Bottom Roughness Research Articles

Measurements of temporal variability of acoustic scattering from the seafloor in shallow-water sandy sites.

In the ocean, the performance of active sonar systems depends on the acoustic properties of the seafloor. Daily to monthly variations in near-bottom hydrodynamics and benthic biological activity may affect seafloor properties which then influence the acoustic response of the seafloor. The dependence of seafloor scatter on evolving environmental parameters was investigated using high-frequency active acoustic systems. Seafloor scattering measurements were analyzed in a series of experiments (two weeks to five months in duration) from downward-looking sonars oriented at 20° grazing angle with respect to the seafloor. Data were obtained in two shallow water locations near Portsmouth, New Hampshire, USA: a wave-dominated site and a site dominated by tidal currents. The bottom type for both sites was gravelly sand. The experimental set-up consisted of a tripod placed on the seafloor equipped with three transducers operating at 38, 70, and 200 kHz, a wave-sensing CTD, and underwater cameras. Scattering strength time series were obtained taking into account the local seafloor slope. Results show that there is variability in scattering strength (both in mean levels and distributions). Large variations often coincided with storm events, suggesting that this variability may be driven by changes in bottom roughness caused by storm-related hydrodynamics.

Variation in Community Structure and Abundance of Fish in Simple Structured Shallow Sandy Habitats

ABSTRACTSandy beaches and their surf zones characterise many of the world's open coastlines. They are important breeding, nursery and feeding areas for many species of fish. Despite the commonness and importance of sandy beach surf zones, the dynamics, space occupancy and diversity patterns of residing fish is in many places poorly understood. The aim of this study was to (1) characterise the fish community structure in 11 simple structured sandy surf zones of the northern Baltic Sea and (2) relate variation in fish abundance and community structure to a set of chosen abiotic variables. Using beach seine, weekly or biweekly sampling was conducted at fixed sites at 10 occasions throughout a summer season. A total of 60,006 fish individuals belonging to 20 species were recorded. Changes in abundance and community structure were mainly driven by the variation of only five species reflecting species‐specific recruitment patterns and different spatial responses to abiotic variables. Dominating groups were Gasterosteidae, Ammodytidae and Gobiidae that together formed 86% of the total adult fish catches. Larval numbers were completely dominated by Gobiidae. Multivariate analyses indicated species‐specific responses to measured environmental variables, most important being a combination of wave exposure, beach slope, bottom roughness, and temperature. The present study shows that changes in fish abundance on simple structured sandy sublittoral beaches in the northern Baltic Sea are large over the course of a breeding season. It also reveals that variation in adult and juvenile fish are driven by a set of abiotic factors that influence on the fish assemblage structure through mainly species‐specific, rather than through generic responses. Unravelling the degree to which the sandy shore fish community vary in the northern Baltic Sea will help in managing coastal environments that are increasingly being threatened by many anthropogenic stressors.

Femtosecond Pulsed Laser Machining of Fused Silica for Micro-Cavities with Sharp-Corners

Abstract Fused silica is the preferred material for applications requiring high temperature resistance, low thermal expansion coefficient and excellent optical properties. The machining of micro-cavities on fused silica surfaces is of particular interest for microfluidic manipulation, mechanical locking, and miniaturization of high-quality optical waveguides, but it still remains technically challenging via traditional manufacturing techniques especially for micro cavities with sharp corners. In the present study, the machining of micro-cavities in fused silica by a femtosecond laser-based method has been investigated numerically and experimentally. The effects of laser machining conditions, including laser power, laser scanning speed, laser incidence angle, and laser-off delay time, on the sidewall slope and bottom surface roughness of the micro-cavities were comprehensively investigated. The results indicated that laser power played a crucially important role in determining the sidewall slope of the micro-cavity, and the laser scanning speed had a significant influence on the bottom surface roughness. Furthermore, the sidewall slope of the micro-cavity was linearly increased as the laser incidence angle increases. By using a laser incidence angle of 10° and a laser-off delay time of 280 ms, a micro-cavity with sidewall slopes close to right angles (10°) was fabricated. This study confirms that femtosecond laser machining is an effective method for fabricating micro-cavities with sharp corners on fused silica surfaces, and the appropriate laser machining conditions should be considered based on practical application scenarios.

