Corals require efficient heat and mass transfer with the overlying water column to support key biological processes, such as nutrient uptake and mitigation of thermal stress. Transfer rates are primarily determined by flow conditions, coral morphology and the physics of the resulting fluid-structure interaction, yet the relationship among these parameters is poorly understood especially for wave-dominated coral habitats. To investigate the interactive effects of these factors on fluxes of heat and mass, we measure hydrodynamic characteristics in situ over three distinct surface morphologies of massive stony corals in a Panamanian reef. Additionally, we implement a numerical model of flow and thermal transport for both current and wave conditions past a natural coral surface, as well as past three simplified coral morphologies with varying ratios of surface roughness spacing-to-height. We find oscillatory flow enhances rates of heat and mass transfer by 1.2-2.0× compared with unidirectional flow. Additionally, increases in Reynolds number and in surface roughness ratio produce up to a 3.3× and a 2.0× enhancement, respectively. However, as waves begin to dominate the flow regime relative to unidirectional currents, the underlying physical mechanisms mediating transfer rates shift from predominantly turbulence-driven to greater control by inertial accelerations, resulting in larger heat and mass transfer for small surface roughness ratios. We show that for rough corals in wave-dominated flows, novel trade-off dynamics for heat and mass transfer exist between broadly spaced roughness that enhances turbulence production versus narrowly spaced roughness that produces greater surface area. These findings have important implications for differential survivorship during heat-induced coral bleaching, particularly as thermal stress events become increasingly common with global climate change.

Seasonal Growth and Senescence of a Zostera marina Seagrass Meadow Alters Wave-Dominated Flow and Sediment Suspension Within a Coastal Bay

Tidally driven flows, waves, and suspended sediment concentrations were monitored seasonally within a Zostera marina seagrass (eelgrass) meadow located in a shallow (1–2 m depth) coastal bay. Eelgrass meadows were found to reduce velocities approximately 60 % in the summer and 40 % in the winter compared to an adjacent unvegetated site. Additionally, the seagrass meadow served to dampen wave heights for all seasons except during winter when seagrass meadow development was at a minimum. Although wave heights were attenuated across the meadow, orbital motions caused by waves were able to effectively penetrate through the canopy, inducing wave-enhanced bottom shear stress (τb). Within the seagrass meadow, τb was greater than the critical stress threshold (=0.04 Pa) necessary to induce sediment suspension 80–85 % of the sampling period in the winter and spring, but only 55 % of the time in the summer. At the unvegetated site, τb was above the critical threshold greater than 90 % of the time across all seasons. During low seagrass coverage in the winter, near-bed turbulence levels were enhanced, likely caused by stem–wake interaction with the sparse canopy. Reduction in τb within the seagrass meadow during the summer correlated to a 60 % reduction in suspended sediment concentrations but in winter, suspended sediment was enhanced compared to the unvegetated site. With minimal seagrass coverage, τb and wave statistics were similar to unvegetated regions; however, during high seagrass coverage, sediment stabilization increased light availability for photosynthesis and created a positive feedback for seagrass growth.

Wave and sediment dynamics along a shallow subtidal sandy beach inhabited by modern stromatolites

To help define the habitat of modern marine stromatolites, wave-dominated flow and sediment transport were studied in the shallow subtidal region (1-2 m depth) along the slightly concave, windward face of Highborne Cay, Exuma, Bahamas - the only face of the cay that includes a population of stromatolites concentrated near the region of highest curvature of the beach. Wave energy impacting this island's most exposed beach was driven by local wind forcing which increases largely in response to the passage of atmospheric disturbances that typically affect the region for periods of a few days. Although some wave energy is almost always noted (maximum horizontal orbital speeds at the bottom are rarely <10 cm s(-1)), wave conditions remain comparatively calm until local winds increase above speeds of approximately 3-4 m s(-1) at which point maximum wave speeds rapidly increase to 50-80 cm s(-1). Stromatolites, which are largely restricted to the shoreward side of a shallow platform reef, are sheltered by the reef beyond which wave speeds are one to four times higher (depending on tidal stage). Moreover, stromatolite populations are predominantly found along a region of this wave-exposed beach that experiences comparatively reduced wave energy because of the curved morphology of the island's face. Maximum wave speeds are 1.4 to 2 times higher along more northern sections of the beach just beyond the locus of stromatolite populations. A quantitative model of sediment transport was developed that accurately predicted accumulation of suspended sediment in sediment traps deployed in the shallow subtidal zone along this beach. This model, coupled with in situ wave records, indicates that gross rates of suspended sediment deposition should be two to three times higher northward of the main stromatolite populations. Regions of the beach containing stromatolites nevertheless should experience significant rates of gross suspended sediment deposition averaging 7-10 g cm(-2) day(-1) ( approximately 4-6 cm day(-1)). Results suggest that one axis of the habitat of modern marine stromatolites may be defined by a comparatively narrow range of flow energy and sediment transport conditions.

