In Antarctica, the temperature of icebergs is in the range of -15oC to -20oC. During the melting process icebergs often calve and fracture; it creates „trails‟ or „halos‟ of growlers. The action of sea waves icebergs is generally the main source of iceberg melting and means that melt occur more often at the waterline. The main sources of decay of icebergs are the water temperature and storms. Erosion is maximal when the iceberg is affected by both mechanical erosion and thermal effects of air and water. Icebergs can melt into saddle, teeth or bipeaked shapes because of unequal erosions between their axes. The parts most affected by erosion rise which creates motion. During the melting process the iceberg shape will be modified as seen in Figure 3.30.
Figure 3.30 Field Observation of Iceberg Deterioration (credit Veitch et al.62, 2001 p. 1)
For each shape the iceberg will find a new physical stability condition passing through a transitory equilibrium state. In warm waters, icebergs often become very unstable. The indentation at the waterline stimulates calving of the suspended and underwater parts of icebergs. However, melt and break-up rates vary with water state and temperature. Melting
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conditions are accelerated or decelerated by daily and seasonal oceanographic and atmospheric variations and also by the route followed by the iceberg (Jansen, 2009). Ocean conditions usually have a strong impact on the melting process (Veitch and Daley, 2000). The waterline is stressed by sea waves and by warm water currents and consequently compression and dilatation of ice take place. When the air temperature is warmer than the sea temperature (by more than 2°C) the emerged parts of the iceberg will melt faster (Jansen, 2009). When the water temperature is warmer than the air temperatures (such as in the case of the Gulf Stream‟s „cold wall‟), the melting rates are faster within the submerged area. During the iceberg drift, the wind and the sea state (waves, sea ice) act as driving forces (Smith and Donaldson, 1987; Smith, 1992 and 1993). These driving forces can be implemented in a numerical model, and can be related to iceberg length reduction (Gladstone et al., 2001; Schodlock, 200563). The influence of solar radiation (White et al., 1980) on the melting of icebergs is neglected because it is very small, less than 0.2 MJ/m² (Veitch et al., 2001).
Numerical and experimental physical models of iceberg deterioration have been proposed in the last ten years (Martin et al., 1977; Veitch and Daley, 2000; Liang et al., 2001; Moores et al., 2001). These models have successfully integrated a number of critical factors responsible for iceberg deterioration, such as iceberg orientation, floatation, stability and mass evolution due to continuous melting and morphological changes. The main focus was put on studies related to wave accelerated melting which have a determinant influence on calving fragmentation. At the waterline of an iceberg, wave erosion produces protruding underwater rams and cantilevered shelves above water. Veitch et al. (200164 p. 4) noted:
The buoyancy forces on the former and the gravity forces on the latter frequently lead to large scale fractures of the iceberg. Removal of mass by a fracture changes the stability of the iceberg and exposes cold interior surfaces to warmer water, leading to further stresses and fractures. The resulting fragmentation reduces the mass of an iceberg at a much higher rate than wave erosion alone would achieve.
Figure 3.31 illustrates the fragmentation of an iceberg as recorded by a video compared with the results from a corresponding simulation. The sequence of pictures in Figures 3.32 to 3.34 shows the wave erosion on an iceberg during 47 ¼ hours, from May 27 to May 29, 2001. The iceberg was observed in the village of Little Harbour - Canada.
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Figure 3.31 Fragmentation of an Iceberg (credit Veitch et al., 2001 p. 12)
Figure 3.32 Outline Profile of the Iceberg Showing Wave Erosion (credit Veitch et al., 2001 p. 14) in green the erosion compared to the original profile
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Figure 3.33 Wave Erosion on an Iceberg during 47 ¼ hours, from May 27 to May 29, 2001. Little
Harbour, Canada. The Arrow Surface Indicates Waves Erosion (credit Veitch et al., 2001 p. 15)
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The orientations changes of the iceberg, its profiles and waterlines are recorded and show the eroded surface lines. According to Bruneau (201065) “a large berg may take 90 days to fully deteriorate in water temperatures around 0oC, whereas the same berg may only last 11 days in 10oC water”. A 120 m long large iceberg (for Arctic icebergs) melted in 36 hours in 27oC water (Bruneau, 2002).
