Rough Seabed Research Articles

Incorporating Non-Equlibrium Ripple Dynamics into Bed Stress Estimates Under Combined Wave and Current Forcing

We present direct measurements of seafloor ripple dimensions, near-bed mean flow Reynolds stresses and near-bed turbulent sediment fluxes on a sandy inner shelf subjected to strong wave and tidal current forcing. The measurements of ripple dimensions (height, wavelength) and Reynolds stresses are used to evaluate the performance of a methodology for the incorporation of non-equilibrium ripple dynamics into the calculations of the drag exerted by the bed on the overlying flow (i.e., the bed stress) using a boundary layer model for wave–current interaction. The methodology is based on the simultaneous use of existing models for the time-dependent evolution of ripple geometry and for the wave–current boundary layer that enable a continuous feedback between bottom drag and small-scale seabed morphology, which determines seabed roughness. The model-data comparison shows good agreement between modeled and measured bed stresses and bedform dimensions. Moreover, the proposed methodology is shown to give better results than combining the wave–current interaction model and standard equilibrium ripple predictors, both in terms of bed stresses and ripple dimensions. The near-bed turbulent vertical sediment fluxes show good correlation with the combined wave–current stresses and are used as a proxy for the resuspension of fine sediments (d < 64 μm) from the sandy seabed matrix. Implications for the modeling of the resuspension processes and erosional fluxes are discussed in light of our findings.

Characteristic Analysis of Vertical Tidal Profile Parameters at Tidal Current Energy Site

Many mathematical models have been proposed to estimate vertical tidal current profiles. However, as previous studies have shown that tidal current energy sites have different characteristics in their vertical tidal current profiles, it is necessary to estimate the profiles from field-measured data for practical purposes. In this study, we measured layered tidal currents over two months using an acoustic Doppler current profiler (ADCP) to analyze the characteristics of vertical tidal current profiles at the Jangjuk Strait, a candidate site for tidal current energy. As a result, the power law parameter α and bed roughness β were estimated as 4.51–12.41 and 0.38–0.42, respectively. Additionally, the maximum roughness length representing seabed roughness in the logarithmic profile was estimated as 0.221 m, and the estimated friction velocity was 0.038–0.194 m/s. Furthermore, a high correlation was observed between the depth-averaged tidal current velocity and friction velocities at all sites during flood and ebb tide conditions. A high correlation was also found between the bed roughness, roughness length, and power law exponent at relatively deeper sites. Tidal current energy sites display distinct characteristics compared to other sea areas. Therefore, it is essential to account for field conditions when conducting numerical modeling and design.

Turbulent Flow Structures Over a Gobi Surface and Their Impact on Aeolian Sand Transport

AbstractHere, we present the results of turbulent flow structures and their influence on aeolian sand transport over gobi surfaces based on quadrant analysis. The results show that the classified turbulent structures over gobi accounted for 43%–44% of the time frequency and contributed ∼97% to the Reynolds stress, yet the contribution rates to sand transport rate ranged from 45% to 52%. Sweeps (Quadrant 4) and ejections (Q2) were the two most frequent events and accounted for 70% of the total frequency for the classified turbulent structures. Sweeps made a major contribution of 51%–57% to sand transport rate over gobi. Outward interactions (Q1) were relatively rare, but they individually entrained as much sand as sweeps of comparable magnitude and duration. The turbulent structures over gobi show similar behaviors comparable to those of water flows over gravel riverbeds or rough seabeds yet different from those of air flow over flat sand surfaces.

Exploring regional coastal sediment pathways using a coupled tide-wave-sediment dynamics model

Knowledge of sediment transport pathways is important for coastal management as well as for offshore infrastructure such as wind farms and other renewable energy installations. Here a three-way coupled model of tides, waves and sediment dynamics is presented, which extends the approach of Davies and Robins (2017) for the prediction of the locally-varying seabed roughness ks resulting from the interaction between the flow (tides and waves) and the bed sediment (grain size mixture) subject to wave-current interaction (WCI) at the seabed. The model was applied to the North Wales coastal area, with particular emphasis on an extensive shore-connected sand bank (Constable Bank), the stability of which was investigated through the study of residual sediment circulation patterns. Multi-beam (MBES) and seabed rig (AWAC) observations have allowed validation of the predicted ks and hydrodynamics, respectively. With due allowance for the supply of mobile sediment, the agreement between predicted ks and observed bedform height was generally good. The model has been used to determine residual velocities and sediment pathways, both with and without wave effects included. Wave influence and WCI are predominant in nearshore areas, affecting the magnitude and direction of the residual fluxes. A key modelling outcome offshore is a clockwise residual circulation pattern for water and sediment around the end of Constable Bank, set within a generally eastward net drift of sediment in the wider study area.