Soluble settlement cue in slowly moving water within coral reefs induces larval adhesion to surfaces

Larvae of many benthic marine animals are induced to settle and metamorphose by dissolved chemical cues released by organisms on the substratum. Can dissolved cues released into the turbulent, wave-dominated flow typical of many shallow coastal areas affect the adhesion of settling larvae to benthic surfaces? We addressed this question using larvae of the nudibranch, Phestilla sibogae, which settle and metamorphose in response to a water-borne, species-specific metabolite of their prey, Porites compressa, abundant corals forming reefs in shallow habitats in Hawaii. Field measurements of water velocities showed oscillatory wave-driven flow above reefs with peak instantaneous velocities of 0.10–0.40 m/s, much slower back-and-forth water movement through the spaces within reefs with peak velocities of 0.02–0.04 m/s, and net shoreward transport of water through reefs of ∼0.01 m/s. We used a water channel in the laboratory to measure the wall shear stresses required to dislodge larvae of P. sibogae from various surfaces. We found that cue from P. compressa is necessary for the larvae to attach to surfaces and, if cue is dissolved in water bathing the larvae, they can adhere to surfaces other than living P. compressa. After 2 h of exposure to cue and a surface, the adhesive strength of the larvae reached its peak value and did not change during the next 20 h. The mean nominal wall shear stress required to dislodge larvae of P. sibogae attached to P. compressa tissue (1.59±0.64 Pa, n=10 experiments) was not significantly different from that necessary to wash them off coralline algae (2.53±2.45 Pa, n=8) encrusting coral skeleton collected within reefs, but they stuck more tightly to glass (4.26±1.04 Pa, n=13). It is likely that most P. sibogae larvae initially settle onto surfaces within reefs because (1) settlement cue released by P. compressa and sinking larvae of P. sibogae accumulate in the slowly moving water within a reef; (2) larvae exposed to cue are able to stick to surfaces such as coralline algae that are common within the reef; and (3) P. sibogae develop their full attachment strength slowly, and the magnitude and frequency of peak shear stresses due to turbulent sweeps that might wash larvae off surfaces are much lower within than at the top of the reef.

Accuracy of a Pulse-Coherent Acoustic Doppler Profiler in a Wave-Dominated Flow

Abstract The accuracy of velocities measured by a pulse-coherent acoustic Doppler profiler (PCADP) in the bottom boundary layer of a wave-dominated inner-shelf environment is evaluated. The downward-looking PCADP measured velocities in eight 10-cm cells at 1 Hz. Velocities measured by the PCADP are compared to those measured by an acoustic Doppler velocimeter for wave orbital velocities up to 95 cm s−1 and currents up to 40 cm s−1. An algorithm for correcting ambiguity errors using the resolution velocities was developed. Instrument bias, measured as the average error in burst mean speed, is −0.4 cm s−1 (standard deviation = 0.8). The accuracy (root-mean-square error) of instantaneous velocities has a mean of 8.6 cm s−1 (standard deviation = 6.5) for eastward velocities (the predominant direction of waves), 6.5 cm s−1 (standard deviation = 4.4) for northward velocities, and 2.4 cm s−1 (standard deviation = 1.6) for vertical velocities. Both burst mean and root-mean-square errors are greater for bursts wit...

