Journal of Coastal Research
SI 56
287 - 291
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Beach Nourishment and Foredune Restoration: Practices and Constraints along the Venetian Shoreline, Italy. A. Bezzi†, G. Fontolan†, K.F. Nordstrom‡, D. Carrer† and N.L. Jackson ∞ † Dipartimento di Scienze Geologiche Ambientali e Marine Università degli Studi di Trieste, Trieste 34127 Italy
[email protected] [email protected] [email protected]
‡ Institute of Marine and Coastal Sciences Rutgers University, New Brunswick, NJ 08901, USA
[email protected]
∞ Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA
[email protected]
ABSTRACT BEZZI A., FONTOLAN G., NORDSTROM K.F, CARRER, D. and JACKSON N.L., 2009. Beach nourishment and foredune restoration: practices and constraints along the Venetian shoreline, Italy. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 287 – 291. Lisbon, Portugal, ISSN 07480258 Artificial beach nourishment is commonly practiced along the North Adriatic coast of Italy to fight beach erosion. About 10 million m3 of sand has been placed along 45 km of coast in 10 years. The strong wind regime and the need to protect coastal habitat have drawn attention to problems associated with aeolian transport and dune reconstruction. This study analyses three nourished sites, where morphological and sedimentological data were gathered along complete beach/dune profiles. The beach/dune sediment budget over a period of 2, 6 and 10 years after the interventions and the elevation and distance of the reconstructed dunes from the shoreline were examined and compared with a natural control area. A grain size suitability evaluation was carried out, together with a verification of the effects of grain size selection in transport from the beach to the dune. The study enabled evaluation of the potential for nourishment practices to reconstruct and restore the beach/dune system. The renewed sediment availability yielded different results in term of dune supply, ranging from the total lack of dune accretion to the spontaneous formation of embryo foredunes. Dune changes do not depend on sediment used for fill, because the relatively fine grain size does not limit aeolian transport. The beach management practices (fences, raking etc.) seem to be critical, together with shoreline orientation and the consequent aeolian transport potential. In many cases, the practices are not suitable and should be redirected to achieve a satisfactory compromise between environmental protection and recreational use. ADITIONAL INDEX WORDS: Sediment budget, sedimentology, shore protection
INTRODUCTION Artificial beach nourishment is usually conducted to address reduction in sediment sources, to protect shorefront buildings and to provide space for recreational use; less common is the idea that foredunes must be included in nourishment projects as a fundamental natural coastal defense against storms. Artificial beach nourishment is rarely used as an opportunity to restore natural beach / dune landforms and habitats (Nordstrom 2008). Wind blown sand transport in coastal environments is an important part of the coastal sediment budget. Aeolian sand drift should not be neglected in a nourishment program, whatever the purpose of the intervention. Part of the nourished sand is subject to aeolian processes and may produce a sediment loss and cause adverse effects on recreation and buildings, both on the beach and in the hinterland (van der Wal, 2004). When adjacent dunes are present, aeolian drift may produce a net input of sand onto the dunes and promote dune growth by vegetation trapping sediment. Alternatively, dunes may be
reconstructed by earth moving equipment or by fencing (Carter and Wilson, 1990). The results of nourishment for coastal defense and environmental restoration are governed by several factors: the characteristics of the sediment, the rate of sediment exchange between the beach and the dune after nourishment (van der Wal, 1998; Matias et al., 2005), the techniques used to trap the sand, the space available seaward of human structures, and the politics of coastal management (Nordstrom and Jackson, 2003). Information on the restoration potential of beach nourishment projects is often lacking because agencies responsible for these projects rarely conduct post-nourishment evaluations or they concentrate on losses of fill sediment rather than evolution of natural features (Nordstrom et al., 2000). The coast of North Adriatic Italy provides an opportunity to improve this knowledge. A total of 10 million m3 of sand has been placed along 45 km of the coast in 10 years. The strong wind regime and the need to protect coastal habitat have drawn attention to problems associated with aeolian transport and dune reconstruction. In this study, three beach nourishment sites
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(Pellestrina, Cavallino and Eraclea) and a natural control area (Ca’ Roman) were selected and compared to assess potential advantages and limitations of nourishment practices aimed at reconstructing and restoring the beach/dune system.
