Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial III, 1445-1448
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Morphological impacts of Christina storm on the beaches of the central western Portuguese coast Impactos morfológicos da tempestade Christina nas praias do litoral centro-oeste de Portugal Z. Diogo3, A. Bastos3, C. Lira1,2,3*, R. Taborda1,2, C. Freire de Andrade 2,3, T. M. Silveira 2,3, M. Ribeiro1,2,4, A. N. Silva2,3, M. M. Carapuço1, C. A. Pinto5, M. C. Freitas2,3 Artigo Curto Short Article © 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: Christina storm hit the Portuguese coast on January 2014 and was characterized by strong winds, high and long sea waves and intense rainfall. This work aims to characterize and evaluate the effects of the Christina storm on the morphology of cross-shore profiles describing the beaches of central western coast of Portugal. The storm lashed the Portuguese coast resulting generally in strong beach erosion and damage to coastal structures and facilities. However, results show that the erosion pattern and damages were not consistent across the study area. In about half of the surveyed profiles, the sand volume in the emerged section of the beach attained minimum values recorded in the 2011/2014 period. This behavior is consistent with the intensity of the event, considering the exceptional wave period. In contrast, some profiles indicate accretion that is interpreted in relation to local morphological and sediment supply constraints. Keywords: Sea storms, Coastal erosion, Beach morphology. Resumo: A tempestade Christina atingiu a costa Portuguesa em janeiro de 2014 e foi caracterizada por ventos fortes, ondas com altura e período elevados e chuva intensa e persistente. O objetivo principal deste estudo é caracterizar e avaliar os efeitos provocados por este evento na morfologia das praias do litoral centro-oeste de Portugal, avaliada através de perfis transversais de praia. De forma geral a tempestade causou danos significativos nesta faixa costeira que se traduziram em acentuada erosão e estragos em infraestruturas de proteção costeira e equipamentos de praia. No entanto, os resultados obtidos refletem a inexistência de um padrão de erosão consistente ao longo da área de estudo. Em cerca de metade dos perfis estudados, a retenção volumétrica na praia emersa atingiu os valores mínimos observados no período de 2011/2014 em concordância com a intensidade deste evento, considerando os valores extremos do período. Porém, outros apresentaram acreção, devido a constrangimentos locais de natureza morfológica e de alimentação sedimentar. Palavras-chave: Tempestades marítimas, Erosão costeira, Morfologia da praia.
1
Instituto Dom Luiz. Departamento de Geologia, Faculdade de Ciências da Universidade de Lisboa. 3 Centro de Geologia da Universidade de Lisboa. 4 Instituto Hidrográfico. 5 Agência Portuguesa do Ambiente, I.P. * Corresponding author / Autor correspondente:
[email protected] 2
1. Introduction The 2013/2014 winter was characterized by strong winds and very high waves, produced by several extratropical cyclones developed in the North Atlantic (IPMA, 2014). The related wave fields reached the western Portuguese coast resulting in damages on coastal structures, overtopping of coastal dunes and defense structures and flooding of coastal areas. Intense coastal erosion in several beaches along the coast was also identified. The most energetic event, the Christina storm – designated by the Free University Berlin (2014) – impacted Western Europe including the central western Portuguese coast on early January. The main goal of the present work is to characterize and evaluate the effects of this storm event on cross-shore profiles describing the morphology of beaches and indicate damages on coastal structures of the central western Portuguese coast. According to the Leixões buoy data (IH, 2014), storm conditions (herein defined when offshore significant wave height (Hs) exceeds 5m) began at 14:30pm on 03/01/2014 and ended at 11:50pm on 07/01/2014, with a total duration of 3.90 days. Offshore waves presented mean Hs of 5.8m, Hs maximum of 9.0m and maximum wave height of 13.5m. The average maximum period (Tmax) was 18.6s with a maximum of 27.3s. The mean wave direction was 295o. According to data of the Sines buoy (IH, 2014), storm conditions began at 00:40pm on 04/01/2014 and ended at 17:20pm on 07/01/2014. The duration was 3.70 days, with mean Hs of 5.3m and Hs maximum of 9.5m. The maximum wave height recorded was 14.9m, the average maximum period was 19.7s, with a maximum of 28.1s. The mean wave direction was 298o. In mainland Portugal, Christina storm reached its peak on 06/01/2014 at 02:50pm corresponding to the transition between neap tide and spring tide. 2. Methods The study area extends from the Liz river mouth (Marinha Grande) to Lagoa de Albufeira (Sesimbra), a 260km long coastal stretch, that includes beaches with different
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backshore contents (resistant and soft cliffs, dunes and artificial hard structures), contrasting exposure to waves (sheltered and exposed) and different geomorphological settings (embayed, swash-aligned and linear, drift-aligned) (Fig. 1).
