--> The Effect of Tides on Deltaic Morphology and Stratigraphy in River-Dominated Conditions

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The Effect of Tides on Deltaic Morphology and Stratigraphy in River-Dominated Conditions

Abstract

Deltas are dynamic and sensitive systems that undergo changes of morphology, channel network, and stratigraphic architecture in response to variations in coastal processes, e.g., waves and tides. These changes need to be properly understood in order to make reliable subsurface predictions. Numerical modeling has been extensively used to study delta evolution in response to a single dominant coastal forcing, but rarely to examine the sensitivity of delta to mixed energy forcing. Therefore predictions on reservoir modeling based on conventional models could be highly misleading when used in mixed-energy delta systems. This study uses Delft3D to investigate the influence of tidal currents on river-dominated deltas in terms of deltaic stratigraphic architecture and sediment partitioning. We conducted 24 modeling runs with a range of tidal amplitude and initial sediment composition of the substrate. The modeling results show that deltas formed under river-dominated condition without tidal currents (control runs) develop delta foresets with concave profiles, whereas with increasing tidal amplitude the delta foreset profiles become more convex or have compound geometries. In the control runs, distributary channels avulse and bifurcate frequently, resulting in the complete reworking of deltaic lobes. As a result, coarse sediment is stored in the proximal delta plain. In contrast, the presence of strong tidal currents creates deeper and stable distributary channels. These channels do not rework previously deposited deltaic lobes, but act as an efficient conduit for ebb-current sediment to bypass across the delta. Furthermore, the analysis of sediment fluxes across the delta shows that ebb tidal currents increase suspended and bedload sediment fluxes by at least 3 times compared to cases without tidal currents. The enhanced sediment transport leads to deposits with higher net-to-gross ratios than in their river-dominated counterparts. This study highlights how tidal currents, even under river-dominated conditions, have strong effects on delta surface morphology, stratigraphic architecture, and sediment partitioning. Therefore it is critical when trying to model paralic reservoirs to consider the changes that varying tidal influence may have in reservoir geometry, net-to-gross distribution, and flow path barriers.