Initial morphological responses of coastal beaches to a mega offshore artificial island

Mega offshore artificial islands (MOAIs) can lead to large-scale and significant coastal responses, which have been poorly documented. In this study, coastal changes of an 8 km-long beach along Haikou Bay, China, were analysed to advance the understanding of the potential morphological responses to a MOAI with a diameter of 2.3 km. Field surveys of beach profiles and shorelines in different years were used to reveal the initial coastal changes after the construction of the MOAI. The results showed that the beach in the leeside of the MOAI accreted seaward, whereas the adjacent beaches retreated, which was revealed to be closely related to the alteration in longshore sediment transport patterns induced by the presence of the MOAI. As predicted by empirical equations, a large salient (hundreds of metres of amplitude) rather than a tombolo would be formed on the leeside of the MOAI if there is sufficient sediment supply. From the initial morphological observation, it was indicated that the final total beach length affected by this MOAI will be larger than 6200 m. However, based on predictive equations derived from small-scale offshore obstacles (SCOOs), this value was less than 6200 m. It is likely that the geometric properties induced by MOAIs may not be estimated by these empirical equations derived from SCOOs. Moreover, a great difference in salient equilibrium time was highlighted between them. The timescale of salient equilibrium for this MOAI was likely on an order of hundred years of magnitude, while that for SCOOs were generally less than 10 years, which was related to the water depth of the obstacle position. This study provides new insights into the differences in coastal change between the MOAI and SCOOs and will help local governments adopt appropriate coastal sediment management strategies from a regional scale and a longer timescale in the face of increasing human activities.