Many Italian beaches are today subjected to erosion and several techniques can be suggested for their defence and reconstruction. In general, approaches producing minimal environmental degradation are presently favoured, such as artificial nourishment of the beach using sandy sediments. The purpose of this study is to demonstrate how to locate potential deposits along the continental shelf and how to articulate the chances of a successful beach replenishment program. The area where potentially extractive sandy bodies were searched, is located in the central Tyrrhenian continental shelf, between the rocky headland of Ansedonia and the coastal lake of Lago di Burano (Southern Tuscany). The beaches selected for possible artificial nourishments are those of Tombolo di Feniglia, Marina di Tarquinia, Ladispoli and Ostia (Central Italy). In the above-mentioned shelf area, high resolution seismic and sedimentological data highlight a Holocene depositional sequence, which is bounded by the present bathymetric surface and by the unconformity of the last-glacial lowstand. This sequence comprehends highstand (HST) and transgressive (TST) systems tracts. The HST is relatively thin and in the study area consists of a lateral succession of three sedimentary facies: (1) distal part of the modern coastal prism, (2) Posidonia mats, and (3) recent drape deposit (sandy and muddy). The underlying TST is bounded at the bottom by the lowstand unconformity and is abundantly made up of mineable sandy deposits. These deposits are organized in three vertical sequence of facies, each located on a specific shelf zone parallel to the coast. (1) Facies Sequence A, on the external shelf zone, is 1-3 m thick and consists of a sand sheet, deriving from the destruction of an ancient beach complex via shoreface retreat processes.(2) Facies Sequence B, on the middle shelf zone, is 3-5 m thick and comprises three facies. The lower facies (above the unconformity) is a 1 m thick muddy coastal-marsh deposit, the middle one consists of a 2-4 m thick sandy body deriving from the coalescence of several washover fans, and the upper facies is the same sand sheet belonging to the Sequence A. (3) Facies Sequence C, on the inner shelf zone, refers only to the lack of transgressive deposits. Where present, they consist of very sporadic and scattered palimpsest sands (belonging to the sand sheet facies). Further TST evidence, not linked to the above facies sequences, is a partially eroded beach lithosome, which is present close to the Formiche di Burano island. The Sand extraction from the TST deposit was evaluated calculating the volume of material required (a) for 50 m enlargement of all the selected beaches and (b) for their decennial maintenance by minor nourishments. These estimes were obtained respectively by using the surface/volume ratio (one square foot of the beach surface corresponds to one cubic yard of material) and the deficit in the sedimentary budgets (previously deduced by the recent shoreline variations). As TST and beach sediments were different in grain size, the resulting values were then corrected by the James fill factor (estime of point a) and the nourishment factor (point b). This additional procedure is based on mean grain size and sorting statistics of composite samples, representative of the TST and each beach sediment. The final volumes for both reconstruction and decennial maintenance of Tombolo di Feniglia (SW and SE sectors), Marina di Tarquinia, Ladispoli and Ostia beaches were estimated at 1,461,951 m3, 1,745,303 m3, 899,000 m3, 2,411,014 m3 and 6,249,111 m3 respectively. Geometry and facies distribution of the TST suggest that the best zone for mining is located past the external limit of the Posidonia oceanica colony. Five possibilities are proposed for material extraction: (1) an extensive excavation over the entire TST, obtaining an amount of 106,000,000 m3; (2) a superficial excavation approximately 2 m in depth over the sand sheet facies, for an amount of 84,600,000 m3; (3) mining on the entire TST, between the external limit of the Posidonia area and the 35 m isobath (zone 1), to obtain 52,200,000 m3; (4) a surface drag in the area previously described, but only 2 m in depth, producing 30,900,000 m3 of sediment from the sand sheet facies, and (5) mining, 2 m in depth and seaward the 35 m isobath (zone 2), creating 53,800,000 m3 from the sand sheet facies. Any of the above mentioned scenarios could satisfy the sediment quantities required for the reconstruction of all the selected beaches. In any case, potential damage to the nearby Posidonia oceanica colony strongly requires an environmental impact investigation.

Sandy shelf deposits as source material for artificial nourishment of modern erosive beaches: an integrated approach using high resolution seismic and sedimentological analysis / Tortora, Paolo. - In: GIORNALE DI GEOLOGIA. - ISSN 0017-0291. - STAMPA. - 56/2:3a(1994), pp. 275-277.

