Abstract Tidal inlets are crucial landforms that control the exchange of water and sediment between the open sea and back-barrier basins. Despite the well-established relationship between tidal prism and inlet cross section area, some aspects of tidal inlet geometry remain unconstrained. What processes set their width to depth ratio (or aspect ratio)? What controls the presence of a single-thread versus a compound channel within the same inlet? Do these relationships change with engineering activities? These questions were investigated by compiling and analyzing a database with 226 inlets worldwide. We found that the aspect ratio has a weak dependency on tidal range and wave height. The aspect ratio varies by at least an order of magnitude, and this variability is likely associated with the variability in the widening process, e.g., the bank erodibility. Engineered inlets tend to have smaller widths and aspect ratios than natural inlets. Within natural inlets, multiple channels tend to form when the inlet width exceeds approximately 1 km. We developed an idealized barrier-inlet system using the detailed 2D hydro-morphodynamic model Delft3D to investigate the long-term morphological evolution of inlets. Two parameters greatly affect the aspect ratio: the transverse bed gradient factor for sediment transport (αbn) and the dry cell erosion factor (θsd), which allows for erosion of dry cells adjacent to a wet cell. Removing the dry cell erosion effectively “armors” the side of the inlets and thus produces inlets with a small aspect ratio, which resembles the geometry of engineered inlets. From model sensitivity analysis, we found that αbn = 5 – 10 and θsd = 0.3 – 1.0 provides inlet configurations that best agree with observations. Despite the dry cell erosion and the transverse transport parameters being a simplified representation of bank erosion processes, their calibration allows to reproduce realistic inlet aspect ratio.

中文翻译：

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自然和工程潮汐入口的几何形状

摘要 潮汐入口是控制公海和背障盆地之间水和沉积物交换的重要地貌。尽管潮汐棱柱和入口横截面面积之间的关系已经确立，但潮汐入口几何形状的某些方面仍然不受限制。哪些过程设置了它们的宽深比（或纵横比）？是什么控制同一入口内单螺纹与复合通道的存在？这些关系会随着工程活动而改变吗？通过编译和分析具有全球 226 个入口的数据库来调查这些问题。我们发现纵横比对潮差和波高的依赖性很弱。纵横比至少有一个数量级的变化，这种变化很可能与加宽过程中的变化有关，例如，银行的可侵蚀性。工程入口往往比自然入口具有更小的宽度和纵横比。在天然入口内，当入口宽度超过约 1 公里时，往往会形成多个通道。我们使用详细的二维流体形态动力学模型 Delft3D 开发了一个理想化的屏障入口系统，以研究入口的长期形态演变。两个参数极大地影响纵横比：沉积物运输的横向床梯度因子 (αbn) 和干细胞侵蚀因子 (θsd)，允许与湿细胞相邻的干细胞侵蚀。去除干细胞侵蚀有效地“装甲”了入口的侧面，从而产生了具有小纵横比的入口，类似于工程入口的几何形状。从模型敏感性分析中，我们发现 αbn = 5 – 10 和 θsd = 0.3 – 1。0 提供最符合观察结果的入口配置。尽管干细胞侵蚀和横向传输参数是堤岸侵蚀过程的简化表示，但它们的校准允许重现真实的入口纵横比。