Scenarios modelling on coastal barriers of the Rio Grande do Sul - Brazil in response to sea-level changes Lucas T. Lima¹, Cristina Bernardes¹ ¹Geoscience Department, University of Aveiro, Portugal. lucasterres@ua.pt, cbernardes@ua.pt. Abstract The coastal plain of Rio Grande do Sul, southern of Brazil, was formed during the Quaternary period (that start about 2600 years before present), and comprise four evolution phases of sand barrier lagoon systems, associated to sea level changes. The barriers show regressive, transgressive and gradational characteristics in response to regional sediment supply, coastal morphology and oceanographic processes; presently the barrier systems show active dunefields. The objective of the present study is to simulate future barrier changes in Rio Grande do Sul coastal plain in response to sea level rise (2050 and 2100) including de coastal lagoons behavior. To achieve this goal a geomorphic model of sand barriers (GEOMBEST - Plus) and geoprocessing tools are used. The impacts of morphological changes on human activities and land uses are also analyzed. Introduction Sand barriers evolution is driven by the oscillations of mean sea level, which, through sediment transport by waves and currents, cause the landward or the seaward translation of the barrier in response, respectively, to an increase or a drop of mean sea level. However, the processes responsible for barrier evolution span several time scales and depends of the physical processes, interacting with the volume of erodible sediments. The GEOMBEST – Plus, Geomorphic Model of Barrier, Estuarine, and Shoreface Translation (Walters et al. 2015) is a morphological-behavior Matlab model that simulates the evolution of coastal morphology and stratigraphic framework. To perform the analysis along the coastal plain of Rio Grande do Sul twelve profiles was extracted in the different types (regressive, transgressive and gradational) and geomorphological context of the barriers (barrier width, lagoon extension and land use) (Figs 1 and 2). The model integrated other data, such as accretion and erosion rates, sea level change, sediment discharge into the lagoon, transport of sediments (overwash) rate from the shoreface to the back-barrier area, during storms, and erosion and resuspension depth in the lagoons. The analysis of impact of morphological changes on human activities and land uses in future decades is being perform by predictions models and geoprocessing technics, which simulate and estimate the potential land use scenarios to 2050 and 2100. Results and conclusions Model simulations indicate that the sand barrier translation was not uniform but occurred as a function of bottom topography (shelf slope) and sediment availability; in general, the foredune and dunefields systems are, nowadays, very active. In the past, the barriers have been formed during periods of lower sea level but they now show, in some cases, significant landward translation. The most critical situations are observed near urban areas and in the narrow barrier stretch. However, in the average volume of sediments moved in back barrier area by overwash may be not sufficient to maintain a uniform migration. In another situations, the flat shelf terrace seems to favor the landward migration of the barrier, followed by sediment deposition of the bottom. The impacts of sea level rise in future decades point to important alterations on urban areas and economic activities as well as on coastal ecosystems (Fig. 3). References Walters, D., Moore, L., Duran, O., Fagherazzi, S., Mariotti, G., (2015). “GEOMBEST-Plus, version 1.0”, Virginia Institute of Marine Science, USA, DOI: 10.5281/zenodo.16687. Acknowledgment Thanks are due for the financial support to CESAM (UID/AMB/50017 - POCI-01-0145-FEDER-007638), to FCT/MEC through national funds, and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020. To National Council of Technological and Scientific Development – Brazil (Process number 234815/2014-0). 1 2 3 4 5 6 7 8 9 10 11 12 Fig 1 / Location of study area. Green lines show the location of extracted profiles in different types of coastal barriers. Fig. 2 / Profiles 1, 3 and 5 created in GEOMBEST-Plus. Fig. 3 / Examples of critical scenarios of losses of land use with high sea level rise (2m).