Report No. : GEO Report No. 106

Report Title : Suspended Sediment in Hong Kong Waters (2000), 68 p.

Author : S. Parry

Abstract

Hong Kong is situated at the estuary of the third largest river in China and is affected by changing oceanic currents, tidal effects, wave action, extreme climatic conditions and significant anthropogenic influences. In comparison with the complicated hydrological conditions affecting Hong Kong's waters today, the conditions in the recent geological past were even more extreme. The coastline was well to the south of Hong Kong 9500 years ago and the majority of the present day marine sediments were deposited between 4500 and 1000 years ago. Whilst sedimentation in Hong Kong is now limited, this is due to the seabed being in equilibrium rather than a decrease in available sediment. The main source of sediment is the Pearl River, which, on average, discharges around 36 million tonnes of sediment each year. A further factor affecting suspended sediment concentrations is the resuspension of sediments due to wave action. On average 5 typhoons a year affect Hong Kong, with elevated suspended sediment concentrations in excess of 350 mg/l being recorded and the impact of the typhoon lasting over 60 hours. The northeast monsoon has a similar, albeit reduced, effect. In addition to the naturally occurring sediments, human activities such as trawling, sewage discharge, shipping, dredging and reclamation all generate suspended sediments. However, in comparison with natural events, these increases in suspended sediment are usually relatively limited in their extent and impact. Consequently, the use of monthly or bi-monthly data, which may be suitable for determining the trends of anthropogenic pollutants, is not sufficiently frequent to capture the highly variable naturally occurring suspended sediment concentrations where large scale effects occur over much shorter periods.

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