Fundamental study for morphodynamic modelling: Sand mounds in oscillatory flows

Experiments on sand mounds in oscillatory flow, undertaken in controlled, large-scale laboratory conditions, have produced well-defined data sets for model comparison. Three bathymetries with different levels of submergence, including a surface-piercing case, were tested. The maximum slope was about 1:5.5. Sediment transport is due to bed load with ripple formation. The principal time-dependent bulk parameters are the vertical distance of the centre of gravity above the base and the volume of the mound. A semi-implicit finite-volume depth-averaged hydrodynamic model is used to drive morphodynamics, using van Rijn's sediment flux model generalized to take account of bed slope, and some justification is given for depth-averaged modeling in these conditions. Starting the model runs with the conditions at the end of the first cycle avoided initial atypical physical behaviour. In general good predictions were obtained with an angle of repose reduced from the standard value of about 30° for stationary beds to 15°. For these situations, morphodynamics was largely unaffected by a hydrodynamic roughness height in the range 2.5D₅₀ to 51D₅₀, with larger values accounting for ripple roughness. The reduced angle of repose may be physically expected with mobile beds but this specific value is only expected to be suited to this form of bed motion. In one case an exaggerated ripple formed near the top of the mound reducing agreement with experiment. For the submerged case with normal ripple structure excellent predictions were obtained. For the initially surface-piercing mound, the time of submergence was better predicted with a 30° angle of repose, presumably due to the prominent influence of the near stationary bed near the wet/dry interface, although long term predictions were better predicted with 15°. The occurrence of vortex shedding in the first cycle modeled was in agreement with experimental observation.     

Experimental investigation of water impact on axisymmetric bodies

The results of an elaborate experimental investigation on bottom slamming of axisymmetric objects are presented. Drop tests have been performed on a hemisphere and two conical shapes with different deadrise angles. The test setup is designed so as to prevent small rotations of the test objects which cause scatter in the measurement data. The pressure distribution and evolution as well as the body motion parameters are measured during impact. By means of a high speed camera the water uprise is visualized and the wetting factor is determined for the cones. The results are compared with a three-dimensional asymptotic theory for axisymmetric rigid bodies with constant entry velocity. The ratio between the registered peak pressures and the asymptotic theory are in accordance with comparable experiments in the literature. The asymptotic theory, however, is found to be quite conservative, since the measured peak pressure levels appear to be approximately 50% to 75% of the theoretical levels.     

An analysis of regional climate model performance over the tropical Americas. Part I: simulating seasonal variability of precipitation associated with ENSO forcing

Sea surface temperature (SST) anomalies associated with El Niño/Southern Oscillation (ENSO) constitute a major source of predictability in the tropics. We evaluate the ability of a regional climate model (the Rossby Centre Atmospheric Model; RCA) to downscale SST and large-scale atmospheric anomalies associated with ENSO. RCA is configured over the tropical east Pacific and tropical Americas and runs for the period 1979–2005, using European Centre for Medium-Range Weather Forecasts (ECMWF) lateral and surface boundary conditions. We study the ability of RCA to represent regional patterns of precipitation, with respect to both the climatology and interannual variability associated with ENSO. The latter is achieved by grouping the simulations into El Niño and La Niña composites and studying the delayed response of precipitation to SST forcing in four regions of Central and South America.
In this paper, we concentrate on seasonal mean timescales. We find that RCA accurately simulates the main features of the precipitation climatology over the four regions and also reproduces the majority of the documented regional responses to ENSO forcing. Furthermore, the model captures the variability in precipitation anomalies between different ENSO events. The model exhibits a wet bias over the northern Amazon and slightly overestimates the magnitude of ENSO anomalies over Central America.     

A predictive numerical model for potential mapping of the gas hydrate stability zone in the Gulf of Cadiz

This paper presents a computational model for mapping the regional 3D distribution in which seafloor gas hydrates would be stable, that is carried out in a Geographical Information System (GIS) environment. The construction of the model is comprised of three primary steps, namely: (1) the construction of surfaces for the various variables based on available 3D data (seafloor temperature, geothermal gradient and depth-pressure); (2) the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples; and (3) the calculation of the thickness of the hydrate stability zone. The solution is based on a transcendental function, which is solved iteratively in a GIS environment.The model has been applied in the northernmost continental slope of the Gulf of Cadiz, an area where an abundant supply for hydrate formation, such as extensive hydrocarbon seeps, diapirs and fault structures, is combined with deep undercurrents and a complex seafloor morphology. In the Gulf of Cadiz, the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions, and explains the geometry and distribution of geological structures derived from gas venting in the Tasyo Field (Gulf of Cadiz) and the generation of BSR levels on the upper continental slope.     

