Mg. Villagran-Valenzuela, Mauricio

The coastal evolution of the microtidal Tubul-Raqui wetland in south-central Chile (36° S), which historically has been affected by large earthquakes and tsunamis, particularly the 1960 (Mw = 9.5) and 2010 (Mw = 8.8) subduction earthquakes and their associated tsunamis, is analyzed. Historical aerial photographs and topographic and bathymetric surveys from the 1961–2017 period, as well as salinity, sediment, and flora data obtained following the 2010 earthquake were used for comparison with data from prior to the event. A steady state of the shoreline was established, with an average erosion rate of −0.016 m/year in the 1961–2017 period. However, erosion predominated in the period between these two large earthquakes (1961–2009), with an average rate of −0.386 m/year. The wetland dried up, partially recovered saline intrusion a year later, and recovered the salinity conditions it had before the earthquake two years later. The postearthquake effects on the floristic composition were not significant, with the species Spartina densiflora, which presented a high tolerance to these types of changes, predominating. Moreover, 75 percent of the taxa in pre- and postearthquake conditions coincided, with the halophyte species Spartina densiflora, Sarcocornia fructicosa, and Cotula coronopifolia predominating, while the best-conserved community was Spartina-Sarcocornia association located in the saltmarsh. Seven years after the earthquake, the shoreline presented an accretion rate of 2.935 m/year; if the current tectonic conditions prevail, an erosive trend can be expected in the coming decades. The morphological variability and the changes associated with the shoreline in this wetland are strongly controlled by tectonic factors. Criteria aimed at integrated coastal management to promote its occupancy and use in accordance with its evolutionary dynamics are proposed.

Coastal evolution is an important research topic worldwide and has become increasingly relevant due to growing anthropogenic pressure on the coast and a climate change scenario (Masselink et al., 2016). The Gulf of Arauco covers an area of roughly 40,000 km2 and has a sandy-rocky coastline located in a very seismic environment. The area has suffered several major earthquakes during the last century (Valdivia 1960, Maule 2010) and seismic displacement has widely affected the coastline (Béjar-Pizarro et al., 2010). Despite these findings, the morphological evolution of sandy coastlines is mainly caused by wave-driven littoral processes. In this paper, using numerical modeling (Delft3D), we aim to characterize the longshore sediment transport (LST) direction at several spots (7 beaches) spread along the coastline of the Gulf of Arauco. Wave patterns were identified at each study site, revealing the importance of Santa Maria Island, located at the entrance to the gulf, despite the approach direction of deep water waves. The island acts as a moderator of wave patterns, softening the highly energetic swell that comes from the Antarctic Ocean and sorting the wave propagation inside the gulf. Moreover, LST patterns were characterized at each site for dominant wave conditions (SW swell and NW winter storms) and it was possible to explain how each condition has a different response at each spot, even under similar co-seismic displacements. Adaptation capabilities differ from site to site, suggesting a dynamic equilibrium of beaches in the area.