These data were compared with equivalent information systematically acquired since March 2010 under the framework of the project CISML (see acknowledgments). Table 1. List of surveyed beaches, code, beach profiles and survey date. Conventions: Standard - increased volume; (Brackets) - decreased volume; Italic – no volumetric change; Bold – extreme values; Underlined code – structure damage. Beach profiles are coded 1…n starting at the north (or east) section of the surveyed beach. Tabela 1. Listas das praias monitorizadas, código, perfis de praia e data de monitorização. Convenções: Padrão: aumento de volume; (Parênteses): diminuição de volume; Itálico: sem alteração volumétrica; Negrito: valores extremos; Código sublinhado: danos em estruturas. Perfis de praias codificados 1…n distribuem-se de norte (este) para sul (oeste).
3. Results Fig. 1. Location map. Identification of the 12 selected beaches and main features: back beach boundary, damage in coastal structures and beach volume changes in at least one of the beach profiles. Fig. 1. Mapa de localização. Identificação das 12 praias seleccionadas e características principais: limite terreste da alta-praia, danos em estruturas costeiras e variações volumétricas verificadas em pelo menos um perfil.
In order to evaluate the impact of Christina storm, an aftermath topographic survey was conducted between 07/01/2014 and 20/01/2014 in 12 sandy beaches (Table 1) designed to acquire the following information: − 2D-cross-shore beach profiles; − dune, cliff or structure toe lines; − 3D-beach topography (at Baleal-Peniche, Coxos and Costa da Caparica). The field survey was conducted using DGPS (Differential Global Positioning System) equipment operated using RTK (Real-Time Kinematic) corrections, with a precision better than 0.05m. In the specific case of Coxos beach, the topographic survey was done using terrestrial LiDAR (Light Detection And Ranging). Beach volumetric retention was computed above mean sea level (MSL) and considering the width between the lowest surveyed location and the landward boundary of the beach (dune, cliff, and structure).
The analysis of the post-storm surveyed profiles includes systematic comparison with the June 2013 situation, taken as representative of the fair-weather morphodynamic stage at each sampling point. In addition, the post-storm profiles were compared with all previous surveys in order to evaluate if this extreme oceanographic forcing corresponded with extreme morphological responses. The Pedras Negras profile PPN1 (Fig. 2) indicates a significant lowering of the beach surface, severe erosion of the backshore and pronounced retreat of the dune toe (about 7m), corresponding to the minimum volume recorded throughout the monitoring period. The beach stairs access was destroyed. A dune scarp with variable height was observed reaching over 2m northward of profile PPN1 (Fig. 3A). In contrast, profile PPN2, located ca. 500m southward, showed a moderate increase in sand retention, essentially affecting the foreshore, and slight erosion of the backshore not affecting the dune toe. At Paredes da Vitória beach, both the northern PPV1 and PPV2 profiles showed decrease in sand volume corresponding to general lowering of the beach surface, while PPV3 (south sector) showed no significant volume change, though sand has been transferred from the backshore
Morphological impacts of Christina storm
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to the foreshore. The dune toe retreated (maximum 6.7m, just northward of PPV3) creating a dune scarp (over 2m). The storm also destroyed some of the beach accesses.
Fig. 4. Representation of beach profile PNZ2 (Nazaré) and seasonal variation of volumetric retention for the period between March 2011 and June 2013 and post-storm January 2014 beach profile.