Peak force coefficients on small-diameter spanning pipelines under waves

This paper reports an experimental investigation undertaken to measure the peak force coefficients for small-diameter pipelines (cables and umbilicals) spanning over rocky, uneven seabed under regular wave conditions. The free spans formed by the uneven bed are investigated by modelling multiple regularly spaced free spans (referred to as multi-span) in a flume. Approximately 600 physical tests were conducted to quantify the effects of gap height to pipeline diameter ratio G and the span length to pipeline diameter ratio S on the peak force coefficients. The Keulegan–Carpenter number KC and Reynolds number Re p tested were in the range 20 ≤ KC ≤ 2000 and 5 × 10 3 ≤ Re p ≤ 10 5 , respectively. The ratios between the peak force coefficients for the multi-span configuration and a uniform-span configuration ( S → ∞) are demonstrated to be a function of S /[( S + C ) G ], where C is the contact ratio, defined as the distance between two adjacent free spans divided by the pipeline diameter. Combined with the peak force coefficients under uniform-span conditions obtained from the present experiments, this finding enables the peak force coefficients under a multi-span condition to be reasonably estimated. • Peak force coefficients for fully spanning small-diameter pipelines subject to waves are provided based on a number of physical tests with KC up to 2000. • Quantification method for the peak force coefficients under the effect of multiple free spans is proposed. • The scattered seabed roughness and the multiple free spans lead to significant reductions in the peak force coefficients.

Effects of Sea Level Rise on Tidal Dynamics in Macrotidal Hangzhou Bay

Sea level rise (SLR) due to climate change is expected to alter tidal processes and energy transport, disproportionately affecting coastal communities. Utilizing a nested hydrodynamics model, we provided an integrated investigation of tidal responses to SLR in the Hangzhou Bay (HZB). The scenarios of SLR in the next hundred years count for both non-uniform trends based on historical altimetry data and uniform trends from the latest IPCC projections. In a comparison of model results under different SLR scenarios, we found that the tidal range is amplified by SLR in HZB with stronger amplification at the shallow southern coast. Tidal range change generally increases with the SLR scale; however, neglecting the heterogeneities in the spatial distribution of SLR tends to overestimate the SLR effects. The harmonic analysis illustrates that SLR exaggerates the dominated semidiurnal tides (M2 and S2) but dampens their overtides and compound tides (M4, M6, and MS4), of which M2 amplitude amplification explains 71.2–90.0% of tidal range change. SLR tends to promote tidal energy entering HZB through the Zhoushan Archipelago (ZA) compared to the prototype, while dampened sea-bed roughness and reduced tidal velocity come with a less dissipative environment in HZB, resulting in 6–18% more tidal energy exported upstream. Numerical experiments indicate ZA has significant effects on tidal responses and energy flux generation, therefore, its quantitative influences and physical mechanism are also discussed in this paper.

Turbulence measurements: An assessment of Acoustic Doppler Current Profiler accuracy in rough environment

The deployment of tidal turbines requires a precise hydrodynamic characterisation of the production site. Acoustic Doppler Current Profilers (ADCP), usually employed for measuring the time-mean characteristics of environmental flows, could also be used for assessing the main features of turbulence. ADCP measurements are sensitive to many sources of uncertainties associated mainly with the spreading of the beams or the assumptions made on flow homogeneity. The ability of ADCPs to accurately measure the hydrodynamic parameters of a given flow can be tested on a synthetic dataset. However, it is difficult to generate a dataset representative of a real environmental flow. In this work, large-eddy simulation of a high Reynolds flow over a rough seabed is performed and used to assess the accuracy of two, coupled, 4-beam ADCP systems forming an 8-beam arrangement. The study confirms the relevance and efficiency of the tested 8-beam configuration for the characterisation of turbulence. The results near the seabed are of a lower quality, with up to 50 % error on the Reynolds stresses for elevations under twice the roughness height, which questions the interpretation of ADCP measurements in the lower part of the water column. Also, the spatial averaging over ADCP cells leads to an underestimation of the turbulence intensity of 10 % to 20 %.

