Environmental Engineering Reference
In-Depth Information
The whole catchments area of the lagoon covers about 1880 km
2
with 65 municipalities and
a rivers network of 743 km. Rivers flow into the lagoon mainly in the western sector
(Marano) draining waters coming from the spring line. The estimated overall amount of
average freshwater discharge is about 70-84 m
3
s
-1
(ARPA-FVG, 2006; Ret, 2006) and is
mostly due to the Stella River, particularly important for its peculiar estuarine habitats, the
Cormor River and the Corno River, that receives the water of the Aussa River before
entering the lagoon. Salinity is strongly influenced in the areas close to the river mouths
where very low values (from 2 to 7) are often recorded. Conversely, a sudden increase is
observed moving towards the tidal inlets, where values between 24 and 36 (typically
marine) were recorded (Brambati, 2001). Temperature gradients and other biogeochemical
parameters (i.e., dissolved oxygen, nutrients, chlorophyll) are characterized by high spatial
and temporal variability (Acquavita et al., 2010a), with some extreme phenomena such as
ipoxic/anoxic conditions recorded in the more confined areas especially in case of a high
organic carbon content coupled to a scarce water renewal.
The sedimentary input of small rivers consists mostly of fine suspended material washed
out from the groundwater table (Brambati, 1972), whereas the most important source of
sediments comes from the sea, through the tidal inlets, as a contribution of the two river
deltas (silty and clayey particles) and from erosion of the barrier islands (sands) (Brambati,
1970). Grain-size distribution observed for the whole lagoon is characterized by medium to
fine sands prevailing along the both main and secondary channels. The fine pelitic fraction
is widely dispersed in the basin and varies from 52% to 100%. The silty fraction (62.5-3.9 µm)
normally comprises a higher percentage than the clay fraction (<3.9 µm) in the pelitic
component. Generally, the ratio fine-silt/clay (2-16/<2 µm) identifies areas affected by
freshwater inputs into the lagoon: a high ratio indicates an area of fresh water input and
vice versa (Piani & Covelli, 2000; Piani et al., 2005).
The main sub-basins identified in the past are characterized by several morphologies. The
presence of barriers, sandbanks, saltmarshes, channels, intertidal/subtidal flats and tidal
creeks are strictly dependent from both tidal and hydraulic regimes. The Marano Lagoon is
a semi-enclosed shallow tidal basin, with a few marshes and several channels, receiving
freshwaters from several adjacent rivers. The Grado Lagoon is shallower, has a series of
morphological relieves (islands) and marshes, and receives freshwater from a single
tributary, the Natissa river (Marocco, 1995).
As already reported for other lagoon environments this system is clearly dynamic, being the
morphology subjected to relevant changes derived from both natural and human influences.
Fontolan et al. (2009) observed a remarkable decrease of saltmarshes surface, which
occurred especially in the last decades (1954-1990), in the central-western areas (i.e. S.
Andrea, Grado and Porto Buso sub-basins), probably due to relative sea level rise, erosive
processes and land reclamation. Actually, the main salt marshes are located between
Marano and S. Andrea sub-basins and in Morgo. Fringing marshes are also located in the
inner parts of the Marano sub-basin.
However, the influence of human activities on lagoon morphology cannot be neglected. In
order to protect the main inlets breakwaters were constructed, along with the creation of the
Porto Nogaro harbor, which is connected by means of a lagoon channel (Aussa Mare), up to
7.5 m of depth, with the open sea for shipping purposes. In addition, stabilization of the
barrier islands and several docks for marinas were realized.
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