LE BARENE DELLA LAGUNA DI MARANO E GRADO: ANALISI DEGLI ASPETTI MORFO-EVOLUTIVI NELLA PROSPETTIVA GESTIONALE.

2012/2013Lo studio delle barene della laguna di Grado e Marano è stato affrontato attraverso due differenti approcci: 1) una raccolta di dati morfologici e sedimentologici in 13 siti campione 2) un’indagine a macroscala attraverso il confronto di foto aeree (1954, 1990, 2006) e l’applicazione di differenti tecniche di analisi in GIS. L’integrazione dei due diversi approcci ha permesso in primo luogo di proporre una classificazione delle barene in base alle loro caratteristiche morfologiche e sedimentologiche: barene di margine lagunare, margine di canale, retro barriera, bacini paralagunari recenti, isolate. Inoltre si è giunti a una definizione e quantificazione dei processi evolutivi in atto e delle loro cause. Il declino significativo nelle superfici a barena corrispondente a 145ha (16% dell’estensione del 1954), una volta esclusa la forzante quantitativamente più rilevante (azione antropica diretta con 175ha), appare molto ridotto e limitato al secondo intervallo di tempo. Le variazioni areali riscontrate sono infatti il risultato di perdite e guadagni a scale differenti, spesso in grado di compensarsi, le quali sono state classificate in diverse tipologie morfoevolutive, associate ad altrettante forzanti. La metodologia proposta è originale e si mostra adatta all’analisi delle aree che presentano una certa scarsità di dati. Dall’analisi topologica sulle singole barene emerge che i fenomeni erosivi che si manifestano con rilevanza maggiore sono in ordine decrescente: l’annegamento (effetto combinato di eustatismo, subsidenza regionale e autocompattazione), erosione da ondazione indotta dal transito dei natanti, azione del moto ondoso da vento, i processi legati alla dinamica costiera. I processi di accrezione sono invece imputabili agli apporti fluviali, agli apporti legati alle correnti di marea, ai processi accrescitivi nei bacini paralagunari recenti e in ex valli da pesca. La mancanza di un comportamento unitario dell’intera laguna e la differenziazione temporale fanno intendere come siano predominanti le forzanti a breve termine rispetto alle forzanti che agiscono sul lungo termine, prima fra tutte l’eustatismo. Il confronto con i dati ipsometrici ottenuti dal confronto batimetrico 1966-2011 conferma però la tendenza trasgressiva in atto nell’intera laguna con approfondimenti diffusi soprattutto a carico dei fondali intertidali, ma anche la differenziazione esistente tra i bacini. I dati di bilancio sedimentario mostrano inoltre una relazione con la variazione areale delle barene per ogni singolo bacino. Parte integrante del lavoro è costituita da un geodatabase contenente tutti i dati e le informazioni relative alle barene; esso può rappresentare un valido strumento di supporto nei processi decisionali di gestione e di pianificazione territoriale. A tal fine sono state individuate una prima serie di filosofie gestionali e strategie d’intervento, associate alle differenti tipologie erosivo / accrescitive individuate.XXVI Ciclo197

Fluxes of settling sediment particles and associated mercury in a coastal environment contaminated by past mining (Gulf of Trieste, northern Adriatic Sea)

Purpose: As the result of historical mining at Idrija (Slovenia), mercury (Hg) contamination in the Gulf of Trieste (northern Adriatic Sea) is still an issue of environmental concern. The element has been conveyed into the coastal area by the Isonzo/Soča River inputs of freshwater and suspended particles for centuries. This research aims to investigate the occurrence of Hg bound to the settling sediment particles (SSP) in the coastal water and to assess the sedimentary Hg fuxes. Methods: Settling sediment particles were collected at four sites located in the innermost sector of the Gulf, a shallow and sheltered embayment where the accumulation of fine sediments is promoted. Six sampling campaigns were performed under different environmental conditions in terms of discharge from the Isonzo River and 12 sediment traps were installed in the upper and bottom water column for SSP collection. Settling sediment particles (SSP) were collected approximately every 2 weeks and analysed for grain size and total Hg. Results: Settling sediment particles (SSP) consisted predominantly of silt (77.7±10.1%), showing a concentration of Hg ranging overall between 0.61 and 6.87 µg g−1. Regarding the daily SSP fuxes, the minimum (7.05±3.26 g m−2 day−1) and the maximum (92.4±69.0 g m−2 day−1) values were observed under conditions of low and high river discharge, respectively. The daily Hg fluxes displayed a notable variability, up to an order of magnitude, both in the surface water layer (3.07–94.6 µg m−2 day−1) and at the bottom (11.3–245 µg m−2 day−1), reaching the maximum values following periods of high river flow. Conclusions: The Isonzo River inputs of suspended particulate matter continue to convey Hg into the Gulf of Trieste, especially following river flood events, which represent one of the most relevant natural factors affecting the variations of the Hg flux in the investigated area

Modern sedimentary facies in a progradational barrier-spit system, Goro lagoon, Po delta, Italy

