Report of the scientific discussion meeting on the physical oceanography of the Great Barrier Reef region : held at the University of New South Wales 5-6 July 1982 with support from the Great Barrier Reef Marine Park Authority and the Australian Academy of Science

A scientific discussion meeting was held in July 1982 to generate a statement concerning the physical oceanography of the Great Barrier Reef region. The statement was to be framed by answering the questions below: . . (a) What is the present state of knowledge? (b) What appear to be the most important unanswered questions? (c) With regard to (a) and (b), what methods would be most effective in answering the questions posed in (b)? (d) What routine monitoring measurements would appear to be of most scientific value in increasing our knowledge of the region? This report summarises the discussions of the meeting

Changes in wave climate over the northwest European shelf seas during the last 12,000 years

Because of the depth attenuation of wave orbital velocity, wave-induced bed shear stress is much more sensitive to changes in total water depth than tidal-induced bed shear stress. The ratio between wave- and tidal-induced bed shear stress in many shelf sea regions has varied considerably over the recent geological past because of combined eustatic changes in sea level and isostatic adjustment. In order to capture the high-frequency nature of wind events, a two-dimensional spectral wave model is here applied at high temporal resolution to time slices from 12 ka BP to present using paleobathymetries of the NW European shelf seas. By contrasting paleowave climates and bed shear stress distributions with present-day conditions, the model results demonstrate that, in regions of the shelf seas that remained wet continuously over the last 12,000 years, annual root-mean-square (rms) and peak wave heights increased from 12 ka BP to present. This increase in wave height was accompanied by a large reduction in the annual rms wave- induced bed shear stress, primarily caused by a reduction in the magnitude of wave orbital velocity penetrating to the bed for increasing relative sea level. In regions of the shelf seas which remained wet over the last 12,000 years, the annual mean ratio of wave- to (M-2) tidal-induced bed shear stress decreased from 1 (at 12 ka BP) to its present-day value of 0.5. Therefore compared to present- day conditions, waves had a more important contribution to large-scale sediment transport processes in the Celtic Sea and the northwestern North Sea at 12 ka BP

Observations of turbulent fluxes and turbulence dynamics in the ocean surface boundary layer

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2008This study presents observations of turbulence dynamics made during the low winds portion of the Coupled Boundary Layers and Air-Sea Transfer experiment (CBLAST-Low). Observations were made of turbulent fluxes, turbulent kinetic energy, and the length scales of flux-carrying and energy-containing eddies in the ocean surface boundary layer. A new technique was developed to separate wave and turbulent motions spectrally, using ideas for turbulence spectra that were developed in the study of the bottom boundary layer of the atmosphere. The observations of turbulent fluxes allowed the closing of heat and momentum budgets across the air-sea interface. The observations also show that flux-carrying eddies are similar in size to those expected in rigid-boundary turbulence, but that energy-containing eddies are smaller than those in rigid-boundary turbulence. This suggests that the relationship between turbulent kinetic energy, depth, and turbulent diffusivity are different in the ocean surface boundary layer than in rigid-boundary turbulence. The observations confirm previous speculation that surface wave breaking provides a surface source of turbulent kinetic energy that is transported to depth where it dissipates. A model that includes the effects of shear production, wave breaking and dissipation is able to reproduce the enhancement of turbulent kinetic energy near the wavy ocean surface. However, because of the different length scale relations in the ocean surface boundary layer, the empirical constants in the energy model are different from the values that are used to model rigid-boundary turbulence. The ocean surface boundary layer is observed to have small but finite temperature gradients that are related to the boundary fluxes of heat and momentum, as assumed by closure models. However, the turbulent diffusivity of heat in the surface boundary layer is larger than predicted by rigid-boundary closure models. Including the combined effects of wave breaking, stress, and buoyancy forcing allows a closure model to predict the turbulent diffusivity for heat in the ocean surface boundary layer.This work was supported by Office of Naval Research grants N00014-00-1-0409, N00014-01-1-0029, and N00014-03-1-0681, the Woods Hole Oceanographic Institution Academic Programs Office, and National Aeronautics and Space Administration grant NAG5-11933

