人工岛影响下养护海滩演变特征研究——以海口湾为例

Study on the evolution of nourished beaches under the influence of artificial islands:taking Haikou Bay as an example

海滩养护作为一种广泛应用的软式海岸防护手段,可以有效缓解人工岛建设引起的海滩侵蚀问题。本文以海口湾人工岛波影区两侧养护海滩为例,基于补沙后两月一次的剖面地形和岸线监测数据,利用经验正交函数、数字岸线分析系统等方法,分析了海滩蚀淤变化特征及其控制因素。经验正交函数分析结果表明,前两个特征值方差总占比超过90%,可以反应海滩的主要时空变化。第一空间模态的极值区域位于滩肩外缘线至平均海平面之间,且波影区两侧养护海滩与波影区天然海滩演变模式存在显著差异,时间模态显示养护海滩在补沙后4个月变化最为显著,之后剖面变化减弱;第二空间模态显示台风对海滩剖面造成了一定影响,导致波影区海滩滩肩上的低洼位置由于泥沙淤积被填平。研究发现,海口湾养护海滩在人工岛影响下的演变特征可分为两个阶段,第一阶段是人工补沙的海滩剖面向平衡状态不断调整,平衡时间为4个月,该阶段以横向输沙为主;第二阶段是受人工岛的影响,形成由人工岛东西两侧向波影区运移的沿岸输沙,该阶段以纵向输沙为主。当前海口湾养护海滩岸线仍未达到平衡状态,未来人工岛后方波影区两侧海滩仍将不断蚀退,而波影区海滩将向海淤积约690 m,直至达到平衡稳定状态。本研究可为相似类型的海滩演变分析及保护修复提供一定的参考。

Beach nourishment,as a widely-adopted soft solution to coastal erosion,can effectively alleviate the beach erosion induced by the construction of artificial islands.This paper takes the nourished beaches on both sides of the wave shadow zone of an artificial island in Haikou Bay as an example.Based on the bimonthly surveys of beach profiles and shorelines since nourishment,the characteristics of beach erosion and accretion and the associated driving factors were analyzed by using empirical orthogonal function(EOF)and digital shoreline analysis system methods.The EOF results showed that the first two eigenvalues accounted for more than 90%of the total variance,which could reflect the main spatio-temporal changes of the beaches.The area with conspicuous changes,corresponding to the first spatial mode,was located between the outer edge of the beach berm to the mean sea level,in which there were significant differences between the nourished beaches on both sides of the wave shadow zone and the natural beach in the wave shadow zone.The temporal mode showed that the most significant changes of the nourished beaches occurred in the initial four months after nourishment,followed by mild profile changes.The second spatial mode demonstrated the impact of typhoons on the beaches,resulting in the accumulation and leveling of some low-lying sites on the beach berms in the wave shadow zone.This study found that the evolution of nourished beaches under the influence of an artificial island in Haikou Bay could be divided into two stages.The first stage was the adaptation period,during which the artificially designed beach profile adjusted to the equilibrium state for adapting to the local hydrodynamics.The equilibrium time of nourished profiles was approximately 4 months,during which beach changes were dominated by cross-shore sediment transport.The second stage was dominated by the longshore sediment transport induced by the artificial island,resulting in sediment transport from the eastern and western sides of the artificial island to the wave shadow zone.At that time,the nourished beach shoreline in Haikou Bay had not reached the equilibrium state.The beaches on both sides of the wave shadow zone would continue to retreat in the future,while the shoreline in the wave shadow zone would advance into the sea for about 690 m until it reaches the equilibrium.This study would be helpful for the analysis of the beach evolution and beach restoration of similar cases.