STUDY AREA Until the 1950s, the Veneto coast (Figure 1) was almost entirely fronted by dunes up to 10 m high, but only a small portion of these natural features survives (Bezzi and Fontolan, 2003). Since 1995 nourishment projects have been carried out to maintain and restore the coast with sand derived from the sea floor. The entire 11 km length of Pellestrina Island was protected by a revetment since the 18th century and the beach was eliminated. The 5-m-high revetment protects a shorefront road and urban community just landward of it. Nourishment and structural defense operations were performed in two phases, thus creating 18 beach compartments or cells, bounded by 18 stone groins. 4.6 million m3 of sand were used to fill Cells 1-10 (in the South) during 1995-97, and Cells 10-18 (in the North) during 1999. Submerged sills, 300 m from the shoreline, were built to help hold the sediment in place on the steep shoreface. In 1999 sandtrapping fences were placed on the new backshore and Tamarix gallica trees were planted to form a wind break and keep sand from blowing over the revetment into the urban area. In Cavallino an 11 km-long segment of beach, protected by an ancient revetment, was nourished in 1994-1999 using 2 million m3 of sand and 32 stone groins, spaced 300 m apart, were built. For the first time in Italy, a protective dune (4-m-high, 50 to 90 m wide) was constructed for flood protection along a discontinuous 4.8-km-long tract, covering the revetment. The protective dune was constructed using bulldozers and then planted with European beach grass (Ammophila littoralis). Elevated dune walkovers were built for access to the beach and sand fences were placed on the seaward side. Since 1953, the eroding beach at Eraclea was protected by a low seawall and a series of groins. Landward, a dune remains as the remnant of a dune destroyed by big storms in 1966-69. The beach was nourished in the spring of 2004, when 1.6 million m3 of sand dredged from offshore was placed along 1.7 km of shore, creating an enlarged beach and completely burying the old seawall. The old groins were removed and new groins were built at greater size and spacing. The beach concessionaires rake the surface of the beach to remove litter (wrack) to improve recreation. An effective comparison with a natural beach/dune model is quite difficult for the Veneto shorelines, due to the intense urbanization and development and the scarcity of natural dune fields. A good compromise is provided by the beach of Ca’ Roman, which is accretionary due to trapping of sediment by the jetty at the southern end of Pellestrina Island. The accretion began at the end of the first phase of jetty construction in 1911 and has been ongoing, resulting in a series of low dune ridges. The site is protected from human development as a result of a biotope protection measure (European pSIC IT3250023) and has evolved without human manipulation of the beach or dune or vegetation plantings.
METHODS Field data were gathered at the four sites in 2006 in early October. Topography was measured along three shore-normal transects at each of the four sites using a Thales Pro Mark 3 GPS system having a horizontal and vertical measurement error of 20 mm. Transects in the three nourished sites (Figure 2) extended from wading depths to the landward dune toe or the first human
Figure 1. Field site llocation and wind rose for the period 19732002 (elaboration from data by APAT: www.idromare.com). structure. In an attempt to evaluate the aeolian potential for the different sites, the wind data from the Lido station (Figure 1) were analysed for the period 1999-2006 (APAT, www.idromare.com). The onshore component of wind velocity for the different coastline orientations were calculated according to the formula proposed by Davidson-Arnott and Law (1996). Two datasets of profiles were used to determine morphometric parameters (distance from shoreline and crest elevation) and compare volumetric sand budgets: Annual profiles carried out after the nourishment operations, courtesy of Magistrato alle Acque of Venice (MAV); Topographic monitoring (4 profiles) carried out in 1999/2000 and 2005/2007 in Ca’Roman by Fontolan et al (2008). The MAV profiles located in dune areas were chosen (33 for Pellestrina, 15 for Cavallino and 4 for Eraclea) after removing profiles with evident accidental errors. Four years were compared: the nourishment year (1997 for South Pellestrina and Cavallino, 1999 for North Pellestrina), 1999, 2000 and 2006. For Eraclea the comparison is limited to the period 2004-2006. An attempt was made to identify the aeolian sediment transport rather than the hydrodynamic sediment transport, although most parts of the supratidal beach developed by the combination of waves and wind. The adopted boundaries are the mean sea level and +1.4 m above it, which corresponds to the mean elevation of the embryo foredune toe at natural sites along the Veneto coast. The landward boundary is either the point of profile closure, or a vertical line in the foredune backslope. In order to verify the granulometric compatibility between native and nourished sediment, surficial (few millimeters thick) samples were collected at different morphological locations (Figure 2) along the profiles. The sand samples were analyzed by settling tube, and the grain size statistical parameters calculated according to Folk and Ward (1957). For the Pellestrina beach, comparison with native sediment is not possible because there was not a beach before nourishment. Thus sediment data from the neighbouring Ca’Roman were used.