Modelling an energetic tidal strait: investigating implications of common numerical configuration choices

Representation of the marine environment is key for reliable coastal hydrodynamic models. This study investigates the implications of common depth-averaged model configuration choices in sufficiently characterising seabed geometry and roughness. In particular, applications requiring a high level of accuracy and/or exhibiting complex flow conditions may call for greater detail in marine environment representation than typically adopted in coastal models. Ramsey Sound, a macrotidal strait in Pembrokeshire, Wales, UK is considered as a case study. The site contains various steeply inclined bathymetric features, including a submerged pinnacle named Horse Rock and a rocky reef called “The Bitches”. The available energy in Ramsey Sound’s tidal currents has attracted attention from tidal energy developers. There is interest in accurately modelling the energetic hydrodynamics surrounding its pronounced bathymetry. The coastal flow solver Thetis is applied to simulate the flow conditions in Ramsey Sound. It is shown that notable prominent bathymetric features in the strait influence localised and, most importantly, regional hydrodynamic characteristics. “The Bitches” consistently accelerate flow in the strait while Horse Rock induces a notable wake structure and flow reversals. The model is calibrated against bed- and vessel-mounted Acoustic Doppler Current Profiler (ADCP) observations, by altering seabed roughness parameterisations. A spatially variable and locally scaled Manning coefficient based on diverse seabed classification observations is found to improve model performance in comparison to uniformly applied constants, the latter a more common approach. The local impact of altering the Manning coefficient configuration is found to be greatest during spring flood periods of high velocity currents. Meanwhile, the effect of coarsening the computational mesh around bathymetric features towards values more typically applied in coastal models is investigated. Results indicate severe misrepresentation of seabed geometry and subsequent wake hydrodynamics unless refined to a mesh element size that adequately represents Horse Rock and “The Bitches”.

CFD modelling of wave damping over a fringing reef in the Colombian Caribbean

The understanding of physical processes over submerged reefs represents an important ongoing research topic when considering wave energy dissipation and coastal protection that these environments provide. Detailed analyses are required to assess wave damping based on the contribution of reef roughness and wave breaking. For this purpose, the CFD (computational fluid dynamics) toolbox OpenFOAM® is applied to simulate the wave energy dissipation process over reefs with explicit accounting for the complexities of coral shape instead of commonly applied parameterized approaches for bottom roughness and wave breaking. Model validation was performed through comparison with field measurements over a reef profile of Tesoro Island in the Colombian Caribbean. Quantitative analysis of wave damping caused by wave breaking and reef roughness was conducted for (1) moderate and extreme wave conditions, (2) smooth and rough seabed configurations and (3) changes in the water depth over the reef crest. Wave height attenuation is found to vary along the reef profile reaching differences of up to 55% between smooth and rough reef surface scenarios, particularly for moderate wave conditions. Wave breaking, high turbulent flows and detachment of undertow currents are among the reef roughness effects on hydrodynamics. The fore-reef terrace and the reef crest are identified as the most critical zones where dissipation takes place. Wave breaking from rough seabeds provides a global wave attenuation of 75.4–94.8%, with the reef roughness alone accounting for ~ 4–14%. Under extreme wave height scenarios, the wave damping from reef roughness is not significant. Further predictions regarding roughness effects on the reef hydrodynamics, wave set-up and undertow currents for moderate and extreme wave climate conditions are also shown. Directions for future research using CFD are presented to address limitations that arise from the limited span-wise domain in our approach that prevents development of large lateral coherent structures.