Fully developed self-aerated flow in steep chute with various bottom roughness

Fully developed self-aerated flow in steep chute with various bottom roughness

Anisotropic turbulence modeling for natural channel flow: A numerical approach with finite element method

In this study, we propose a numerical model aimed at improving the understanding and prediction of flow velocities in natural channels. This model specifically focuses on the challenges presented by anisotropic turbulence and bottom roughness. By incorporating the mixing length concept into an algebraic turbulence model and employing the finite element method with Streamline-Upwind/Petrov–Galerkin (SUPG) stabilization, our model seeks to refine the simulation of axial velocity distribution and secondary motions. Validation was achieved through Acoustic Doppler Current Profiler (ADCP) measurements in three sections of the Canal do Rodeador, showing our model’s predictions of discharge and average velocity to have a deviation of approximately 9.34% from experimental data. The results underline the significance of secondary currents and turbulence anisotropy in shaping channel flow behaviors, offering new insights into the interactions between flow characteristics and channel bed features. This model stands out as a robust tool for hydraulic structure design and hydrokinetic potential evaluation, providing a non-intrusive, cost-effective alternative to traditional methods.

Diseño de estructuras hidráulicas en régimen supercrítico con sedimentos: un criterio matemático para calcular la rugosidad del fondo

In some channels with high gradients, heavy scouring and erosion, as well as overflow, is highly common to occur, thus it is required a water flow velocities regulation. An option for achieving this, is to significantly increase the channels’ s bottom roughness through the installation of rapid hydraulic structures. However, in sedimentary density fluids, the change of velocity generates the deposition of solids which could be consolidated by changing the geometric design of these structures. This study aims to estimate the degree of confidence expected when modifications take place to artificial roughness geometries in the bottom of the channel with turbulent flow and sediment flow. This modification requires to transverse ribs into ramps using an experimental mathematical analysis. The study allows us to conclude that the newly generated bottom roughness causes more stable water flows, and it is a way to reduce flow velocities.

Onshore sediment transport enhancement and evolution of bedforms: Laboratory experiments of beach ploughing

Progressive coastal retreat has been an issue exacerbated in recent years due to climate change. Sand is eroded from beaches during the winter and partially recovered during summer by slow accretion processes. The development of new working with nature techniques that produce enhanced beach accretion could help recover most of the sand lost during winter and thus reduce the impact of climate change on beaches. The presence of bedforms contribute to increasing onshore sediment transport, but few studies have been performed to quantify their effect. In this study, the evolution and effect of artificially created bedforms on onshore sediment transport were analysed in prototype-scale laboratory experiments. The tested bedforms mimicked a beach ploughing of the intertidal area, with a wavelength of 1.6 m and height of 0.25 m, corresponding to the ploughing dimensions that a tractor can perform. Two tests were performed with the same initial morphology, medium sand (D50 = 0.318 mm), sea state conditions (Hs = 0.3 m, Tp = 7 s) that produced accretion, and different water levels that represent two tidal states. The experimental flume was longitudinally split into two equal channels of 1 m wide, allowing the simultaneous simulation of a natural control geometry and a ploughed geometry, facilitating the comparison and assuring the very same sea conditions. The presence of ploughed bedforms produced two effects: (1) an acceleration of natural accretion rates reaching 40%, and (2) onshore sediment transport due to the migration of the bedforms. The acceleration of natural accretion was explained by the extra bottom roughness induced by the bedforms, which produced more wave dissipation through bottom friction and thus more accretive conditions. The ploughed height decreased exponentially as waves broke over the crest of the ridges, which almost disappeared after 2–3 h of wave action. As a result, the extra bottom roughness also decreased as time passed. Consequently, the nature-assisted beach enhancement technique of ploughing should be applied at each low tide to produce a cumulative effect. Plough bedforms migrated onshore at a rate of approximately 0.2 m/h during the first hour, mobilizing onshore up to 61 kg m−1 h−1 of sediment. Ripples appeared on the tops of the ridge crests and migrated faster onshore, contributing to the migration of the ploughed bedforms. These results demonstrated the importance of considering bedforms while studying accretion processes and the potential of ploughing as an innovative strategy of working with nature to enhance beach recovery.