Barriers and spits connected to fluvial sedimentary sources represent environments which tend to evolve rapidly and experience sudden transformations, mainly driven by changes in sediment supply and path. As a consequence, the variability of facies is significant even within small sedimentary records. The 7 km long barrier-spit system facing the Goro Lagoon, and fed by the mouth of the Po di Goro, is a typical example of an accretionary coastal morphotype, suitable to describe adjacent nearshore depositional environments and their stratigraphic signatures, variability, and relationships. Thirteen short cores of sediment were sampled in order to represent the variable depositional subenvironments from the shoreface (prodelta-delta front) to the back barrier, crossing the active barrier-spit and the ancient spit arms and relative swales. The description of the modern sedimentary records, improved upon using core X-rays, has been coupled with information on the morphological changes which occurred during the period of maximum spit development (1955\u20132000), based on available aerial photos and a cartographic/topographic dataset. The results obtained allow for the description and interpretation of the depositional environments changing at the human-scale. Sediments of the upper shoreface are quite uniform, composed by evenly laminated sands; the transition between delta front and prodelta at a depth of 6m is marked by the alternation of sand and mud beds. These reflect the periodic changes in sediment supply by the river, as well as storm events. The most recent spit branch and the relative back barrier-swale environment are the results of the rapid progradation of the spit system, which implies phases of rapid longshore growth, hooked spit development, cannibalization, overwash, and breaching. Morphodynamic changes have resulted in an overlap of short sedimentary records where stratigraphic signatures are linked either to phases of sediment transport and selection by waves and tidal currents (cross-bedding, foreset, and planar laminated sands, shell imbrication, massive beds) or to phases of sedimentary stasis when biological activity is predominant (algal mat and bioturbation). Human signature is also well marked inside the stratigraphic record. Clam harvesting is carried out within the lagoon, causing the physical disturbance and winnowing of the superficial sediment, thus inducing the local formation of graded beds and shell rehash

Saltmarshes in a backbarrier lagoon: sediment source and variability

In backbarrier lagoons, saltmarshes represent a depositional sub-environment with peculiar characteristics and management demands. With saltmarshes covering approximately 760 ha, the Grado and Marano Lagoon is an important wetland in the Northern Adriatic (Italy). Here, the extent of saltmarshes has decreased over the last fifty years due to a combination of natural and anthropogenic factors. Two short cores of sediment were collected from each of the twelve sample sites, which were chosen to represent different saltmarsh environments. Their stratigraphic description was combined with the recent morphological evolution (1910–2010), as determined by cartography, aerial photos, and topographic surveys. The facies description has been improved by using grain size and mineralogical-compositional data determined by a laser diffraction particle size analyser and the powder diffraction method, respectively. In the inner part of the lagoon, silt content is dominant even in cores situated in different morphological contexts, such as isolated and channel-fringing saltmarshes or abandoned fish farms. The facies transition from the tidal flat sub-environment to the saltmarsh sub-environment is frequently discernible and is marked by small grain size differences, both fining and coarsening upward. These different trends attest to the role of local changes in environmental and hydrodynamic conditions, as well as human action. Sand is the main component in the backbarrier saltmarshes, where the grain size variability along the core is evident due to the mixing of tidal and coastal processes. The mineralogical composition is geographically distributed in accordance with the morphology and hydrodynamics of the lagoon. The highest carbonate concentrations were only found in saltmarshes near or directly connected to tidal inlets, thus emphasising the importance of the external sedimentary source (Isonzo and Tagliamento rivers), with calcite playing a discriminant role. Our findings and interpretations contribute to a better understanding of the depositional model of saltmarshes in a back-barrier lagoon, particularly in different erosive or depositional contexts and with significant human interferences

Inventory and conservation assessment for the management of coastal dunes, Veneto coasts, Italy

Management of coastal dunes on developed coasts could effectively take advantage of comprehensive and multi temporal georeferenced data collection, which offers the possibility to relate dune data with the natural and cultural characteristics of the beach and hinterland. The recent implementation of a coastal management geodatabase for the Veneto region provides the opportunity for improving knowledge on coastal dunes on developed littoral as well as a basis for appropriate future coastal planning in the study area. The geodatabase gathers data concerning different physical, evolutionary and human aspects of the coastal zone, with a special focus on coastal dunes. Established foredunes, human-altered dunes and relict dunes are irregularly distributed along 59 km, 38% of the entire coastal length. Their distribution and characteristics are the result of favourable natural conditions as well as long-lasting tourism exploitation (evaluated through an index of Land Use Pressure) and fragmentary and diversified uses of beaches (evaluated through an index of Tourism Pressure on the beach). At the same time, beach/dune nourishment intervention allowed the presence of artificial or sand fenced dunes along 17 km of coast. High dune elevation up to 8\u201310 m is promoted by the onshore exposition of the beach to dominant wind (from ENE), by stable-to-slowly negative sedimentary budget or by the re-activation of high relict foredunes in the case of shoreline retreat associated with strong negative budget. Present sedimentary budget (evaluated through the code ASPE \u2013 Accretion, Stable, Precarious, Erosive) is the tool used for dividing coasts in sedimentary compartments or cells. Past and present sedimentary budget and different human responses to erosive cases (hard and soft interventions) give the foredunes different means to form, grow, survive and evolve over time. The assessment of human impact and active management practices on the dunes allows a first evaluation of the management effectiveness, which shows strong shortcomings for 81% of the dunes. The great variability of beach usage, human impact and management practises on the different dune stretches highlight the lack of effective and systematic management actions being correctly scheduled and performed

