Beach Face and Berm Morphodynamics Fronting a Coastal Lagoon

This study documents two different modes of berm development: (1) vertical growth at spring tides or following significant beach cut due to substantial swash overtopping, and (2) horizontal progradation at neap tides through the formation of a proto-berm located lower and further seaward of the principal berm. Concurrent high-frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes. In mode 1 sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent. The net transport of this sediment is landward only, resulting in accretion onto the upper beach face and over the berm crest. The final outcome is a steepening of the beach face gradient, a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm. In mode 2 sediment is eroded from the lower two-thirds of the active swash zone during the rising tide and is transported both landward and seaward. On the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide. The net result is relatively slow steepening of the beach face, a change of the profile shape towards convex, and horizontal progradation through the formation of a neap berm. The primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face. This depends on the current phase of the spring-neap tide cycle, the wave runup height (and indirectly offshore wave conditions) and the height of the pre-existing berm. A conceptual model for berm morphodynamics is presented, based on sediment transport shape functions measured during the two modes of berm growth.本研究记录了两种不同的滩肩（berm）发育模式：(1) 大潮期或因显著冲流越浪导致海滩发生严重冲刷后的垂向增长；(2) 小潮期通过在主滩肩下方更向海侧形成初始滩肩（proto-berm）实现的侧向进积。同步开展的床面高程与伴随的波浪爬高分布高频测量，揭示了这两种滩肩发育模式的细节。在模式1中，泥沙从冲流作用盛行的内碎波带与低滩冲流带被侵蚀。该泥沙的净输运仅为向陆方向，最终淤积于上部滩面及滩肩脊之上。其最终结果为滩面坡度变陡、剖面形态向凹形转变，以及滩肩快速垂向与侧向增长。在模式2中，涨潮期间泥沙从活动冲流带的下三分之二区域被侵蚀，并同时向陆与向海方向输运。落潮期间，泥沙从内碎波带被侵蚀并向陆输运，以回填涨潮期间被侵蚀的区域。其最终结果为滩面坡度相对平缓地变陡、剖面形态向凸形转变，以及通过形成小潮滩肩实现侧向进积。决定滩肩发育模式的核心因素，是滩面泥沙堆积阶段是否存在冲流越浪。这一条件取决于当前所处的大小潮周期阶段、波浪爬高（间接关联离岸波浪条件）以及先期存在的滩肩高程。本研究基于两种滩肩发育模式下测得的泥沙输运形态函数，提出了滩肩地貌动力学的概念模型。