Caspian Sea bottom scouring by hummocky ice floes

Abstract

Forms of ice gouging were documented for the first time at 3 to 12 m depth in the North Caspian Sea. Gouges and systems of scouring gouges formed by drifting hummocky bodies frozen in ice floes have been discovered as a result of simultaneous hydroloca� tion survey and echosounding. The length of the larg� est and most welldefined gouges exceeds a few kilo� meters; the width of individual gouges is up to 5 m, and the width of gouge systems is up to 200 m. Apart from linear forms, local pits remaining from stamukhas sit� ting at the bottom have been discovered. Work on development of the existing hydrocarbon reserves in the North Caspian Sea has become active in the last 20 years; this work is supported by engineering and construction of stationary platforms, underwater pipelines, and other oil and gas infrastructure. For this reason estimation of the intensity of the ice impact that belongs to the category of hazardous natural pro� cesses is the key to geotechnical safety of the oil and gas infrastructure and environmental safety of the water areas. The Caspian Sea belongs to partially freezing seas. The ice conditions of the Caspian Sea are very com� plex and variable. Its northern part, which is shallow, freezes every year, in the central portion ice appears along the coast during severe winters only, while in the southern portion there is no ice at all (1). The ice sea� son in the North Caspian Sea lasts from November through March. Complete freezing and formation of fast ice usually take place north of the line from Chechen' Island to Kulaly Island (Fig. 1). During cold and extremely cold winters, fast ice can settle down to the 20 m isobath. Unlike the seas in the Arctic Region and the Far East, ice formation in the North Caspian Sea at a typ� ical water salinity of 2 to 11‰ takes place at the freez� ing point of -0.2 to -0.6°C (2). The density of the sea ice with no impurities is lower than in the Arctic Region and is ~920 kg/m (3). The ice temperature conditions strongly depend on the ambient air tem� perature. In typical conditions, the ice temperature is -1 to -2°C with homoeothermic distribution, while during long periods of -20°C frosts, the temperature