Geometry of natural and engineered tidal inlets

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.