Experimental Observations And Prediction Of Wave Attenuation Using A Coral Reef Restoration Approach

The large bottom roughness typical of coral reefs can be effective at reducing wave energy incident to coastlines through the dissipation induced by how wave-driven oscillatory flows interact with the roughness to determine hydrodynamic forces (i.e., drag and inertial). A physical understanding of these fluid-structure interaction processes is essential in designing coral reef restoration projects that can enhance coastal protection as well as deliver other beneficial ecosystem services, as a more sustainable alternative over conventional engineered structures (e.g. breakwaters and seawalls). In this study we quantify both wave attenuation and hydrodynamic forces across progressive stages of a coral reef restoration solution developed by Mars Sustainable Solutions. The Mars Assisted Reef Restoration System (MARRS) involves propagating coral fragments onto hexagonal steel structures called Reef Stars, which are connected in tessellating patterns over degraded reef flats (Figure 1).

Spatially quantifying and mapping the riverbed roughness of a tropical river via acoustic Doppler current profiler (ADCP) measurements to develop a 2-D numerical model

Field measurement of the bed roughness of tropical tidal rivers is a long and expensive process. In this study, we quantified the roughness coefficient from the acoustic Doppler current profiler (ADCP) measurements considering flow characteristics along a 240 km length of the Vu Gia Thu Bon River system, in central Vietnam. Our approach entailed the application of the logarithmic velocity distribution trend of turbulent flows to determine the bottom roughness height. Then, we applied the equation suggested by Strickler (1923) to calculate the Manning roughness coefficient. The TELEMAC-2D hydraulic model was then used to simulate the effect of the roughness coefficient on the estimated water level. Despite its uncertainty, the estimated bed roughness was sufficient for the accurate calculation of the hydraulic variables. Compared to those in previous research, the statistical indicators showed good performance in the first simulation process during both the calibration and validation periods along the river system.

Laboratory Investigation of Energy Dissipation and Turbulent Mixing During Internal Solitary Wave Breaking on the Rough Shelf Slope

AbstractWhen internal solitary waves (ISW) propagate to the continental shelf, they typically runup and break on the forereef. This process might lead to periodic temperature drops at the reef slope, which has potential to protect coral reefs from bleaching threats. A series of laboratory experiments were conducted to investigate turbulence characteristics and energy dissipation during the ISW breaking events on the shelf slope with different bottom roughness to mimic the forereef environment. Particle image velocimetry (PIV) and planar laser‐induced fluorescence (PLIF) were coupled to obtain simultaneous high‐resolution velocity and density fields, based on which we then calculate the energy budget and turbulent dissipation rate. We found that the ISW breaking can be divided into two turbulent processes, the production of shear instability and coherent structures. Ratio of energy loss is weakened with the increase of bottom roughness and the collapse height, while the rough structure substantially enhanced the turbulent dissipation rate. The mixing efficiency is between 0.1 and 0.3, close to its maximal rate at 0.25. Our results reveal that the bottom roughness has impact on the mixing efficiency and turbulent process during the ISW breaking events. We found that rough structures at the shelf slope inhibit the mixing efficiency, especially in cases with small amplitude incline ISW. This finding may help us to better understand the reef damage and coral reef decline and support the management of coral protection strategy.

