A two-layer model of the Rhine plume

The outflow of Rhine water into the shallow Southern Bight of the North Sea leads to almost discontinuous vertical density distributions and sharp frontal structures around the river mouth. Strong tidal motion, wind and baroclinic effects have large influence on the dynamics and dispersion of river water. A three-dimensional tidal model, including advective and diffusive transport of salinity, is used in the two-layer mode for simulation of Rhine water outflow to quantify the interaction of the different processes and the effect on dispersion and mixing of river water. Layer depths are adjusted in a way that no advective transports between upper and lower layer take place in case of sufficiently stable stratification. In case of weak or no stratification the upper layer depth is fixed, and advective transports between layers are computed. Model results show frontal eddy development and (limited) growing internal waves due to baroclinic instability. Comparisons with observational data are presented.     

Meroplankton abundance in the Northeast Water Polynya: Insights from oceanographic parameters and benthic abundance patterns

We investigated meroplankton (planktonic larvae of benthic organisms) abundance and distribution in the Northeast Water (NEW) Polynya, located on the northeast coast of Greenland, from July 15 to August 15, 1992. Meroplankton was present at all sites visited (0.03–84.83 individuals per m³); at one station meroplankton comprised 8.28% of total zooplankton. Total meroplankton abundance was correlated with total zooplankton abundance and total benthic infaunal abundance but was not correlated with either microscopic carbon concentration or primary productivity. Examination of distribution data for barnacle nauplii and adults indicated that both adults and larvae were concentrated at the same locations. Patterns of distribution were also examined for stelleroid plutei, polychaete larvae and trochophores. There were distinct geographic patterns in total and class-specific meroplankton distributions, with maximal abundances occurring over the Belgica Bank and in the eastern regions of the Westwind Trough and minimal abundances in the Belgica Trough. The apparent control of meroplankton distribution by the hydrography of the region, coupled with the correlation between meroplankton, zooplankton and adult infaunal abundance, reinforces the hypothesis that hydrography plays a major role in controlling the distribution of biota in the NEW polynya (Ambrose and Renaud, 1995; Ashjian et al., 1995, 1997-this volume; Smith et al., 1995; Piepenburg et al., 1997-this volume).     

A new formulation of the impact mechanics of ship collisions and its application to a ship–iceberg collision

A new formulation is proposed for the analysis of the impact mechanics of ship collisions that can be applied to both 2D and 3D cases. It is assumed that the impact force is large, and all other forces except the impact forces are neglected. The equations of motion are solved in a local coordinate system, and a transformation matrix between the global and the local coordinate system is proposed. The mass and inertia properties are formulated in the local coordinate system. The orientation of the local coordinate system is determined by the hull shape of the struck ship at the contact point. A closed form solution of the external mechanics of ship collisions is derived. Excellent agreement with an alternative 2D formulation for ship–ship collisions is achieved. The features of the proposed 3D method are demonstrated by numerical examples. An application of the method to estimate the required energy dissipation in ship–iceberg collisions is included. Results and discussions are presented and finally, conclusions are made.     

Hydrography and biogeochemistry of the north western Bay of Bengal and the north eastern Arabian Sea during winter monsoon

The north eastern Arabian Sea and the north western Bay of Bengal within the Indian exclusive economic zone were explored for their environmental characteristics during the winter monsoons of 2000 and 2001 respectively. The two regions were found to respond paradoxically to comparable intensities of the atmospheric forcing. There is an asymmetry in the net heat exchange of these two basins with atmosphere because of the varying thickness of barrier layer. During winter, the convective mixing in the Arabian Sea is driven by net heat loss from the ocean, whereas the Bay of Bengal does not contribute to such large heat loss to the atmosphere. It appears that the subduction of high saline Arabian Sea water mass is the mechanism behind the formation of a barrier layer in the northeast Arabian Sea; whereas that in the Bay of Bengal and the southeast Arabian Sea are already established as due to low saline water mass. The weak barrier layer in the Arabian Sea yields to the predominance of convective mixing to bring in nitrate-rich waters from the deeper layers to the surface, thereby supporting enhanced biological production. On the other hand, the river discharge into the Bay of Bengal during this period results in the formation of a thick and stable barrier layer, which insulates vertical mixing and provide oligotrophic condition in the Bay.     