Fig. 2. Representation of beach profile PPN1 (Pedras Negras) and seasonal variation of volumetric retention for the period between March 2011 and June 2013 and post-storm January 2014 beach profile. Fig. 2. Representação gráfica do perfil de praia PPN1 (Pedras Negras) e da variação sazonal do volume de praia para o período entre março de 2011 e junho de 2013 e perfil pós-temporal de janeiro 2014.
Fig. 3. Storm effects in coastal structures, facilities and beach morphology. A – Pedras Negras beach and B - Coxos beach. Fig. 3. Efeitos da tempestade em estruturas costeiras, apoios de praia e morfologia de praias. A - Pedras Negras e B – Coxos.
The Nazaré embayed beach, showed minor morphological changes either with a small accretion at PNZ2 (Fig. 4) or no change at PNZ1 and slight erosion at PNZ3. There were no signs of damage in coastal structures and the berm was only partly overwashed with addition of a sand wedge at the berm crest. The coastal stretch Lagoa de Óbidos - Baleal experienced a slight increase in sand volume at PLOB1 and PLOB2, and decrease in sand volume at PLOB3 and PLOB4, the latter recording the minimum retention throughout the survey period, despite the survey being conducted 13 days after the storm event. The shape of all profiles, which reduce to a beach face, recorded no significant changes in morphology.
Fig. 4. Representação gráfica do perfil de praia PNZ2 (Nazaré) e da variação sazonal do volume de praia para o período entre março de 2011 e junho de 2013 e perfil pós-temporal de janeiro 2014.
The embayed stretch Baleal – Peniche, showed generalized beach accretion (~200 000m3) along with retreat and escarpment of the foredune (8.8m near PBP2), the beach profiles remaining invariant in shape. Most of the sand retention affected the central and western sectors (PBP3 to PBP5, with PBP4 reaching the maximum value recorded during the monitoring period). In the east sector (PBP1, PBP2), minor erosion spots were detected with profile PBP1 reaching a minimum in volume. In addition, beach accesses and facilities were damaged. In the PSC3 Santa Cruz beach profile no significant change in sand volume was recorded although overwash of the seawall occurred, damaging beach accesses. The Coxos beach, a deeply incised pocket beach, experienced a remarkable decrease in volume (which exceeded half of the modal retention) reaching the minimum value recorded during the monitoring period (Fig. 3B). The storm eroded a berm and an embryo dune observed in all previous surveys and damaged beach facilities and the access road. All Tamariz beach profiles (PTM1 to PTM4) experienced reduction in sand volume, recording minimum volumetric retention throughout the monitoring period. The berm was obliterated in all profiles and coastal structures and facilities were damaged. South of the Tagus estuary, the Costa da Caparica ribbon includes a hardened (southern) and a semi natural (northern) section. The storm effects on the northern section (São João beach, Fig. 1) were very significant. Extensive overtopping of both the dune ridge and seawall caused partial destruction of coastal structures and beach facilities, and scarping of the upwind foredune slope with maximum dune toe retreat of 15m. The beach experienced asymmetric behavior, the southward profiles showing in general lowering of the surface, in cases down to minimum volumetric retention (PCC3 to PCC6, Fig. 5). In contrast at its northernmost tip, controlled by the Cova do Vapor groin, the beach remained stable (PCC2) or accreted (PCC1) and this is interpreted as related to local sand retention promoted by the groin effect.
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Fig. 5. Representation of beach profile PCC6 (Costa da Caparica – São João) and seasonal variation of volumetric retention for the period between March 2011 and June 2013 and post-storm January 2014 beach profile. Fig. 5. Representação gráfica do perfil de praia PCC6 (Costa da Caparica – São João) e da variação sazonal do volume de praia para o período entre março de 2011 e junho de 2013 e perfil pós-temporal de janeiro 2014.
