The study focuses on the impact of human interventions on the natural behaviour of estuaries in response to sea-level rise. In particular it focuses on the affect of managed realignment on the critical sea level rise (SLRcrit) for the Poole Harbour. Numerical model ASMITA (Aggregated Scale Morphological Interaction between a Tidal inlet and the Adjacent coast, Stive and Wang, 1996) is used to study the response of the system to the external forcing (both natural and anthropogenic). ASMITA model schematises the tidal-inlet system and characterises each model element by a single variable volume. Volumetric data of the Poole harbour inlet elements obtained through the digitisation of the historical admiralty charts was used to calibrate the model. The model assumes that the each inlet elements tends towards an equilibrium volume which can be defined using the empirical equations. These equilibrium equations consist of equilibrium parameters that depend on the regional properties like hydrodynamics and topography of the Poole harbour. Once the model is calibrated, simulations were performed with various sea level rise scenarios (historic and future SLR) to observe the natural behaviour of the Poole harbour. Managed realignment is being introduced in the model and a range of values of critical sea-level rise was obtained and compared with the predicted sea-level rise rate given in the literature for the next century. Results suggest that the “hold the line” management scenario (current situation) would not result in the complete loss of inter-tidal flats and the coastal flooding is unlikely to occur over the next century, as a result of critical sea-level rise values exceeds the predicted rate. However research suggests, introducing realignment (coastline retreat management scenario) reduces SLRcrit values for the estuary; thus making the estuary more sensitive to the predicted future SLR rates.