Water level time series and sediment grain size model

The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 µm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions.障壁岛系统(barrier island systems)间的落潮三角洲(ebb-tidal delta)环境以复杂地貌为典型特征，发育落潮主导与涨潮主导水道、浅滩(shoals)及冲流坝(swash bars)，上述地貌要素将落潮三角洲平台与邻近岛屿相连。此类地貌特征对应特征性的表层沉积物粒度分布，且始终处于对主流水动力条件的持续适应过程中。本研究选取北海南部东弗里西亚群岛(East Frisian barrier islands)中朗厄奥格(Langeoog)与施皮克奥格(Spiekeroog)之间的混合能量潮汐通道(mixed-energy tidal inlet)奥茨默湾(Otzumer Balje)作为典型研究区域，旨在识别控制地貌与沉积学过程的关键水动力驱动因子。本研究采用基于过程的数值模型(process-based numerical model)，对比高能量波浪主导的风暴条件与中期潮汐主导的晴好天气条件对潮汐通道地貌及沉积学特征的影响；同时采用包含150至450微米五个粒级的多组分方法(multi-fractional approach)，实现对应表层沉积物粒度分布的模拟。研究识别了不同条件下的净沉积物输运通量：风暴条件下，较浅落潮三角洲浅滩上的推移质(bed load)输运整体指向岸向，而细颗粒悬浮沉积物(suspended sediment)则通过波浪驱动的环流绕过潮汐通道；晴好天气条件下，沉积物输运主要集中于通道喉道与边缘涨潮水道。本研究阐明，混合能量潮汐通道观测到的沉积物粒度分布与地貌响应，是低频波浪主导的风暴条件与中期潮汐主导的晴好天气条件共同作用的结果。