Seafloor Slope Research Articles

Discussion of assumptions behind the external dynamic models in ship collisions and groundings

ABSTRACTIn the past decades, a prevailing way for the assessment of responses in ship collisions and groundings has been to decouple the problem into two parts: external dynamics and internal mechanics. The external dynamics deals with global motions of the two interacting bodies prior to, during and after the collision. The main outcome of an external dynamics assessment is the energy loss during the collision, which will be dissipated by structural deformations in the assessment of internal mechanics. The decoupled method works well for right-angle collisions in general, but the accuracy can be much less in many scenarios, e.g. skew collisions with small collision angles and collisions with long durations. This is mainly because the assumptions in the decoupled method are violated in the studied cases. This paper reviews the assumptions and simplifications behind the external dynamic models and discusses validity of the assumptions by comparing with coupled simulation results. Decoupled and coupled models in both planar 3DOF and full 6DOF are addressed. Various collision scenarios are studied, including colliding with oblique plates, grounding on a sloping sea floor, crushing into rigid plates with normal vectors misaligned with coordinate axes, and collision with a submersible platform. In some scenarios, cases with different attack angles and impact velocities are simulated. The outcome will help understand potential limitations of the decoupled method, which should be used with care.

Oceanographic currents, differential subsidence, and physiography control three-dimensional clinothem growth in the Gulf of Papua, Papua New Guinea

A regional high-resolution CHIRP seismic survey in the Gulf of Papua (GoP) extends the geographic scope of previous studies by 125 km and reveals that the Holocene clinothem along-margin geometries, surfaces of lap, and steepness of foreset slopes appear less dependent on eustatic sea level changes and sediment supply than previously suggested. Clinothems imaged by CHIRP profiles and sampled by sediment cores include two older, relict clinothems and a younger Holocene clinothem divided into three units by two surfaces of lap. New radiocarbon ages from this study establish that depositional timing is more recent than previously proposed for the transgressive deposits, the Holocene clinothem units, and surfaces of lap. While previous ages suggested that observed stratal relationships could coincide with meltwater pulses or sea level stillstands, the younger ages from this study question these results and suggest the dominant control on Holocene clinothem architecture may be inherited physiography, SW-NE differential subsidence, along-margin currents, and seasonal variations in wave energy. Inherited physiography underlying the Holocene clinothem consists of valleys incised during relative lowstands of Marine Isotope Stages (MIS) 4 and 2. Valleys in the relict clinothem potentially served as across-margin conduits for an older Holocene clinothem unit. After the older Holocene unit filled valleys and accommodation, younger Holocene units preferentially grew along- rather than across-margin as bottom shear stresses increased. Relict clinothems have undergone differential subsidence since deposition due to foreland basin loading, which has engendered more accommodation in the northeast GoP that systematically decreases toward the southwest. Areas with less accommodation are more likely to experience toplap as they are exposed to increased shear stresses. Therefore, the surfaces of lap are caused by interactions between accommodation and oceanographic currents. Examining stratal relationships along the Holocene clinothem has broad implications for sequence stratigraphy, as many clinothems exhibit a three-dimensional stacking pattern. Measurements of seafloor slopes yield complicated results, illustrating that seafloor steepness within lobes is influenced by a complex interplay of accommodation, sediment supply, underlying physiography, and oceanographic currents. This suggests that clinothem architecture in the GoP cannot simply be interpreted as a rate-related problem between rates of sediment supply and relative sea level.

