Assessment of Catastrophic Wave Impact in Apulia Region (Southern Italy)

Assessment of catastrophic wave impact in Apulia region (Southern Italy)

G. Mastronuzzi1, C. Pignatelli2 & P. Sansò3 1Department of Geology and Geophysical, University of Bari, Italy 2Phd School of Geomorphology and Environmental Dynamic, Department of Geology and Geophysical, University of Bari, Italy 3Department of Materials Science, University of Lecce, Italy

Abstract

The aim of this study is determining the degree of vulnerability along the southern coast of Apulia (Italy) using historical reports and field evidence. Apulia is a low seismic region surrounded by highly seismic zones which form the borders of the Apulian microplate in the Adria plate: the coast of Albania and Ionian Islands are located to the east, the southern Apennines to the west, and Gargano promontory to the north [16]. Seismic activity has been responsible for the occurrence of numerous earthquakes in this area during the last millennium; some of them have been responsible for the generation of the historical tsunamis which hit the Adriatic and Ionian Apulian coasts. Recent geomorphological research has pointed out the occurrence both along the rocky coasts and beaches of landforms and deposits related to the occurrence of catastrophic waves generated by low frequency sea storm events and tsunamis. The available data point out for the coasts of southern Italy a recurrence period of tsunamis of about 50 years and a maximum intensity of the III-VI grade on the Sieberg-Ambraseys scale [20, 22, 23]. This information has been collected in a database for updating a G.I.S. containing geomorphological features of the region. It allows one to calculate the actual range of shore resistance to the stress generated by extreme events (tsunami, sea storm, seaquake, etc.), and to map the vulnerability of the Apulian coast related to the catastrophic sea events. Keywords: catastrophic waves, vulnerability, Adria plate, Apulia.

1 Morphological types of coast

Apulian region stretches for 350 km in the southern part of Italy, between the Adriatic and the Ionian Seas [4] (Figure 1); it is the emerged part of the foreland domain of both Apenninic and Dinaric orogens, slightly deformed and is affected by Apenninic and anti-Apenninic trending faults [4,15,16]. The coast of Apulia is made of an alternance of cliffs, rocky sloping coasts and beaches (Figure 1). Beaches are generally less then 40 meters wide and 8 km long, and are invariably bordered by a dune belt, often covering a former mid – Holocene cemented dune, and by reclaimed and urbanised swamps [4,15,16].

Figure 1: Localization of studied area and morphological type of coast recognised along the Ionian coast of Salento peninsula. Legend: a) beaches; b) gently sloping and convex rocky coasts; c) cliffs.

In the studied coastal area, stretching from Brindisi to Taranto, cliffs are placed to the south of Brindisi, to the north of Otranto and near Porto Miggiano (Lecce) [4]. The sloping coasts, flat or convex, are the most widespread morphological types in the region. The coast with convex profile are constituted by subaerial slope, in some places mantled by slope deposits as along the Otranto – Capo S. M. di Leuca coastal tract. Along the Adriatic side the flat sloping coast are made up of a low, gently platform cut through Plio-pleistocene calcareous sandstones as well as Mesozoic limestones. The eastern area of Gulf of Taranto,

Risk Analysis IV, C. A. Brebbia (Editor) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1 Risk Analysis IV 683 is characterised by gently slope rocky coast shaped on Pleistocene algal calcarenite, locally named panchine, and interbedded pocket beaches [19]. Pocket beaches of different size characterize both the Apulian side [18].

2 Tsunami evidence

Along the gently slope rocky coast typical features are ascribed to tsunami occurrence. The main features of the boulder accumulations surveyed along the coast of southern Apulia are: a) the boulders often consist of slabs of calcareous sandstones carved out along strata planes and joints in the nearshore and accumulated farther inland. These slabs are often imbricated in small groups of several elements or form parallel rows which are not oriented at right angles to the coastline, but instead are usually aligned in accordance with the direction of wave approaching to the coast responsible for their deposition. b) most of boulders have been carved from an area close to the supratidal zone, which can be ascertained from the presence of wide, flat potholes and by barnacles on the surface of the boulders; due to the tilting of transported boulders, potholes have been subsequently modified by new, horizontal solution pools formed because of karstic processes. c) some boulders were carved out from the mid- sublittoral zone as it is testified by the presence of biogenic encrustation (Vermetids, Serpulids and Briozoa colonies) and boring bivalves (Lithophaga, Pholas) which have colonized their surface d) boulders detachment and transport occurred after the Middle Age since most boulders rest on medieval quarries or cover other man-made structures. In some cases, boulders have been found inside Messapic graves dating back to the VI century B.C. [16]. Several tracts of Ionian rocky coast of Apulia are characterised by large boulders, up to 80 tonnes in weight scattered some meters above present sea level. Elongated boulders show a remarkable narrow range of orientation of the long axis as well as of the imbrication axis, suggesting that they had been transported by a single catastrophic event, most likely a tsunami, connected to a wave train approaching the coast from south of Mediterranean Sea. Stratigraphic, morphological and historical data suggest that this event occurred during the late Holocene. Conventional radiocarbon age determination performed on Lithophaga shells collected from a large boulder in Torre Squillace (Lecce) locality yielded a calibrated age between 1421 and 1568 AD (Figure 2). Then, the catastrophic wave train, probably a tsunami, could be a local effect, may be a landslide, of the strong earthquake which hit southern Italy the 5th December 1456. The continental shelf of Calabria is in fact characterised by a number of landslide marks since the platform of Calabria is very short and the border of continental shelf abyssed at the depth of about 2000 meters in the Ionian Sea no more than 700 meters from shoreline. Large boulder accumulation have been recognized also along the Adriatic coast. An accumulation of about 80 boulders has been found at the coastal village of Torre Santa Sabina, which lies

Risk Analysis IV, C. A. Brebbia (Editor) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1 684 Risk Analysis IV approximately 30 Km to the northwest of Brindisi. The collated data suggest that two tsunamis could have struck this area: the first one was produced by the 6th April, 1667 earthquake which destroyed Ragusa (modern Dubrovnik), on the Dalmatian coast; the second accompanied the strong earthquake that struck southern Apulia the 20th of February, 1743 [16].

