Journal of Coastal Research
SI 56
1199 - 1203
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Integrating Geological Knowledge in Planning Methods for Small Islands Coastal Plans R. Coutinho†, J. Pacheco†, N. Wallenstein†, A. Pimentel†, R. Marques† and R. Silva† †Centro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, Portugal
[email protected]
ABSTRACT COUTINHO, R., PACHECO, J., WALLENSTEIN, N., PIMENTEL, A., MARQUES R. and SILVA, R., 2009. Integrating Geological Knowledge in Planning Methods for Small Islands Coastal Plans. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 1199 – 1203. Lisbon, Portugal, ISSN 0749-0258. The Azores archipelago lies in the junction of the North-American, African and Eurasian lithospheric plates. The geodynamical setting is therefore complex and, consequently this area is affected by intense seismic and volcanic activity. This study addressed the coastal geology of four Azorean islands: Santa Maria, Graciosa, Flores and Corvo. The coastlines studied exhibited a diversity of littoral types, with distinct lithologies and morphologies. Coastlines are socio-economically very important in any archipelago. Therefore, it is imperative that land-use management methodologies are implemented in these vital areas. This need is further justified by the natural seismic and volcanic vulnerabilities of these islands, which makes hazard assessments absolutely fundamental. It is this knowledge that will later inform our policies and methodologies of risk mitigation, sustainable development and land management planning. This study analyses two aspects of this problematic: geological characterization and hazard assessment. Special attention must be paid to the geomorphology, volcanic features, tectonic structures, lithology, hydrology, coastal stability and erosion patterns of these areas. Risk assessments of seismic, landslide and tsunami events were undertaken and used as a reference in the formulation of the Coastal Zone Management Master Plans for the above-mentioned islands. After the Hazard Assessment was concluded, the Geology team along with the Planning team, built up a Management Map, a Constraints Map and the corresponding legal rules that ought to be applied in these domains. ADITIONAL INDEX WORDS: Coastal zones, Geological hazards, Management Master Plans, Azores.
INTRODUCTION The Azores archipelago is located on the North Atlantic, between 36º 55’ and 39º 44’ N latitude, and 24º 46’ to 31º 17’ W longitude. It consists of nine volcanic islands dispersed along more than 600 km in a NW-SE trend. The Azores lies in the junction of the North American, Eurasian and African lithospheric plates (Figure 1). Due to this peculiar location some of the islands are affected by seismic and volcanic activity. Since its settlement, in mid 15th century, the Azores have been affected by more than 20 major earthquakes that caused more than 5000 deaths and severe destruction. There are also records of at least 27 historical volcanic eruptions, either inland or at sea. The last volcanic events occurred in 1957-1958 at Capelinhos, Faial Island, and in 1998-2001 offshore Terceira Island. Annual rainfall of 1000- to 5000- mm is distributed over the year, with a dryer period from June to September. Landslides caused by heavy rainfalls are not uncommon. One of the more recent events affected all of the eastern part of S. Miguel Island, after a succession of rainy days that culminated with a record rainfall of more than 200 mm in 24 hours, causing 27 fatalities. In other situations the natural weathering of the rocks combined with some events like heavy rainfalls, strong winds or earthquakes may potentiate critical situations.
The islands have witnessed considerable socioeconomic development. This led to increasing pressure on coastal areas and surrounding territories. In order to execute an adequate land-use management plan, within the present legal framework, the government of The Azores issued the Regional Legislative Decree Nº 18/98/A, dated November 9th that sets the principles to be observed for Coastal Zone Management Master Plans (CZMP) for the Azores Autonomous Region. Its main recommendations are: protection of the biophysical integrity of the space; evaluation of the coastal resources and preservation of landscape and environmental resources. The Regional guidelines concerning coastal interventions are defined in the Resolution Nº138/2000, dated August 17th, which sets as a priority the protection and safety of human lives and belongings. The present work results from the investigative efforts of its authors in the geological characterization and evaluation of natural hazards. The authors of this study are members of the team that carried out the CZMP of several islands of the Azores.
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Figure 1. Geographical location of the Azores Archipelago and main tectonic features: MAR- Mid Atlantic Ridge, TR- Terceira Rift, EAFZ- East Azores Fractures Zone (bathymetry from LOURENÇO et al., 1998).