In the Antarctic, most tabular icebergs take several seasons to deteriorate, and some are grounded for years in shallow bays. The average life of the other types of large icebergs is around six years (Hellmer, 2004). Tabular icebergs from ice shelves are bigger and stronger in the beginning, and therefore have a much longer life (up to 11 years) than smaller density icebergs. Iceberg drift and decay depend on whether icebergs are driven completely, partially or stay close to their original location (Schodlok et al., 2005b). Freshwater flux from icebergs depends on iceberg decayed time. Net precipitation input can compensate icebergs freshwater flux output (Schodlok et al., 2005b). These elements are important for the understanding of the harvesting techniques of icebergs and the study of their potential impacts.
3.8 Conclusion
In this chapter the properties of icebergs have been analysed. Specific characteristics of icebergs, including their mechanical properties, underwater volume, dielectric and optical properties provide important information which would be necessary to develop an iceberg transportation system. For example, variables such as the suitable volumes of ice that could be used, the types of icebergs which are the most stable, the time for icebergs harvesting, the ideal locations to source icebergs, the techniques to operate icebergs, the environmental properties which can be used and the environment characteristics which have to be protected. In Antarctica, the annual precipitation would correspond to 5 mm of sea level rise. This accumulation is compensated partially by ice discharge into floating ice shelves that very often break up to form icebergs (Church et al., 200166). The size of the global iceberg resource is estimated at 3,000 billion tonnes/year and is equivalent to more than half of the world‟s water consumption. In Antarctica, the harvestable tabular iceberg resource is about 200,000 icebergs/year. Using 1% of the resource would not overtake the regeneration of the resource (up to 10 km3/year, 0.03 km3/day). A theoretical global ice daily volume of up to 3
65 http://www.icebergfinder.com/iceberg-guide/iceberg- faq.aspx,http://www.caperace.com/stories/where-do-icebergs-really-come-from/ 66 www.grida.no/climate/ipcc_tar/wg1/pdf/TAR-11.pdf
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km3 (up to 1,200 km3/year) would be available providing that environmental impacts of their harvesting could be minor.
According to Church et al. (2001 p. 650):
Changes in ice discharge generally involve response times of the order of 102 to 104 years, the time scales are determined by isostasy, the ratio of ice thickness to yearly mass turnover process [up to 4,500 km3], affecting the velocity the physical and the thermal processes at the sea bed.
Melting produces iceberg instability. During the melting process icebergs often calve and fracture into many small pieces. The action of sea waves icebergs is generally the main source of iceberg melting and means that melt occur more often at the waterline. Their melting rates vary according to their environment. Antarctic tabular icebergs are the most appropriate for a project of freshwater supply because of their properties. The temperatures of tabular icebergs are between -15°C and -20°C. Between two to five years are needed for total iceberg disintegration of a tabular iceberg. Therefore tabular icebergs, have the required characteristics to be successfully selected for transportation. They have the most reliable physical stability, which minimises risks associated with harvesting. They offer large volumes and could therefore be considered as semi fossil water supply resources as the ratio between the scale of the resource and the times of regeneration is small. Icebergs are natural ice outputs into the sea. The natural routes of iceberg movement are favourable for a maritime transportation system. Locations with a reasonable accessibility to tabular icebergs could take advantage of the transportation of icebergs as a solution to local freshwater shortages. This is a key point of the thesis. New operating techniques would still need to be designed to stabilise icebergs for transportation purposes. And the environmental impacts of their use would require a more detailed investigation on the characteristics of the environments of iceberg which will be undertaken in the next chapter.