Morfometria e geometria idraulica dei canali della Laguna di Grado (Friuli-Venezia Giulia)

Morphometry and hydraulic geometry of channels of the Grado Lagoon (Friuli – Venezia Giulia)

Abstract

The aim of this paper is to carry out a deeper study of tidal systems and in particular of lagoon hydrographic networks, with reference to the Lagoon of Grado (Northern Adriatic Sea). The scope of the study is to underline, by analysing the morphometry of the riverbeds, the meanders and the hydraulic geometry of the channels, the similarities and differences between tidal and fluvial canalization, in order to apply this knowledge to reconstruct the environmental evolution or for the recognition of buried paleochannels.
The analysis of the hydrographic network as a whole points out that the methodology used to study fluvial systems can only be applied in part to lagoon systems, as the latter present particular features that can be sommarized as the bidirectional flows and as normal outflooding of channels waters.
Tidal network is composed of main channels, that conduct river or marsh waters towards the sea, and secondary channels, that distribuite and receive waters from tidal planes, according to a system of hierarchization of the network, which is very similar to that proposed by ASHLEY & ZEFF (1988).
Quantifying the development of the canalization of the Grado Lagoon, it is seen that, no an average, one square km of lagoon basin subtends 1 km of the main channel and a secondary channel of the same length. The analysis of the morphometry of the channels and lagoon meanders has shown that these do not differ greatly from meandering fluvial channel. It is to be pointed out that the relation between different dimensions of the beds and the lagoon meanders are defined by empirical laws that differ only minimally from those summarized by WILLIAMS (1986) for channel in general.
The only important difference seen, is that concerning the development of the meanders (Am), which is greater in fluvial channel than in lagoon channels. This difference seems to be only partly related to the different characteristics of the flows, whereas, it is greatly attributable to the different conditions of stability of the embankments, given that lagoon banks are set by the colonization of halophyte plants. On examining the equations that link channel flow and channel morphometry, there appear to be important connections between the wavelength of the meander, the width of the channel and the flow. The study of the hydraulic geometry of the lagoon channels has shown that there is great analogy between the innermost segments of lagoon network and of the fluvial canalizations, whereas the main segments are different.