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Pre-nourishment data were not available for Cavallino, so we used sediment data collected in 1995 in the dune field of Punta Sabbioni, 2.5 km south of the Cavallino site. Sedimentological data collected in 1999 were available for Eraclea. All the sediment datasets are derived from our field work and are unpublished.
RESULTS AND DISCUSSION Wind regime and potential aeolian transport The effectiveness of wind for onshore sand transport depends on the different orientations of the coastline with respect to the dominant wind, blowing in the North Adriatic from ENE (“Bora” wind). Southerly winds (the “Scirocco” from SE) are frequent, but they rarely exceed 20 km/h (the threshold velocity for sand movement at the study sites) and are often associated with rain and storm surge, which strongly affect aeolian transport on the beach. Despite the limitations of the method (Davidson-Arnott and Law, 1996), the onshore component calculated for the examined period reveals a high aeolian potential for Ca’Roman and Pellestrina, whereas Cavallino and Eraclea have a lower potential due to the longshore orientation. Morphological evolution The morphological analysis reveals the main modification that occurred in the beach/dune system during the monitored period and the current (2006) morphological features for each different site (Figures 2, 3, 4). Beach The first morphological variation observed in the nourished areas was the significant retreat of shoreline that occurred after the nourishment operations. The highest erosion rates at Pellestrina occurred during the first two post-nourishment years, with a mean retreat of 16.7 m/yr in the South and 17 m/yr m in the North.
During 2000-06 shoreline retreat diminished, with values of 1.5, and 2.7 m/yr, respectively Currently, the shoreline has a pronounced parabolic indentation inside each groin cell, where the beach may reach a minimum width of 25 m, with an average of 53 m. Maximum beach width occurs at cell extremities, with a mean of 72 m in the north and 74.5 m in the south of each cell. The shoreline evolution in Cavallino is highly variable, mainly due to the historical tendency of the beach to rotate, with erosion along the northern shores, stability along the central pivot area and accretion along the southern end, according to the longshore drift. Trend values are further influenced by location of the profiles inside the beach cells: in the examined profiles the beach maintained large widths, between 71 and 150 m, even 9 years after the nourishment. Similar to Pellestrina, Eraclea revealed high erosion rates between 16.1 and 18.5 m/yr during the first two post-nourishment years. In 2006 the beach widths, including the dunes, varied between 78 and 103 m. Embryo dunes In a natural beach system the sediment transported landward by winds is trapped by vegetation to form embryo dunes at a certain distance from the shoreline, as is occurring at Ca’Roman (Figure 2). At Pellestrina the embryo dune accretion occurred only where the beach is wide enough to allow the growth of the pioneer vegetation, out of the influence of salt water, along the wider beach close to the groins. In general the elevation of the embryo dunes at Pellestrina is similar to that of Ca’ Roman, but they grow at a significantly shorter distance from the shoreline. The presence of embryo aeolian deposits at a narrow distance (about 20 m) from the shoreline is limited, due to a rapid beach retreat in the period 2005/06 that caused an increase in the foreshore steepness, without erosion of the dune. At Cavallino the formation of embryo dunes has been observed only in three MAV profiles, where beach raking is not performed by managers. At Eraclea only the formation of ephemeral unvegetated deposits has been observed. Foredunes In Pellestrina and Cavallino the fence lines were successful in creating a linear dune ridge, with different heights in relation to wind exposure at variable distances from the shoreline.