Topographic Rossby Waves in the Arctic Ocean's Beaufort Gyre

AbstractA 5‐year long time series of temperature and horizontal velocity in the Arctic Ocean's Beaufort Gyre is analyzed with the aim of understanding the mechanism driving the observed variability on timescales of tens of days (i.e., subinertial). We employ a coherency/phase analysis on the temperature and horizontal velocity signals, which indicates that subinertial temperature variations arise from vertical excursions of the water column that are driven by horizontal motions across the sloping seafloor. The vertical displacements of the water column (recorded by the temperature signal) show a bottom‐intensified signature (i.e., decay toward the surface), while horizontal velocity anomalies are approximately barotropic below the main halocline. We show that the different characteristics in vertical and horizontal velocities are consistent with topographic Rossby wave theory in the limit of weak vertical decay. In essence, a linearly decaying vertical velocity profile implies that the whole water column is stretched/squashed uniformly with depth when water moves horizontally across the bottom slope. Thus, for the uniform stratification of the deep water column, the response in the relative vorticity field (ensuring conservation of potential vorticity) is also uniform with depth, leading to the observed barotropic horizontal velocity changes. The prevalent topographic Rossby wave activity is discussed in context with Beaufort Gyre spin‐up, dissipation, and stabilization.

Seamounts as hot-spots of large pelagic aggregations

The distribution of four top predators in the Tyrrhenian Sea, a sub-basin of the Mediterranean Sea, was investigated by means of random forest regression considering depth, distance from the coast, seafloor slope, and distance from seamounts as habitat descriptors on a 2x2–nautical mile regular grid. RF results are processed to estimate variable importance and model performance. Random forest architecture reached optimal sensitivity and specificity, thus providing a consistent support tool for identifying suitable habitats. The considered species are characterized as having patched suitable habitats with a number of hot-spot areas where the different species’ habitats overlap. These hot-spot areas’ locations correspond to those of specific seamounts identifying the attraction effect of these topographic structures. The mean features typifying the most attractive seamounts are investigated and found to be shallow peak and base depths but wide base area and high relative elevation.

Evaluating habitat, prey, and mesopredator associations in a community of marine birds

Abstract Offshore wind energy development on the US Atlantic Continental Shelf has brought attention to the need for marine spatial planning efforts to reduce potential conflict between wind turbines and marine animals, including seabirds. We evaluated the effects of marine mammals, fishes, and habitat characteristics on the distribution and relative abundance of marine birds off the coast of Delaware, Maryland, and Virginia. From May 2012 to 2014, we collected line transect data from 14 shipboard surveys, and novel high-resolution digital videography data from 14 aerial surveys. We compiled five habitat covariates: three static (distance to shore, sea floor slope, and sediment grain size), and two dynamic (sea surface temperature, salinity). We additionally analysed two seabird community covariates: the density of observed marine mammals and detected fish. Using zero-altered models, we tested our hypothesis that plunge-diving seabird species would show positive associations with marine mammals. Our results provide statistical evidence that, alongside competition, facilitative interactions occur among pelagic communities, where subsurface predators improve the detectability and accessibility of prey to surface-feeding seabirds. This study highlights the importance of quantifying community and ecological influences on avian abundance, particularly in predicting the potential exposure of marine birds and mammals to offshore development.

Ridges, Seamounts, Troughs, and Bowls: Topographic Control of the Dianeutral Circulation in the Abyssal Ocean

AbstractIn situ observations obtained over the last several decades have shown that the intensity of turbulent mixing in the abyssal ocean is enhanced toward the seafloor. Consequently, a new paradigm has emerged whereby dianeutral downwelling dominates in the ocean interior and dianeutral upwelling only occurs within thin bottom boundary layers. This study shows that when mixing is bottom intensified the net abyssal dianeutral transports and the stratification can depend on subtle features of the seafloor geometry. Under an assumption of depth-independent net dianeutral upwelling, small changes in the curvature of the seafloor can result in interior stratification that is bottom intensified, uniform, or surface intensified. Further, when the net dianeutral transport is allowed to vary in the vertical, changes in the seafloor slope and bathymetric contour length with height can drive lateral exchange between the boundary layer and interior, with particularly strong lateral outflows predicted at the crests of midocean ridges. Finally, using a realistic neutral density climatology the authors suggest that the increase in the perimeter of abyssal neutral density surfaces with height drives much of the dianeutral upwelling at depths greater than 4 km, while the increase in the slope of the seafloor at shallower depths acts to oppose upwelling. These results add to a growing body of literature highlighting the key control of seafloor geometry on the abyssal overturning circulation.

