Beach erosion control along the Golfo di Follonica (Southern Tuscany): actual hard protection vs. potential soft solutions

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1 Littoral 2002, September. Porto, Portugal. Beach erosion control along the Golfo di Follonica (Southern Tuscany): actual hard protection vs. potential soft solutions Pierluigi Aminti 1, Luigi E. Cipriani 2, Pierluigi Iannotta 3 and Enzo Pranzini 3 1 Dipartimento di Ingegneria Civile - Università degli Studi di Firenze. Via di S. Marta, Firenze, Italy. Tel , Fax aminti@dicea.unifi.it 2 Regione Toscana - Dipartimento delle Politiche Territoriali e Ambientali. Via di Novoli, Firenze, Italy. Tel , Fax , l.cipriani@regione.toscana.it 3 Dipartimento di Scienze della Terra - Università degli Studi di Firenze. Via Iacopo Nardi, Firenze, Italy. Tel , Fax , epranzini@unifi.it Abstract The main rivers feeding the 21-km-long Golfo di Follonica beach were diverted for land reclamation in 1830, inducing a limited but chronic erosion. After World War II the town of Follonica expanded towards the coast, and sea-front buildings began to be threatened by shoreline retreat: several detached breakwaters were therefore built in front of the town. Their effectiveness was limited yet enough to induce erosion in the adjacent beaches, where additional structures were constructed. From 1954 to 2000 mean shoreline retreat was of approximately 8.7 m for the entire beach. Globally, a net sediment drift from the unprotected parts of the Gulf to the protected ones has been acknowledged. During this time, 5 km of hard structures were constructed, at a present-day cost of A new 5.4-km-long submerged detached breakwaters is now under construction at a cost of Thus, the hard protection of the beach will have a cost of A gross estimation of sediment deficit gives a value of m 3 which could have been balanced with artificial nourishment at a cost of / (marine / terrestrial sediments) THE STUDY AREA The Golfo di Follonica (Southern Tuscany, Italy) is located on the Tyrrhenian Sea between Piombino and Portiglione rocky headlands (Figure 1). Its beach stretches for 21 km from north-west to south-east and is sheltered to the west by the Elba Island. The coastal plain hosted, up to the XVIII Century, several lagoons and marshes, which were partly reclaimed by the diversion of the two main rivers feeding the beach; the Cornia River, on the north-western side, with and estimated sediment load of approximately m 3 /yr, and the Pecora River, on the south-eastern one, with and estimated sediment load of approximately m 3 /yr (Cavazza and Nardi, 1976). Both these rivers were diverted in 1830; the former returned to flow directly into the sea in 1957, but was not able to efficiently feed the beach because of intensive river bed quarrying undertaken during the XX Century, the latter is still filling the Scarlino marsh. Data on subsidence exist for the western side of the Gulf, where, from the Mid-Pliocene to the Present a rate of approximately 0.03 mm/yr was estimated. From 1890 to the 1950 s the rate increased to 1 2 mm/yr. In recent decades, intensive water extraction for agriculture and for the industry located in this part of the coastal plain increased land subsidence to the present rate of 5 10 mm/yr. (Bartolini et al., 1977 and 1989). Shoreline retreat brought to light a discontinuous beach-rock layer, at the sea-level, which englobes wastes of the Etruscan iron industry, thus proving its years age, together with a stability of the coast during this time span. Figure 1 Position of the study area.

2 Littoral 2002, September. Porto, Portugal Figure 1 Location map of the study area. Numbers refers to section for which longshore transpost has been calculated (see Tab. 1). Whereas the western side of the Gulf, near Piombino, hosts an industrial district (steel and power plants), the central and eastern parts are mostly uninhabited, except some tourist villages and the town of Follonica, born as an industrial settlement in the late XIX Century, but converted to tourism after World War II. In addition to the Piombino commercial harbor, located on the western extremity, two more facilities are present along the coast: the small harbor built in the late 1970 s on the eastern side of the Cornia River outlet, in order to serve the power plant at Torre del Sale, and the marina in progress of construction since 2000 at the eastern tip of the Gulf at Scarlino (Figure 1). 2. WAVE DATA AND LONGSHORE TRANSPORT Wave data collected by the Royal Dutch Meteorological Service (KNMI) from 19** to 19** were integrated with direct wave records by a non directional buoy located inside the Gulf since 1978, in order to characterize deep water wave climate of the study area. Based on directional wave frequency distribution in deep water, correspondent distributions in shallow water at breaking point were calculated using a wave refraction spectral model. Potential longshore sediment transport was calculated for 13 sections (Figure * and Table 1) along the Gulf utilizing the CERC (1984) formula. Potential longshore sediment transport can be assumed representative of actual longshore sediment transport only along stretches of coast characterized by the absence of coastal structures. I risultati evidenziano un flusso prevalente di sedimenti dalla zona centrale del golfo verso le estremità. Va notato che i valori netti sono molto limitati e che sono dello stesso ordine degli apporti di materiali da parte del fiume Cornia il cui trasporto solido è stimato in mc anno (Cosma,2001) Frequency (%) Figure * - Wave frequency distribution (KNMI data)

