A comparison of the post-storm recovery of two sandy beaches on Hong Kong Island, southern China

摘要

Typhoon Hagupit was one of six tropical cyclones that affected Hong Kong in 2008. When the typhoon made landfall, the combined effect of the waves, storm surge and seasonal high tides resulted in a maximum sea level of 3.53 m at the nearest tide gauge to the study site, the highest since 1962. The event caused considerable damage to coastal infrastructure and large-scale erosion on many sandy beaches in Hong Kong. To study the degree of erosion and recovery of Hong Kong beaches, we surveyed two sandy beaches: Tai Long Wan (TLW), a relatively high-energy exposed beach system, and To Tei Wan (TTW), a relatively low-energy Sheltered beach system. We examined the two beaches for changes in their beach profiles and sediment properties. TLW recovered quickly, with significant post-storm accretion of fine- to medium-grained sand. Within four months after the typhoon, TLW regained >700 m~3 sand, forming a new, ～1.0 m high, berm, with the beach face prograding approximately 5.0 m seaward. In contrast, within the same four months after the typhoon, TTW showed little evidence of sand accretion or recovery, with only minor changes in the beach profile. In fact, instead of post-storm sand accretion, the landward migration of the high tide mark shown in the pre- and post-storm satellite images suggested that the post-storm beach elevation at TTW decreased. The results, which reflect the different background energy regimes of each beach, indicate that different coastal management strategies should be applied. In the case of relatively high-energy beaches of Hong Kong, we suggest not interfering with their natural state. In contrast, in the case of relatively low-energy beaches of Hong Kong, which appear unlikely to recover rapidly from erosion during storms, we suggest further investigation before applying any long- to medium-term management strategies.