Renaissance of the historical Lentini Lake

G. Baldovin 1 – E. Baldovin 1 – M.Toti 2 1 Geotecna Progetti – Milano, Italy 2 Consultant Engineer E-mail: [email protected]

Abstract

The historical Lentini Lake (Sicily) has been re-created with a perimetrical earth rockfill dam enclosing the area of an old internal natural pond. The new reservoir, of 127x106 m 3 capacity, has solved a secular problem for the Provinces of Catania and Siracusa, supplying water for civil, irrigation and industrial needs. The flooded area is a tectonic graben, constituted by a pleistocenic formation of marine clay 200 m thick. The main dam is 33 m high and its axis, divided in two sections, is 9 km long. The embankment is formed with calcarenite and basalt rockfill; its upstream face is lined with a bituminous concrete membrane. The foundation of the dam requested various important works Figure 1: Site position for the reclamation of the sediments of the old lake.

Due to the deepness of the lacustrine and alluvial sediments catchment basin originated and was used for various storage overlaying the marine clay, a complex network of drainages, purposes(Fig. 2); it grew gradually, after the construction of a both natural and with pumps, safeguards the inspection small dam, up to a surface of several thousands of hectares. galleries and the embankment, requiring a compelling and In the last centuries the reservoir was progressively neglected systematic maintenance in operation. and the area was drained and reclaimed. The seismicity of the area is rather high, the main seismogenetic source being the "Malta Escarpment", a submarine fault with direction parallel to the Eastern Sicily coast, 20 km far. A recent 5.4 Magnitude earthquake caused some fissures in the bituminous lining, when the reservoir was still empty, so that they have been easily restored.

Historical introduction

The creation of large reservoirs is often obtained by widening or regulating existing natural lakes or by the rehabilitation of old water storages which in the course of time have been abandoned or drained. Similar situations are frequent in Italy. The most recent is the new Lentini Reservoir (Fig.1), which has been formed again in Sicily in an historic area known in the age of ancient Rome as the granary of the Empire, and which was sung by Homer and described by Cicero, Sicily Praetor . Figure 2: The ancient "Biviere" In fact, in the Middle Age, due to the convergence of some local rivers, 10-15 km upstream of the mouth of the present In the 1970-2000 period, a new large artificial reservoir has San Leonardo River, a little natural pond without a significant been realized in that location with the construction of a peripheral earth-rockfill dam along three sides of a trapezium including the ancient impound. The reservoir will be utilized as a multipurpose storage for irrigation, industry, drinkable water supply and probably for energy production, and will contribute to the economic development of the Eastern Sicily solving the secular problem of local scarcity of water [1].

Geology of the area

The area of the reservoir has an average elevation 15 m a.s.l.. Its geology is dominated by a blue marine silty clay formation dated to the Inferior Pleistocene (Sicilian or Calabrian), 200 m thick, covered by 10-12 m of terraced and recent alluvial deposits and lacustrine brown silts, rich in organic substances, deposited in the old lake. The clay formation lies on a deep substratum of calcarenites and old basalts, belonging to the

Inferior Vulcanites Formation. The calcarenite is a yellow organogen deposit, coarse and stratified, with 70-80% percent Figure 4: Reservoir Plan of CaCO 3, transgressive on the vulcanites. The embankment From a tectonic point of view, the area is characterized by a 6 3 natural tectonic depression (graben) originated by some The embankment has an overall volume of 9x10 m and is displacements of the substratum along two main faults formed with compacted calcarenite and basalt rocks coming systems, the most important one aligned E-W (Fig.3). from the surrounding hills. The waterproofing is ensured by an upstream bituminous lining which allows the control of the seepage and is therefore composed by two impervious layers, 8 and 6 cm thick, of bituminous concrete, including a draining layer. The total thickness of the lining is 32 cm; its surface extension 385000 m 2. The upstream face has a slope 1.8/1 and is geometrically shaped at the toe with a circular stretch ending at the contact joint of the inspection gallery (Fig.5).

