Seasonal water table dynamics and effects of a rising sea-level in the low-lying sandy coast of Cassino Beach, southern Brazil

The water table is one of the primary components of the coastal hydrological system, being regulated by the precipitation input and action of meteo-oceanographic forcing. The monitoring of the water table amplitude is crucial in a permeable substrate, such as low-lying sandy coasts. In scenarios of sea-level rise (SLR), these environments will be impacted by the direct action of marine flooding and, indirectly, by the rising of the groundwater. In 2018, seasonal monitoring of the water table was carried out at Cassino beach, located on the south Brazilian coast. The Cassino beach is an exposed coast dominated by waves, inserted in a low elevation barrier (Holocene lagoon-barrier system), with a humid subtropical climate and the action of extratropical cyclogenesis. The objectives were to determine the water table variation throughout the year and to identify the main forces that act in the high-frequency fluctuations. The low-cost electronic sensor systems Arduino-based were installed in tubular wells for monitoring. In parallel, indirect surveys were performed using the GPR method. The results demonstrated the significant contribution of the infiltration and recharge pulses in the shallow freshwater lens. The contributions of energy waves and winds in the S quadrant were identified, promoting the stacking of water on the shore. The results also suggest a relationship between the water table depth and the morphology of ridges and swales. The projections performed with the groundwater level from SLR scenarios on the IPCC showed a higher susceptibility of the Cassino beach to direct and indirect floods for the second half of this century. The intensification and increase of the frequency of the storms caused by climate change may result in a situation of urban infrastructure collapse, with the progressive occurrence of compound flooding (rainfall and storm surge).