Figure 2: A view of the largest boulder (80 tonns) recognized at Torre Squillace locality.

3 Effects of severe wave storms

The Adriatic coast of Apulia shows important hazard scenery: the 4th January 2002, shortly after a detailed topographical survey of Torre Santa Sabina coastal area (Brindisi), a severe sea storm was produced by strong NE winds whose velocity exceeded 30 knots. The National Wave Measuring Service (Servizio Ondametrico Nazionale) buoy at Monopoli [10, 11], which is stationed few kilometers to the NW of Torre Santa Sabina area, recorded waves marked by significant height up to 4.8 m and peak period of 8.3 seconds propagating with N43E direction. These wave statistics indicate this extreme event as among the most severe storms at sea recorded along this coastal area since the nineties. During this sea storm one single boulder was detached, transported landward for about 1.6 m and eventually deposited at 0.5 m of altitude. The boulder has triangular shape (2.2 m at its base, 1.3 m in height and about 0.7 m thick) and weighs 1.4 tonnes. The biogenic colonization present at the base of the boulder

Risk Analysis IV, C. A. Brebbia (Editor) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1 Risk Analysis IV 685 indicates that it was carved out from the midlittoral-sublittoral zone, and that it overturned during transportation [16].

Figure 3: In Torre Santa Sabina locality a row of embricated boulders is formed by tsunami (B46 and B47) and sea storm-accumulated boulders (B49 and B45).

The place where the boulder was detached from has been identified by means of scuba survey. During the second storm, occurred the 12th of January 2003 the boulder was transported for about 20 meters and deposited on a row of three boulders. In the same area Dendropoma samples collected on form imbricated boulder, that made a row, were submitted to 14C analysis. The obtained age indicate that rows formed in four different phases (Figure 3). 4 Vulnerability assessment

The coast between Capo San Vito (Taranto) and Egnatia-Savelletri (Brindisi) has been subdivided into UCU (unit-condition-unique) (Figure 4).The division in UCU has been executed considering the different morphological types of Apulian Coasts. Firstly, for each UCU was estimated the susceptibility S. It is the tendency of a UCU to suffer and/or contrast a particular hazard event. By the spatial analysis in G.I.S. environment have been assigned different range and score to obtain thematic map of susceptibility. Moreover, overlapping these maps, every UCU has a specific and unique range of resilience to stress determinate by an extreme event. The range and score obtained aren’t absolute but they are relative value for distinguish the UCU marking areas with maximum and minimum risk. Consequently has been evaluated the vulnerability V for each sector.

Figure 4: UCU limit along the studied area in red arrows; grey arrows indicate respectively areas where effects due to tsunami and sea storms have been recognised; black arrows shows UCU with maximum number of boulders.

An array A has been implemented: in the rows are collected UCU subdivisions and in the columns are placed the geomorphological elements of vulnerability (regional slope of UCU, dimensions of dunes, distance between shoreline and dune, Beniawski’s parameters for rocky coasts [1], ecc.). Another step has been the implementation of vulnerability map by statistical analysis of the data contained into array A. The areal and statistical distribution allow to score a classification denominated equal range wherewith have been fixed the areas much more exposed to risk. The degree of vulnerability for every UCU is maximum value of this classification. Another important parameter is the return times RT of the extreme events. Specifical statistical distribution, Gumbel’s distribution [9] law and Weibull distribution [17], have been used to estimate return times but only for extreme waves because the elevate number of datas (intensity and direction of the wind, high waves).

5 Conclusion

The impact of catastrophic waves and mainly of tsunamis has been neglected in coastal geomorphology even if such high-magnitude low-frequency events are likely to have played an important role in the evolution of many coasts [24]. Main morphological effects of catastrophic events on rocky coast are represented by the detachment of large boulders in the nearshore zone and their deposition farther inland, and by the sculpturing of bedrock with the formation of smooth, small scale form and large scale features [2,3] this implic an important role of these extreme events in front of human settlements and human lives.

Table 1: Example of vulnerability array with parameter studied for each UCU.

Beniawski' Distance Number Dimensions Regional s Dune- of of Dunes Slope Parameters Shoreline boulder UCU1 (Capo S.Vito-T.Castelluccia) - X X - X UCU2 (T.Castelluccia-Campomarino) X X X X X UCU3 (Campomarino-Punta X - X X X Prosciutto) UCU4 (Punta Prosciutto-Porto X - X X X Cesareo) UCU5 (Porto Cesareo-T. X X X X X dell'Inserraglio) UCU6 (T. dell'Inserraglio-Gallipoli) X X X X X UCU7 (Gallipoli-Capo Santa Maria - X X - X Leuca) UCU8 (Capo Santa Maria Leuca- - X X - X Otranto) UCU9 (Otranto-San Cataldo) X X X X X UCU10 (San Cataldo-Brindisi) X X X X X UCU11 (Brindisi-Torre Guaceto) X X X X X UCU12 (Torre Guaceto-Torre S. X X X X X Leonardo) UCU13 (Torre S. Leonardo-Torre X - X X X Canne) UCU14 (Torre Canne- X X X X X Savelletri/Egnatia)

At the first state of this study a maximum value of V has been recognized for the studied area. The implementation of the data available with others (human activity, karst or chemistry activity, ecc.) will allow to obtain a more precise assessment of the coastal vulnerability.

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