MATERIALS AND METHODS This study was carried out on the islands of Corvo and Flores, the westernmost islands of the archipelago, Graciosa, located on the central group, and S. Maria, on the easternmost end of the archipelago (see Figure 1). Geological studies of coastal regions are a fundamental aspect to be taken into consideration when a diagnosis of existing problems is needed as well as a tool for decision making in terms of a sustainable management. Therefore, two main aspects were taken into account: geological characterization and hazard assessment. The requirements of the work implied the study of coastal areas of the islands (at a 1:15000 scale), limited by the bathymetric line of 30 m and a 500 m wide inland strip. A comprehensive compilation of bibliographic data concerning the study areas was elaborated prior to the field trips. Special attention was given to historical information. Fieldwork was complemented with boat trips around the islands as well as helicopter or plane observation flights over the islands. Corvo is the smallest island of the Archipelago, with an area of 17 km2 and 720 m of maximum altitude. Flores has an area of 143 km2 and an elevation of 915 m. Graciosa, has an area of 61.6 km2 and a maximum elevation of 401 m. Santa Maria has an area o 97.2 km2 and an elevation of 590 m. For each island, a geomorphological map was prepared in which all of the relevant features were considered, like the case of Flores Island (Figure 2), where some important landslides have occurred in recent times (Figure 3). Tectonic maps were prepared, showing regional and local tectonic trends, as it is the case of Graciosa Island (Figure 4), where faults trending NW-SW expressing the Terceira Ridge alignment whilst the faults around the Caldeira volcano present a more radial pattern. The age of the islands ranges from more than 0.62 Ma for Graciosa (GASPAR, 1996) and over 10 Ma for Santa Maria Island which is the oldest of the Archipelago (SERRALHEIRO et al., 1987 and MADEIRA, 1989).
Figure 2. Geomorphological map of Flores Island for the studied area (PACHECO et al., 2006a).
Figure 3. Landslide on the south-western coast of Flores Island, near Lajedo, occurred in 1985 (AZEVEDO, 1999). The lithological maps elaborated for each island were based on the volcanostratigraphic sequences and aimed at a clear understanding of the mechanical/geotechnical properties of the materials found in the coastal outcrops.
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Slope maps were also elaborated for the studied islands, as well as hydrographic and groundwater maps containing the aquifer systems and springs and/or wells location (Figure 6).
Figure 4. Tectonic map of Graciosa Island for the studied area (PACHECO et al., 2006b, modified from GASPAR, 1996). In the case of Santa Maria (Figure 5) the geological record consists of a successions of volcanic materials interbeded with sedimentary deposits that result from several sea transgressions and regressions, in a sequence from bottom to top: submarine pyroclasts; subaerial scoria cones; basaltic lava flows alternating with thin pyroclastic layers;, sedimentary breccias (like conglomerates) overlaid by sub aerial basaltic lava flows, followed by sedimentary (limestone, sandstone, siltstone, mudstone and conglomerate) and volcanic (lava flows) deposits with submarine characteristics; submarine lave flows an sandstones containing fossils from the Pliocene (< 4.5 Ma); submarine and subaerial volcanic deposits, including thick lava flows and dykes; Quaternary deposits (alluvial deposits; sand and cobble beaches; conglomerate and landslide deposits).
Figure 6. Aquifer systems and springs location of Flores Island for the studied area (PACHECO et al., 2006b).
RESULTS
Figure 5. Lithological map of Santa Maria Island for the studied area (PACHECO et al., 2006c, modified from SERRALHEIRO et al., 1987). For mechanical/geotechnical purposes, the materials found in the outcrops were grouped as lava flows, basaltic pyroclasts, submarine pyroclasts, coherent sedimentary deposits, landslide deposits, alluvium and incoherent deposits, sand and fill embankments.
The information provided by the geological characterization enabled to evaluate the hazard for the studied areas. Seismic hazard assessment was based on historical records and instrumental data and allowed the definition of maximum historical seismic intensity maps (Figure 7). Volcanic hazards are also of great concern in the Azores. Although none of the studied islands had historical eruptions, at least 10 eruptions of different eruptive styles occurred at Graciosa in the last 12 ka, and the last eruption has less than 2 ka. (GASPAR, 1996). Considering the eruptive history of Graciosa one can expect an eruption to occur either at the north-western platform of the island or at the central volcano. The geological record includes basaltic (s.l.) and trachytic (s.l.) nature eruptions, effusive or explosive, magmatic or hydromagmatic. Flores has very well preserved volcanic features that are not more than 2 or 3 ka old. According to the eruptive history of Flores records, one can admit that future eruptive events may have explosive and hydrovolcanic characteristics. These eruptive styles may originate lava flows, pyroclastic emission with ballistic projection, pyroclastic flows, mudflows, and gas emission.
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Figure 7. Seismic intensity map for Graciosa Island for the studied area (PACHECO et al., 2006a, modified from SILVA, 2005). Volcanic hazard for Santa Maria has to be considered low. However this island can be affected by explosive events that may occur in the neighbour Island of São Miguel. In what concerns landslide susceptibility was established through the calculation of the safety factor (infinite slope model), and maps were executed for each island (Figure 8). The occurrence of tsunamis in the Azores has also to be taken into consideration. Historical records show that the 1755 earthquake, with its epicentre at the Gorringe Bank, caused a low magnitude tsunami on the islands (ANDRADE, et al., 2006). Tsunami run-up susceptibility maps were prepared for all the islands (Figure 9).