A framework to quantify uncertainties of seafloor backscatter from swath mapping echosounders

Multibeam echosounders (MBES) have become a widely used acoustic remote sensing tool to map and study the seafloor, providing co-located bathymetry and seafloor backscatter. Although the uncertainty associated with MBES-derived bathymetric data has been studied extensively, the question of backscatter uncertainty has been addressed only minimally and hinders the quantitative use of MBES seafloor backscatter. This paper explores approaches to identifying uncertainty sources associated with MBES-derived backscatter measurements. The major sources of uncertainty are catalogued and the magnitudes of their relative contributions to the backscatter uncertainty budget are evaluated. These major uncertainty sources include seafloor insonified area (1–3 dB), absorption coefficient (up to > 6 dB), random fluctuations in echo level (5.5 dB for a Rayleigh distribution), and sonar calibration (device dependent). The magnitudes of these uncertainty sources vary based on how these effects are compensated for during data acquisition and processing. Various cases (no compensation, partial compensation and full compensation) for seafloor insonified area, transmission losses and random fluctuations were modeled to estimate their uncertainties in different scenarios. Uncertainty related to the seafloor insonified area can be reduced significantly by accounting for seafloor slope during backscatter processing while transmission losses can be constrained by collecting full water column absorption coefficient profiles (temperature and salinity profiles). To reduce random fluctuations to below 1 dB, at least 20 samples are recommended to be used while computing mean values. The estimation of uncertainty in backscatter measurements is constrained by the fact that not all instrumental components are characterized and documented sufficiently for commercially available MBES. Further involvement from manufacturers in providing this essential information is critically required.

Spatio-temporal distributions of piscivorous birds in a subarctic sound during the nonbreeding season

Understanding bird distributions outside of the breeding season may help to identify important criteria for winter refuge. We surveyed marine birds in Prince William Sound, Alaska, USA over nine winters from 2007 to 2016. Our objectives were twofold: to examine the seasonal patterns of piscivorous species overwintering in Prince William Sound, and to explore the relationships between spatial covariates and bird distributions, accounting for inherent spatial structure. We used hurdle models to examine nine species groups of piscivorous seabirds: loons, grebes, cormorants, mergansers, large gulls, small gulls, kittiwakes, Brachyramphus murrelets, and murres. Seven groups showed pronounced seasonal patterns. The models with the most support identified water depth and distance to shore as key environmental covariates, while habitat type, wave exposure, sea surface temperature and seafloor slope had less support. Environmental associations are consistent with the available knowledge of forage fish distribution during this time, but studies that address habitat associations of prey fish in winter could strengthen our understanding of processes in Prince William Sound.

Pulse propagation in turbidity currents

AbstractSubmarine turbidity currents are a key mechanism in the transportation of clastic sediments to deep seas. Such currents may initiate with a complex longitudinal flow structure comprising flow pulses (for example, by being sourced from retrogressive sea floor slope failures) or acquire such structure during run‐out (for example, following flow combination downstream of confluences). A key question is how far along channel pathway complex flow structure is preserved within turbidity currents as they run out and thus if flow initiation mechanism and proximity to source may be inferred from the vertical structure of their deposits. To address this question, physical modelling of saline flows has been conducted to investigate the dynamics of single‐pulsed versus multi‐pulsed density driven currents. The data suggest that, under most circumstances, individual pulses within a multi‐pulsed flow must merge. Therefore, initiation signatures will only be preserved in deposits upstream of the merging point and may be distorted approaching it; downstream of the merging point, all initiation signals will be lost. This new understanding of merging phenomenon within multi‐pulsed gravity currents broadens our ability to interpret multi‐pulsed turbidites.

Predicting Breaking Wave Conditions Using Gene Expression Programming

The forces and loading resulting from shallow water breaking waves are one of the most important drivers in coastal engineering design and morphological change. The importance of accurately and precisely predicting breaking wave conditions cannot be overstated. Using a novel dataset of laboratory and field scale breaking wave conditions, this study assesses the performance of widely applied empirical relationships for breaking waves and uses newly available artificial neural networks and gene expression programming (GEP) numerical methods to develop an accurate and easily applied predictor of breaking conditions for coastal engineers and planners. A novel GEP model is developed and shown to: provide excellent predictive ability at all scales, greatly improve prediction compared with previous works at laboratory scale, and clearly identify the relevant importance of seafloor slope and the water depth to wavelength ratio.