Sandy shelf deposits as source material for artificial nourishment of modern erosive beaches: an integrated approach using high resolution seismic and sedimentological analysis

TORTORA, Paolo
1994

Abstract

Many Italian beaches are today subjected to erosion and several techniques can be suggested for their defence and reconstruction. In general, approaches producing minimal environmental degradation are presently favoured, such as artificial nourishment of the beach using sandy sediments. The purpose of this study is to demonstrate how to locate potential deposits along the continental shelf and how to articulate the chances of a successful beach replenishment program. The area where potentially extractive sandy bodies were searched, is located in the central Tyrrhenian continental shelf, between the rocky headland of Ansedonia and the coastal lake of Lago di Burano (Southern Tuscany). The beaches selected for possible artificial nourishments are those of Tombolo di Feniglia, Marina di Tarquinia, Ladispoli and Ostia (Central Italy). In the above-mentioned shelf area, high resolution seismic and sedimentological data highlight a Holocene depositional sequence, which is bounded by the present bathymetric surface and by the unconformity of the last-glacial lowstand. This sequence comprehends highstand (HST) and transgressive (TST) systems tracts. The HST is relatively thin and in the study area consists of a lateral succession of three sedimentary facies: (1) distal part of the modern coastal prism, (2) Posidonia mats, and (3) recent drape deposit (sandy and muddy). The underlying TST is bounded at the bottom by the lowstand unconformity and is abundantly made up of mineable sandy deposits. These deposits are organized in three vertical sequence of facies, each located on a specific shelf zone parallel to the coast. (1) Facies Sequence A, on the external shelf zone, is 1-3 m thick and consists of a sand sheet, deriving from the destruction of an ancient beach complex via shoreface retreat processes.(2) Facies Sequence B, on the middle shelf zone, is 3-5 m thick and comprises three facies. The lower facies (above the unconformity) is a 1 m thick muddy coastal-marsh deposit, the middle one consists of a 2-4 m thick sandy body deriving from the coalescence of several washover fans, and the upper facies is the same sand sheet belonging to the Sequence A. (3) Facies Sequence C, on the inner shelf zone, refers only to the lack of transgressive deposits. Where present, they consist of very sporadic and scattered palimpsest sands (belonging to the sand sheet facies). Further TST evidence, not linked to the above facies sequences, is a partially eroded beach lithosome, which is present close to the Formiche di Burano island. The Sand extraction from the TST deposit was evaluated calculating the volume of material required (a) for 50 m enlargement of all the selected beaches and (b) for their decennial maintenance by minor nourishments. These estimes were obtained respectively by using the surface/volume ratio (one square foot of the beach surface corresponds to one cubic yard of material) and the deficit in the sedimentary budgets (previously deduced by the recent shoreline variations). As TST and beach sediments were different in grain size, the resulting values were then corrected by the James fill factor (estime of point a) and the nourishment factor (point b). This additional procedure is based on mean grain size and sorting statistics of composite samples, representative of the TST and each beach sediment. The final volumes for both reconstruction and decennial maintenance of Tombolo di Feniglia (SW and SE sectors), Marina di Tarquinia, Ladispoli and Ostia beaches were estimated at 1,461,951 m3, 1,745,303 m3, 899,000 m3, 2,411,014 m3 and 6,249,111 m3 respectively. Geometry and facies distribution of the TST suggest that the best zone for mining is located past the external limit of the Posidonia oceanica colony. Five possibilities are proposed for material extraction: (1) an extensive excavation over the entire TST, obtaining an amount of 106,000,000 m3; (2) a superficial excavation approximately 2 m in depth over the sand sheet facies, for an amount of 84,600,000 m3; (3) mining on the entire TST, between the external limit of the Posidonia area and the 35 m isobath (zone 1), to obtain 52,200,000 m3; (4) a surface drag in the area previously described, but only 2 m in depth, producing 30,900,000 m3 of sediment from the sand sheet facies, and (5) mining, 2 m in depth and seaward the 35 m isobath (zone 2), creating 53,800,000 m3 from the sand sheet facies. Any of the above mentioned scenarios could satisfy the sediment quantities required for the reconstruction of all the selected beaches. In any case, potential damage to the nearby Posidonia oceanica colony strongly requires an environmental impact investigation.
1994
beach nourishment; continental shelf; seismic; sedimentology; methods.
01 Pubblicazione su rivista::01a Articolo in rivista
Sandy shelf deposits as source material for artificial nourishment of modern erosive beaches: an integrated approach using high resolution seismic and sedimentological analysis / Tortora, Paolo. - In: GIORNALE DI GEOLOGIA. - ISSN 0017-0291. - STAMPA. - 56/2:3a(1994), pp. 275-277.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/85187
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