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Projeto de investigação
ASSINATURAS DEPOSICIONAIS DE BLOCOS A MEGACLASTOS ORIGINADAS POR TSUNAMIS OU TEMPESTADES NO LITORAL DE PORTUGAL
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Boulder deposits related to extreme marine events in the western coast of Portugal
Publication . Oliveira, Maria Alexandra; Andrade, César Freire de, 1955-; Neves, Ramiro Joaquim Jesus, 1955-
This work describes a complex boulder accumulation found in the Portuguese western coastline, N of Ericeira, containing over 1600 boulders sitting on a structural platform and low cliffs at 2-13 m above mean sea level (amsl). The main objective of this work consisted in the identification (storm vs tsunami), age estimation and the computation of recurrence intervals of extreme events responsible for the transport and deposition of this boulder accumulation. Several proxies were used to interpret the origin of the deposit under analysis, such as: morphological characteristics of the boulder accumulation and of individual boulder positions and arrangement; monitoring boulder movement by storms, numerical modelling of storm wave propagation and with statistical analysis of extreme values applied to the wave regime; application of numerical approximations describing boulder movement; age estimation based on aerial photographs, lichenometry, erosion rates and optically stimulated luminesce (OSL) of finer sediments of marine origin. Distribution of source layer, mass and directional properties indicate short-lived boulder transport following a flow directed from the W-NW, compatible with the modern-day wave regime rather than the major tsunamigenic source, located SW of Portugal. Statistical modelling of extreme values applied to the wave regime and comparison with storm wave parameters which generated observed boulder movement, indicate 2-5-years recurrence interval for the occurrence of storms capable to generate boulder transport and deposition in the W-NW facing segments, and a higher than 20 years in the WSW-facing segments of the study area. Moreover, the application of numerical solutions to the boulder dataset indicate wave periods and significant wave heights compatible with a storm origin and with the wave regime affecting the western Portuguese coastline. A strong geomorphological signature is observed in the morphological characteristics of the boulder accumulations. This is reflected in local changes of directional properties of boulders which mimic the coastline configuration, with emphasis in areas of the structural platforms where NW-SE aligned indentations exist. These indentations function as natural channels and generate a funnelling effect increasing the potential to detach and transport boulder-size particles. Age estimation results based on the comparison of aerial photographs and lichenometry indicate that boulder transport frequently occurs, thus implying a storm origin to at least part of the deposit. Additionally, it indicates an extremely recent age for the boulder accumulation from around the 14th-15th centuries. Moreover, age estimation results obtained for lichenometry show that this boulder accumulation mainly comprises boulders recently deposited (past ~140 years), which is confirmed with results from OSL age estimation of marine sand patches found within a boulder cluster, of 230-290 years. The fact that older boulders are scarce and that boulder transport and deposition frequently occurs suggests that: the boulder deposit under analysis is highly dynamic and that particles typically present a residence period lower than 200 years; the AD 1755 tsunami, which reached a minimum height of 9 m amsl, partly obliterated a previously existing boulder accumulation, having mostly generated an essentially erosive signature. The presence of older and sometimes heavier boulders located higher up and further inland in WSW facing segments of the coastline indicates boulder deposition on these locations during higher storminess periods, probably associated with periods of negative NAO index. This work contributes to the discussion regarding sedimentological/erosional signatures of boulder-size accumulations associated with storms and tsunamis in rocky coastlines and to a better understanding of the geomorphological controls and physical processes occurring within this context. Another relevant contribution was the development of a lichen growth model for the species Opegrapha durieui Mont. (Roux and Egea, 1992), which allows estimating the age of limestone surface exposure of up to ~500 years.
Estimating the age and mechanism of boulder transport related with extreme waves using lichenometry
Publication . Oliveira, Maria Alexandra; Llop, Esteve; Andrade, C.; Branquinho, Cristina; Goble, Ronald; Queiroz, Sónia; Freitas, Maria da Conceição; Pinho, Pedro
Tsunamis and storms cause considerable coastal flooding, numerous fatalities, destruction of structures, and erosion. The characterization of energy and frequency associated with each wave contribute to the risk assessment in coastal regions. Coastal boulder deposits represent a physical proof of extreme inundation and allow us to study the effects of marine floods further back in time than instrumental and historical records. Age estimation of these deposits is challenging due to lack of materials (such as sand, shells, corals, or organic matter) that retain information about the passage of time. Lichenometry, a simple age estimation method, which is cost-effective, quick to apply, and non-destructive, is here proposed as a solution. A lichen growth model for a calcium-tolerant lichen species was developed and used to estimate the age of a boulder deposit related to extreme marine inundation(s) in Portugal. Estimated ages indicate several very recent events (<700 years) for most of the boulders’ stabilization and agree with results obtained with optically stimulated luminescence of marine sands found beneath boulders. Frequent and recent boulder transport implies a storm-origin for this deposit. These conclusions contrast with other works describing identical deposits that are attributed to paleotsunamis. This study presents a methodology using lichenometry as a successful alternative for age estimation in rocky coastal settings. These results offer an alternative explanation for coastal boulder deposits found on the west coast of Portugal.
Morphological controls and statistical modelling of boulder transport by extreme storms
Publication . Oliveira, Maria Alexandra; Scotto, Manuel; Barbosa, Susana; Andrade, C.; Freitas, Maria Da Conceição
The study of coastal boulder accumulations generated by extreme marine events, and of the energy and frequency involved in boulder transport, is of paramount importance in understanding the risk associated with extreme marine inundations. One of the frequently asked questions is whether the deposits are storm or tsunami-related, both events being characterized by different return periods. Boulder transport by storms was monitored on the west coast of Portugal. Significant changes were detected in boulders' position as a result of extreme inundation by the 2013/2014 winter storms. Results presented in this work indicate that the wave power associated with the “Christina” and “Nadja” storms occur once every three years. However, this interval is not supported by field observations of boulder displacement, which suggests that wave power over-predicts boulder movement in the study area. Furthermore, wave parameters from the “Christina” and “Nadja” storms were very similar, but have generated different impacts in the boulder accumulation described herein. Differences include the magnitude and direction of boulder movement, and are most likely associated with distinct tidal levels during the events. Higher tide levels generated an increase in the sea surface level and thus in the reach of waves, which generated displacement of larger boulders and consequent cross-shore contribution in boulder transport. Regardless, the combination of monitoring campaigns, wave data, and statistical modelling of extreme values indicate that boulder transport by storms is more frequent than initially expected. Based on recorded boulder movements, we present a conceptual model for boulder ridge formation and development and identify significant control of incoming flow by local geomorphological/topographical features. Storm events, not less frequent tsunamis, are identified as the events responsible for modulating this rocky coastline. These results question a direct attribution of coastal boulder deposits to tsunamis in coastal regions with a high risk of tsunami inundation.
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Fundação para a Ciência e a Tecnologia
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SFRH/BD/66017/2009