Observations of turbulence in the ocean surface boundary layer : energetics and transport

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 39 (2009): 1077–1096, doi:10.1175/2008JPO4044.1.Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.The Office of Naval Research funded this work as a part of CBLAST-Low

Numerical Experiment of Sediment Dynamics over a Dredged Pit on the Louisiana Shelf

Sediment transport over Sandy Point dredge pit in the northern Gulf of Mexico was examined using field measurements and a finely resolved numerical model. Delft3D model with well-vetted computational grid and input parameters was used. Numerical experiments were performed to examine the effect of wind-generated waves, wind-driven currents and their interaction on sediment dynamics in our study area during a cold front in November 2014 and fair-weather conditions between July and August of 2015. Sediment dispersal from the lower Mississippi River, sediment resuspension, transport and deposition with high spatial and temporal resolution were simulated. A reliable satellite-derived near-surface suspended particulate matter (SPM) map was employed to provide an initial condition in a numerical model and support the model calibration/verification. To prepare SPM maps, short-wave infrared (SWIR) and near-infrared atmospheric correction algorithms on remote sensing reflectance (Rrs) products from Landsat-8 OLI and Management Unit of the North Sea Mathematical Models (MUMM) and SWIR.NIR atmospheric correction algorithms on Rrs products from MODIS-Aqua were evaluated. Results indicated that SWIR atmospheric correction algorithm was the suitable algorithm for Landsat-8 OLI and SWIR.NIR atmospheric correction algorithm outperformed MUMM algorithm for MODIS. Delft3D Flow, wave and sediment transport were validated using LSU WAVCIS (Wave-Current-Surge Information System) and NDBC (National Data Buoy Center) data for both events. Results suggested that the primary source of sediment for the Sandy Point dredge pit during a cold front was re-suspension due to the fortified bottom shear stress (BSS) by wind-induced waves and currents. Strong southward wind-driven currents during the cold front passage dispersed sediments from the Mississippi River passes and inhibited riverine sediment supply from the Sandy Point dredge pit. Results also showed that total cold front passages in a year (30-40 passages per year) contribute to the sedimentation thickness over Sandy Point dredge pit from 16% to 24% of the total sedimentation thickness annually. Results indicated that during the fair-weather event, Mississippi River plays a pivotal role in providing sediment for Sandy Point dredge pit and about 60% of deposited sediments are from the Mississippi River plume

