Morphodynamics of a wave dominated embayed beach on an irregular rocky coastline

Abstract Wave-dominated embayed beaches on irregular rocky coastlines are highly appreciated for their landscapes and tourism potential. Yet our understanding about the interaction of the oceanographic processes with the geological inheritance in this type of beaches still needs further investigation to better understand their natural balance. In this study, we apply the classical approach of morphodynamics to describe for the first time the variations in the Boiçucanga beach morphology, an embayed beach located in São Sebastião City on an irregular rocky coastline that is well known for its beautiful beaches embedded in the valleys of Serra do Mar. Field surveys were performed over 15 months, between April 2008 and September 2009, seeking to evaluate the interactions among the morphology, sediments and hydrodynamic characteristics at Boiçucanga. All data generated during the field surveys have supported the application of the following morphodynamic models: Classification of morphological beach stages [O]; Number of nearshore sandbars [B*]; Relative tide range [RTR]; Beach exposure on embayed coasts [Ro/a and ß]; and Embayment beach scaling parameter [d] to determine the geomorphological behavior of the beach on monthly and annual time scales. From these models, Boiçucanga is classified as a deep embayed beach exposed to high-energy waves, with a predominant reflective profile, which favors the development of beach cusps and topographically controlled rip currents. The combined results of the field observations and applied models allowed us to better understand the role of geological heritage in the two morphodynamic signatures found at the same beach arch. For future studies, we will focus on numerical models to increase the understanding of the hydrodynamic processes that govern the sediment transport in Boiçucanga.

摘要 不规则岩质海岸上受波浪主导的湾滩以其优美景观与旅游开发潜力广受赞誉。然而，当前学界对该类海岸上海洋动力过程与地质继承性的相互作用机制仍缺乏充分认知，亟需开展深入研究以明晰其自然平衡状态。本研究首次运用地貌动力学（morphodynamics）经典研究方法，对博伊库坎加海滩（Boiçucanga beach）的形态变化展开系统描述，该湾滩坐落于圣塞巴斯蒂安市（São Sebastião City）境内的不规则岩质海岸带，以嵌入马尔山脉（Serra do Mar）山谷间的绝美海滩景观闻名遐迩。研究团队于2008年4月至2009年9月期间开展了为期15个月的野外实地勘测，旨在解析博伊库坎加海滩的地貌形态、沉积物特征与水动力条件间的相互作用关系。野外勘测获取的全部数据支撑了下述地貌动力学模型的应用：海滩地貌阶段分类模型[O]、近岸沙坝（nearshore sandbars）数量模型[B*]、相对潮差（relative tide range, RTR）模型、湾岸海滩暴露度模型[Ro/a与ß]以及湾滩尺度参数模型[d]，以此开展该海滩月尺度与年尺度的地貌行为分析。基于上述模型分析结果，博伊库坎加海滩被归类为受高能波浪作用的深水湾滩，其地貌剖面以反射型为主，该特征有利于海滩尖棱（beach cusps）与受地形控制的裂流（rip current）的发育。野外观测与模型应用的综合分析结果，进一步阐明了地质遗产在同一海滩拱曲处发现的两种地貌动力特征中的调控作用。未来研究将聚焦于数值模拟模型，以深化对控制博伊库坎加海滩沉积物输运的水动力过程的认知。