Marano and Grado Lagoon: Narrowing of the Lignano Inlet

Abstract

The morphological evolution of a lagoon tidal inlet over a medium-long period is a very important research topic since it can greatly affect both the hydrodynamic balance of the coastal environment and all the several human activities related to its proper functioning. The morphodynamic balance, which is the result mainly of the complex interaction of tidal currents and wind waves, can also be deeply influenced by the presence of maritime structures that are required for sea defence. This is the case of the Lignano inlet, which has undergone a progressive narrowing during last decades. In order to investigate the causes of this process and to evaluate possible solutions for the consequent filling of the port access canal, a morphodynamic-spectral coupled model has been applied to this context. Results are presented and discussed confirming that the numerical modelling can be used as a useful engineering tool for the correct management and the integrated planning of coastal zones.

An Integrated Approach to Study the Morphodynamics of the Lignano Tidal Inlet

“…Figure 3 shows that until the beginning of the 1980s, when the Marina Punta Faro port was built, the width of the inlet was close to 660 m; after the construction of the protective pier, it was reduced to approximately 500 m. The Lignano tidal inlet is undergoing a progressive narrowing phenomenon, which is due to the formation and growth over time of a sand deposit on the external side of the protective pier of Marina Punta Faro port. In order to determine the causes of the inlet shrinking, an in-depth historical reconstruction has been carried out by Petti et al [9] with a focus on the morphological evolution of the easternmost part of Lignano Sabbiadoro barrier spit. Figure 3 shows that until the beginning of the 1980s, when the Marina Punta Faro port was built, the width of the inlet was close to 660 m; after the construction of the protective pier, it was reduced to approximately 500 m. Since 1988 the increase of the sand deposit, delineated by the dotted line, has led to a considerable narrowing of the inlet, which has been further reduced to 300 m with an overall loss of 55% compared to the 1988 width.…”

“…The historical reconstruction of the morphological development of the sand deposit provided by Petti et al [9] is based on a variety of cartographic, aerial and satellite images of this area. The first available image, dated 1891, highlights the rounded shape of the spit end of the Lignano barrier, which remained almost unchanged even after a series of groynes was built in 1938.…”

“…During the years after 1991, when the harbor was completed, no other structures were built in this area; however, the shoreline progressively moved from the pier towards the center of the inlet, as shown in Figure 4. The historical reconstruction of the morphological development of the sand deposit provided by Petti et al [9] is based on a variety of cartographic, aerial and satellite images of this area. The first available image, dated 1891, highlights the rounded shape of the spit end of the Lignano barrier, which remained almost unchanged even after a series of groynes was built in 1938.…”

“…Tidal inlets act primarily as hydraulic regulators of water exchange between the sea and the back-barrier basin; they also function as both source and sinks of sediment in this manner governing the morphological, ecological and environmental balance of the adjacent coastal system [8]. Moreover, in many cases, they represent preferential waterways towards harbor sites located at the back-barrier basin, increasing the socio-economical value of these natural connections [9]. For these reasons, the morphological evolution of tidal inlets is the focus of a decadal detailed research, which has attempted to understand the main processes governing their behavior and their consequent prediction over a medium or long term.…”

“…The complex dynamism of this transitional area is highlighted by significant morphological changes that have occurred over the last few decades, during which a narrowing trend has been outlined. Petti et al [9] provided an accurate historical reconstruction of the coastline evolution pointing out that the protective pier of the port beside the inlet has greatly affected the border of the Lignano barrier spit. Moreover, some preliminary numerical simulations were conducted on two configurations, before and after the construction of the port respectively.…”