3 Littoral 2002, September. Porto, Portugal. Table 1 Mean resultant longshore transport along the Golfdo di Follonica (see Fig. 1 for section location; + = westward; = eastward) Section. [m 3 /yr] Figure 1 Sectors in which the coast was divided to compute mean shoreline displacement. 3

4 Littoral 2002, September. Porto, Portugal Figure 1 Mean shoreline displacement along the Golfo di Follonica beach from 1954 to 1983/84 (average sectors length is 500 m). 3. BEACH EROSION The diversion of the Cornia and Pecora Rivers in 1830 for land reclamation dramatically reduced sediment input to the Gulf and induced a limited but chronic beach erosion. Unreliable data are available to quantify shoreline retreat before 1954, however Mori (1940) identifies alternate phases of beach erosion and accretion since For the study of the last 50 years are available several aerial photos taken in: 1954, 1973, 1976, 1979 and 1983/84, from which the shoreline position has been mapped through a photogrammetric procedure. In 1996 the shoreline of the western side of the Gulf was surveyed with traditional topographic methods, and in 2000 a full survey was carried out, together with a bathymetric survey and beach and nearshore sediment sampling. A map at the scale of 1:5.000 has been produced with all available shorelines. To analyze shoreline evolution, the coast has been divided in 44 sectors, each one approximately 500-meter-long (Figure 2), for which mean shoreline displacement (m) and shoreline displacement rate (m/yr) were retrieved for the time intervals in which data were available for all the coast. Data for the period /84 show that most of the coast was eroding at that time (Figure 3), with values of more than 50 m in the western sectors, although the Cornia River was diverted again to the sea just in The reason for this severe erosion must be found in the intense quarrying activity performed on the beach itself to provide sand to the construction of the industrial plants, and dredging carried out along the river bed to meet the needs of material for the post-war reconstruction. In addition, a dyke was built on the western extremity of the beach in order to reclaim from the sea a large area for the commercial port activity. The filling was never achieved, but in the sheltered area silting occurred with sediments coming from the nearby eastern beach (Figure 4), which eroded for more than 70 m (sector n. 2). 4

5 Littoral 2002, September. Porto, Portugal m m Figure + Figure * - The dyke built on the western side of the Golfo di Follonica in order to create a large yard for the commercial harbor. The filling was never carried out, but silting behind the barrier occurred at the expenses of the eastern beach. 5

6 Littoral 2002, September. Porto, Portugal m m m Figure 1 Beach evolution at Prato Ranieri after the construction of the breakwaters. Photograps: In 19** (bottom left) and in 1982 (bottom right) 6

7 Littoral 2002, September. Porto, Portugal Figure 1 Mean shoreline displacement along the Golfo di Follonica beach from 1983/84 to 2000 (average sectors length is 500 m). Figure * - The first wooden buildings on the Follonica beach in

8 Littoral 2002, September. Porto, Portugal Figure * - Follonica: the eastern beach in 19** (left) and in 1996 (top), after the construction of the detached breakwaters. Figure * - Deposition of very fine sand behind the detached breakwaters on the Follonica eastern beach. Limited changes occurred in the shoreline position along the central part of the Gulf, but significant shoreline displacement took places in the eastern part (Figure 3). Here, the town of Follonica was expanding on the beach itself and shore protection works were performed to prevent sea-front buildings collapse: they consisted in offshore detached breakwaters, which reduced beach erosion but increased this process in the adjacent sectors, mainly in the eastern ones. Here, new structures were requested by the local Authority and built by the Ministry of Public Works. In the 80 s and 90 s more coastal defenses were constructed to protect tourist villages built over the foredune bordering the coast in the central part of the Gulf (Baia Toscana, Figures 5 and 6; Pratoranieri, Figures 7 and 8). Also these structures were effective in halting beach retreat, but again they increased beach erosion on the unprotected neighboring sectors (Figure 9). In front of Follonica more detached breakwaters were built and the oldest, which had gradually sunk, were reshaped to the original design. Again these structures were effective and in the sheltered sectors silting occurred, locally with the formation of tombolos and deposition of very fine sand, hosting more pollutants and making seawater turbid when people wade in the shoreface. Mean shoreline displacement from 1983/84 to 2000 is very limited in respect to that performed in the previous period, and erosion and accretion are concentrated along beaches where old structures had not reached an equilibrium configuration or where new one were built. 8

