• NEANIAS Underwater Research Community

Abstract Mesoscale eddies are ubiquitous in the ocean and play a key role in exchanges across continental slopes. In this study the properties of wind-driven baroclinic turbulence are investigated using eddy-resolving process simulations, focusing on the case of retrograde winds that arises around the margins of the subtropical gyres. In contrast to a flat-bottomed ocean, over steep slopes eddies develop from baroclinic instabilities are confined to the top few hundred meters. Deeper in the water column baroclinic instability and vertical momentum transfer are suppressed, so wind-input momentum is exported toward the open ocean by eddies before traversing down to the ocean bed. Close to the sloping topography, eddy energy sourced from the upper ocean is converted to potential energy, steepening isopycnals and driving bottom-trapped prograde flows. This process is associated with upgradient lateral buoyancy fluxes and downgradient isopycnal potential vorticity fluxes, and cannot be reproduced via linear stability calculations. These properties of wind-driven shelf/slope turbulence are contrasted against simulations with flat bathymetry. The key differences described above hinge on the flow close to the steep topographic slope, which may be sensitive to the model’s vertical coordinate system. The simulations are therefore replicated using models that employ geopotential coordinates, terrain-following coordinates, and isopycnal coordinates. Quantitative inter-model discrepancies in the momentum and energy budgets are much more pronounced in the presence of a steep bottom slope. However, the key findings of this study are consistent across the models, suggesting that they are robust and warrant incorporation into parameterizations of eddy transfer across continental slopes.

The aim of the study is to understand the characteristics of a volcano occurred in marine environment, as Weh Island where Sabang City located is still demonstrated its volcanic cone morphology either through satellite imagery or bathymetric map. Methods used were marine geology, marine geophysics and oceanography. Results show that surface volcanism (sea depth less than 50 m) take place as fumaroles, solfataras, hot ground, hot spring, hot mud pool and alteration in the vicinities of seafloor and coastal area vents. Seismic records also showed acoustic turbidity in the sea water column due to gas bubblings produced by seafloor fumaroles. Geochemical analyses show that seafloor samples in the vicinities of active and non-active fumarole vent are abundances with rare earth elements (REE). These were interpreted that the fumarole bring along REE through its gases and deposited on the surrounding seafloor surface. Co-existence between active fault of Sumatra and current volcanism produce hydrothermal mineralization in fault zone as observed in Serui and Pria Laot-middle of Weh Island which both are controlled by normal faults and graben. Keywords: submarine volcano, hydrothermal mineralization, Sabang-Weh-Aceh. Tujuan kajian adalah memahami karakteristik suatu gunungapi yang berada dalam lingkungan marin, sebagaimana Pulau Weh dimana Kota Sabang terletak masih menunjukkan morfologi kerucut volkaniknya baik melalui citra satelit maupun batimetri. Metoda yang digunakan adalah geologi kelautan, geofisika kelautan dan oseanografi. Hasil menunjukkan bahwa volkanisma permukaan (kedalaman laut kurang dari 50 m) terdapat dalam bentuk fumarola, solfatara, lahan panas, mata air panas, kolam lumpur panas dan alterasi sekitar lobang kepundan dasar laut dan pantai. Rekaman seismik juga menunjukkan turbiditas akustik dalam kolom air laut akibat gelembung gas yang dihasilkan oleh fumarola dasar laut. Analisis geokimia menunjukkan bahwa contoh-contoh dasar laut sekitar lobang kepundan fumarola yang aktif maupun tidak aktif kaya akan logam tanah jarang. Ini ditafsirkan bahwa proses fumarola tersebut membawa REE melalui gas-gasnya dan mengendapkannya pada permukaan dasar laut di sekitar. Ko-eksistensi antara Sesar Sumatera aktif dan volkanisma Resen menghasilkan mineralisasi hidrotermal dalam zona sesar seperti teramati di Serui dan Pria Laot - bagian tengah Pulau Weh yang keduanya dikontrol oleh sesar normal dan graben. Kata kunci: gunungapi bawah laut, mineralisasi hidrotermal, Sabang-Weh-Aceh.

Wave height variability along the crest of breaking waves is shown to be a significant factor in the assessment of surf zone dynamics. Variations in excess of 50 percent of the maximum wave height can occur along a single crest without significant variations in bathymetry. The horizontal scale of this longshore variability in crest height corresponds to the wave length of incident breaking waves. Four possible mechanisms for this variability are postulated and then evaluated individually on the basis of field observations. A major result of these evaluations is that two-dimensional shallow-water wave equations appear to be inappropriate for expressing natural surf zone wave transformations and water motions even under the condition of waves encroaching on a plane sloping bottom. Consequently, three-dimensional equations of surf should be used for describing most natural surf zone dynamics.

