半遮蔽型海滩剖面长期时空演化过程的经验正交函数分析

EMPIRICAL ORTHOGONAL FUNCTION ANALYSIS OF THE LONG-TERM SPATIOTEMPORAL EVOLUTION OF A SEMI-SHELTERED BEACH

半遮蔽型海滩由于一侧海岬的差异作用而影响海滩的演变过程。本文以青岛石老人海水浴场海滩为例,基于对3条典型剖面连续11年现场监测的高程数据,利用经验正交函数(empirical orthogonal function,EOF)分析方法将该海滩各剖面高程数据组成的原始距平矩阵依次分解为相应的空间和时间特征函数,分析了石老人海滩的形态演化特征、蚀淤趋势及驱动因素,并采用方差总占比超过77%的前3个经验正交函数来反映此类海滩的基本演化情况,分别为:表征长时间尺度砂质海滩滩面最主要蚀淤变化趋势的第一模态、表征季节性蚀淤变化及其引起的滩肩剖面形成增长或削弱消亡的第二模态和表征人类活动、偶发因素或海岸地形等引起的具有随机性的不规律蚀淤变化的第三模态。结果表明:11年来石老人海滩经历了三个阶段,第一阶段为平稳期(2009年1月—2011年7月),第二阶段为突变期(2011年7月—2012年9月),该阶段之后至今,是第三阶段的平稳期(2012年9月—2019年12月)。海滩剖面在自然和人类活动影响下的高程变化范围在0.1—2.0 m范围内。波浪是影响该海滩形态演化的最主要水动力因素,岬角对半遮蔽型海滩的差异庇护作用导致不同位置的剖面演化存在差异。高频率台风的影响、海平面阶段性升高并长期保持较高水平导致了远离岬角的开放岸段存在明显阶段性调整,剖面滩肩宽度减少约30 m,岬湾内部剖面则相对稳定,目前海滩剖面形态在第二阶段突变期后达到了新的稳定状态。

The evolution of a semi-sheltered beach is affected by the headland on one side.The morphological evolution characteristics,erosion and deposition trend,and driving factors of Shilaoren Beach in Qingdao,Shandong,was analyzed as an example based on elevation data monitored for 11 consecutive years at 3 typical sections.By using the Empirical Orthogonal Function(EOF)analysis method,the original anomaly matrix of the elevation data was decomposed into corresponding spatial and temporal characteristic functions,and the morphological evolution,erosion and deposition trends,and the driving factors were examined.More than 77%of the total variance of first three empirical orthogonal function was used to reflect the basic evolution of the beach.The first mode represented the most important erosion and deposition trend of sandy beach surface on a long time scale.The second mode indicated the seasonal erosion and deposition change and the growth or decay of the beach shoulder formation caused by seasonal deposition and erosion.The third mode reflected the irregular erosion and deposition change caused by human activities,accidental factors,or coastal topography.Results show that in the past 11 years,Shilaoren Beach has experienced three stages:the first stage is the stable period(January 2009 to July 2011),the second stage is the sudden change period(July 2011 to September 2012),and afterward was the third stage,a new stable period(September 2012 to December 2019).The average elevation variation range of the beach in the past 11 years was 0.1—2.0 m,which was affected by natural and human activities.Wave was the most important hydrodynamic factor of the morphologic evolution of the beach.Different sheltering effect of headland on semi-sheltered beach led to evolutional difference at different sections under the impact of typhoon and the resultant temporal sea level rise that stayed quite a long time.During typhoon events,the open shore section away from the headland was significantly adjusted and the width of section shoulder was reduced by about 30 m,while the headland inside the bay remained relatively stable.At present,the beach morphology is in a new stable state after the sudden change period.