REGIONAL COASTLINE CHANGES AND FUTURE PREDICTED SCENARIO ON SOUTHERN BRAZIL

Joao Nicolodi,Elirio Toldo,Salette De Figueiredo,Lauro Calliari

doi:10.23854/07199562.2021571esp.nicolodi76

Abstract

The Rio Grande do Sul (RS) coastline consists of a 615 km long sandy barrier, with a NE-SW orientation. It is characterized by alternating slightly concave and convex segments extending from the rocky headlands at Torres up to the Chuí stream, at the Uruguayan border. In general, the genesis of this geographical area is related to changes in sea level in the Quaternary, when a complex Lagoon-barrier system was created. This study presents a compilation of decades of studies about short-term monitoring and long-term projections conducted by the two local main research centers on the subject. In this study, trends in shoreline behavior between 2002 to 2013 were analyzed. Results indicated that 43,5% of the coastline was stable, 41% was under moderate erosion, 9% in moderate progradation, 3% under pronounced progradation, and 3,5% under severe erosion. The specific causes of these variations are not yet fully understood. However, some factors are likely to be responsible for coastline behavior in the long and short term: geological inheritance which influenced coastline orientation, regional sediment budget, as well as shoreface gradient and bathymetry. The main erosive hotspots found along the RS coastline are influenced by wave energy focus due to refraction caused by the local bathymetric features and coastline orientation. The Cassino Beach sector, however, has a pronounced progradation which in the long term can be mainly related to its coastline orientation, and more recently due to jetties built at the Patos Lagoon inlet, which provided more sediment retention of the prevailing net longshore drift to the northeast. This study also offers a review about shoreline changes along the four existing inlets. Coastal vulnerability to sea-level rise (SLR) analyzed through coastline recession distances in long-term scenarios (2030 and 2100) indicated that the most vulnerable sectors are those located along coastal embayments, which coincide with low gradient shorefaces.

Highlights

Variations in the coastline positions are part of a natural cycle of continental adjustment to sea-level fluctuations, regardless of their causes
Erosion is typically seen as a problem, coastal erosion occurs unnoticed on coasts that are unoccupied by human communities, where the shoreline displacement cannot be seen by untrained eyes
Projected coastline positions from the random shoreface translation model (RanSTM) modeling in the present study indicate an average retreat of about 695 m (50%), resulting in coastline retrogradation rates of about 8.1 m.yr-1 by 2100

Summary

Introduction

Variations in the coastline positions are part of a natural cycle of continental adjustment to sea-level fluctuations, regardless of their causes. Erosion is typically seen as a problem, coastal erosion occurs unnoticed on coasts that are unoccupied by human communities, where the shoreline displacement cannot be seen by untrained eyes. This phenomenon is considered natural and has no adverse effects on social communities. According to Muehe (2006), fluctuations in coastline positions occur largely due to a lack of sediments, caused by a depletion of the sediment source, usually the continental shelf. Depletion occurs when sediments are carried to dune fields farther inland or through human action, the building of dams or other infrastructure that retains sediment and prevents its flow along the coast (Komar, 2018)

Methods

Results

Discussion

Conclusion