Dynamics of suprabenthos-zooplankton communities around the Balearic Islands (western Mediterranean): Influence of environmental variables and effects on the biological cycle of Aristeus antennatus

Dynamics of suprabenthos and zooplankton were analyzed in two areas located in the NW (off Sóller harbour) and S (off Cabrera Archipelago) of Mallorca (Balearic Islands, western Mediterranean) at depths ranging between 135–780 m. Four stations situated respectively at 150 m (shelf-slope break), and at bathyal depths of 350, 650 and 750 m were sampled at bi-monthly intervals during six cruises performed between August 2003 and June 2004. Suprabenthos showed maximum biomass in both areas from late spring to summer (April to August), while minimum biomass was found in autumn (September–November). Though variable, temporal dynamics of zooplankton showed peaks of biomass in late winter and summer (February and June), while minimals occurred in autumn (August–September) and, at bathyal depths, in April. Suprabenthos (abundance; MDS analyses) showed a sample aggregation as a function of depth (3 groups corresponding to the shelf-slope break, upper slope — over 350 m; and the middle, deeper part of the slope — over 650–750 m), without any separation of hauls by season. By contrast, zooplankton samples were separated by season and not by depth. There was evidence of three seasonal groups corresponding to summer (June 2004–August 2003), autumn–winter (September and November 2003, February 2004), and spring (April 2004), being especially well established off Sóller. In general, suprabenthos was significantly correlated with the sediment variables (e.g. total organic matter content (% OM), potential REDOX), whereas zooplankton was almost exclusively dependent on Chl a at the surface, which suggests two different food sources for suprabenthos and zooplankton. The increase of suprabenthos abundance in April–June was paralleled by a sharp increase (ca. 2.8 times) in the %OM on sediment during the same period, coupled ca. 1–2 months of delay with the peak of surface Chl a recorded in February–March (from satellite imagery data). Suprabenthos biomass was also correlated with salinity close to the bottom, suggesting a link between suprabenthos abundance and changes in the oceanographic condition of water masses close to the bottom. It is suggested that a higher suprabenthos biomass recorded off Sóller in comparison to that off Cabrera in June could, in turn, be related to a seasonal inflow of Levantine Intermediate Water (LIW) in April–June in this area at mid bathyal depths (350–650 m). This trend would be based on: 1) it was evident only at mid-slope depths between 350–750 m, coinciding with the LIW distribution, and 2) it was not recorded among zooplankton (collected throughout the water column). The possible effect of the fluctuations of suprabenthos and zooplankton on higher trophic levels has been explored studying the diet and food consumption rates of the red shrimp Aristeus antennatus, as indicator species by its dominance in bathyal communities. A. antennatus increased its food consumption from February to April–June 2004 off Sóller, which in the case of large (CL > 40 mm) specimens was found in both areas. In addition, there was a shift of diet from winter to spring–early summer. In this last period, A. antennatus preyed upon euphausiids and mesopelagic decapods and fish, while benthos (e.g. polychaetes and bivalves) decreased in the diet. This indicates an increase in the food consumption and probably in the caloric content of the diet in pre-spawning females in April–June 2004, which is synchronized with the period when gonad development begins in A. antennatus females (May–June). Anyway, macrozooplankton, and not suprabenthos, is crucial as a high energetic food source in the coupling between food intake and reproduction in the red shrimp.     

A statistical investigation of time-variant hull girder strength of aging ships and coating life

Risk- and reliability-based approaches are increasingly being applied in assisting inspection and maintenance planning. One of the keys to such approaches is properly predicting the hull girder strength (HG) of gradually degradated hull structures. The development has been limited by the lack of data on aging ships—validation of the developed methods has not been possible. To fill the gap of knowledge on hull strength of ships in service, this paper presents a database of as-gauged hull structures and a statistical study of the time-variant HG of tankers. The expanded data set was collected from 2195 as-gauged girth belts (transverse sections) of 211 single-hull tankers that were 12–32 years old. It was intended to (1) provide actual data on hull girder section modulus (HGSM) of tankers, (2) investigate the general trends of the change in the HG over ships’ service life, and (3) propose formulations for presenting time-variant HGSM and coating life.

The data set demonstrated a high variation of HGSM that changed over time. The mean value and standard deviation of HGSM loss were derived as functions of time. The probability density function (pdf) of coating life was also derived. Comparisons were made between previous studies on HG and the current data set. It was found that almost all previous studies showed much greater HG loss than what this database revealed. The refinement of existing calculations appears to be needed. The data set and statistical study were expected to form the basis for validating formulations of HG that are key components in risk- and reliability-based approaches.     

Transportation benefits from containerized cargo movements on the Trinity River, Texas

The future of waterborne commerce on the Trinity River, Texas, depends on the extent to which the waterborne transport system constitutes a definite cost saving over alternate transportation modes. This paper outlines the results of a study conducted in the area, the purpose of which was to establish potential transportation savings from containerized commodity movements on the Trinity River.