Figure 3: Geology of the reservoir area

The reservoir and the dam

Dimensional data The reservoir has a trapezoidal shape, a surface of 10 km 2 with maximum water depth of about 20 m and a net capacity of 127x 10 6 m 3. It is enclosed by a long dam divided in two Figure 5: Typical dam cross section sections: the main one develops its axis along three sides (respectively indicated N, E, S), has a maximum height on the Such gallery collects the water of the possible seepages foundation of 33 m and a length of 7250 m; a second section, through the lining and conveys it to the draining system. Its S side, is up to 20 m high and 1550 m long. structure is connected with a cut-off, some meters deep, The W side of the reservoir is formed by the natural slope of inserted in the foundation clay. The internal structure of the the area (Fig.4). embankment consists of: • an upstream draining zone, 1.5 m thick, supporting the Foundation and deep waters drainage system impervious lining, and followed by a granulometric A large and more than 10 m deep excavation has been carried transition 0.5 m thick; out to eliminate the fine poor alluvium constituting the • the central zone formed with compacted selected sedimentation of the ancient lake and to prepare the calcarenite, named C1 type when placed on the foundation foundation of the embankment. Consequently, a complex and under the upstream face and C2 type in the core of the network dedicated to the drainage, collection and evacuation dam. The C2 type has a higher content of fine sand and of the water during operation, has been created. The outflow silt, which gives to the fill a low permeability (k=3x10 -6 is obtained, by natural gradient or by pumping apparatus, m/s) and therefore makes it possible to the core to assume along all the length of the dam. Nine transversal galleries the function of subsidiary waterproof structure in case of connect the upstream toe peripheral inspection gallery and the serious damaging of the upstream lining. The core zone is downstream general collector, external to the embankment. limited downstream by a geotextile and by an inclined The drainage system, otherwise, not only evacuates the drainage layer 1 m thick; seepages coming from the reservoir, but also protects the dam • the downstream zone, which is a basaltic rockfill with and its foundations from the superficial and deep water external facing slope 1.8/1 with two banks. inflowing from the surrounding lands, conveying most of it, A very wide system of filters, transition zones and drains partially in two tunnels, to San Leonardo and Simeto Rivers. controls any water-flow inside the embankment and in the The final point where the drained water is delivered, for the S foundation. As above said, since the retention capacity of the and E side, is 5 km far from the toe of the dam; for the N side, inclined draining layer was found to be insufficient, especially the distance is 1.5 km. for the smaller diameters of the calcarenites, an additional and For the monitoring of the dam, about 200 instruments and appropriate geotextile filter was adopted. The same was done topographical stations are installed in 17 transversal sections, at the downstream toe of the embankments, where the to register the deformations of the embankment and of the geotextile was used as an anti-contamination defence to foundation and the piezometric levels in the dam and in the protect the drains from external water in case of flooding [2, surrounding area. 3]. As from Fig.6, where the mean grain-size curves resume Several water discharge measure stations are placed along the the results of the analyses during the construction, the main drainage conduits. calcarenites cover a wide grain interval with d 15 =0.063 mm, d 50 =1.8 mm, d 85 =45 mm, while the draining layer 2-70 mm Conveyor channels, spillway, outlet presents a typically restrained curve. and release conduits

The conveyor channels supplying the reservoir (Fig.7) are two: a first one from Simeto River on N side has a length of 20 km, is partially in tunnel and has a maximum discharge of 24 m 3/s. The second one, coming from four tributaries of San Leonardo River, on S side, is 9 km long and has a maximum discharge of 50 m 3/s. All the hydraulic system of water catchment, transfer and inflowing in the storage capacity is monitored and controlled in the central station installed in the guard-house of the dam. The spillway is placed on a natural promontory between the two longitudinal sections of the dam; it consists of a free sill dimensioned for a maximum discharge 160 m 3/s, i.e. for the total inflow from the conveying channels and for the flood peak of the little concerned watershed of the reservoir. Figure 6: Mean grain-size curves The bottom outlet and the water releasing intakes are included in a tower building, followed by a tunnel excavated in the blue clay; the outlet is intercepted by two slide gates 3 The geotextile used as a filter is a needle-punched non-woven and can discharge 200 m /s. In the invert of the tunnel two polyester with mass per unit area of 350 g/m 2 and a thickness pipes are placed for the release of the water to be used for of 2.6 mm. Its opening size D f is 54 µm and it has an average irrigation and industry in the areas of Lentini, Catania, permeability coefficient k=2.7x10 -3 m/s under a load of 2 Augusta, Siracusa. kPa, with a permittivity ψ equal to 0.3 s -1. Both the retention Downstream of the tunnel two coupled stilling basins return and the permeability criteria are widely fulfilled. Overall, the the water into a canal 2300 m long, which reaches Trigona extension of the used geotextile is about 400000 m 2. River, one of the tributaries of San Leonardo River.

