DINAMICA MORFOLOGICA DEL LITORALE DEL DELTA DEL FIUME TAGLIAMENTO (ADRIATICO SETTENTRIONALE) IN RELAZIONE AI POSSIBILI INTERVENTI DI RIPASCIMENTO

Abstract

Gordini E., Marocco R., Ramella R., Morphological dynamics of the littoral zone of the Tagliamento River delta (Northern
Adriatic Sea) related to eventual nourishment works. (IT ISSN 0394-3356,2006).
The morphodynamic analysis of the Tagliamento River delta beaches, where two of the most important holiday sites of the upper
Adriatic Sea are located, revealed some areas undergoing endemic erosion, some of which are presently under accretion, and others
that appear to be stable. Sixtysix topographic and bathymetric profiles orthogonal to the emerged, inter-tidal and submerged portions
of the beaches were obtained during 2002 and 2003. Comparison to results from earlier studies showed that seasonal and multiyear
variations in the submerged areas are very low, except for the zones located in correspondence to the river mouth and the tidal inlet.
Here, the thickness of the sea floor sediment cover increases due to the deviation of the alongshore sediment transport as a consequence of both the tidal and fluvial outflows from the lagoon. In the most dynamic areas of the submerged beaches (bars and troughs), the sediment thickness variations are mainly represented by the outward and inward movements of the bars without net sediment volume losses or gains. A different situation has been identified in correspondence to the inter-tidal and emerged zones, where the variations are more conspicuous and are due not only to natural phenomena (limited alongshore feeding, waves etc.) but also to several coastal protection structures (constructed in the past for different purposes) that are not very effective. The worst situation has been found at the Tagliamento River mouth where most of the river levees and the apical beach areas are undergoing intense erosion.
Instead, the adjacent submerged beaches are in general presently accreting (a global increase of 490.000 m3
). The areas characterized by sediment accretion are mainly located in the central sector of the delta front and at its eastern side, southeast of a groin. The volumetric variations in the area have been analyzed over both short and long time-scales, and the resulting information has been confronted with those obtained from previous studies carried out in 1969 and in 1985 (BRAMBATI, 1987). This defined that, after a first accretion phase of the inter-tidal and exposed parts and of a deepening of the submerged portions of the beaches (1969 -1985), a completely different regime set in (1985-2002) which appears to characterize the present situation. A sediment supply in the order of about 50.000 m3 /yr is urgently necessary to replenish the eroded beaches. Sediment could be extracted from the river fan which presently has to be re-shaped and deepened for safe navigation (68.000 m3 or 166.000 m3 depending on whether a water depth of 2,5 m or 3,5 m is required, and a width roughly ranging from 100 m to 150 m). Part of this material (90.000 m3
) could also be obtained from the accretion areas, characterized currently by annual feedings, that were identified in the present study.