Estuaries provide a wide range of environmental, cultural, social, and economic services. However, sea level rise (SLR) is increasingly threatening these services in estuaries due to their low-lying topography and proximity to the open ocean. As such, the sustainable management of estuaries requires an evidence-based understanding of how different estuaries may respond to SLR over time and space. Assessing SLR impacts in estuaries can either be undertaken on an individual site basis or via broader approaches that may be relevant to many estuaries. This thesis utilises the latter approach and, to this aim, a large ensemble of idealised estuarine hydrodynamic simulations were developed and tested. The results are then analysed, (i) to present a systematic hydrodynamic understanding of different estuary types in present-day and future conditions, (ii) to detail the influence of SLR on tidal energy dynamics, (iii) to establish a framework for assessing SLR impacts on estuaries worldwide, and (iv) to guide decision-makers in establishing holistic, evidence-based management plans for estuaries. Initially, over 2000 idealised estuary models were simulated to determine how SLR influences the tidal dynamics of various systems with different boundary conditions (e.g., different estuary length, depth, convergence, entrance condition, roughness, river discharge). These results indicate that SLR can lead to tidal range amplification in some estuaries, although entrance restriction effects may offset this phenomenon. Further, SLR is likely to alter the distributions and magnitudes of tidal currents, tidal prism, tidal asymmetry, tidal energy, and the location of tidal energy harvesting sites. To link the idealised findings with real-world estuaries, 26 real-world estuaries were simulated and analysed under present-day and future sea level conditions. This information was used to identify sites that are most vulnerable to SLR-induced tidal variations. These results were consistent with the idealised findings and highlighted their transferability to certain estuaries worldwide without an existing, detailed hydrodynamic model. Long, weakly convergent estuaries with higher friction, river inflows and shallower water depths will likely experience significant changes in tidal dynamics due to SLR. The numerical approach and learnings presented in this thesis could be used to assess and predict cumulative SLR impacts on estuaries globally.