Research On The Man-made Island’s Influence On Beach Erosion Under Typhoon

Coastal sediment movement will inevitably influence on beach erosion under wave action,so there is great practical significance to study on erosion-deposition variation for coastal engineering construction.Both storm surge induced by typhoon and man-made island have influence on beach erosion.This paper,taking beach erosion in Haikou Bay under Typhoon Kalmaegi as an example.With theoretical mechanism,field investigation,example comparison and numerical simulation,the influence of man-made island on beach erosion under typhoon is discussed.The research work and results are as follows.1.By the third-generation wave model SWAN combining wind-generated waves,refraction,diffraction,reflection and bottom friction,with four different scales from deep sea to Haikou Bay.Typhoon Kalmaegi wave field in Haikou Bay,wave transforming of pre-and post man-made island construction are simulation calculation.The validation results show that the calculation results of wave height and wave period are in good agreement with the measured values.The wave height is over 10 year frequency.After man-made island construction,the wave height behind the island is significantly decreased 17%,and 4%on other positions.2.The following characteristics and basic cause can be carried out through analyzing the field investigation on beach erosion in Haikou Bay under Typhoon Kalmaegi.Sea area of each section is changed from deposition in 2012-2013 to erosion in 2013-2014 after man-made island construction.The biggest difference is rough sea action induced by strong typhoon in 2014.Field photos and section comparison showed that beach erosion in Haikou Bay is“storm profile”and erosion in revetment anterior.With record high tidal level,wave height is over 10 year frequency,the maximum wave height and the maximum tide level appeared simultaneously during typhoon period.The rapid deformation in beach was caused by both rough sea action and tidal level inflation.3.Beach evolution is calculated by means of the longshore sediment transport and the modelling of coastline deformation.During the rough sea caused by Typhoon Kalmaegi,longshore sediment discharge is less than 12,000 m~3,owing to man-made island,the longshore sediment discharge behind it was significantly decreased.After the island construction,decreased erosion amount by 5,184 m~3,coastline recession amount by 0.6 m;and increased erosion amount by 400-2,000 m~3,coastline recession amount in 0.2 m.4.This paper reports that the qualitative analysis of sediment lateral motion and man-made island’s influence on beach erosion in Haikou Bay.Beach erosion in Haikou Bay is“storm profile”and erosion in revetment anterior,which was caused by sediment lateral motion with on-and-off shore direction,related to incident wave,the bigger the wave height,the larger the change of sea section.After man-made island construction,wave height behind the island is more significantly decreased than others.The“storm profile”beach erosion is reduced by the island.5.With theoretical mechanism,field investigation,example comparison and numerical simulation,the influence degree of man-made island on beach erosion under typhoon is comprehensive analysis.The significant erosion in Haikou Bay was mainly caused by both rough sea action and tidal level inflation,with on-and-off shore direction.And the man-made island has some influence on the beach erosion.Although complex beach evolution under typhoon wave action,it is quite difficult to calculate the sediment transport with on-and-off shore direction,and a further study on the calculation of“storm profile”.But the conclusion of this paper is reasonable and reliable,which can provide scientific basis for beach repair and island construction.