Distribution and spatial variation of hydrothermal faunal assemblages at Lucky Strike (Mid-Atlantic Ridge) revealed by high-resolution video image analysis

Whilst the fauna inhabiting hydrothermal vent structures in the Atlantic Ocean is reasonably well known, less is understood about the spatial distributions of the fauna in relation to abiotic and biotic factors. In this study, a major active hydrothermal edifice (Eiffel Tower, at 1690 m depth) on the Lucky Strike vent field (Mid-Atlantic Ridge (MAR)) was investigated. Video transects were carried out by ROV Victor 6000 and complete image coverage was acquired. Four distinct assemblages, ranging from dense larger-sized Bathymodiolus mussel beds to smaller-sized mussel clumps and alvinocaridid shrimps, and two types of substrata were defined based on high definition photographs and video imagery. To evaluate spatial variation, faunal distribution was mapped in three dimensions. A high degree of patchiness characterizes this 11 m high sulfide structure. The differences observed in assemblage and substratum distribution were related to habitat characteristics (fluid exits, depth and structure orientation). Gradients in community structure were observed, which coincided with an increasing distance from the fluid exits. A biological zonation model for the Eiffel Tower edifice was created in which faunal composition and distribution can be visually explained by the presence/absence of fluid exits.     

Near-surface seismic facies at the Korea Strait shelf margin and trough region

High-resolution seismic profiles across the shelf margin and trough region of the Korea Strait reveal five shallow, near-surface facies units. These are relict coastal deposits, relict delta deposits, slumps and slides, and trough lag deposits. Most deposits represent a lowstand systems tract, formed during the last lowstand of sea level. Relict coastal deposits represent a linear sediment body along the present shelf margin at water depths of 120–150 m, whereas relict delta deposits occur on the gentle, southwestern slope of the trough at water depths of about 150–200 m. Slumps and slides are dominant at the base of slope in the central trough region. Sediments on the central trough floor were partly eroded and redistributed by strong currents, resulting in lag deposits.     

Spatial and temporal variability of the ratios between the nitrate and phosphate concentrations in the Sea of Okhotsk

The relation between the nitrate and phosphate concentrations in the Sea of Okhotsk and the bordering waters of the Pacific Ocean were studied. The surveys were carried out in the autumn, spring, and summer of 2001–2002. For the deepwater part of the sea, the relation [NO^? ₃] = ((14.88 ± 0.07) × [PO^3? ₄] ? 5.46 ± 0.17) was found. The coefficients in the equation given are statistically different from those in the similar equation for the Pacific waters: [NO^? ₃] = (16.05 ± 0.15) × [PO^3? ₄]-(7.23 ± 0.36). In the northern part of the sea; on the shelf; in the slope area; and, especially, in the deep waters of the TINRO Depression, the linear dependence between the phosphate and nitrate concentrations was distorted. This feature was described in terms of nitrate deficiency. The maximum values of this deficiency were found in the near-bottom waters. The principal processes that might cause the nitrate deficiency were considered: the difference in the oxidation rates of the nitrogen and phosphorus organic compounds, the matter transfer between the continent and the sea, the different efficiency of the biogenic burial of nitrogen and phosphorus in the bottom sediments, and the denitrification in the upper layer of the bottom sediments. It was shown that the most probable cause of the nitrate deficiency was the denitrification. The loss of inorganic nitrogen owing to the supply of the waters of the Sea of Okhotsk to the Pacific Ocean was estimated as ～2.5 × 10¹¹ mol N/year.     

Geochronology of the American-Antarctic Ridge

A new map of chrons for the American-Antarctic Ridge area has been compiled. Its analysis and the calculations performed showed that the seafloor spreading with respect to its axis started before 85 My B.P. The spreading directions were 115° (chrons C34-C29), 145° (chrons C29-C21), 110° (chrons C21-C5C), and 85° (chrons C5C-C1). The maximum rates of about 4 cm/year were reached earlier than 52 My B.P.; subsequently, a progressive general decrease in the spreading rate has been observed. According to our forecast, the spreading may cease in the following 3.5 My.     

Estimation of the long-term variability of hydrophysical characteristics of the Black Sea based on the assimilation of the climatic hydrological fields L and altimetry data

To study the long-term variability of the thermohaline and dynamic characteristics of the Black Sea, we use three versions of climatic fields, namely, the fields reconstructed in the model according to the old (1903–1982) and new (1903–2003) hydrological climatic data arrays of temperature and salinity and according to the data of satellite altimetry. The analysis of the altimetry-based climatic fields confirms the distinctions (established earlier according to the old and new data arrays) in the seasonal variability of the integral characteristics of temperature and salinity and in the structures of hydrophysical fields in the sea. It is shown that, in the winter-spring season, the thermohaline fields reconstructed according to the new and altimetry data arrays are characterized by a small elevation of the halocline (pycnocline) and the upper boundary of the cold intermediate layer. In all seasons, the altimetry-based surface geostrophic currents contain numerous mesoscale eddies with different signs of rotation. Moreover, in all seasons, the Rim Current reconstructed according to the altimetry data is characterized by a narrower jet almost along the entire its length. This jet is especially intense near the coasts of West Anatolia. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 3–17, July–August, 2006.