RIASSUNTO 

San Fruttuoso di Camogli, che rappresenta l’emergenza più caratteristica del Parco di Portofino, è soggetto a diversi pericoli naturali come l’erosione costiera, le frane e gli eventi alluvionali: in questo studio si è concentrata l’attenzione sulle colate detritiche, già verificatesi con l’evento del 1915 che ha danneggiato l’Abbazia benedettina del X secolo ed ha determinato la formazione di una spiaggia. Gli studi condotti mettono in risalto che il bacino di San Fruttuoso presenta tutti i fattori favorevoli all’innesco di colate detritiche, paragonabili a quelle tipiche dell’ambiente alpino. Il fenomeno citato si è manifestato con una precipitazione di circa 310 mm rilevata a Chiavari e superiore a 400 mm nelle zone più vicine, per una durata inferiore a 4 ore: un evento simile dovrebbe presentare un tempo di ritorno quantomeno di 100 anni, ma le sfavorevoli caratteristiche geomorfologiche dell’area, in relazione anche alle tendenze climatiche in atto ed al cambiamento d’uso del suolo, rendono l’approccio statistico poco aderente alla realtà. Alla luce di quanto elaborato, appare necessario attuare un piano per l’individuazione, il monitoraggio e la mitigazione del rischio idrogeologico sulla base del modello della gestione adattiva, affrontando le problematiche a diverse scale, da quella progettuale fino al livello tipico della pianificazione territoriale, rendendo il bacino di San Fruttuoso un laboratorio sperimentale per l’analisi delle pericolosità geomorfologiche con approccio multiscalare e multirischio.  

TERMINI CHIAVE: Pericolosità geomorfologica, Colate detritiche, Piogge intense, Parco di Portofino, Italia.

ABSTRACT

Geomorphological hazard due to debris flow evaluation in the San Fruttuoso di Camogli catchment (Portofino Park, Italy).

San Fruttuoso of Camogli is not only the most typical topic of Portofino Park -  internationally well-known for  the historical and architectural high value of the hamlets and for naturalistic and environmental incomparable qualities of the landscape, underwater too - but presently the village can also be recognised as a UNESCO World Heritage site. Various natural hazards - such as coastal erosion, landslides and flood events - threaten these cultural and landscape assets. This study focuses on the debris flows that already occurred here on the 25th September 1915 when the Abbey complex was damaged and a beach was formed in less than 24 hours. A number of methods were used so as to reconstruct the event, following both the scientific approach and the historical-geographical model. Archive research allows to say that the event displayed a rainfall of about 310 mm at Chiavari and of more than 400 mm on nearby areas (Cervara and S. Margherita Ligure) for less than 4 hours. San Fruttuoso Bay displays all the factors that can foster fast debris flows: slope and water courses steepness, presence of debris on riverbed, heavy rainfall; the collected data allow to think that the 25th September 1915 rainfall triggered a debris flow whose geometrical, sedimentology and magnitude features can be compared to those typical of the alpine environment. The debris flow originated on the upper slope sectors and quickly canalized along Vallone dei Fontanini - representing the transportation  area – including its tributary on its orographic left, dubbed ad hoc Fosso dell’Alluvione. The deposits only starts about 10 m a.s.l. and is spread as far as the present beach; it is  bigger at the end of the event and in subsequent withdrawal. According to the thermo-pluviometrical data proce According to the thermo-pluviometrical data processing recorded at the Chiavari ultracentenarian weather station, such an event should have a recurrence time of at least 100 years. However, the valley unfavourable geomorphological features  - also related to the present climate trends and to the land-use change – make the statistical approach scarcely real. If the mean annual rainfall does not show significant differences, the mean annual temperature  - together with dry days  – shows a positive trend. Therefore the daily rainfall rate causes a rainfall tropicalization effect thus the possibility of heavy rainfall including triggering shallow instabilities thresholds. The present trend is confirmed by recent events that occurred nearby. The multi-temporal maps and photographs comparison allowed to highlight different land uses related to the abandoned farming during the first half of the  20th century. This rapidly deteriorated the terraced areas and all land protection works, causing hydrogeological instability. Only very recently measures for a suitable land environmental policy have been encouraged, involving the reclamation and settlement of original hamlets.  In conclusion we would like to highlight the need for carrying out a studies plan involving determination, monitoring and reduction of the hydrogeological risk, taking into account that damage has increased due to some negative anthropogenic actions and that the area  natural hazard is related to a number of factors, not only debris flows but also gravitational, fluvial and marine processes.

The studies involving land protection must be undertaken at different levels, from the project perspective to the level typical of land planning, converting San Fruttuoso basin into an experimental laboratory for analysing geomorphological hazards by following a multi-scalar and multi-risk approach. Actions for reducing the risks can be carried out following the model of adaptive management, once hazards have been defined.  

KEY WORDS: Geomorphological hazard, Debris flow, Heavy rainfall, Portofino Park, Italy