Should tsunami simulations include a nonzero initial horizontal velocity?

Tsunami propagation in the open ocean is most commonly modeled by solving the shallow water wave equations. These equations require initial conditions on sea surface height and depth-averaged horizontal particle velocity or, equivalently, horizontal momentum. While most modelers assume that initial velocity is zero, Y.T. Song and collaborators have argued for nonzero initial velocity, claiming that horizontal displacement of a sloping seafloor imparts significant horizontal momentum to the ocean. They show examples in which this effect increases the resulting tsunami height by a factor of two or more relative to models in which initial velocity is zero. We test this claim with a “full-physics” integrated dynamic rupture and tsunami model that couples the elastic response of the Earth to the linearized acoustic-gravitational response of a compressible ocean with gravity; the model self-consistently accounts for seismic waves in the solid Earth, acoustic waves in the ocean, and tsunamis (with dispersion at short wavelengths). Full-physics simulations of subduction zone megathrust ruptures and tsunamis in geometries with a sloping seafloor confirm that substantial horizontal momentum is imparted to the ocean. However, almost all of that initial momentum is carried away by ocean acoustic waves, with negligible momentum imparted to the tsunami. We also compare tsunami propagation in each simulation to that predicted by an equivalent shallow water wave simulation with varying assumptions regarding initial velocity. We find that the initial horizontal velocity conditions proposed by Song and collaborators consistently overestimate the tsunami amplitude and predict an inconsistent wave profile. Finally, we determine tsunami initial conditions that are rigorously consistent with our full-physics simulations by isolating the tsunami waves from ocean acoustic and seismic waves at some final time, and backpropagating the tsunami waves to their initial state by solving the adjoint problem. The resulting initial conditions have negligible horizontal velocity.Graphical abstractGRAPHICAL ABSTRACT CAPTION

Effects of wave-current interaction on storm surge in the Taiwan Strait: Insights from Typhoon Morakot

The effects of wave-current interaction on storm surge are investigated by a two-dimensional wave-current coupling model through simulations of Typhoon Morakot in the Taiwan Strait. The results show that wind wave and slope of sea floor govern wave setup modulations within the nearshore surf zone. Wave setup during Morakot can contribute up to 24% of the total storm surge with a maximum value of 0.28m. The large wave setup commonly coincides with enhanced radiation stress gradient, which is itself associated with transfer of wave momentum flux. Water levels are to leading order in modulating significant wave height inside the estuary. High water levels due to tidal change and storm surge stabilize the wind wave and decay wave breaking. Outside of the estuary, waves are mainly affected by the current-induced modification of wind energy input to the wave generation. By comparing the observed significant wave height and water level with the results from uncoupled and coupled simulations, the latter shows a better agreement with the observations. It suggests that wave-current interaction plays an important role in determining the extreme storm surge and wave height in the study area and should not be neglected in a typhoon forecast.

Numerical simulations of T-wave generation and conversion at shores: Influence of slope angles and of the SOFAR channel

The term T waves is generally associated to acoustic signals generated by earthquakes which end up traveling horizontally in the ocean at the speed of sound in the water. After traveling over long ranges they reach the shore, where they can be reflected or converted into (visco)elastic waves. For almost 90 years, T waves have been the subject of numerous studies in order to identify their generation mechanisms. Generally, it is supposed that seismic waves are converted to horizontally propagating T-waves by a sloping sea bottom before entering the SOFAR channel. In this study, we present a two-step parametric study of the influence of the slope on a typical T-wave generation/conversion scenario. We use a 2D spectral-element method that allows for full-waveform modeling of wave propagation. In a first step we study the amount of acoustic energy channeled from a deep earthquake below a sloping sea-floor. We then model the reflection/transmission of T waves at the shore. Our results suggest that an optimum slope angle exists for T-wave generation, and that a non-negligible amount of the channeled energy can be reflected at a sloping shore. We find no evidence of a significant influence of the SOFAR channel.