Current and wave dynamics in the shallow subtidal: implications to the ecology of understory and surface-canopy kelps

Current and wave properties were studied in the semi-protected waters of the San Juan Archipelago, Washington, at 6 shallow subtidal sites chosen to include a wide range of variability in exposure to both tidal currents and waves. Within each site, 4 to 6 plots measuring roughly 50 to 100 m 2 each were established on nearly horizontal rock platforms at mean depths ranging from 6.1 to 11.0 m, with most plots at mean depths of 7.0 to 9.2 m. Plots were established nominally to minimize within-site variability in hydrodynamics, and were locations of several species of kelp (reported in a companion paper). For more than a year replicate measurements of flow and pressure (mean depth and wave signals) were collected at all sites in 2 Hz bursts 25 cm above the substratum, a height relative to understory kelps, providing a unique, detailed spatial and temporal characterization of a shallow subtidal hydrodynamic regime pertinent to these plants. Despite the intended similarity of plots within sites, local-flow microhabitat remained substantial at scales relative to understory plants, and largely ameliorated differences in tidal signals among most sites. Greater than 50 % of the spatial variability in maximum tidal current speed, and 31 to 44 % of the variance in the duration of periods of calm flow (speeds consistently <20 cm s -1 ), occurred within sites at scales of meters, and not at the larger (km) scales that separated sites. In contrast, wave effects at 7 to 11 m depth were predictable spatially. Significant wave impacts were recorded only at 2 sites characterized by a large, open fetch. Wave-dominated flows were recorded by replicate sensors multiple times within both of these sites, and the strongest instantaneous wave speeds approached 150 to 200 cm s -1 . Maximum wave-generated speeds recorded just above the substratum at these 2 sites were ∼2 to 3x maximum tidal currents. These results help to explain patterns noted in concurrent studies of the population dynamics and morphology of several species of kelp.

Velocity and turbulence distributions in combined wavecurrent flows over a rippled bed

This paper describes an experimental programme carried out in a laboratory channel, to investigate the velocity distributions over a rippled bed in combined wave-current flows. In this study, waves propagate against a turbulent current. Velocity measurements were made with a laser-Doppler anemometer. The Nikuradse roughness length k s , of the bed has been estimated from the traditional logarithmic velocity profile for current alone. The most significant effect of the superposition of waves on a current is the increase of the apparent roughness of the bed with increasing wave height. This effect is well described by Sleath's [49] model. As for rough flat beds, the turbulence intensity over ripples varies inversely with height at sufficiently large distance from the bed for wave-dominated flows.

Transgressive facies sequence of a high energy, wave-tide-storm-influenced shoreface: A case study of the east Frisian Barrier Islands (Southern North Sea)

Published information and recent observation of physical and biogenic structures as well as grain size patterns along the East Frisian barrier islands (southern North Sea) have been synthesized to develop a facies model for a high energy, wave-tide-storm-influenced transgressive shoreface. As exemplified by Spiekeroog Island, three major shoreface facies (Upper, Central and Lower) are distinguishable, each of which can be subdivided into a Proximal and a Distal subfacies. A transgressive shoreface sequence in the rock record, in ascending order, will show the Upper Shoreface facies (USF) at the base. This is characterized by very well-sorted fine sands exhibiting predominantly plane lamination, which is consistent with its wave-dominated flow regime. The Central Shoreface facies (CSF) overlying the USF shows a high proportion of shells, a broad range of sediment sizes (fine sand to pebbles), and marked diversity in sedimentary structures. Graded storm beds, tidal cross-strata, mud-sand couplets, and horizontal to subhorizontal lamination are common. Swaley/hummocky (?) strata may also occur in its Distal subfacies. The sequence is topped by the Lower Shoreface facies (LSF). It consists mostly of bioturbated fine sands, although graded beds do occur. Distal sabfacies sands are consistently finer, better sorted, and more positively skewed than their Proximal counterpart. Anomalies in both coastwise and cross-shore textural trends are noted. In contrast to most other coastal settings, both fair-weather (wave- and tidal-current) and storm deposits are likely to be represented in subequal proportion in the rock record.

Boundary layer growth of a micropolar fluid

The unsteady boundary layer flow of a micropolar fluid along an infinite plate is analysed when the plate undergoes an impulsive motion in its plane. The solution is obtained by Laplace transform technique. The microstructure of the fluid is found to induce a wave-dominated flow pattern, there being two modes of wave-propagation. The characteristics of these waves, during the initial and final stages of the boundary layer growth, are discussed.