Iterative dynamics-based mesh discretisation for multi-scale coastal ocean modelling

Flow in coastal waters contains multi-scale flow features that are generated by flow separation, shear-layer instabilities, bottom roughness and topographic form. Depending on the target application, the mesh design used for coastal ocean modelling needs to adequately resolve flow features pertinent to the study objectives. We investigate an iterative mesh design strategy, inspired by hydrokinetic resource assessment, that uses modelled dynamics to refine the mesh across key flow features, and a target number of elements to constrain mesh density. The method is solver-agnostic. Any quantity derived from the model output can be used to set the mesh density constraint. To illustrate and assess the method, we consider the cases of steady and transient flow past the same idealised headland, providing dynamic responses that are pertinent to multi-scale ocean modelling. This study demonstrates the capability of an iterative approach to define a mesh density that concentrates mesh resolution across areas of interest dependent on model forcing, leading to improved predictive skill. Multiple design quantities can be combined to construct the mesh density, refinement can be applied to multiple regions across the model domain, and convergence can be managed through the number of degrees of freedom set by the target number of mesh elements. To apply the method optimally, an understanding of the processes being model is required when selecting and combining the design quantities. We discuss opportunities and challenges for robustly establishing model resolution in multi-scale coastal ocean models.

On the maximum velocity profile of wave boundary layer flows in the very rough turbulent regime

The maximum velocity profile in turbulent wave boundary layer flows has been experimentally investigated under both regular and irregular wave conditions. Four types of seabed models are adopted, i.e., smooth, sand-covered, uniform-sphere-covered and nonuniform-stone covered. The results show that the maximum overshoot increases with the decreasing A/ks (A is the semi-excursion of fluid particles in the free stream and ks is the bottom roughness), but it is not notably influenced by the irregularity of the waves nor the nonuniformity of bottom roughness elements. Two-dimensional numerical simulations are carried out to reveal the physics behind the large overshoot behavior. It is found that the gap flow accelerates around roughness elements and subsequently contributes to the overshoot. A three-parameter defect function is proposed that well describes the maximum velocity profile, and all three model parameters are correlated with the governing flow parameter A/ks.

Turbulence Across the Antarctic Circumpolar Current in the Indian Southern Ocean: Micro‐Temperature Measurements and Finescale Parameterizations

AbstractTurbulence structures across the Antarctic circumpolar current (ACC) in the Indian Ocean at around 50°E were revealed using microstructure measurements. Depth‐averaged turbulent energy dissipation rates (ε) in 320 m segments estimated using a conductivity‐temperature‐depth (CTD)‐attached micro‐temperature fast‐response thermistor (FP07) were well reproduced using existing finescale parameterizations excluding near‐surface and near‐bottom 160 m. The spatial variations of ε are explained by density stratification N2, bottom topographic roughness, and depth‐averaged current speed U. Diapycnal diffusivity Kρ(=0.2εN−2) is found proportional to the squared internal wave energy () and increases toward the bottom, especially over rough bottom topography. The decay scale height is >1000m and the turbulence remains large even in the upper pycnocline. Existing shear‐based finescale parameterizations were generally consistent with the FP07 measurements but tended to overestimate in the areas with large shear‐strain ratio Rω and low ε in the Continental Zone (CZ) south of the ACC fronts (58–65°S). This overestimation is improved by using the strain‐based finescale parameterization with a constant Rω = 3.

