Offshore Extent Research Articles

Surface Buoyant Jets in Steady and Reversing Crossflows

Experimental data on surface buoyant jets in steady and reversing crossflows is presented. The experiments were conducted in a 130 ft (40 m) by 81 ft (25 m) by 1.5 ft (0.45 m) deep basin, which was large enough to ensure negligible boundary effects while allowing controlled ambient, conditions. The test results are presented nondimensionally and referenced to controlling length scales. The steady crossflow results are trajectories and longitudinal temperatures, which show the effects of the discharge buoyancy and initial flowrate. Comparing these results with those of other experimental studies leads to the suspicion that boundary effects may be present in some of the latter. The reversing crossflow tests furnish surface temperature patterns and plume depths for a range of conditions. These results can be used to evaluate mathematical models. In addition, the effects of tidal reversals on the offshore extent and depth of the plume are examined. Dispersion tests reveal that buoyancy played an important role in plume spreading. This factor is important as buoyant spreading is frequently omitted in two dimensional farfield mathematical models of thermal plumes.

Edge waves on complex beach profiles

In order to interpret field data in terms of edge wave modes, investigators usually assume that the beach profile is linear, allowing the use of simple analytical solutions for edge wave structure and wavelength. The validity of this assumption is checked by using a numerical model to find the edge wave modes on a typical concave beach. Results show that estimates of edge wave wavelength using the plane beach assumption can be wrong by ±100% at a fixed mean sea level with a further error of ±50% introduced by the tides. Greatly improved estimates can be made if the value of beach slope is chosen to reflect suitably the offshore extent of the edge wave. A rule of thumb for calculating the effective beach slope βe is , where h is the water depth at distance x′ from shore, L is the edge wave wavelength, and n is the edge wave mode number. The motion of the offshore profile of the edge wave due to a change of tidal elevation is dramatically different on a concave beach than on a plane beach. The tendency for the profile to move offshore for a drop in tide is compensated by the decrease in mean beach slope and hence reduction in wavelength and offshore extent of the wave. For a fixed instrument a drop in tide may lead to a profile shift onshore or offshore or no shift at all. Edge wave damping on a concave beach is shown to change as a function of tide. At low tide the surf zone width is increased while edge wave wavelength is decreased, both factors increasing damping. Thus edge wave spectra taken at low tide may be less energetic than those at high tide, particularly at high frequencies.

Background water temperatures and power station discharges

An analylical treatment of the background or far-field temperature distribution associated with the cooling water discharge from a direct-cooling coastal power station is presented. Factors considered in the formation of this temperature field include residual tidal currents, horizontal mixing, the cooling water flow rate, separation of the outlet and intake and heat transfer across the water surface. A criterion is derived which specifies conditions under which the circulation of water between the outlet and the intake is expected to have a significant influence on background temperatures. As an illustration, the analysis suggests that under conditions typical of the U.K. coast in the southern North Sea this recirculation, while affecting the operation of the power station, has little effect on the far-field temperature for plant of less than 3–5 GW electrical capacity. At many coastal locations there will be a mean, or residual tidal current running parallel to the shore. The effect of this is to limit the off-shore extent of the temperature field while transporting the heat a considerable distance along the coast. These factors are quantified and it is demonstrated that the ‘area’ of the far-field is very sensitive to the threshold temperature which is taken to define its boundary. The results obtained with this model are greatly influenced by the values adopted for residual currents and mixing. The paper therefore stresses the need for reliable data, obtained from site surveys, on which the parameters must be based.

Geological structure of the Malin Sea

Geophysical surveys in the Malin Sea have revealed the presence of two asymmetric fault bounded basins, here termed the Loch Indaal basin and the Rathlin trough. They extend in a NE–SW direction between Northern Ireland and Western Scotland, and are thought to be largely Permo-Triassic in age. Four major fault belts have also been identified, together with a large number of igneous bodies, the latter having been confirmed by dredge sampling. In addition the offshore extents of the Fanad and Thorr Plutons have been delineated.

Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.