CFD-simulations of wave-wind interaction

Apart from solar energy, wind energy is the renewable energy which has the greatest potential. Offshore wind power is expected to have an annual growth of approximately 30 % in the decade to come. Even though the offshore wind industry tends to use larger turbines than over land, the standards used in designs, for the rotor-nacelle assembly, are similar to those used for onshore wind turbines. Recent studies by Kalvig et al. and Obhrai et al. (2012) reveal weaknesses in the simplifications made regarding the marine boundary layer (MBL) in the governing industry guidance and standards. Precise knowledge of wind speed is generally important for wind farm design and operations such as design basis, wind site assessment, energy yield assessment and power prediction. Wind profile and turbulence characteristics depend on the wave state, but this is usually ignored and the surface thought of as level and smooth. Field experiments and numerical simulations by Sullivan et al. (2008) and Smedman and Semedo et al. (2009) show that wave state need to be taken into account. The goal of this study was to develop and use OpenFOAM to improve the understanding of the interactions between atmospheric wind field and surface waves. A Reynold’s averaging Navier Stokes (RANS) standard k-ε model with the capability to resolve a moving sinusoidal wave at its lower boundary was implemented. It was set up as a 2-dimensional and grid independent case. It was used as a basis for testing several boundary conditions and averaging procedures. Since a transient model is used it is important to know what to do when interpreting the results. What can one get out of snapshots, what should be averaged and how is the averaging done? Interesting patterns in the velocity profile and the turbulence characteristics were looked for in sensitivity studies where different input parameters on the wind speed and wave state were used., A comparison with the LES experiments of Sullivan et al.’s (2008) was performed in order to investigated if the wave modified wind field will be captured with the simpler CFD code? In order to answer the questions in Kalvig’s PhD work to some extent the following research question was defined: “In which way does the sea state influence the wind field in the MBL?” The answer to this is that surface waves impact the flow field and “footprints” are visible in the whole height of the domain. A “knee” is present as a result of speed up over the wave trough, supported by measurements from Smedman et al. (1999, 2009) and simulations from Sullivan et al. (2008). A good way of averaging was found as there is a need for averaging when studying varying wave parameters and when examining high wind speeds and rough wave states. Wind opposed with the wave propagation is decelerated close to the surface in accordance with Sullivan et al. (2008) and Smedman et al. (1999) and Kudryavtsev and Makin (2004). This implies highest vertical wind and resulting in the highest turbulent kinetic energy in an opposed situation. Although the LES experiment gives the most precise picture, the k-ɛ model used highlights many of the same features. Using OpenFOAM requires a steep learning curve but the hard work pays off as there are no expensive licenses which other similar programs have. With the results from the sensitivity studies and comparisons with Sullivan et al. (2008) the interdependence of wave and winds, and the ability of the former to influence the flow field, are reflected. This can be used by wind park developers, professionals involved in the offshore industry, and last not least in the further PhD work of Kalvig

Across-shelf sediment transport modeling and its application to storms at Duck, North Carolina

To understand the morphodynamics of the inner shelf, a benthic boundary layer tripod supporting 6 point-measuring current meters, an acoustic Doppler current profiler, and three near-bed profiling acoustic backscatter sensors documented storm and swell conditions during October, 1996, at a depth 13 in on the inner shelf off Duck, North Carolina. The relationship between eddy viscosity and eddy diffusivity during storm and swell conditions was examined using data collected in October 1996 on the inner shelf off Duck, NC. Sediment suspension models, including Rouse-type diffusion models, combined advection and diffusion models, and a Rouse model with a thickened wave boundary layer, were compared to determine which model best reproduces observed sediment concentration profiles. A physics-based morphodynamics model was then developed to determine which components of hydrodynamic forcing and resulting sediment transport are predicted to be most significant to morphological change outside the surf zone on the inner shelf of the Middle Atlantic Bight. The simplest possible analytical solutions were sought for depth-dependent currents driven by the along- and across-shelf components of the wind and by waves via Stokes return flow and boundary layer streaming. Predicted currents and sediment concentrations were compared with observations collected at 13 m depth off Duck, NC, during October, 1996. Sediment transport and morphologic change were modeled and the morphologic change model was applied to 24 significant storms, which were documented by before-and-after shoreface profiles collected by the Field Research Facility of the US Army Corps of Engineers at Duck, NC, between 1987 and 1993. Significant correlations were found between observed shoreface volume change between 600--800 in offshore and predicted depth change on the inner shelf due to across-shelf sediment flux. Overall, correlations between observed and predicted change were higher for wave-driven components of sediment flux than for wind-driven components

Wave resource characterization and co-location with offshore wind in the Irish Sea

One barrier affecting progress in the wave energy sector is detailed knowledge of the spatiotemporal distribution of waves in shelf sea regions, including their inter- and intra-annual variability. Here, a recent decade (2012-2021) of waves is simulated at high-resolution in the Irish Sea - a region with much offshore energy infrastructure. The spectral wave model SWAN is forced with ERA5 wind fields. There is a strong seasonal cycle in wave height and power. In all months except for July, large waves (significant wave height greater than 5 m) can penetrate into the northern part of the Irish Sea, but the most energetic region is the Celtic Sea, where monthly mean wave power exceeds 30 kW/m in December. In this region, wave power strongly correlates with the North Atlantic Oscillation (NAO) from September to March. To investigate the potential for co-location, i.e. to reduce costs through shared infrastructure, wave and wind power were compared at a leased floating wind site in the Celtic Sea. Over the simulated decade, r^2 ~ 0.5, demonstrating modest potential for co-location of wind and wave energy technologies in this part of the Irish Sea - considerably less favourable than other sites in the North Atlantic that experience greater swell