9 Littoral 2002, September. Porto, Portugal. The beach expansion in sectors 1 and 2 (Fig. *) is due to silting behind the dyke of the commercial harbor area, which still induces erosion in the sectors 3 7. Beach accretion from sector 19 to sector 24 is the effect of the construction of ****; the same is for sectors and sectors All these are flanked by eroding sectors. Along the eastern side of the Gulf the effects induced by the new marina were recently evident The marina shelters sectors 42 and 43 where sediments coming from sectors 40 and 41 are deposited, silting the harbor entrance such as a first dredging operation just performed has demonstrated. 4. BEACH EROSION MANAGEMENT CONSIDERATIONS Comparing shoreline position in 1954 and in 2000, a mean beach erosion of approximately 8.7 m (0.19 m/yr) for the entire beach is retrieved; whereas from 1984 to 2000 beach accreted for 0.54 m (0.34 m/yr). The narrowness of the original beach (due to the limited wave energy present in this area) and the construction of houses on the foredune and, frequently, on the beach itself, made a limited beach erosion a crucial problem. Detached breakwaters were built with the help of a law enacted in 1907, which made financial support available to protect seaside resorts from marine erosion as a consequence of storm events. Project and realization were performed by the Ministry of Public Works staff, originally harbor engineers and thus more suited to plan breakwaters instead to consider more soft solutions which were tested abroad (Copa Cabana, Virginia Beach, Ocean City, Redondo Beach and so on). This hard solution was facilitated, in Italy, by the availability of low cost stone rocks, present along the rocky coast or in the nearby mountain quarries. Due to the lack of river sediment input into the physiographic unit, confirmed by the petrographic maturity of the sand (Gandolfi and Paganelli, 1975), each protected segment of the coast trapped sediments from the neighboring beaches. The early breakwaters were efficient and able to collect sand from the adjacent segments of the beach; however, the late ones were not able to perform similar results, because most of the available sand was already trapped behind the former structures. therefore, slight erosion rates gave rise to a strongly impacting engineering system that now asks for more and more hard structures, as proved by the last project, now under construction, with a continuous 5-km-long submerged detached breakwater to replace the present structures. In order to defend the Golfo di Follonica beach, 5 km of hard structures were constructed, at a present-day cost of approximately The new submerged detached breakwaters, long 5.4 km and with a 25 m wide berm, is now under construction at a cost of A gross estimation of sediment deficit (considering a fixed beach slope and the presence of beach-rock outcrops) gives a value of m 3. The present day cost of beach nourishment with inland borrow material is approximately 20,00 per cubic meter; and the studied beach s deficit could therefore be fully covered with ,00. On the other hand, using offshore borrow sediments (approximately 12,00 per cubic meter) the cost in order to restore the 1950 s shoreline position is approximately ,00. Soft shore protection systems should have had a lower cost, however the hard solution was preferred, although it had a strong environmental impact and it determined the reduction of the beach value as a consequence of a decrease in beach and sea water quality. REFERENCES Taveira-Pinto, F; Proença, M.F. & Veloso- Gomes, F Experimental analysis of the energy reflected from submerged breakwaters, Coastal Structures 99, Vol. 2, Balkema, Rotterdam; pp ISBN Aiello E., Bartolini C., Caputo C., D'Alessandro L., Fanucci F., Fierro G., Gnaccolini M., La Monica G.B., Lupia Palmieri E., Piccazzo M., Pranzini E. (1975) - Il trasporto litoraneo lungo la costa toscana tra la foce del F. Magra e i Monti dell'uccellina. Boll. Soc. Geol. It., 95: Aiello E., Bartolini C., Conedera C., Pranzini E., Taccini S. (1980) - Il litorale della Provincia di Livorno tra Marina di Castagneto e Follonica. Amm. Prov.le Livorno. 44 pp. Albani D., Griselli A., Mori A. (1940) - Ricerche sulle variazioni delle spiagge italiane. II. Le spiagge toscane. Tipogr. del Senato, 155 pp. Barsanti D., Rombai L. (1986) La guerra delle acque in Toscana. Edizioni medicea, Firenze. 169 pp. Bartolini C., Palla B., Pranzini E. (1988) - Studi di geomorfologia costiera: X - Il ruolo della subsidenza nell'erosione litoranea della pianura del Fiume Cornia. Boll. Soc. Geol. It., 108: Bartolini C., Pranzini E., Lupia Palmieri C., Caputo C. (1977) - Studi di geomorfologia costiera: IV - L'erosione del Golfo di Follonica. Boll. Soc. Geol. It., 96, Cavazza S., Nardi M. (1976) - Trasporto solido in 9

10 Littoral 2002, September. Porto, Portugal sospensione e di fondo dei corsi d'acqua. In: Ricerche sul Regime e la Conservazione dei Litorali. Area Campione Alto Tirreno. Rapporto Quad. Ric. Scient., 92: Cosma M. (2001) - Studio sulla dinamica litoranea del Golfo di Follonica Università di Firenze- Dipartimento di Ingegneria Civile, unpublished degree thesis. Gandolfi G., Paganelli L. (1975) - Il litorale toscano fra Piombino e la foce dell Ombrone (Area campione Alto Tirreno). Composizione provenienza e dispersione delle sabbie. Boll. Soc. Geol. It., 94: