Evolution of coastal transgressive aeolian sand sheets over 75 years (1948-2023) at Concheiros Barrier - Southern Brazilian coast

Abstract

Transgressive sand sheets (TSS) and transgressive dunefields (TDf) over time often present multiple phases of development and various morphologies controlled by biogeomorphological interactions related to abiotic processes, vegetation, and human interference. While dunefields have experienced increased vegetation cover over the last decades worldwide, a few places have shown increased dune mobility. In this study, a GIS analysis was conducted using aerial photographs and satellite imagery to investigate the evolution of a TDf, foredune collapse, new TSS formation, and vegetation cover on a Holocene coastal barrier in southern Brazil over the last 75 years (from 1948 to 2023). Wind speed (1994–2009), significant wave height (Hs) and peak wave period (Tp) (1940–2022) were analyzed to examine these potential drivers that could have caused geomorphological changes on the foredunes and dune system. Vegetation cover exhibited an increasing trend, and both an increase and decrease in TDf movement and TSS expansion were concurrent. Correlation analyses revealed a strong negative correlation between vegetation cover and TDf movement. Three transgressive sand sheet formation phases were identified between 1948 and 2023 (1964 to 1975,1996 to 2000, and from 2010 until 2023). Post-2003, a poorly vegetated nebkha-dominated foredune subsequently collapsed and was replaced by a TSS in less than a decade, being fully formed in 2010 and continuing to move inland at rapid rates. In the third phase, the highest total TSS expansion exceeded 700 m in 12 years (from 2010 to 2022). A small increase in wind velocity and subsequent decrease may possibly have contributed to the formation of the TSS second phase and the foredunes respectively. Other possible drivers for TSS formation include climatic modes, negative precipitation anomalies and groundwater level lowering, in addition to anthropogenic actions (especially water extraction) that might have caused a negative feedback in foredune sand-biding vegetation.