New insights on tsunami genesis and energy source

AbstractConventional tsunami theories suggest that earthquakes with significant vertical motions are more likely to generate tsunamis. In tsunami models, the vertical seafloor elevation is directly transferred to the sea‐surface as the only initial condition. However, evidence from the 2011 Tohoku earthquake indicates otherwise; the vertical seafloor uplift was only 3–5 m, too small to account for the resultant tsunami. Surprisingly, the horizontal displacement was undeniably larger than anyone's expectation; about 60 m at the frontal wedge of the fault plate, the largest slip ever recorded by in situ instruments. The question is whether the horizontal motion of seafloor slopes had enhanced the tsunami to become as destructive as observed. In this study, we provide proof: (1) combining various measurements from the 2011 Tohoku event, we show that the earthquake transferred a total energy of 3.1e + 15 joule to the ocean, in which the potential energy (PE) due to the vertical seafloor elevation (including seafloor uplift/subsidence plus the contribution from the horizontal displacement) was less than a half, while the kinetic energy (KE) due to the horizontal displacement velocity of the continental slope contributed a majority portion; (2) using two modern state‐of‐the‐art wave flumes and a three‐dimensional tsunami model, we have reproduced the source energy and tsunamis consistent with observations, including the 2004 Sumatra event. Based on the unified source energy formulation, we offer a competing theory to explain why some earthquakes generate destructive tsunamis, while others do not.

지형 기울기에 의한 항공 수심 라이다 수심 측정 오차 보정

Light detection and ranging (LiDAR) is one of the most efficient technologies to obtain the topographic and bathymetric map of coastal zones, superior to other technologies, such as sound navigation and ranging (SONAR) and synthetic aperture radar (SAR). However, the measurement results using LiDAR are vulnerable to environmental factors. To achieve a correspondence between the acquired LiDAR data and reality, error sources must be considered, such as the water surface slope, water turbidity, and seafloor slope. Based on the knowledge of those factors’ effects, error corrections can be applied. We concentrated on the effect of the seafloor slope on LiDAR waveforms while restricting other error sources. A simulation regarding in-water beam scattering was conducted, followed by an investigation of the correlation between the seafloor slope and peak timing of return waveforms. As a result, an equation was derived to correct the depth error caused by the seafloor slope.

Mesophotic Coral Ecosystems: A Geoacoustically Derived Proxy for Habitat and Relative Diversity for the Leeward Shelf of Bonaire, Dutch Caribbean

Current trends demonstrate coral reef health in serious decline worldwide. Some of the most well preserved coral reefs in the Caribbean basin are located in the waters surrounding Bonaire, in the Dutch Caribbean. In many places on the leeward side on islands dominated by trade winds, the shallow reef systems extend into deeper water where they are known as Mesophotic Coral Ecosystems (MCE). Autonomous Underwater Vehicles (AUVs) were used to collect geoacoustic data of these leeward reefs at multiple sites as part of an ocean exploration project. AUV swath bathymetry and side-scan sonar data were analyzed for depth, acoustic backscatter intensity, seafloor slope and rugosity. These geomorphic metrics were then used as inputs to generate a composite synthetic index of bottom-type to delineate MCE features. A confusion matrix statistical analysis of the acoustic class map showed an overall accuracy of the acoustic classes at 66%, with accuracy of the hard coral class the highest at 83% and the sandy-bottom class the lowest at 55. The hard coral class was also the statistically most reliable, at over 80%, with the noise class coming in as the least reliable. This morphologic habitat index is a potentially useful new tool in quantifying the extent of MCE located in proximity to Marine Protected Areas (MPAs).