Low-Tech and Low-Cost System for High-Resolution Underwater RTK Photogrammetry in Coastal Shallow Waters

Monitoring coastal seabed in very shallow waters (0–5 m) is a challenging methodological issue, even though such data is of major importance to many scientific and technical communities. Over the years, Structure-from-Motion (SfM) photogrammetry has emerged as a flexible and inexpensive method able to provide both a 3D model and high-resolution imagery of the seabed (~cm level). In this study, we propose a low-cost (about USD 1500), adaptable, lightweight and easily dismantled system called POSEIDON (for Platform Operating in Shallow-water Environment for Imaging and 3D reconstructiON). This prototype combines a floating support (typically a bodyboard), two imagery sensors (here, GoPro® cameras) and an accurate positioning system using Real Time Kinematic GNSS. Validation of this method was deployed in a macrotidal zone, comparing on the foreshore the point cloud provided by POSEIDON “SfM bathymetry” and by classical terrestrial SfM survey. Mean deviation was 5.2 cm and standard deviation was 4.6 cm. Such high-resolution SfM bathymetric surveys have a great potential for a wide range of applications: micro-bathymetry, hydrodynamics (bottom roughness), benthic habitats, ecological inventories, archaeology, etc.

Internal wave activity in the deep Gulf of Mexico

Internal wave activity in the Gulf of Mexico (GoM) is investigated using a fleet of profiling floats. The floats continuously measured temperature and salinity as they drifted at a parking depth of 1500 dbar, allowing for the reconstruction of 2615 time series of isopycnal displacements. Thanks to the dense sampling of the eastern part of the GoM (east of 90°W), the geographical distribution of the internal waves displacement variance and available potential energy (APE) is revealed. The Loop Current (LC) influence region, between the Yucatan shelf to the west and the southern West Florida shelf to the east exhibits increased displacement variance and APE both in the continuum and near-inertial bands, while the north-eastern and central GoM show reduced internal wave activity. As the LC position fluctuates between a retracted and extended mode, we assessed the impact of the presence or absence of the LC in the increased internal wave activity region. It is shown that in the LC influence region, APE is increased (decreased) when the LC is present (absent), suggesting a strong control of the LC on deep internal waves activity. The 1500 dbar flow velocity, bottom roughness, and float altitude also seem to contribute to increased internal waves APE, but their influence is more subtle. Oppositely, no correlation with wind speed or wind intermittency is found.

Cavitating flows in microchannel with rough wall using a modified microscale cavitation model

Cavitation can induce drastic disruption and significantly enhance heat transfer, especially in microchannels. However, due to the scale effect, the cavitation characteristics in micro-scale can be significantly different from those in conventional scale. A modified cavitation model according to the Rayleigh-Plesset bubble kinetic equation is formulated by including the effects of surface tension, and temperature difference between the local environment and the far-field environment. The impacts of latent heat of vaporization and condensation are introduced into the energy equation. The micro-scale cavitation model proposed in the paper is verified through comparison with experimental data. The influence of surface roughness on cavitation flow in microchannel with restrictor is then investigated. The results demonstrate that bottom surface roughness in microchannels significantly increases flow resistance, suppresses cavitation flow and causes high frequency pressure fluctuations.

The impact of rough topography on behaviors of mesoscale eddies as revealed by submesoscale resolving simulations

Mesoscale eddies are ubiquitous in the world ocean and are a key factor in maintaining the global oceanic energy balance. Geophysical turbulence theory predicts that the eddy length scale should increase to Rhines scale as it inherits the inverse energy cascade from smaller scales that help to stabilize the ocean circulation system. However, satellite-observed mesoscale eddies are much smaller than predicted, which indicates that unclear forward energy cascades prevail in their life cycles. In this study, we explore the role of finite rough topography in modulating eddy kinetic energy and horizontal length scales by using a series of submesoscale resolving numerical simulations with parameters typical of the Drake Passage. Our results show that, when compared with those generated over a flat bottom, the presence of rough topography can substantially reduce the horizontal length scales of eddies by approximately 60%–70%, even in upper layers 2–3 km above the bottom. Rough topography also consumes significant amounts of eddy kinetic energy, as the volume-mean eddy kinetic energy over a rough bottom decreases by about 70%–80% when compared with that over a flat bottom. By using a dynamics-based decomposition method, we subtracted the submesoscale motions from total flow and found that, in addition to energetic internal lee waves, deep-ocean submesoscale processes (integrated over the bottommost 1 km) contributed roughly 22% of the total viscosity dissipation across the whole research domain over a rough bottom. Therefore, this study indicates that the role of deep-ocean submesoscale motions in the energy cascade and abyssal mixing should be considered in ocean model parameterizations in the future.

