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Environmental Geological Maps of San Fruttuoso Bay (Portofino Park, Italy) FRANCESCO FACCINI1 , MAURO PICCAZZO2 and ANDREA ROBBIANO3 1 University
of Genoa, DISAM (Environmental-Geography Science Section), Via Balbi 2, 16126 Genoa, Italy;
[email protected] 2 University
of Genoa, DIPTERIS, Corso Europa 32, 16132 Genoa, Italy;
3 Geological
consultant, Via Capolungo 46, 16167 Genoa, Italy;
(Received 16th March 2008; Revised 11th September 2008; Accepted 20th October 2008)
Abstract: This paper presents environmental geological maps (applied geomorphology, morpho-sedimentology, ecological set-up, geomorphological hazard geothemes) of San Fruttuoso Bay (Portofino Park, Italy). The hamlet of San Fruttuoso is the most popular attraction of Portofino Natural Park and is internationally known. This charming village is located at the mouths of two steep and tiny valleys, which have headwaters at about 600 m a.s.l.; their catchments are developed entirely on conglomerate rocks. The value of the landscape and cultural heritage of this area are immense, but they are at risk from geo-hazards. The geomorphological hazards fall into three categories: landslides, debris flows and coastal erosion. Our detailed maps are based on geological, geomorphological and ecological data collected according to the following methods: a) analysis of aerial photographs at 1:13,000 scale; b) geological field survey; c) observation and comparison of historical images and maps; d) granulometric analysis of soil samples; and e) underwater survey. We believe that such geo-maps are important tools in land management studies, especially in areas threatened by natural hazards. In the case of San Fruttuoso Bay, this kind of study is even more important, given the need to preserve a world-class natural heritage site.
431 ISSN 1744-5647 http://www.journalofmaps.com
Journal of Maps, 2008, 431-443
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Faccini, F., Piccazzo, M. et al
Introduction
San Fruttuoso village is located in the middle of the Portofino Promontory and can be reached by sea or by foot along hiking trails. It represents the most typical and charming site of the Portofino Park Area since it combines both high historical-architectural quality with valued natural environments, both on-land and underwater (Figure 1).
Figure 1. Panoramic view of San Fruttuoso Bay (Photo A. Girani).
Coastal areas are crossed by well-known hiking trails starting from Camogli, Portofino and San Rocco (Brandolini et al., 2006; Faccini et al., 2007). San Fruttuoso village belongs to the Italian Environmental Fund (Fondo Ambiente Italiano), and procedures have recently been started to recognise this hamlet as a UNESCO World Heritage site. The bay waters are a significant tourist attraction. They offer visitors the possibility of viewing by boat a highly venerated underwater statue of Christ (Cristo degli Abissi), as well as the entire bay from a distance. In the bay, it is also possible to observe important ecological-environmental areas because of the geological-structural and geomorphological setting of the Promontory (Limoncelli and Marini, 1969; Terranova, 1995). This paper identifies the geomorphological features of the slopes behind San 432
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Fruttuoso village, as well as the morpho-sedimentological features of the seabed to a depth of 60 m. The ecological aspects of the seabed are described, together with their relationship to the geography of the area. Finally, the geomorphological hazard map was produced by overlaying these data sets.
2.
Geological and Climatic Settings
The Portofino Promontory covers 18 km2 and has a coastline 13 km long; it presents a landscape of high ecological and environmental value. The Portofino Mount on land was officially declared a protected area in 1935 (Law n◦ 1251/1935). It is included in a Regional Park whose boundaries were defined in 2001 (Regional Law n◦ 29/2001). Subsequently, the “Portofino Protected Marine Area” was created by an Environment Ministry Decree in 1999 (Law n◦ 979/1982). The San Fruttuoso river catchment is developed on the Conglomerate of Portofino (Oligocene); the conglomeratic rocks consist of thick layers ranging from 0.5 to 7 m thick with an average cobble size of 5 to 50 cm alternating with thinner (up to 1 m thick) layers of muddy and sandy rocks (Figure 2). These dip to the SE at roughly 15◦ (Giammarino et al., 1969). The conglomeratic rocks are cut by many tectonic lineations that influence the geomorphological landforms, such as the drainage network, the position and form of the ridge and the coastline (De Stefanis et al., 1984). The climate is typically Mediterranean, with hot summers, mild winters and long periods of insolation. At the local scale, however, factors such as altitude, exposure, air humidity and vegetation cover create different microclimates (Girani and Olivari, 1986). The maximum rainfall is recorded in autumn and the minimum in summer. Faccini et al. (2005) analysed a variety of meteorological data and note an annual mean precipitation varying from 1100 to over 1500 mm, while annual mean temperature ranges from 13 to 15◦ C (data for the period 1922-1995). A number of springs have been recognized, both at the base of the slope and at altitudes of 300 to 500 m a.s.l. in the San Fruttuoso catchment; those at altitude show fairly good flows (more than 2 l/min). It has been suggested that the spring water in the conglomeratic rocks originates from 433
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(a)
m a.s.l. 500 400 300 200 100 0
1.