On the role of high frequency waves in ocean altimetry

This work mines a coastal and open ocean air-sea interaction field experiment data set where the goals are to refine satellite retrieval of wind, wind stress, and sea level using a microwave radar altimeter. The data were collected from a low-flying aircraft using a sensor suite designed to measure the surface waves, radar backscatter, the atmospheric flow, and turbulent fluxes within the marine boundary layer. This uncommon ensemble provides the means to address several specific altimeter-related topics. First, we examine and document the impact that non wind-driven gravity wave variability, e.g. swell, has upon the commonly-invoked direct relationship between altimeter backscatter and near surface wind speed. The demonstrated impact is larger in magnitude and more direct than previously suggested. The study also isolates the wind-dependence of short-scale slope variance and suggests its magnitude is somewhat lower than shown elsewhere while a second-order dependence on long waves is also evident. A second study assesses the hypothesis that wind-aligned swell interacts with the atmospheric boundary flow leading to a depressed level of turbulence. Cases of reduced drag coefficient at moderate wind speeds were in evidence within the data set, and buoy observations indicate that swell was present and a likely control during these events. Coincidentally, short-scale wave roughness was also depressed suggesting decreased wind stress. Attempts to confirm the theory failed, however, due to numerous limitations in the quantity and quality of the data in hand. A lesson learned is that decoupling atmospheric stability and wave impacts in field campaigns requires both a very large amount of data as well as vertical resolution of fluxes within the first 10--20 m of the surface

Microseismic monitoring of the controls on coastal rock cliff erosion

The aim of this thesis has been to improve understanding of the controls on coastal rock cliff erosion, utilising microseismic ground motion. Coastal cliff erosion has remained poorly understood, in part confounded by the challenges associated with monitoring changes to and controls upon steep slopes in the coastal zone. As a result the relative contribution of marine to subaerial and episodic to iterative forcing is based upon models with only limited field validation. For two years, from July 2008 to July 2010, cliff top microseismic ground motions were monitored using a broadband seismometer, installed on top of a 70 m high hard rock cliff of Jurassic mudstone, shale and sandstone, on the North York Moors National Park coast, UK. Concurrently cliff face erosion was monitored using high-resolution 3D terrestrial laser scanning. Regional-scale marine and weather data for the monitoring period and modelled nearshore wave conditions were used to establish the conditions under which cliff microseismic ground motions were generated. Distinct ground motion frequency bands were found to correlate with a range of marine and subaerial processes that transfer energy to the coastline and cliff. Fundamentally, microseismic sources were identified both at the cliff face from, for example, direct wave impact during cliff toe inundation, but also at more distal locations resulting from the transfer of energy from gravity and infragravity waves. Further analysis demonstrates statistically significant correlations between rockfall and cliff ground motion generated by wave impacts and wind at the cliff face, but also surprisingly waves across the nearshore and offshore, implying direct environmental controls on cliff erosion rate. The significance of longer period ground motion, representative of ocean gravity and infragravity waves also identifies an almost constant dynamic loading of the cliff rock mass, highlighting a potential for progressive deterioration of the cliff rock. The analysis demonstrates that cliff top microseismic ground motion provides a valuable proxy for marine and atmospheric forcing at coastal cliffs, overcoming the limitations in quantifying and testing controls on cliff erosion. The findings of this study are used to develop a new conceptual model of the environmental processes and failure mechanisms that control rock cliff erosion