The Effects of Tidal and Seasonal Scale Flooding and Soil Texture on Surface Water‐Groundwater Interactions Along a Fluvio‐Deltaic Floodplain: The Meghna River, Bangladesh

Abstract River corridors in coastal and fluvio‐deltaic plains (FDPs) experience tidal fluctuations and seasonal flooding which result in variable sediment transport regimes and a mix of sand, silt, and clay deposits in the channel and floodplain. In FDPs, river fluctuations influence the extent and dynamics of hyporheic zone mixing and groundwater flow relative to the river. We analyzed the combined effects of external forcing by tidal and seasonal flooding and internal controls by sediment texture on groundwater‐surface water (GW‐SW) interactions in the Meghna River in Bangladesh. We conducted numerical simulations to assess the relative importance and interaction of these factors on GW‐SW mixing. We assessed the influence of complex river stage fluctuations, capturing both semi‐diurnal and seasonal‐scale variations in the mid dry period, early wet period, and the early dry period following the wet season. Sensitivity analyses were conducted to assess the influence of sand, silt, and clay percentage and their predicted van Genuchten soil‐water retention parameters on mixing dynamics. The simulations reveal that mixing zone dynamics are highly sensitive to seasonal periods and soil texture. We found that vertical infiltration of river water was limited to <1 m depth in most scenarios and most promoted in the early wet period. In contrast, lateral infiltration was favored during the early dry period. Higher sand content promoted greater lateral and vertical infiltration, while relatively higher silt and clay contents contributed to thicker capillary fringes and greater water retention closer to the surface. Generally, GW‐SW mixing appears to be limited in low‐energy and tidally influenced fluvio‐deltaic settings.