Figure31. Height and cross-shore location of embryo dunes (empty symbols) and foredunes (filled symbols) in 2006 for the different sites. Figure 2. Example of beach-dune morphology for the different sites. Points represent sediment samples along each profile. Journal of Coastal Research, Special Issue 56, 2009 289
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Figure 4. Foredune growth rates at different sites. Zero reference is the foredune elevation at the time of the first survey
This morphological variability is due to the fixed position of the fences, that prevents the possibility of adaptation of the dune to a new equilibrium condition farther landward when erosion occurs. The North Pellestrina dune deposits have the highest accretionary rate, whereas in the South accretion is similar to that occurring at Ca’Roman (Figure 4). This difference is likely to be related to the construction of the first set of fences along the southern tract after the former phases of nourishment works. Despite the morphological irregularity, the vegetation consists of species and habitat types of community interest according to the EU Habitat Directive. At Eraclea, dunes did not show any dimensional variation two years after the nourishment, thus implying that few sedimentary transfers from the beach occurred. The lack of morphologic change is certainly due to management practices. Provisional fences are used during winter to allow the sand to accumulate, but the deposits are mechanically removed and re-distributed over the beach during late spring for the tourist season. Secondary dunes At Cavallino the presence of fences at the dune toe concentrates sediment in that location and prevents the transfer of sediment farther inland, isolating the original reconstructed crest and adding no sediment to that part of the dune system. The result is that the psammophilous species used for planting the reconstructed dune were not compatible with the stable environment caused by the sand trapping fences and did not survive. Beach / dune budget Data collected at Pellestrina (Figure 5) show that the sediment budget, which is negative for the beach, is counterbalanced by a positive dune budget. Aeolian potential is high and the wind barriers used to prevent sand transport inland are effective as sand traps. The total sediment budget is positive at Pellestrina, in spite of the considerable erosion of the backshore. Dune growth now occurs due to the new backshore, but this situation cannot continue because the progressive reduction of beach width will unavoidably cause the erosion of the dune, that will be unable to migrate landward due to the presence of the revetment. At Cavallino, the beach-dune budget is variable, due to the great differences in beach width between profiles. Only the central tract features a total positive budget; along the southern and northern shores the total budget is negative, mainly due to beach erosion, since dune budget is slightly positive. The budgets at Eraclea, dune included, are all negative, although aeolian potential is greater than at Cavallino and
Figure 5. Total, dune and beach budget and aeolian onshore component (U tot) for Ca’Roman (1999/2006), South Pellestrina (1997/2006), North Pellestrina (1999/2006), Cavallino (1997/2006) and Eraclea (2004/2006). sediment availability is still high, a few years after the nourishment. The results confirm that beach management practices recycle sediment back to the beach and not to the dune. When post-nourishment beach retreat occurs, as normally happens, sand is irreversibly lost, because the potential sand storage by dune accretion is prevented. Sedimentology Sediment grain-size data (Table 1) indicates that the fill material is on the whole suitable compared to the native material. Where the system has had sufficient time and space, the evolution has been similar to that of the natural sites. As revealed in previous studies (Bezzi and Fontolan, 2003), there is a sediment selection in the beach-to-dune transition i.e. a dimensional selection between 0.10 and 0.25 phi towards the finer fractions and a frequent reduction in sorting value. Table 1. Mean size (Mz) and sorting (SD) characteristics of sediments in phi units. foreshore backshore foredune site Mz SD Mz SD Mz SD 2.44 0.32 2.31 1.18 2.50 0.26 Ca’ Roman 2.18 0.24 2.10 0.28 2.19 0.25 Pellestrina 2.26 0.30 --2.36 0.27 Cavallino native 1.90 0.30 2.24 0.34 2.26 0.29 Cavallino 2.06 0.24 2.11 0.32 2.20 0.24 Eraclea native 1.93 0.30 2.21 0.30 2.29 0.32 Eraclea The sediment on the Pellestrina beach seven years after the nourishment is totally artificial, because an external sediment supply is unlikely. It is very well sorted, with a mean grain size coarser than that of the naturally evolving beach at Ca’ Roman, and this contributes to the stability of the nourishment. Compared to the reference natural site of Ca’ Roman, Pellestrina beach is a highly compressed system where the fill sediment is already well sorted and thus the transport process cannot create differences between the beach-dune sub-environments over such short distances. At Cavallino the results show sediment similar to the native, a little coarser but very well sorted. The sediment trapped by the fences is finer and better sorted than that on the beach, as naturally occurs by vegetation trapping at Punta Sabbioni. The beach profiles are less compressed than those of Pellestrina and thus the
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longer fetch allows for more preferential selection and differentiation of grain sizes. At Eraclea the nourished sediment is not clearly distinguishable from the native sediment and the dune sand is on average finer than on the beach.