DISCUSSION AND CONCLUSIONS The integration of hazard assessments leads to the definition of high risk zones along the coastal areas and the establishment of specific legal rules. In some situations, e.g. the one observed at Ponta da Fajã, Flores, an important landslide (Figure 10) triggered by heavy rainfall occurred on the 19th December 1987 and special rules were adopted for the area, as we can see from the transcription of article 19 of the regulatory document: “High Risk Areas of Ponta da Fajã. 1- The high risk areas are located in Ponta da Fajã, municipality of Lajes das Flores, were affected by landslides and rock falls, due to rainwater infiltration and the riverside. These activities were prohibited in the high risk areas: a) Civil construction-building activity; b) Inhabiting or reoccupying existing houses. 2 - The following measures should be undertaken in the high risk areas of Ponta da Fajã: a) Ask occupants of houses to leave immediately; b) The restitution of any housing assistance funds granted by the Regional Government of the Azores; c) Ending provision of electricity and water supplies.”
Figure 8. Landslides susceptibility map for Flores Island for the studied area (PACHECO et al., 2006a).
Figure 9. Tsunami run-up susceptibility map for the studied area on Graciosa Island (PACHECO et al., 2006a).
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Figure 10. Orthophotomap (modified from Instituto Nacional de Intervenção e Garantia Agrícola, 1989), with the landslide observed at Ponta da Fajã, Flores, on the 19th December 1987. Therefore, it is quite evident that geological characterization and hazard assessment should be mandatory tools for an appropriate coastal land-use management planning.
LITERATURE CITED AZEVEDO, J.M., 1999. Geologia e hidrogeologia da ilha das Flores, Açores. Tese de Doutoramento, Universidade de Coimbra. 403 p. (not published). ANDRADE, C., BORGES, P. AND FREITAS, M.C., 2006. Historical Tsunami in the Azores archipelago (Portugal). Journal of Volcanological and Geothermal Research, 156, 172-185. GASPAR, J.L., 1996. Ilha Graciosa (Açores). História Vulcanológica e Avaliação do Hazard. Tese de Doutoramento em Vulcanologia, Departamento de Geociências, Universidade dos Açores. 256 p. (not published). LOURENÇO, N., MIRANDA J.M., LUÍS, J.F., RIBEIRO, A., VICTOR, L.A.M., MADEIRA, J. and NEEDHAM, H.D., 1998. Morphotectonic analysis of the Azores Volcanic Plateau from a new bathymetric compilation of the area. Mar. Geophys. Res., 20(3) pp. 141-156.
MADEIRA, J., 1986. Geologia estrutural e enquadramento geotectónico da ilha de Santa Maria (Açores). Provas APCC, Dept. Geologia, Faculdade Ciências Universidade Lisboa, 107 p. (not published). PACHECO, J.M., COUTINHO, R., PIMENTEL, A., and MARQUES, R., 2006a. Proposta de plano de ordenamento da orla costeira da Ilha das Flores – Geologia, 1ª Fase – Caracterização e diagnóstico. Documento Técnico-científico 41/CVARG/06, 138 p. PACHECO, J.M., COUTINHO, R., PIMENTEL, A. and MARQUES, R., 2006b. Proposta de plano de ordenamento da orla costeira da Ilha Graciosa – Geologia, 1ª Fase – Caracterização e diagnóstico. Documento Técnico-científico 40/CVARG/06, 162 p. PACHECO, J.M., COUTINHO, R., PIMENTEL, A. and MARQUES, R., 2006c. Proposta de plano de ordenamento da orla costeira da Ilha de Santa Maria - Geologia, 1ª Fase - Caracterização e diagnóstico. Documento Técnico-científico 39/CVARG/06, 159 p. SERRALHEIRO, A., ALVES, C.A.M., FORJAZ, V.H. and RODRIGUES, B., 1987. Carta Vulcanológica dos Açores – ilha de Santa Maria. Edição do Serviço de Protecção Civil, Universidade dos Açores e Centro de Vulcanologia do INIC, 2 folhas na escala 1:15000. SILVA, M., 2005. Caracterização da sismicidade histórica dos Açores com base na reinterpretação de dados de macrossísmica: contribuição para a avaliação do risco sísmico nas ilhas do Grupo Central. Tese de Mestrado em Vulcanologia e Riscos Geológicos, Departamento de Geociências, Universidade dos Açores. 146 p. (not published). WALLENSTEIN, N., SILVA, R., and COUTINHO, R., 2006. Proposta de Plano de Ordenamento da Orla Costeira da ilha do Corvo – Geologia. 1.ª Fase – Caracterização e Diagnóstico. Documento Técnico-Científico 42/CVARG/06, 101 p.
ACKNOWLEDGEMENTS We thank the Portuguese Air Force Command based at Terceira Island for the facilities with air flights over the studied areas. The Regional Direction of Land Use Management and Water Resources, Regional Secretary of Environment and Sea, Regional Government of the Azores supported this study. Prof. Helena Calado, Section of Geography, Biology Department, Azores University, provided help with legislation to be applied.
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