Flood-dominated Fan Delta System Induced by the A.D. 79 Somma-Vesuvius Eruption at Amalfi Coast, Southern Italy
by
Crescenzo Violante
CNR - National Research Council - Istituto per L'ambiente Marino Costiero (IAMC)
Coauthors: Marco Sacchi, Eliana Esposito, Giuseppe Gargano, Sabina Porfido, Eutizio Vittori.

The Stream Canneto and the adjacent Stream Dragoni have deposited a coalescing fan delta system along the sidewall of Amalfi Coast, a rocky coastal area on the southern side of the Sorrento peninsula. The submerged delta system has an area of about 7 × 105 m2 with a delta-front gradient averaging 25-27^° to water depths of 50 m, about 500 m from the shore. The intertidal and supratidal delta are narrow areas where the settlements of Amalfi and Atrani developed because of water resources and low topographies.

Both Streams drain high-relief basins and have high-gradient steep-sided profiles cutting into carbonate bedrock discontinuously covered by reworked fallout deposits of the A.D. 79 Somma-Vesuvius eruption. These latter occur as well lithified flood/landslide accumulations (locally called Durece) up to 40 m thick along the Stream valleys.

Large floods and sliding phenomena have frequently occurred in this area, suggesting rapid slope morphodynamics. Slides are triggered by intense rainfalls over a range of magnitudes, inducing displacement of the sedimentary covers rapidly flowing down slopes (earth/debris flow). These events induced severe flood-flow in the main Streams: the first delivery area for the displaced materials then ultimately transported to the delta area and adjacent inlets. Indeed, the floods also caused major damage to property and loss of livelihoods, as testified by historical analyses, a mandatory step for the evaluation of the hydrogeological hazard.

Side-scan sonar imagery, multibeam bathymetry, ultra-high resolution seismic profiling and diving inspections on the subaqueous delta area, allowed for reconstruction of morphology and patterns of sediment distribution indicative of high energy sediment transport processes. Seaward of the main subaerial channels, clinoforms composed of coarse sand, gravels and submetric boulders, prograde towards deeper water. At shallow depth they crop out at seafloor, otherwise they pass upward into "massive" deposits about 2 m thick, by means of an unconformity surface. At present, the upper slope of the subaqueous delta is characterized by hard grounds and mattes of living Posidonia Oceanica, testifying a reduced flood control. A volcaniclastic level found offshore in core samples and recognized elsewhere as the underwater expression of A.D. 79 fall out event, develops at the base of the prograding units thus providing a lower chronological constrain.

The discovery of a fan-delta system offshore Amalfi coastal area testifies the recurrence of extreme floods as a consequence of the famous A.D. 79 Somma-Vesuvius eruption. These events were mainly fed by unstable accumulation of volcaniclastic fall-out deposits on a steep rocky coast, and induced a maximum seaward shift of the shoreline of about 500 m. The emplacement of the Amalfi fan-delta implied very critical conditions for human activities, lasting many decades after the destructive A.D. 79 eruption and after, as testified by flooding events occurred over the last century in this area.

References

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Date received: July 22, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-53.