MORPHODYNAMIC CHANGES OF BAY BARRIER BEACHES ON THE SOUTH COASTS OF LANTAU ISLAND, HONG KONG
Wang Wei, Zhang Dian
1997, 52 (s1):
Three bay barrier beaches in Pui O, Mui Wo and Cheung Sha bays on the south coast of Lantau Island, Hong Kong were selected as examples for the study of morphodynamic changes of bay barrier beaches on the Hong Kong coasts. The selected beaches were studied by means of geomorphology and sedimentology including the analysis of two years longshore and cross-shore variations of beach morphology in association with the changes of grain size and cusp spacing along the beaches in the same period.The study shows that firstly, according to the directions of arriving refracted waves, the sediment process on the bay barrier beaches is different in the deep and shallow indented bays. In deep indented bays,the sand movements on the bay barrier beaches are mainly in an on-shore/offshore direction with the longshore movement near the stream outlets. Both lateralsediment movements and onshore/off shore ones are of similar importance for the beach process in a shallow bay.Secondly, under the condition of a tidal range larger than wave heights on the Hong Kong coasts, the profiles vary along each beach in the bays due to a longshore variation in wave heights, offshore width and sediment grain size. In places where the wave heights are small, sediments are not well sorted and the sediment supply is sufficient to build up a wide and shal-low offshore, the beach profiles are characterized by a high tide beach, which has a relatively steep beach face, termimated rather abruptly at its base by a wide low-tide terace, a low tide beach. The high tide beach slope becomes gentle connecting at its foot to the low tide beach without the abrupt change, while the wave heights increase reducing the difference between the tidal range and wave heights. If the sediment supply is not sufficient supply is not suffi-cient , the beach slope will become steep, even the difference between the tidal range and wave height is small.Finally, under the local geological, monsoon climatic and wave conditions, the barrier beaches in Hong Kong have a very regular yearly change in sand movements which can be summarized as a tendency for late summer cut followed by a stable period in winter and early summer fill. However, the barrier beaches in Hong Kong may also have irregular changes in response to abrupt changes in coastal environment, such as big storm waves attacks or human impacts on the beaches, because of the beaches developing based mainly on the materials from the inland strems, which are limited resulting in the bay barriers and beaches on the Hong Kong coasts having formed in small size.
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