2019 CSDMS meeting-105

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Cross-Sectional Geometry of Natural and Engineered Tidal Inlets

Shamim Murshid, Louisiana State University Baton Rouge Louisiana, United States. smursh1@lsu.edu
Giulio Mariotti, Louisiana State University Baton Rouge Maine, United States. gmariotti@lsu.edu

Tidal inlets are crucial land-forms that control the exchange of water and sediment between the open sea and the back-barrier basin. Despite the well accepted relationship between tidal prism and inlet cross section area, some questions about the geometry of tidal inlets remain open. What processes set their width to depth ratio (or aspect ratio)? What control the presence of a single-threaded versus a compound channel within the same inlet? Do these relationships change with human activities? These questions were investigated by creating and analyzing a database of inlet geometry and numerical modeling. We found that inlet’s aspect ratio has a weak dependence on tidal range, wave height. Despite the scatter, we developed relationships between inlet width and aspect ratio for three separate types of inlets, i.e. engineered inlet, natural inlet with single channel and natural inlet with multiple channels. From the observation, we found that multiple channels tend to form when the width of natural inlet exceeds 1 km. To investigate the long-term morphological evolution of inlets, we developed an idealized barrier-inlet system in Delft3D (a 2D hydromorphodynamic model). We found that two parameters affect the aspect ratio the most: the transverse bed gradient factor for sediment transport (αbn) and the global/maximum dry cell erosion factor (θsd), which controls erosion of dry cells adjacent to a wet cell. Including dry cell erosion is necessary to widen an initially narrow inlet; removing the dry cell erosion effectively “armors” the side of the inlets and thus produces inlets with small aspect ratio. From model sensitivity analysis, we found that αbn = 10 and θsd = 0.8 provides inlet configurations that best match with observations. When inlets were able to widen, we found that aspect ratio has a weak dependency on the initial width. Despite the dry cell erosion and the transverse parameters being a simplified representation of bank erosion processes, their calibration allows to reproduce realistic inlet geometries.