ENG-2006 — Page 360

296 The Environment

During the last two million years, the Quaternary Period, several major glaciations affected the polar regions. These caused successive lowering of world sea level of up to 120 metres, leaving the site of present day Hong Kong as much as 130 kilometres from the coast. During the interglacial periods, such as the present time, global sea level returned to its present level and marine sediments were deposited.

Information about the geology of Hong Kong is presented in a series of 15 1:20 000-scale geological maps and six accompanying geological memoirs that were produced by the Hong Kong Geological Survey, a part of the Geotechnical Engineering Office of the Civil Engineering and Development Department. The Hong Kong Geological Survey has also published two summary memoirs and a set of 1:100 000-scale geological and thematic maps that synthesise current interpretations of the geology of Hong Kong.

Hydrography and Oceanography

Hong Kong's waters are characterised by the interaction of oceanic and estuarine water masses, which vary in relative effects throughout the year. The variable freshwater discharge from the Pearl River has a marked influence on Hong Kong

waters.

During the summer, an oceanic flow from the southwest to the northeast brings. the warm, high-salinity water of the Hainan Current into Hong Kong waters. This interacts with fresh water from the Pearl River and divides Hong Kong into three distinct zones. In the west, where the fresh water influence is greatest, the environment is estuarine and the water is brackish. In the east, the water is mainly oceanic with relatively minor dilution from rainfall and runoff from streams. The limits. of the central transitional zone vary depending upon the relative influence of Pearl River water and marine currents.

During the winter dry season, the Kuroshio oceanic current brings warm water of high salinity from the Pacific through the Luzon Strait. The freshwater discharge of the Pearl River is much lower than in the summer and salinity is more uniform across Hong Kong. The coastal Taiwan current also brings cold water from the northeast down the South China coast, affecting inshore waters.

The normal tidal range in Hong Kong waters is between one and two metres, depending on the relative influence of the moon and sun. The tidal pattern is complex due to the relative effects of the diurnal and semi-diurnal components. The basic pattern during flood tides is for oceanic water to flow north into Mirs Bay and west through Lei Yue Mun into Victoria Harbour and through Kap Shui Mun and the Ma Wan Channel. This flow is reversed during the ebb tide. Maximum tidal currents generally range from 0.5 to 1.5 knots, peaking at up to five knots in narrow channels.

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