CONCLUSIONS This study enabled evaluation of the potential for nourishment practices to reconstruct and restore the beach/dune system, with the Ca’ Roman site providing the morphological and sedimentological perspective on the other more human-altered sites. The beach nourishment carried out at Pellestrina rebuilt the beach, which was lacking for hundreds of years. The nourished sediment, which appears to be suitable compared to the native sediment, formed a sizeable dune aided by the Tamarix gallica trees that were planted to protect the inland from windblown sand. This gave rise to the unexpected potential for environmental restoration, due to spontaneous flourishing of valuable vegetation and formation of embryo foredunes where the beach was wide enough. On the whole, the beach nourishment has improved coastal protection, but the morpho-ecological restoration appears limited, mostly by the lack of space seaward of the revetment, which causes a compression of the system and prevents landform mobility. The beach nourishment carried out at Cavallino was characterized by dune reconstruction for the first time in Italy. The intervention has been positive from a coastal protection perspective. Not only has the reconstructed dune survived, the sediment budget analysis shows that a good portion of the beach material has fed the dune. The artificial dune building techniques and positioning of fences created an unnatural beach/dune system, both from a morphological and vegetation standpoint. The interventions had been carried out without proper management practices on the dune and beach, preventing formation of embryo dunes and thus losing natural habitat types of Community interest revealed in the EU habitat directive. At Eraclea, the beach nourishment, which was carried out only for 1.7 km, covered the existing parallel protection structure, improving sediment availability and widening the recreational backshore surface. However, detrimental management practices (beach levelling, mechanical raking up to the dune toe and placement of fences during winter) had not been changed. These practices prevent feeding of the stabilized dune and formation of embryo dunes, despite the suitability of nourished sediment. Management directions considered valid in the whole investigated area include the following: (1) In areas similar to the North Adriatic, where the sediment is predominantly fine sand and the wind speeds are high, aeolian transport plays a key role in the sediment budget. (2) Beach nourishment practices provide a good opportunity for environmental restoration, but it is necessary to assist this trend, through selection of appropriate aeolian sand retention systems (fences or vegetation). (3) These systems must be designed for both environmental restoration and optimization of the nourishment yield and
incorporate a multidisciplinary monitoring approach that considers geomorphic and ecological principles. (4) Particular attention must be paid to the management of the beach seaward of the dunes because beach raking completely removes the ephemeral landforms and pioneer vegetation, with severe loss of potential habitat types of Community interest.
LITERATURE CITED BEZZI, A. and FONTOLAN, G., 2003. Foredunes classification and morphodynamic processes along the Veneto coasts (northern Adriatic, Italy). In OZHAN, E. (ed.) Proceedings of the Sixth International Conference on the Mediterranean Coastal Environment, MEDCOAST’03, Ravenna, Italy: pp. 1425-1434. CARTER, R.W.G. and WILSON, P., 1990. The geomorphological, ecological and pedological development of coastal foredunes at Magilligan Point, Northern Ireland. In: NORDSTROM K.F., PSUTY N.P. and CARTER R.W.G. (eds.), Coastal dunes: form and process. Chichester, England: Wiley, pp. 129-157. DAVIDSON-ARNOTT, R.G.D. and LAW, M.N., 1996. Measurement and prediction of long-term sediment supply to coastal foredunes. Journal of Coastal Research, 12 (3), 654-663. FONTOLAN, G., BEZZI, A., BOSCOLO, F., PILLON, S. and DELLI QUADRI, F., 2008. Monitoraggio geomorfologico delle dune e della spiaggia dell'Oasi LIPU di Ca'Roman. Unpublished report: 97 p. (in Italian) FOLK, R.L. and WARD, W.C., 1957. Brazos river bar: a study in the significance of grain size parameters. Journal of Sedimentary Petrology, 27, 3-26. MATIAS, A.; FERREIRA, O.; MENDES I.; DIAS J.A. and VILACONCEJO, A., 2005. Artificial construction of dunes in the south of Portugal. Journal of Coastal Research, 21 (3), 472481. NORDSTROM, K.F., 2008. Beach and Dune Restoration. Cambridge, England: Cambridge University Press, 187 p. NORDSTROM, K.F. and JACKSON, N.L., 2003. Alternative restoration outcomes for dunes on developed coasts, In OZHAN, E. (ed.) Proceedings of the Sixth International Conference on the Mediterranean Coastal Environment, MEDCOAST’03, Ravenna, Italy, pp. 1469-1478. NORDSTROM, K.F.; LAMPE R. and VANDEMARK, L. M., 2000. Reestablishing naturally functioning dunes on developed coasts. Environmental Management, 25(1), 37-51. VAN DER WAL, D., 1998. The impact of the grain size distribution of nourishment sand on aeolian sand transport. Journal of Coastal Research, 14(2), 620-631. VAN DER WAL, D., 2004. Beach-dune interactions in nourishment areas along the Dutch coast. Journal of Coastal Research, 20(1), 317-325.
ACKNOWLEDGEMENTS We thank the Magistrato alle Acque - Consorzio Venezia Nuova, that kindly made the post-nourishment monitoring data available. This project was supported by a grant from the National Geographic Society. We thank Laura Caruso, Francesca Delli Quadri, Ulrike Gamper and Simone Pillon, for help gathering field data.
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