Inventory and conservation assessment for the management of coastal dunes, Veneto coasts, Italy

4noreservedmixedBezzi, A.; Pillon, S.; Martinucci, D.; Fontolan, G.Bezzi, Annelore; Pillon, Simone; Martinucci, Davide; Fontolan, Giorgi

The historical practice of disposal of dredged sediments in the Marano and Grado Lagoon: evaluation of geomorphological, evolutionary and functional perspective of the resulting artificial saltmarshes

An inventory was conducted of the present saltmarshes whose origin is due to the dredging and sediment disposal subsequent to an important waterway carried out in the early 1900 in the Marano and Grado Lagoon, a Special Area of Conservation (SAC) located in the northern Adriatic Sea. Morphology, evolution and habitat distribution were investigated by means of field surveys and topological analysis in GIS, using air photo for the period 1954-2011. Considering the overall maintenance of saltmarshes after 57 years since the sediment disposal, the frequent presence of near-natural morphologies and the subsequent development of Natura 2000 habitats, it is possible to state that building saltmarshes with dredged sediment has achieved good results during time. The historical practice of re-using dredged sediments for morphological and habitat restoration could be re-proposed as best practice for an effective conservation and management of the lagoon

Scenari di inondazione costiera: la realtà di Grado

Le zone costiere sono vulnerabili agli eventi di acqua alta causati da una combinazione di alta marea e condizioni meteoclimatiche. Grazie al modello altimetrico delle coste del FVG, preciso e aggiornato, è possibile simulare mappe di allagamento potenziali, sia nelle condizioni attuali che considerando scenari futuri di innalzamento del mare, come fatto ad esempio per Grado

Tidal Flats Morphodynamics: A new Conceptual Model to Predict Their Evolution over a Medium\u2010Long Period

The morphological evolution of tidal flats has been widely investigated in recent years as it represents a very important topic which is highly related to the climate-driven environmental changes. The period over which geomorphological changes can be noted is a multi-year up to pluri-decennial time scale, defined as medium-long period. This work presents a new conceptual model which is able to predict and estimate a limit depth between an erosion condition and a no-erosion condition for tidal flats. The domains of applicability are shallow and confined basins, where tidal flats are characterized by near-horizontal topography, as occurs inside lagoons. The theoretical approach provides a general equation which relates the limit depth of tidal flats to current velocity and critical erosion shear stress. The procedure, followed through to its development, takes into account the important role of the bottom friction dissipation in wind wave generation process for shallow water. The relationship between tidal flat depth, current velocity and critical shear stress is provided in three different configurations, depending on the direction of the wave motion compared to the current. The limit depth compared to the measured depth can suggest if tidal flats tend or not towards an erosion state over a medium-long period. In this sense, the conceptual model provides a relevant contribution to the comprehension of morphodynamics of these important environments. This approach has been validated with its application to a real context and the results are provided in the paper

Geomorphological Changes of a Migrating Sandbank: Multidecadal Analysis as a Tool for Managing Conflicts in Coastal Use

While beach erosion and sand loss are typically of great concern to the tourism industry, managing rapid morphological changes linked to large amounts of moving sediments is the challenge facing Grado, an important seaside resort in the northern Adriatic, Italy. The cause of the unusual management conflict is the presence of the Mula di Muggia Bank, a nearshore depositional system made up of relict and active migrating sandbanks extending up to 2 km seawards from the touristic beachfront. A reconstruction of the morpho-sedimentary evolution of the coastal system over a 200-year period was done using a large dataset which includes historical cartography, topographic maps, aerial photos and topo-bathymetric surveys. The results show the growth of a significant urban development aimed at creating a tourist destination by occupying the waterfront along fetch-limited coastal tracts with very shallow water and scarce hydrodynamics. Furthermore, a number of sandy dynamic landforms (longshore migrating bars, a bypass corridor, an ebb-tidal delta) and accumulation zones attest to a sediment excess which can be mostly attributed to the eastern river supplies. The progressive constant migration rate of 12.6 my-1 allowed the bank to induce the expansion of the low-energy silty backbarrier environment, characterised by abundant seagrass meadows a short distance directly in front of the tourist beaches of Grado. As a result of historical analysis and more current observations, areas with diverse morphosedimentary features and with varying tourist/recreational, ecological, and conservation values have been identified. These can be considered as basic units for future accurate planning and re-evaluation of coastal management choices to balance environmental protection and tourist use. A soft coastal defence approach is proposed which includes either the preservation of specific environments or the proper use of excess sand for beach nourishment via periodic dredging or sediment bypassing