Figure 8: Tectonic lineaments map

Seismic hazard evaluation According to the Italian national seismic classification the area of the works is defined of 2nd category. For the seismic hazard study a probabilistic approach has been adopted using Cornell methodology [5]. The surrounding areas have been divided in zones of a certain homogeneity, assuming for each of them a recurrence law, using the Gutenberg-Richter formula, on the basis of the historical seismicity. The

attenuation laws of the parameters of the motion in the ground Figure 7: Water inlet – outlet scheme as a function of the mean where they propagate and of the local conditions of the site have been then established, adopting the Sabetta and Pugliese approach [6] based on Italian data. The analysis has been developed for 2 return Seismic security of the dam times, respectively 475 years for OBE, and 975 years for MDE. The evaluations have been operated in terms of peak Seismicity of the area values of the motion (PGA, PGV) and of spectral values The Lentini Plain is the result of quite energetic tectonic (PSA) in 5 topographic points, in the centre and at the movements, with strains and faults which can be attributed to cardinal extremities of the reservoir. The maximum peak the last Quaternary Age. values have been obtained in the E site, where for a return As the area is historically interested by strong and relatively time of 975 years (MDE) the PGA is 0.21 g. The Fig. 9 frequent seismic events the evaluation of the seismic security shows the millenary response spectra in acceleration, with of the dam has been carefully examined [4]. damping 5%. They present a quite regular form. The Among the existing systems of faults, those oriented E-W are maximum values of the spectral ordinates are contained in the particularly important: some secondary faults of this system interval of period 0.15-0.4 seconds, with amplification values were supposed to cross directly the abutment of the main dam until 2.7. Also in this case the E site is the most solicited. (Fig.3): during the construction of the works indeed no direct evidence of this situation was found. Dynamic analysis of the embankment Although the bigger concentration of events is located in the A preliminary static analysis has been developed using a area of the Simeto-Scordia-Lentini graben, and particularly in finite elements model, which represents the maximum height the zone of intersection between the transgressive Scicli- section with 234 elements connected through 273 nodes, and Ragusa-Monte Lauro fault and the graben structure, the considering for the main materials an elasto-plastic behaviour strongest earthquakes are usually registered further east, near with a linear elastic phase represented by E and ν and a the Malta Escarpment (Fig.8). plastic shear law of associate type. main embankment (free field at the downstream toe, in axis inside the transversal gallery and on the crest) and of 2 strong motion accelerometric stations located on the crest of the intake works from two tributaries of San Leonardo River. Such a system has been integrated with a micro-seismic network installed to control all the area next to the reservoir perimeter, with particular reference to any differential motion north and south of the cited east-west faults with ancient subvertical dislocation. That network is constituted by a station with triaxial seismometer in the guard-house, equipped with a telemetry system and an aerial, and by 5 monoaxial vertical stations supplied with photovoltaic panels and located in specially built reinforced concrete huts, covered with local Sabucina stone for a better environmental compliance. Figure 9: Design acceleration spectra (Tr=975 years)

For the rockfill also a plastic curve has been used to Operation and maintenance reproduce irreversible volumetric strains in function of the mean effective stresses. A long series of numerical analyses The management of the new hydraulic system has turned out reproducing the construction steps has brought to define the to be rather complex and expensive, due to the relevant geotechnical characteristics of the materials which better fit extension of the water catchment and conveying works, to the the historical settlements and piezometric monitoring data. length of the dam, and to its peculiar characteristics in Also the impounding phase has been simulated. Then the relation to the topographic and hydrographical location, as dynamic characteristics of the materials have been defined on well as to soil geotechnical conditions. the basis of the calculated stress state, using the results of The more binding chapters for the supervision and some experiences on Italian embankments. Both the shear intervention have appeared to be: modulus G and the damping modulus D have been assumed, o the preservation of the upstream bituminous lining and of respectively as a reducing fraction of G 0 at low strains (G 0=a the downstream irrigated grass mantle, which are subject 0.5 +k 2 * ( σ’) ) and as rising from 0.5 until 25 % when the to high seasonal changes of temperature; tangential strains increase. o the operation of the pumping system to raise the drained The adopted mesh is the same as that of the static analysis. water along the peripheral collectors of the dam, 15 km The reference dynamic analysis of the main embankment has long, and in the stations for long distance discharge; been executed with the linear-equivalent model, using 6 o the electrical network and the automatic devices accelerograms, representative of MDE, defined in the seismic maintenance for the control and operation of the water hazard evaluation. The peak values of the motion on the crest catchment works of the inflowing channels apparatus, the result lower than 0.6 g, with a characteristic frequency of the outlet and the release conveyors, the pumping stations, structure of about 3.5 Hz. the lighting and the monitoring of all installations. The calculation of the permanent displacements, developed according to the “sliding block” model of Newmark [7], has An operative staff of some tens units is dedicated to the been executed both with empirical correlations and direct complex of the necessary services. numerical integration. The obtained maximum settlements are An extraordinary intervention was made on the bituminous respectively 0.6 m (in fact reference magnitudes widely lining as a consequence of some local crackings, due to the overtake the determined MDE M=6.5) and 0.3 m. above mentioned M L = 5.4 earthquake in December 1990, Finally the dynamic analysis has been repeated also with a which caused some victims in Lentini area and relevant non linear model, still obtaining an irreversible settlement of damages in all the Siracusa Province. At the moment of the 0.35 m of the crest at the end of the earthquake. The design seismic event the reservoir was empty. Fissures on the lining freeboard of 4.2 m, which was established according to the were observed on the S side, where the dam has its maximum Italian Regulatory Code, is then widely dimensioned to face height. They were continuous and oblique, on the facing, the potential settlements induced by the design seism. from the crest to the toe of the dam, but did not present any relative displacement of the hedges, which were 1-1.5 mm Seismic monitoring open. Only the superior bituminous 8 cm layer was damaged. In order to control the seismicity, either natural or induced by The repair was executed by local milling and reconstruction the formation of the Lentini Reservoir, a dynamic monitoring of the layer, and with extensive spreading of seal, namely in system has been installed, which consists of 3 strong motion proximity of the crest and of the toe of the facing. accelerometric stations in the maximum height section of the Sustainability of the works