Comparison of mechanical disturbance in soft sediments due to tickler-chain SumWing trawl vs. electro-fitted PulseWing trawl

Abstract Tickler-chain SumWing and electrode-fitted PulseWing trawls were compared to assess seabed impacts. Multi-beam echo sounder (MBES) bathymetry confirmed that the SumWing trawl tracks were consistently and uniformly deepened to 1.5 cm depth in contrast to 0.7 cm following PulseWing trawling. MBES backscatter strength analysis showed that SumWing trawls (3.11 dB) flattened seabed roughness significantly more than PulseWing trawls (2.37 dB). Sediment Profile Imagery (SPI) showed that SumWing trawls (mean, SD) homogenised the sediment deeper (3.4 cm, 0.9 cm) and removed more of the oxidised layer than PulseWing trawls (1 cm, 0.8 cm). The reduced PulseWing trawling impacts allowed a faster re-establishment of the oxidised layer and micro-topography. Particle size analysis suggested that SumWing trawls injected finer particles into the deeper sediment layers (∼4 cm depth), while PulseWing trawling only caused coarsening of the top layers (winnowing effect). Total penetration depth (mean, SD) of the SumWing trawls (4.1 cm, 0.9 cm) and PulseWing trawls (1.8 cm, 0.8 cm) was estimated by the depth of the disturbance layer and the layer of mobilized sediment (SumWing = 0.7 cm; PulseWing trawl = 0.8 cm). PulseWing trawls reduced most of the mechanical seabed impacts compared to SumWing trawls for this substrate and area characteristics.

Removal of the airwave effect by main-part decomposition of the anomalous field of MCSEM data

The airwave effect greatly influences the observational data from controlledsource electromagnetic exploration in shallow seas, which obscures the abnormal effects generated by exploration targets and, hence, affects the accuracy of the late exploration data interpretation. In this study, we propose a method to separate the main part from the anomalous field of marine controlled-source electromagnetic method (MCSEM) data based on Stratton–Chu integral transforms to eliminate the airwave effect, which dominates observed electromagnetic (EM) response in shallow seawater. This method of separating the main part from the anomalous field is a type of finite impulse response filter based on a discrete data set. Theoretical analysis proved that the method is stable and able to effectively depress noise. A numerical test indicated that the method could successfully eliminate the airwave effect from the observed EM signals generated by an air–water interface and a seawater layer. This technique is applicable for seawater models with either flat or rough seabeds.

Quantifying pursuit‐diving seabirds’ associations with fine‐scale physical features in tidal stream environments

Summary The rapid increase in the number of tidal stream turbine arrays will create novel and unprecedented levels of anthropogenic activity within habitats characterized by horizontal current speeds exceeding 2 ms−1. However, the potential impacts on pursuit‐diving seabirds exploiting these tidal stream environments remain largely unknown. Identifying similarities between the fine‐scale physical features (100s of metres) suitable for array installations, and those associated with foraging pursuit‐diving seabirds, could identify which species are most vulnerable to either collisions with moving components, or displacement from these installations. A combination of vessel‐based observational surveys, Finite Volume Community Ocean Model outputs and hydroacoustic seabed surveys provided concurrent measures of foraging distributions and physical characteristics at a fine temporal (15 min) and spatial (500 m) resolution across a tidal stream environment suitable for array installations, during both breeding and non‐breeding seasons. These data sets were then used to test for associations between foraging pursuit‐diving seabirds (Atlantic puffins Fratercula arctica, black guillemots Cepphus grylle, common guillemots Uria aalge, European shags Phalacrocorax aristotelis) and physical features. These species were associated with areas of fast horizontal currents, slow horizontal currents, high turbulence, downward vertical currents and also hard–rough seabeds. The identity and strength of associations differed among species, and also within species between seasons, indicative of interspecific and intraspecific variations in habitat use. However, Atlantic puffins were associated particularly strongly with areas of fast horizontal currents during breeding seasons, and European shags with areas of rough–hard seabeds and downward vertical currents during non‐breeding seasons. Synthesis and applications. Atlantic puffins’ strong association with fast horizontal current speeds indicates that they are particularly likely to interact with installations during breeding seasons. Any post‐installation monitoring and mitigation measures should therefore focus on this species and season. The multi‐species associations with high turbulence and downward vertical currents, which often coincide with fast horizontal current speeds, also highlight useful pre‐installation mitigation measures via the omission of devices from these areas, reducing the overall likelihood of interactions. Environmental impact assessments (EIA) generally involve once‐a‐month surveys across 2‐year periods. However, the approaches used in this study show that more focussed surveys can greatly benefit management strategies aiming to reduce the likelihood of negative impacts by facilitating the development of targeted mitigation measures. It is therefore recommended that these approaches contribute towards EIA within development sites.

