Renaissance of the Historical Lentini Lake
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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.