A Possible Impact Crater from Outer Continental Shelf off Cochin, Arabian Sea, India

During the Cruise SR-004, taken up in August 2014 by RV Samudra Ratnakar of the Geological Survey of India, high resolution sea bed survey by swath bathymetry and sub-bottom profiling were carried out in the outer continental shelf off the Cochin coast, 65 km SW of Cochin, an isolated large circular depression was observed in an otherwise gently sloping (0.8) sea floor (Figure 1). Depth of the outer shelf in the eastern part of the depression is 155 m and 160 m on the western side. Part of a submarine channel has also been identified in the outer shelf and slope and indications of a few faults are seen in the vicinity of the shelf margin.

Environmental Drivers of Benthic Flux Variation and Ecosystem Functioning in Salish Sea and Northeast Pacific Sediments.

The upwelling of deep waters from the oxygen minimum zone in the Northeast Pacific from the continental slope to the shelf and into the Salish Sea during spring and summer offers a unique opportunity to study ecosystem functioning in the form of benthic fluxes along natural gradients. Using the ROV ROPOS we collected sediment cores from 10 sites in May and July 2011, and September 2013 to perform shipboard incubations and flux measurements. Specifically, we measured benthic fluxes of oxygen and nutrients to evaluate potential environmental drivers of benthic flux variation and ecosystem functioning along natural gradients of temperature and bottom water dissolved oxygen concentrations. The range of temperature and dissolved oxygen encountered across our study sites allowed us to apply a suite of multivariate analyses rarely used in flux studies to identify bottom water temperature as the primary environmental driver of benthic flux variation and organic matter remineralization. Redundancy analysis revealed that bottom water characteristics (temperature and dissolved oxygen), quality of organic matter (chl a:phaeo and C:N ratios) and sediment characteristics (mean grain size and porosity) explained 51.5% of benthic flux variation. Multivariate analyses identified significant spatial and temporal variation in benthic fluxes, demonstrating key differences between the Northeast Pacific and Salish Sea. Moreover, Northeast Pacific slope fluxes were generally lower than shelf fluxes. Spatial and temporal variation in benthic fluxes in the Salish Sea were driven primarily by differences in temperature and quality of organic matter on the seafloor following phytoplankton blooms. These results demonstrate the utility of multivariate approaches in differentiating among potential drivers of seafloor ecosystem functioning, and indicate that current and future predictive models of organic matter remineralization and ecosystem functioning of soft-muddy shelf and slope seafloor habitats should consider bottom water temperature variation. Bottom temperature has important implications for estimates of seasonal and spatial benthic flux variation, benthic–pelagic coupling, and impacts of predicted ocean warming at high latitudes.

Fine-scale foraging cues for African penguins in a highly variable marine environment

Table S1. Top five candidate generalized linear mixed-effects models (GLMMs) predicting the occurrence probability of African penguins in relation to bathymetry (Bat), bathymetry as a quadratic term (Bat), chlorophyll a (Chla), distance to the nearest colony (Dcol), distance to the nearest neighbouring colony (DNcol), sea floor slope (Sl), sea surface temperature (SST), and sea surface temperature as a quadratic term (SST). AICc scores were used to rank models and those within 4 ΔAICc were model averaged to derive fixed effect estimates used in generating plots (see Fig. 4 in the main article).

Effects of uncertainties in the position and orientation of both the transmitter and receivers on marine controlled-source electromagnetic data

Simulation and interpretation of marine controlled-source electromagnetic (CSEM) data often approximate the transmitter source as an ideal horizontal electric dipole (HED) and assume that the receivers are located on a flat seabed. Actually, however, the transmitter dipole source will be rotated, tilted and deviated from the survey profile due to ocean currents. And free-fall receivers may be also rotated to some arbitrary horizontal orientation and located on sloping seafloor. In this paper, we investigate the effects of uncertainties in the transmitter tilt, transmitter rotation and transmitter deviation from the survey profile as well as in the receiver’s location and orientation on marine CSEM data. The model study shows that the uncertainties of all position and orientation parameters of both the transmitter and receivers can propagate into observed data uncertainties, but to a different extent. In interpreting marine data, field data uncertainties caused by the position and orientation uncertainties of both the transmitter and receivers need to be taken into account.