Measurements and modeling of the variability in normally incident reflections from the seabed

Normal incidence reflection measurements taken during the TREX13 experiment off the coast of Florida show substantial variability. One likely source of variability is due to scattering from the rough surface of the seabed. These data were taken from an omnidirectional source at a relatively low frequency (around 3 kHz). Systems being considered for measuring seabed properties (such as sound speed, density, and roughness) may use commercial echo-sounders that are directional and operate at much higher frequencies. Measurements from the TREX13 data as well as higher frequency systems are analyzed together with modeling to determine how surface roughness effects these measurements. The modeling uses an integral equation approach with a power law roughness spectrum to calculate backscattering of incident plane waves. The effects of changing the model parameters such as seabed type and roughness on the nature of backscattering are examined. The simulated results are compared with the measurements obtained from the experiments. The measurements are further used to calculate the statistical parameters to understand the nature of the normal reflection from a rough seabed.

Assessing the ecological importance of red tree coral thickets in the eastern Gulf of Alaska

Abstract Red tree corals (Primnoa pacifica), the largest structure-forming gorgonians in the North Pacific Ocean, form dense thickets in some areas. These thickets are a dominant benthic habitat feature in the Gulf of Alaska (GOA), yet little is known about the ecosystems they support. In 2005, we used a submersible to study the ecology of thickets inside or near five small areas of the eastern GOA later designated in 2006 as habitat areas of particular concern (HAPCs)―areas closed to all bottom contact fishing. We show that red tree corals are keystone species in habitats where they form thickets (mean density 0.52 corals m−2)—the densest and largest thickets documented anywhere. Measured sponge densities (2.51 sponges m−2) were also among the highest documented anywhere. The corals and sponges in the study areas provide essential fish habitat for some fish species, and we show with logistic regression models modified with a scaled binomial variance that bedrock, while important habitat for some fish, is even more important when paired with corals and sponges. Red tree corals were not equally distributed with regard to habitat characteristics, and we show that their presence was correlated with bedrock substrate, moderate to high seabed roughness, and slope >10°. Most corals and sponges are vulnerable to disturbance from longlining, the principal bottom contact fishing in this region, but the larger corals and sponges are the most vulnerable. We observed evidence of infrequent recruitment events and a strong pulse of predation, apparently from fishing gear-induced trauma, that could exacerbate slow recovery of red tree corals from disturbance. Some red tree coral thickets are provided protection within designated HAPCs and some are not. Modifications to longline gear and an expanded network of HAPCs could help preserve these keystone species and the ecosystems they support.

Physics of backscattering in terms of mode coupling applied to measured seabed roughness spectra in shallow water

Energy conserving coupled integral equations for the forward and backward propagating modal components have been previously developed [J. Acoust. Soc. Am. 130, 2673–2680 (2011)]. A rough seabed surface leads to a backscattered field and modifies the interference structure of the forward propagating field. Perturbation theory applied to the basic coupled integral equations allows for physical insight into the correlation of the roughness spectrum to the forward and backward modal intensities and cross mode coherence. This study applies the Nx2D integral equation coupled-mode approach to 3-D roughness measurements and examines the physics of the coupling of the forward and backward field components and computes the modal intensities as a function of azimuth. The roughness measurements were made in about 20 m of water off Panama City, Florida. [Work supported by ONR Code 322 OA.]

Wave-energy dissipation by bottom friction in the English Channel

The energy dissipation by bottom friction of wind-generated surface-gravity waves is evaluated in relation to the seabed roughness magnitude in the English Channel (western Europe). The investigation is based on the phase-averaged wave model SWAN (Simulating WAves Nearshore) modified to account for a new parameterisation of the wind-drag coefficient at high wind speeds. Two formulations of the bottom-drag coefficient are evaluated: (1) the default constant empirical values derived from the JONSWAP experiment and (2) the eddy-viscosity model of Madsen et al. (1988) integrating the hydrodynamic conditions and the bottom roughness length scale considered successively constant and parametrised according to the grain size of bed sediments. Model performances are evaluated by comparing predictions with available measurements of the significant wave height and the peak period at (1) three offshore lightships and (2) two nearshore wave buoys off Le Havre and Cherbourg harbours. The heterogeneous bottom roughness length scale associated with the grain size of seabed sediments improves globally numerical estimates. Mappings of coastal regions influenced by bottom friction are produced exhibiting significant energy dissipation in areas of pebbles and gravels of the Normano-Breton Gulf and the surroundings of the Isle of Wight exposed to the incoming waves from the North-Atlantic ocean.