S. Fruttuoso
2. (b)
Figure 2. (a) An example of conglomerate outcrop, (b) longitudinal profile of Fontanini stream with geological features: 1) conglomeratic rock; 2) fault or tectonic lineation.
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condensation of vapour in the air mass moving from the south along the cliff of the Promontory (Faccini et al., 2005). The consequent air temperature decrease causes condensation of water in rock discontinuities. The springs’ perennial flows are such that they have been exploited for a long time and continue to be exploited by tourists.
3.
Methods
Initial work involved bibliographic research, including both scientific notes and archive records, with specific attention to village historical photographs and maps. Photointerpretation was then undertaken using 1:13,000 images (acquired from the Ligurian Region) to map the main structural geological features of the Portofino Promontory. Field surveys were carried out on land, with specific attention to mapping geological features and hillslope landforms. The underwater survey focused upon mapping sedimentological and ecological aspects. Several seabed samples were taken using an onboard manual dredge, and a granulometric analysis was carried out in the laboratory. A digital elevation model (DEM) was generated from contour lines produced from the 1:5,000 technical regional map. For the underwater area we used an original survey produced using a boat mounted depth finder. The DEM was used to generate a slope angle map of the bay. The geomorphological hazard map takes into account factors related to both land and underwater processes and landforms: seven hazard classes were qualitatively defined on the basis of geomorphological data, morphosedimentological data and slope angle using a geographic information system (GIS).
4.
Geomorphology, morpho-sedimentology and ecological set up
San Fruttuoso Bay is roughly triangular in shape, and lies between Punta Torretta and Punta Carega, which are about 1 km apart. The village of 435
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San Fruttuoso is located on a 45◦ slope, crossed by 2nd order streams, the Fontanini and San Fruttuoso streams, which have their headwaters at Mt. Portofino (608 m a.s.l.) and Mt. delle Rocche (505 m a.s.l.). At about 25 m a.s.l., at the foot of the interfluve that separates the two valleys, lies a 16th century Tower. The main settlement is located at the end of Fontanini stream, where a 10th century Benedictine Abbey stands. A smaller settlement lies further east, at the end of the San Fruttuoso stream (Pellati, 1934). Slopes mainly show the outcropping conglomerate, often over large areas, sometimes covered by shrubs and rock vegetation and less frequently by pines and holm-oaks. The surface processes that dominate the area are driven by gravity, water run-off and wave action: some concave areas show debris cover and colluvial deposits, usually transformed into anthropogenic terraces supported by containment dry stone walls where olive trees are planted (Brandolini et al., 2005). Events such as debris flows have occurred due to the very steep river beds and the presence of coarse sediments. Historical records note that a debris flow occurred here on the 25th September 1915 (Faccini et al., In press), in which the Abbey complex was damaged and a beach formed over a 24 hour period (Figure 3). The debris flow originated on the upper slopes and was channelled down Vallone dei Fontanini. Debris was mobilized by heavy rainfall, with historical archives recording over 400 mm of rainfall between 7:30 a.m. and 11:00 a.m. The grain size of deposits, estimated from original photos, is mainly made up of sand and gravel with some metric boulders. The contemporary channel shows no evidence of former levees. Currently, the pocket beaches, which are effected by the rough sea blown by the main south-westerly winds, are stable, although artificial accretion works have been scheduled. Active cliffs can be observed along the coast marking the boundaries (both east and west) of the San Fruttuoso Bay: this coastline is linked with sets of normal faults or tectonic lineations (Cevasco et al., 2004). Rock falls and topples occur along the cliffs due to wave and gravity actions. Two planation surfaces at about 30 m a.s.l. can be observed behind the village: one located at the base of the Tower and the other on the heliport stand. These are probably terraced landforms, according to recent work on the marine terraces of Liguria (Biagioni et al., 2007). A cave, named Grotta 436
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Figure 3. The San Fruttuoso Abbey: San Fruttuoso Bay in 1908, from the actual heliport (top right); the partially destroyed church after the event of 25 September 1915, with a thick accumulation of debris cover (bottom right); front of Abbey in 1933, after the restoration works by the State (left) (Source: Archivio F.A.I. San Fruttuoso).