Waves at sea limit both the resolution and accuracy of bathymetric surveys. Vessel roll introduces errors difficult to correct. By limiting operations to calm days according to criteria given, these errors can be controlled. Heave and roll appear as wiggles on the record, masking bottom irregularities. Speed limitations are given to assure seeing irregularities of a given size. Three tidal datum plan are defined, and indirect methods of obtaining offshore tidal data are described.

Abstract Synoptic bathymetric surveys and current meter data collected over a sandwave field in Adolphus Channel (20 m water depth), Australia, yield average estimated celebrities of 0.75 and 0.25 m day−1, respectively. The sandwaves average 3.9 m in height, 102 m in wavelength and are comprised of up to 96% carbonate, consisting primarily of intact and fragmented calcareous alga Halimeda, benthic foraminifers, bryozoans and molluscs. The sand has a modal grain size of 0.8 mm. Current speeds measured 1 m above the bed averaged 0.42 m−1 and reached a peak of 1.36 m−1. Surveys carried out in September and February show that the sandwaves reversed their asymmetric orientation over this time interval, which is attributed to a change in the direction of the wind-driven currents during the monsoon season. The reversal of asymmetry was accompanied by a statistically significant change in the degree of sandwave asymmetry (ratio of stoss and lee slope lengths) whereas no change in mean wavelength was detected. The reversal is estimated to have required 47 days to occur based upon estimates of average sandwave cross-sectional area and bedload transport rates predicted from the current meter data.

There are thousands of shallow lakes on the North Slope of Alaska. They are one of the most obvious manifestations of the hydrological system at work, and they play an important role in physical, biological and biogeochemical processes in the tundra environment. The depth of the lakes is a key determinant of these processes, yet few bathymetric data are available. The authors conducted a pilot study to investigate the ability of interferometric synthetic aperture radar (INSAR) to derive bathymetry of the shallow lakes on the Alaskan North Slope. Because lake ice is transparent in the microwave region, they hypothesize that INSAR can see through the lake ice and reveal the topography beneath the grounded lake ice. The analytical result of a sensitivity analysis indicates that interferometric phase is more sensitive to lake bathymetry under grounded lake ice than to the land surface topography. A series of interferograms are derived for the area near Point Barrow, Alaska, from pairs of ERS-1 3-day repeat SAR images acquired in January through March 1994. Floating ice is identified by the noisy phase pattern and low coherence, which are due to the increase of the ice thickness in the interim. Grounded ice is identified by the clean phase pattern and relatively high coherence, because no further growth occurs once the lake has frozen to the bottom. The phase difference between the grounded ice and the surrounding land reflects the lake bottom topography. The final usefulness of this new INSAR application needs to be verified and validated.

This study explored two-dimensional (2D) numerical hydrodynamic model simulations of Lake Victoria. Several methods were developed in Matlab to build the lake topography. Old depth soundings taken in smaller parts of the lake were combined with more recent extensive data to produce a smooth topographical model. The lake free surface numerical model in the COMSOL Multiphysics (CM) software was implemented using bathymetry and vertically integrated 2D shallow water equations. Validated by measurements of mean lake water level, the model predicted very low mean flow speeds and was thus close to being linear and time invariant, allowing long-time simulations with low-pass filtered inflow data. An outflow boundary condition allowed an accurate simulation to achieve the lake’s steady state level. The numerical accuracy of the linear measurement of lake water level was excellent. QC 20191106. QC 20200129 PhD project