At Costa da Caparica southern sector, all beaches within the groin field significantly reduced in volume, reaching minimum volumetric retention in PCC12 and PCC13. The lowering of the topography related with the total erosion of the subaerial beach, precluded surveying three profiles as the low tide waterline leaned against the seawall. The beach profiles located southward of the groin field showed minor volumetric changes (PCC15 to PCC17); once more this is possibly in relation with longshore drift retention by the groin field. Similarly to the northern sector, the seawall was extensively overwashed, flooding the inland region and damaging facilities sitting on the seawall. Further south, where the coast naturally developed an arcuate shape, four profiles were surveyed at the beaches of Rainha (PRA1), Fonte da Telha (PFT1, PFT2) and Lagoa de Albufeira (PLA1). All profiles show beach erosion and sand depletion reaching minimum retention throughout the monitoring period. Field evidences indicate that the upwind slope of the foredune has been nibbled by wave swash, with litter drift lines being found at the, or slightly landward of, the dune crest indicating that no full overtopping occurred. Minor scarping of the foredune was only locally observed and dune toe retreat was not significant. In the particular case of Fonte da Telha wave run-up scarcely failed to reach the road running parallel to the shore (at approximately 9m above MSL). The storm also damaged the beach bar at Rainha beach.
beaches only at Nazaré the swash failed to entirely sweep the whole of the beach profile and affect the back beach boundary (dune and coastal structure). Despite this common trend, the nature and intensity of the morphological responses of the sand beaches was highly variable in space. For example, the incised pocket beach of Coxos, deeply indented in a cliff coast, nearly vanished, whereas the profiles located at the downdrift (north) end of São João beach accreted. All beaches within the groin field of Costa da Caparica share similarities with Coxos by the existence of a rigid reflective backshore feature and closed lateral boundaries, limiting longshore supply and giving an explanation for the similarity observed in the morphological response. The Baleal-Peniche embayment shares, to some extent, the previous morphological context but the landward boundary consists of a dune ridge, which responded to wave forcing by retreating and supplying a sufficient amount of sand to allow the beach to accrete. In sections where the morphological changes may be dominated by local changes in longshore drift intensity such as Pedras Negras and Paredes da Vitória beaches, the volumetric expression of the cross-shore sediment transfers may be masked and complicated by longshore processes, thus concealing the expected morphological response to storm events. Field data indicate that this storm was exceptional in its ability to promote unusual run-up levels with frequent dune overwash. Previous storms with similar or slightly higher wave height (e.g. January 2013, Fig. 5) produced less intense morphological responses of the foreshore and backshore zones of the beach. Thus, in addition to storm duration and wave height and direction, results show that wave period and site specific morphological and geological constraints are relevant variables in modulating the pattern of beach response to storm events. Acknowledgments This work was supported by the project CISML – Criação e implementação de um sistema de monitorização no litoral abrangido pela área de jurisdição da Administração da Região Hidrográfica do Tejo, awarded by the Agência Portuguesa do Ambiente, I.P./Administração da Região Hidrográfica do Tejo and funded by QREN - POVT (Operação POVT-120233-FCOES-000034).
4. Discussion and Conclusion
References
Christina storm had significant impacts on the beach morphology of the central western Portuguese coast. There was a clear trend for sand loss in the subaerial section of most surveyed beaches which developed a more featureless (dissipative) profile. The high energy of breakers translated in exceptional run-up levels, numerous overwash events and damages. These effects are related not only to wave height but also to the uncommon low frequency of the storm waves. Among the surveyed
Free University Berlin. 2014. Forecast map of the pressure at sea level http://www.met.fu-berlin.de/de/wetter/maps/Prognose_20140103.gif (accessed in 7/1/2014).
IH - Instituto Hidrográfico, 2014. http://www.hidrografico.pt/boiasondografo.php (accessed in 3/1/2014, 6/1/2014). IPMA - Instituto Português do Mar e da Atmosfera, 2014. Detailed information about the storm Christina -
https://www.ipma.pt/pt/media/noticias/newsdetail.html?f=/pt/ media/ noticias/textos/temporal-atlantico-norte-3-6-jan2014.html (accessed in 15/1/2014).
