Seabed gouging by ice

Seabed gouging by ice is a process that occurs when floating ice features (typically icebergs and sea ice ridges) drift into shallower areas and their keel comes into contact with the seabed. As they keep drifting, they produce long, narrow furrows most often called gouges, or scours. This phenomenon is common in offshore environments where ice is known to exist. Although it also occurs in rivers and lakes, it appears to be better documented from oceans and sea expanses. Seabed gouging by ice is a process that occurs when floating ice features (typically icebergs and sea ice ridges) drift into shallower areas and their keel comes into contact with the seabed. As they keep drifting, they produce long, narrow furrows most often called gouges, or scours. This phenomenon is common in offshore environments where ice is known to exist. Although it also occurs in rivers and lakes, it appears to be better documented from oceans and sea expanses. Seabed scours produced via this mechanism should not be confused with strudel scours. These result from spring run-off water flowing onto the surface of a given sea ice expanse, which eventually drains away through cracks, seal breathing holes, etc. The resulting turbulence is strong enough to carve a depression into the seabed. Seabed scouring by ice should also be distinguished from another scouring mechanism: the erosion of the sediments around a structure due to water currents, a well known issue in ocean engineering and river hydraulics – see bridge scour. It appears Charles Darwin speculated in 1855 about the possibility that icebergs could gouge the seabed as they drifted across isobaths. Some discussion on the involvement of sea ice was brought up in the 1920s, but overall this phenomenon remained poorly studied by the scientific community up to the 1970s. At that time, ship-borne sidescan sonar surveys in the Canadian Beaufort Sea began to gather actual evidence of this mechanism. Seabed gouges were subsequently observed further north, in the Canadian Arctic Archipelago, and in the Russian Arctic as well. Throughout that decade, seabed gouging by ice was investigated extensively. What sparked the sudden interest for this phenomenon was the discovery of oil near Alaska's northern coastlines, and two related factors: 1) the prospect that oilfields could abound in these waters, and 2) a consideration that submarine pipelines would be involved in future production developments, as this appeared to be the most practical approach to bring this resource to the shore. Since then, means of protecting these structures against ice action became an important concern. An oil spill in this environment would be problematic in terms of detection and clean-up. Scientists in fields of research other than offshore engineering have also addressed seabed gouging. For instance, biologists have linked regions of the seabed reshaped by seabed gouging by ice to the formation of black pools, seabed depressions filled with anoxic high-salinity water which are death traps for small marine organisms. However, much of it appears to have been documented from an offshore engineering perspective, for the purpose of oil exploration. Seabed gouging by ice is an eminently discreet phenomenon: little sign of it can be observed from above the water surface – the odd evidence includes sea floor sediments incorporated into the ice. Information of interest on these gouges includes: depth, width, length and orientation. Gouging frequency – the number of gouges produced at a given location per unit time – is another important parameter. This kind of information has been gathered by means of seabed mapping with ship-borne instrumentation, typically a fathometer: echo sounding devices such as a side-scan and a multi-beam sonar systems. Repetitive mapping involves repeating these surveys a number of times, at an interval ranging from a few to several years, as a means of estimating gouging frequency. Seabed gouges produced by drifting ice features can be many kilometers in length. In Northern Canada and Alaska, gouge depths may reach 5 metres (16 ft). Most, however, do not exceed 1 meter (3 feet). Anything deeper than 2 meters is referred to by the offshore engineering community as an extreme event. Gouge widths range from a few meters to a few hundred meters. The maximum water depths at which gouges have been reported range from 450 to 850 metres (1,480 to 2,790 ft), northwest of Svalbard in the Arctic Ocean. These are thought to be remnant traces left by icebergs during the Pleistocene, thousands of years ago, when the sea level was lower than what it is today. In the Beaufort Sea, Northern Canada, a 50 km (30 mi) long gouge was shown to exist, with a maximum depth of 8.5 metres (28 ft) and in water depths ranging from 40 to 50 metres (130 to 160 ft). The gouge is not always straight but varies in orientation. This event is thought to be about 2000 years old. Recent episodes of grounding, gouging and fragmentation of large Antarctic icebergs have been observed to produce powerful hydroacoustic and seismic signals that further illuminate the dynamics of the process. In the offshore environment, the gouging features are made up of two kinds of ice: glacial ice and sea ice. Physically and mechanically, glacial ice is akin to lake ice, river ice and icicles. The reason is that they all form from freshwater (non saline water). Ice sheets, ice caps and glaciers essentially consist of glacial ice. Since glacial ice spreads sideways and down-slope (as a result of gravity), in some areas this ice reaches the coastline. Where this happens, depending on topography, the ice may break up into pieces that fall in the sea, a mechanism called ice calving, and drift away. Alternatively, ice sheets may spread offshore into extensive floating ice platforms called ice shelves, which can ultimately also calve. The features produced by these calving processes are known as icebergs and may range in size from meter to kilometer scale. The very large ones, referred to as ice islands, are typically tabular in shape. These may be responsible for extreme gouging events.