International audience; – This synthesis of the morphological and stratigraphical evolutions of the Marennes-Oléron Bay (west coast of France) combines bathymetric data (1824 and 2003) and very high-resolution seismic profiling groundtruthed by vibracore samples. The Marennes-Oléron Bay is characterised by a very high sedimentation rate and appears to be an ideal place to investigate the sedimentary record of the major environmental changes that occurred since the last several millennia. The sediment budget of the Marennes-Oléron Bay, between 1824 and 2003, is clearly positive. The flood-dominated northern Marennes-Oléron Bay displays sediment gain in both intertidal and subtidal areas whereas the ebb-dominated southern Marennes-Oléron Bay displays sediment gain restricted to the intertidal area and deepening of subtidal channels. In addition, human influences such as oyster farming may play a role in the sediment gain of the bay. The sediment-fill of the northern Marennes-Oléron Bay consists of five main phases: (1) lenticular units and flooded intertidal flats recording lower sea level periods before 7500 yr B.P.; (2) tidal channel-fills recording changes in tidal drainage pattern from 7500 to 5000 yr B.P.; (3) a subtidal unit which constitutes the main phase of sediment fill in the northern part of the bay from 5000 to 1500 yr B.P.; (4) a major channelized erosional surface related to huge coastline changes from 1500 to 1000 yr B.P.; and (5) a mud drape emplaced during the last millennia and potentially recording historical human impact (deforestation and land reclamation). The sediment fill of the southern Marennes-Oléron consists of sandbanks, mixed sand-and-mud flats and tidal channels, mainly emplaced under wave-and-tide processes since the last centuries. Despite its relatively thin (20 m at the maximum), recent and rapid sediment fill, the stratigraphic organization and morphological evolution of the Marennes-Oléron Bay is very complex and spatially variable. Like in many other estuaries , sediment fill of the Marennes-Oléron Bay was successively controlled by relative sea level changes, and then by sediment supply driven by hydrodynamic changes related to huge coastline migrations, and finally by human activities. Moreover, this kind of " rocky coast " estuary, where the sediment-fill is very thin and discontinuous, is characterised by a bedrock control at each phases of the sediment fill both in terms of preservation in topographic lows and in terms of control on hydrodynamics and related sediment input. – Cette synthèse sur les évolutions morphologiques et stratigraphiques de la baie de Marennes-Oléron (côte ouest de la France) repose sur des données bathymétriques (1824 et 2003), des enregistrements sismiques à très haute résolution et des carottages. La baie de Marennes-Oléron qui se caractérise par de très forts taux de sédimentation repré-sente un site idéal pour étudier l'enregistrement sédimentaire des changements environnementaux qui se sont produits lors des derniers millénaires. Le budget sédimentaire de la baie de Marennes-Oléron, entre 1824 et 2003 est clairement positif. Dans la partie nord de la baie, dominée par le flot, le gain sédimentaire se situe à la fois dans les zones inter– et subtidales, alors que dans la partie sud de la baie, dominée par le jusant, le gain sédimentaire se limite aux zones intertidales avec un creuse-ment des chenaux subtidaux. L'activité ostréicole semble jouer un rôle dans le gain sédimentaire au niveau des zones in-tertidales. Le comblement sédimentaire de la partie nord de la baie de Marennes-Oléron comprend cinq phases principales, de la base vers le sommet : (1) des unités lenticulaires et des estrans enregistrant des périodes de plus bas niveau marin avant 7 500 B.P. ; (2) des comblements de chenaux témoignant de réorientations dans le drainage de la baie entre 7 500 et 5 500 B.P. ; (3) une unité subtidale mise en place entre 5 500 et 1 500 B.P. et qui constitue la plus grande partie du comblement ; (4) une surface d'érosion majeure liée à une mobilité drastique du trait de côte entre 1 500 et 1 000 B.P. ; (5) un drapage vaseux déposé depuis le dernier millénaire et enregistrant probablement l'impact anthropique de la défo-restation et de la poldérisation. Dans la partie sud de la baie de Marennes-Oléron des bancs de sable, des estrans mixtes, tel : 33 (0) 5 46 45 72 31 ; fax : 33 (0) 5 46 45 82 74. eric.chaumillon@univ-lr.fr Manuscrit déposé le 15 octobre 2008 ; accepté après révision le 15 janvier 2009. sablo-vaseux et des chenaux tidaux constituent un comblement rapide depuis les derniers siècles principalement. L'ensemble des données montre que malgré un comblement sédimentaire peu épais (20 m au maximum), récent et rapide, les évolutions morphologiques et l'organisation stratigraphique de la baie de Marennes-Oléron sont très complexes et variables dans l'espace. Comme dans la plupart des estuaires, le comblement sédimentaire de cette baie a suc-cessivement été contrôlé par les changements relatifs du niveau de la mer, puis par les apports sédimentaires gouvernés par des changements hydrodynamiques en relation avec des migrations importantes du trait de côte et enfin par les acti-vités humaines. Une originalité de ce type d'estuaire de côte rocheuse, caractérisé par une couverture sédimentaire très fine et discontinue, est le contrôle par la morphologie du socle rocheux à toutes les étapes du comblement, tant du point de vue de la préservation des sédiments dans les creux topographiques, que du point de vue de la modification des para-mètres hydrodynamiques et des apports sédimentaires qui leurs sont associés.

Complex principal component analysis (CPCA) is applied to examine medium-term geomorphological behavior of an exposed sandy beach adjacent to a gigantic breakwater of the Port of Sendai located near a river mouth in the Sendai Coast, Japan, using a 12-year series of bathymetry survey data. The results of CPCA and conventional real-PCA demonstrate that erosion and subsequent accretion of the submerged terrace formed in front of the river mouth appear in the first mode of CPCA and have the most significant influence on the medium-term geomorphology of the study area. The first mode is mostly caused by northward alongshore sediment transport driven by wave energy flux, explained from the observed wave data. The second mode of CPCA demonstrates that the sediments previously discharged from the river return again into the nearshore region, and furthermore, the topography changes due to cross-shore sediment transport emerge in the third mode of CPCA.