Influence of rough seabed surface on statistics of modal energy flux

When sound propagates through a random media, the wave properties of the acoustic field may be viewed as stochastic variables. It is thus natural to investigate the relationship between the statistical properties of the acoustic field and the random fluctuations of the waveguide. The interest here is a 2-D ocean waveguide with a rough seabed. Numerical solutions to the 2-way integral coupled mode equations (ICME), for random realizations of the roughness from a wavenumber power spectrum, provide the statistics of the modal intensity and cross-mode coherence with range. The roughness induces mode coupling within the trapped spectrum, between the trapped and the continuum spectrum, and to the back propagating modal spectrum. Instead of a master equation for the modal intensities to study the connection between the statistics of the acoustics and the fluctuations in the waveguide as has been advocated in previous studies, the conservation law for acoustic energy flux is used to develop an expression for the individual modal Poynting vectors. In addition to exact numerical computations of the range derivatives of the modal energy flux vectors for both forward and backward propagation, a Poynting vector master equation is derived for the case where the Born approximation is valid.

Numerical simulation of flow around a circular cylinder close to a flat seabed at high Reynolds numbers using a k– ε model

High Reynolds number flows around a circular cylinder close to a flat seabed have been computed using a two-dimensional standard high Reynolds number k– ε turbulence model. The effects of gap to diameter ratio, Reynolds number and flat seabed roughness for a given boundary layer thickness of the inlet flow upstream of the cylinder have been investigated. Hydrodynamic quantities and the resulting bedload transport have been predicted, and the vortex shedding mechanisms have been investigated. Predictions of hydrodynamic quantities around a cylinder located far away from the bed (so that the effect of the bed is negligible) are in satisfactory agreement with published experimental data and numerical results obtained for the flow around an isolated cylinder. Results for lower Reynolds number flows have also been computed for comparison with the high Reynolds number flow results. Overall it appears that the present approach is suitable for design purposes at high Reynolds numbers which are present near the seabed in the real ocean.

VIV analysis of pipelines under complex span conditions

Spans occur when a pipeline is laid on a rough undulating seabed or when upheaval buckling occurs due to constrained thermal expansion. This not only results in static and dynamic loads on the flowline at span sections, but also generates vortex induced vibration (VIV), which can lead to fatigue issues. The phenomenon, if not predicted and controlled properly, will negatively affect pipeline integrity, leading to expensive remediation and intervention work. Span analysis can be complicated by: long span lengths, a large number of spans caused by a rough seabed, and multi-span interactions. In addition, the complexity can be more onerous and challenging when soil uncertainty, concrete degradation and unknown residual lay tension are considered in the analysis. This paper describes the latest developments and a ‘state-of-the-art’ finite element analysis program that has been developed to simulate the span response of a flowline under complex boundary and loading conditions. Both VIV and direct wave loading are captured in the analysis and the results are sequentially used for the ultimate limit state (ULS) check and fatigue life calculation.

Observations of Bed Roughness of a Coral Reef

Seabed roughness was measured over an area of the south shore of O'ahu, Hawaii, with a boat-mounted acoustic altimetry system. The study site is characterized by a highly inhomogeneous rough seabed. Roughness was resolved in the 40–200-cm range over an area extending from the wave breaking zone to a depth of 20 m. Theory suggests that roughness length scales and distribution both play an important role in wave energy dissipation mechanisms. In this paper, we investigate methods for quantifying bed roughness within this range of scales. Various methods for quantifying roughness are examined, including spectral and statistically derived variables. These methods are compared qualitatively with sidescan imagery and quantitatively via in situ diver-based roughness surveys. Results show that wavenumber spectra can effectively differentiate smooth from rough areas. Results also suggest that the bed roughness in the study site exhibit a high spatial variability within the examined range, with no dominant length scale but with a characteristic spectral slope. In addition, roughness also exhibits variation between the shallow and deep ends of the study site.