Tendencia evolutiva decadal e inundación por estresores naturales en el estuario del Maipo, Región de Valparaíso: incorporación del conocimiento (técnico/científico) en la gestión local del riesgo
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Date
2023
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Abstract
The estuary of the Maipo River corresponds to the river mouth of an Andean basin located in central Chile, which is permanently connected to the sea by river discharge. Its main characteristic is a coastal bar, parallel to the coast, located at the distal part of the estuary that serves as a natural protection of the system against wave action. In recent years, the frequency of extreme events such as river floods and extreme waves has increased. During the period from 2021 to 2023, the Maipo River estuary was affected by different extreme events. In January 2023, an episode of bar closure occurred that caused two mechanical openings, while in July 2023, a large part of it was destroyed due to the increase in the channel caused by intense mountain rains, attributed to El Niño effects. These episodes have caused flooding risk, overflows, and other conflicts, which have required the eneration of methodologies applicable to scientific monitoring, to guide decision-making on a scientific basis. To analyze the short- and medium-term spatiotemporal changes of the coastal bar system, a methodology based on the use of satellite images and aerial photographs was implemented. Five geomorphological maps were made for 1945, 1967, 1976, 2011, and 2023 to establish the evolutionary trend of the estuary. Likewise, a multitemporal analysis of the estuary bar under low tide conditions was carried out, based on PlanetScope satellite images. The supervised classification provided a zoning of dry sand surface, wet sand, dune vegetation, and water, between June 2021 and June 2023. In addition, morphometric parameters of the bar (surface, length, and width) were obtained. On the other hand, through semi-structured interviews, it was determined how the variations of the bar affect the population and how scientific knowledge is incorporated into the local management of the estuary’s flood risk. The geomorphological maps generated in this work, together with the interviews carried out with different local actors, allow to reconstruct the geomorphological history of the estuary over the last 70 years, through different milestones and timeframes. The evolutionary trend of the estuary shows that the bar presents episodes of breakage and destruction associated with river overflow and tsunami events, while the closure of the bar is linked to a significant decrease in river discharge, which is consistent with barrier-type estuaries, which have an intermittent bar at their distal part, influenced mainly by the action of the river and wave action. In the period studied, the average bar parameters are 860 m. long, 254 m. wide and 160,000 m² of surface area. Since October 2022, a considerable increase in the periodic measurements of the surface, length and width of the bar has been observed before its closure in January 2023. After the mechanical opening, the values tend to return to the initial ones, so the bar would be in a "recovery phase". The extreme event of the river flood in June 2023 destroyed the bar, which is evidenced by a significant decrease in its surface area (remaining in a third of the size it had before the flood) and length (from 870 m. to 330 m.), while its width does not show great variations (253 m. to 232 m.). On the other hand, minor variations associated with storms and the subsequent recovery of the bar were observed during the studied period. This information was contrasted with the time series of wave, tide, and flow parameters to complement the behaviour of the bar under natural stressors. The resulting evolutionary model establishes that variations in flow river affect its morphometric parameters, while extreme wave events generate forms of erosion and/or deposition on the bar, which is consistent with classical models of generation and destruction of spits. The flooding of the estuary caused by the increase in the river channel mainly affects the inhabitants of Llolleo, in the sector of the fishermen’s homes, and the informal homes located on the edge of the El Sauce stream, as well as the farmers who grow vegetables on the estuarine banks. Additionally, a lot of anthropogenic waste arrives from the river basin. On the other hand, the closure of the bar produces stagnation and an increase in the level of the estuarine lagoon affecting the inhabitants of Llolleo. The change in water salinity affects the fishermen as it harms fishing and agriculture upstream. On the other hand, although storms are extreme wave events, they are not considered a flood threat by the risk managers of the Maipo estuary, since they only affect the bar, and these events have not had direct interference with the population. Finally, the tsunami flooding affected the inhabitants of Llolleo, and in the 2010 event, the waves reached the Lo Gallardo sector. Therefore, a containment barrier was built on the beach and riverbank as a mitigation measure after the tsunami. Currently, there is a local flood risk management in the estuary, which operates from the municipality of San Antonio, in line with the provincial delegation, when the emergency escapes the scope of the commune. The municipality authorities are clear about what to do during an emergency caused by a tsunami, and by a river flood. In addition, they have the support of SENAPRED, which leads the aid and provides instructions for proper management. Additionally, there are efforts to create a technical coordination committee for the management of the Maipo estuary, which is composed of intersectoral organizations, and is led by the provincial delegation, and its technical counterparts are the Coastal Observatory and the Millennium Institute in Coastal Socioecology, SECOS. This committee has been convened at least twice this year, whose main objective is to develop an opening protocol to future bar closure and to continue integrally managing the estuary. However, it is important to note that this management is top-down, based on relevant institutional and technical actors, and now it does not involve the rest of civil society, so we cannot yet speak of a governance system. Knowledge is a key component in governance systems and risk management, as it is incorporated into decision-making. This is why the choice of scientific knowledge to be produced must meet the needs and concerns of all actors involved, which is why a coproduction approach to knowledge is recommended. The exchange of scientific knowledge reduces disputes among actors and increases transparency, so it is essential to generate instances that promote access to science for the community. These can be generated by formal knowledge-generating institutions, such as government organizations, through seminars, talks, and workshops. The information provided must be relevant, clear, and accurate. The incorporation of scientific knowledge is key to enhance disaster risk management. In the case of the Maipo estuary, the coordination committee is a good opportunity to design and promote guidelines for the generation of new scientific knowledge and its subsequent implementation in estuary management. However, it must establish mechanisms to include the demands of the rest of the community. future scientific research should include the effect of longshore drift on bar recovery and adjust the effect of wave erosion, including bathymetry.
Description
Tesis (Magíster en Geografía y Geomática)--Pontificia Universidad Católica de Chile, 2023
Tesis (M.Sc. Governance of Risk and Resources)--Universidad de Heidelberg, 2023
Tesis (M.Sc. Governance of Risk and Resources)--Universidad de Heidelberg, 2023