Keywords: 
• beach profile /
• size distribution /
• Gaolong Bay /
•  /

Abstract: The material basis of the coast includes water, rock, sediment and organisms. Biodetritus which come from organisms have certain effect on grain size of beach sediment. By adding excess acid and hydrogen peroxide in the beach surface sediment samples from the Gaolong Bay, Hainan Island, the difference of their grain size distribution with and without biodetritus was discussed, and the analysis was made with the height of the beach. It can be seen  from the results that (1) Biodetritus content on beach is mainly determined by marine dynamic, and the biodetritus which have coarse sizes and large mass is brought to the beach by waves during high tidal period.  (2) The distribution of biodetritus in the beach sediment is influenced by landform and main wave direction, biodetritus produced by beach breeding as well as coral reefs flat offcoast. (3) Biodetritus content in the beach sediment changes with the seasons. The biodetritus supplied by sea water would be reduced and further decomposed from winter to spring, which makes biodetritus content decrease in the north area, but the stable supply of coral debris and strong wave dynamics in spring increase the biodetritus content in the south area of the beach. (4) Coarse particles in surface beach sediment in Gaolong Bay contain more biodetritus than that fine particles contain. Grain size of biodetritus is mostly 1~2, namely coarse sand to gravel. The more biodetritus sediment contains the larger changer grain size parameters have.