Random Matrix Theory for Sound Propagation in a Shallow-Water Acoustic Waveguide with Sea Bottom Roughness

The problem of sound propagation in a shallow sea with a rough sea bottom is considered. A random matrix approach for studying sound scattering by the water–bottom interface inhomogeneities is developed. This approach is based on the construction of a statistical ensemble of the propagator matrices that describe the evolution of the wavefield in the basis of normal modes. A formula for the coupling term corresponding to inter-mode transitions due to scattering by the sea bottom is derived. The Weisskopf–Wigner approximation is utilized for the coupling between waterborne and sediment modes. A model of a waveguide with the bottom roughness described by the stochastic Ornstein–Uhlenbeck process is considered as an example. Range dependencies of mode energies, modal cross coherences and scintillation indices are computed using Monte Carlo simulations. It is shown that decreasing the roughness correlation length enhances mode coupling and facilitates sound scattering.

Combining field observations and high-resolution numerical modeling to demonstrate the effect of coral reef roughness on turbulence and its implications for reef restoration design

Coral reefs are effective natural barriers that protect adjacent coastal communities from hazards such as erosion and storm-induced flooding. However, the degradation of coral reefs compromises their ability to protect against these hazards, making degraded reefs a target for restoration. There have been limited field and numerical modeling studies conducted to understand how an increase in coral reef roughness, as would occur due to restoration, can affect wave energy dissipation for a range of real-world wave and water level conditions. To address this knowledge gap, field measurements were collected over adjacent low-roughness and high-roughness reefs off Molokaʻi, Hawaiʻi, USA, subjected to the same oceanographic forcing. Those field data were then used to calibrate and validate OpenFOAM computational fluid dynamics models of the reef. These calibrated models were then used to explore energy dissipation for a range of wave conditions based on measurements from a suite of existing datasets and values from the literature. In general, wave dissipation scales with incident wave conditions, where greater dissipation occurred for shallow depths and shorter-period waves. This tendency for short-period waves to be more readily attenuated is supported by wave energy dissipation factors in the range of 0.1–5, which decline with increasing wave period. Near-bed turbulent kinetic energy dissipation also scales with incident wave conditions, where the greatest difference in dissipation between low and high relief cases occurs for short wave periods. Turbulence becomes less affected by bottom roughness as the wave period increases. Based on this study, wave attenuation and turbulent energy dissipation could be enhanced by 0.5–1 order of magnitude (45% per across-shore meter) if the seabed roughness at the field site were increased by 13%, an achievable goal in coral reef restoration.

LABORATORY STUDY OF WAVE HYDRODYNAMICS IN THE SURF ZONE IN PRESENCE OF ROUGHNESS

In the global context of increasing anthropic pressure on coastal environments, understanding and modelling the physical processes leading to submersion or erosion of the coastline are key factors in anticipating these risks. Many studies of nearshore wave transformation have been conducted mainly on sandy beaches or gentle slopes. On this type of beach, wave transformation models give generally satisfactory results (Thornton and Guza, 1983). Yet, the hydrodynamics of other types of bottoms, specifically rough and/or steep bottoms, are not fully understood and modelled, especially the effect of roughness on frictional dissipation (Péquignet et al, 2011; Monismith et al, 2015; Poate et al, 2018). Thus, a laboratory experiment, in the wave flume (6m long) of Seatech Engineering School, in Toulon, France was conducted both to improve the understanding of physical processes in controlled environment, and to connect the hydrodynamics to the seabed structure.