del Cristo degli Abissi, can be observed on the eastern side of the bay. Its entry is located at sea level and the cave formation is due to tectonic and marine processes (Figure 4). Another cave can be recognized in the underwater cliff, about 20 m deep. Tides have limited extent (20-30 cm), so they do not influence the coastal evolution as much as the hydrodynamics due to wave action. This area shows reduced man-made landforms, at least compared with other sectors of the Ligurian coast: beside a few buildings, including the Doria family complex, only the artificial berthing pier can be seen. The seabed morpho-sedimentology setting shows a well-graded grain size distribution up to the -60 m isobath (Figure 5, Table 1). Moving from the land to open sea, the profile quickly slopes down from the shoreline to -10 m along the conglomerate cliff, interspersed by rock blocks that have slid down due to wave action. Between the -10 and -40 m isobaths, the seabed is mainly sandy, although locally gravelly from -20 to -30 m; below -40 m, after a narrow sandy strip from coarse to fine grained soils, a tract begins composed of fine sand and silt. 437
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Longitudinal profile
Plan
0
5
10
15 m
Figure 4. Grotta Cristo degli Abissi: plan and longitudinal profile of the cave (Surveyors: F. Mariani and G. Repetto; redrawn from Gruppo Speleologico C.A.I. Bolzaneto, 2006).
The ecological set-up is strictly related to the morpho-sedimentological asset: the Posidonia Oceanica seagrass meadows or scattered spot zones between -10 m and -30 m contribute to increased sediment retention and, therefore, reduced seabed erosion in the bay. The surface water temperature varies between 13-15◦ C from January until April and between 20-24◦ C from June until September; deep waters have a constant temperature never below 12-13◦ C and underwater visibility in open waters can reach 30-40 m (Faccini et al., In press).
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SAN FRUTTUOSO 5
12
0
10
75 50
-10
25
-3 0 25
-20
125
100
-50
75
50
-40
-40
-20
-70
0
-60
-60
0
100 m
200 m
300 m
Figure 5. Location map of soil samples.
5.
Geomorphological hazards
We have produced a geomorphological hazards map of the entire San Fruttuoso Bay from an applied geomorphological map, linked to the on-land sector, and from morpho-sedimentological and ecological maps of the seabed for information on the underwater environment. Very high hazard conditions are present in San Fruttuoso village, due to the potential for debris flow and flooding events, which can occur simultaneously, as in the historical case of 25th September 1915. In fact, taking into account the envelope curve for the maximum flood with a return period of 200 years (Cati, 1970), the watercourse flows of the San Fruttuoso and Fontanini streams can reach about 20 m3 /s. In these 439
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Sample
Depth (m)
Gravel (%)
Sand (%)
Silt (%)
1 2 3 4 5 6 7 8 9
+1 +1 -2 -4,5 -5 -7 -24 -38 -51
100 92 87 87 72 100 98 56 10
8 13 13 28 2 42 75
1 15
Table 1. Grain-size distribution of samples (see Figure 5).
conditions, fast events like debris flows can be triggered due to the steep longitudinal profiles and availability of riverbed debris. Both the eastern and the western sectors of San Fruttuoso Bay are affected by very high geomorphological hazards due to wave action. These have to do with both the cliff and the rocky seabed below. It is evident that there are risk conditions resulting in high tourist vulnerability. On the slope over the cliff edge, we identify hazard situations ranging from high to fairly high: several edges of degradational and/or landslide scarps together with the strongly fractured rock conditions can trigger rock falls and topples. The remaining underwater area is characterized by a geomorphological hazard ranging from moderate to very low.
6.
Conclusions
The surveys carried out at San Fruttuoso Bay show several geomorphological hazards posing clear risks not only to buildings and infrastructure but also to tourists visiting this area. San Fruttuoso Bay displays all the factors that can trigger debris flows: slope and water course steepness, debris on the riverbed, and heavy rainfall. These events can be compared to those typical of the alpine environment in terms of their geometries, magnitudes, and sedimentological features (Figure 6). Landslides and sea storms are the other natural hazards that can occur in the bay. In conclusion, the San Fruttuoso catchment can be considered a natural laboratory where geological issues (present and potential) can be studied by 440
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Figure 6. DEM of San Fruttuoso Bay.
following a multi-scalar and multi-hazard approach. This approach is essential to identifying critical sectors and planning risk reduction work while enhancing the value of the village: this cannot be done without ensuring safe conditions for its users.
Software The software CorelDRAW 11 was used to draw the geological-environmental maps. The DEM was produced using contour lines drawn on the technical regional map, processed with Surfer 8 (Golden Software Inc.). Intergraph’s GeoMedia was used for all GIS processing.
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