The sustainability of the new large waterworks related to the environmental conditions, here particularly characteristic for their historic and naturalistic specificity, has been carefully taken into account. At this purpose all the general lines of the design have been chosen following absolutely compatible construction and operation criteria. A special study has been developed in order to face the fact that the reservoir is located along the routes of the migratory birds from Africa to North Europe (Fig.10). It was then considered essential the limitation of the water surface subject to the alternate conditions of submersion and emersion. It would be so possible to organize a suitable portion of the reservoir with permanent modest water depths.

Figure 11: Enlivening the old wet area

areas of Sicily, existing secular problems of scarcity of water due to the highly marked seasonal regime of the precipitations. At the same time the possibility often appears to restore some environmental conditions, like that of the old traditional wet areas, already existing in the past, but partially or totally lost in the course of the time (Fig. 12).

Figure 10: Flamingos in the new wet area Figure 12: View of the Reservoir and of the snowed Etna Therefore a solution has been recently studied which Volcano considers, as final configuration of the W side of the reservoir, to realize a separate little lake with an intermediate Acknowledgements secondary dam (Fig. 11); by this way the birds nidification will be protected and the "natural wet area", with its old The Authors thank the Owner Regione Siciliana and typical vegetation, will be restored, controlled and preserved. Consorzio 10 Siracusa, who made the publication of this Furthermore all the water catchment works on the tributaries paper possible. of San Leonardo River have been built with the use, as much as possible, of local natural materials and with the restoration of all the surrounding grassy and vegetated spaces. References The accessibility to the reservoir and to all works is allowed only in limited measure and is very seriously checked. The [1] Geotecna Progetti (1986). Lentini Reservoir – Notice paper . [2] ICOLD(1986). Geotextiles as filters and transitions in fill dams, central building of the system, i.e. the dam guard-house, has Bulletin 55. been shaped according to the architecture of the historical [3] Baldovin E.(1992). New developments of filters in some recent Italian "Norman Towers", which mark all the most singular points of embankment dams. In: Brauns J., Heibaum M. and Schuler U. (eds), Sicily. Proc. of the 1st Int. Conf. “Geo-Filters”, Karlsruhe, Germany, Balkema, Rotterdam, 1993. [4] ICOLD(1986). Earthquake analysis procedures for dams-State of the Conclusion art , Bulletin 52. [5] Cornell C.A.(1968). Engineering seismic risk analysis , BSSA 58. The re-creation of an ancient lake can involve large efforts [6] Sabetta F. – Pugliese A. (1996). Estimation of response spectra and and rather heavy costs for construction and maintenance [8]. simulation of nonstationary earthquake ground motion, BSSA 86.

The Lentini example, nevertheless, proves that such approach [7] Newmark N.M.(1965). Effects of earthquake on dams and embankments , Geotechnique 15. can be a productive reason to realize some new multi purpose [8] Baldovin G. et al. (2010). Lentini reservoir-Final report of the reservoirs and, by this way, to solve, it is the case of some supervision staff .