Effects of entrance conditions on tidal hydrodynamics in idealized prismatic estuaries under sea level rise

Understanding tidal hydrodynamics response to sea level rise (SLR) in estuaries is essential to predict future potential issues such as shoreline erosion, more frequent and intense flooding, coastal wetland threatening, and their economic and ecological associated losses. By going beyond the classic static or “bathtub” approach, this study uses the RMA-2 modelling software to capture the full energy dynamic resulting from SLR and changes in entrance conditions. Indeed, entrance restrictions are seen as a key factor controlling tidal propagation within the entire system and only little knowledge exists on their effects on the flow under SLR. An innovative numerical approach using idealized prismatic estuaries is undertook to fill this gap. More than 150 simulation cases are performed, varying the channels geometry, restrictions dimensions and tidal forcing to capture most of existing prismatic estuarine dynamics. The resulting tidal flow is described using tidal ranges (TR), but also flow velocity, tidal asymmetry, wave types, tidal prism, etc. to find out potential interesting trends. In short channels, tidal reflection is found to be the dominant process influencing tidal propagation while in long channels the flow is mostly controlled by bottom friction. TR are amplified up to 10% per metre SLR, exacerbating the future flooding problematics linked to a higher mean sea level. However, strong TR attenuation is observed when entrance restrictions are implemented, leading to potential insights on SLR effects mitigation using narrower inlets. SLR acts on the flow velocities distribution and on the tidal asymmetry by increasing medium velocities and decreasing flood dominance. Both will impact sediment transport, leading to stronger coastal erosion. This increases therefore the pressure on sensitive ecosystem already struggling to stay in pace with SLR. SLR can bring the channels closer or further from their natural period of sway. This change in tidal resonance can lead to modifications of the flow parameters behaviour. When channels become more resonant with SLR, amplification of the above phenomena are usually seen, while when they are brought away from resonance, attenuation or even reverse effects can be observed. All these issues are not captured by the bathtub approach and arise from changes in tidal dynamics. Furthermore, instead of focusing on specific estuaries as most of the previous studies, this project is using an idealized approach allowing a global evaluation of the future tidal hydrodynamics. Understanding tidal response to sea level rise in estuaries is crucial to develop efficient coastal management plans able to mitigate this threat on estuarine ecosystems and populated areas. In particular, over 130 port cities having a population higher than 1 million people are concerned by SLR issues in estuaries.