Analysis of Holocene sediment accumulation in the Guadiana estuary (southern Portugal) during sea-level rise since ca. 13 cal. ka BP was used to simulate the long-term morphological evolution of the lower Guadiana estuary and the associated intertidal zone for 21st-century predicted sea-level rises. Three sea-level rise scenarios given by the IPCC (2007) were used in the simulations of morphology using a large-scale behaviour-oriented modelling approach. Sedimentation rate scenarios were derived both from the Holocene evolution of the estuary and from a semi-empirical estimation of present-day sediment aggradation. Our results show that the net lateral expansion of the intertidal zone area would be about 3-5% of the present intertidal zone area for each 10 cm rise in sea level. Under constraints imposed by the lack of fluvial sediment supply, the lateral expansion of the landward boundary of the intertidal zone will occur mainly in the Portuguese margin of the Guadiana estuary, while submergence of the salt marshes will occur in the Spanish margin. Therefore the Spanish margin is highly vulnerable to both sea-level rise and lack of sediment supply.

a b s t r a c tThis paper focuses on simulations of the morphological evolution of an estuary during sedimentary infilling that accompanied Holocene sea-level rise. The simulations were conducted using the Estuarine Sedimentation Model, which uses a behaviour-oriented approach, supported by the chronostratigraphy of the estuary's sedimentary sequence. Behaviour curves were computed to represent the relationship between the estuarine channel depth below maximum high tide and the net accretion at a given location relative to the average sedimentation rate of the estuary during the Holocene. The model was validated by comparing the observed present-day bathymetry of the Guadiana River Estuary, southeastern Portugal, with the corresponding simulated bathymetries for nine control sections across the estuary. The best fit between simulated and actual sediment surface elevations was obtained along the cross-sections in the sheltered, low-energy environments of the estuary. The accuracy of the sedimentary stratigraphy of the best-fit model was further established using 16 radiocarbon ages obtained from five boreholes in the estuary. The present approach is particularly suitable for simulating long-term morphological evolution in sheltered estuarine environments where tidally driven vertical aggradation dominates at centennial to millennium timescales. However, the accuracy of simulated sediment surface elevations and consequently the robustness of behaviour-type models based on Geographical Information System platforms can be enhanced by incorporating (i) the impacts of nearshore hydrodynamic processes and episodic flood events in highly energetic channels, and (ii) the impacts of cross-currents in meandering channel sections.

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