Avisio Torrent, is an Italian stream located in the central eastern part of the Alps and a left tributary of the Adige River. It is about 90 km long with an average gradient of 2.02%, has a basin with an area of 940 km2 and it feeds 4 hydroelectric power plants. Avisio is as an emissary of Fedaia lake (2054 m above s.l.), which receives the melting waters of the Marmolada glacier.The river runs through the valleys of Fassa, Fiemme and Cembra, and flows into Adige River at a height of 196 m above s.l. in the town of Lavis, a few kilometers north of the city of Trento. In Fassa Valley, the stream receives its first tributary, the “Ruf de Contrin”, in the village of Penia, and, continuing downstream, it suddenly widens describing a wide curve towards the south. In Canazei village Avisio receives the “Ruf d'Antermont” tributary from mountain of Sella Group and, moving towards the valley, it greatly increases its reach thanks to the tributaries from Rosengarten group. After Soraga village the stream is barred from Pezzé Dam. In Fiemme Valley the water catchment area extends in its maximum amplitude, receiving numerous streams, such as Travignolo stream, the main tributary. Travignolo generates from a glacier and is barred in Paneveggio valley creating Forte Buso lake; it flows in Avisio torrent near Predazzo village. Moving downstream, after the village of Molina, the Avisio stream is dammed to create Stramentizzo Reservoir, and its waters are conveyed to the hydroelectric plant of San Floriano of Egna (Bolzano Province) and then returned directly to Adige river. Immediately after the artificial lake Stramentizzo, the stream (with significantly reduced flow rate because of withdrawals for hydroelectric purposes) enters into Cembra Valley, through a characteristic and winding canyon in the porphyry rock. The major tributaries in this reach are those of the left side of the basin. Lastly Avisio flows into the Adige River at Lavis, with a very broad delta (width about 1 km), forming a swampy zone that is a protected area, the biotope “Foci dell'Avisio”. From a geological point of view, moving from the Fassa Valley to the Fiemme Valley, Avisio gradually passes the from Dolomite region (with limestone rocks) to the porphyritic atesina platform (with siliceous rocks) and this affects the hydrology. In Cembra Valley, given the geological conformation of the territory (impermeable rocks) and the steep slopes, Avisio has a distinctly torrential character, with minimal flow rates of about 5 m3/s and flood events with over 1000 m3/s. With regard to the climate, Trentino has the characteristics of a typical alpine region, with cold and dry winters and cool and rainy summers. The mean annual precipitation for the Avisio basin settles around 993 mm (PGUAP data).
The two pilot sites on Avisio river are located around the hydroelectric reservoirs Pezzè (46°23'2.09"N - 11°39'50.62"E - Fassa Valley) and Stramentizzo (46°15'50.27"N - 11°22'24.81"E - Fiemme Valley). The reaches of interest are both upstream and downstream of the dams, and the main physical characteristics are described in the table below (Table 1). With respect to the hydrological regime and the ecological state of the watercourse, both aspects are conditioned by the presence of the reservoirs.
Location and map of the pilot area reach
From an hydromorphological point of view, the presence of artificial lakes induces an interruption on sediment transport. Currently the reservoirs management doesn't include efficient methods in order to ensure the sediments continuity between upstream and downstream. Especially in Stramentizzo reservoir all sediment supply from upstream is stored in the reservoir and there aren't sediment flushes planned, transit of sediment occurs only during flood events with obvious environmental problems.
Stramentizzo dam was build between 1954 and 1955, reservoir total capacity is 11,5 million m3 (10 million useful for regulation). The power house is located in the Adige valley (Egna – BZ), in terms of flow the hydropower plant of Stramentizzo deduct from Avisio catchment until 30 m3/s (average 13 m3/s). Recent studies report the presence of 3,5 million m3 of sediment in Stramentizzo lake that, of course, significantly reduce the basin regulation capacity and, moreover, sediment creates considerable problem to the dam bottom discharges.
Pezzé di Moena dam was build in 1952, reservoir total capacity is 460,000 m3 (360,000 m3 available for weekly regulation). The power house and water restitution in Avisio river is located near Predazzo, around 10 km downstream respect Soraga Lake. In terms of flow the hydropower plant deduct until 7,7 m3/s, part of this water come from San Pellegrino river where, with a pipe of 1,2 km, around 1 m3/s of water is derived from the river to Pezzé di Moena reservoir. Reservoir management include periodic sediment flushes in order to ensure reservoir efficiency, on the other hand this practice leads important ecological problems. Actually periodic sediment flushes does not allow to ensure the regulation capacity maintenance, basin sediment deposition create also problems for general public for the presence of a public park on basin banks.
The most important pressure affecting hydromophology of Avisio, with consequent physical and ecological responses, is damming of the river, that induces:
- strong hydrological alteration that affected river dynamics; the alteration are even worse in presence of hydroelectric plants with diversion catchment (in Avisio river Stramentizzo and Forte Buso);
- interruption of upstream sediment supply. Consequences are an increase of erosion processes, an alteration of river dynamics and, more in general and combined with previous point, a reduction of river ecological quality;
- barrier to fish migration that affect local fauna;
- clogging of the dam discharges and storage capacity reduction;
Other pressures that are present are:
- Artificial embankments, located upstream Stramentizzo dam, that induces an inhibition of the natural processes of stream morphological evolution, with a reduction of natural riparial areas;
- Massive presence of river weirs, especially in lateral tributaries, that induces phenomena similar of those caused by river damming in terms of sediment transport reduction;
- Periodic sediment flushes, performed on river dams (for example Pezzé di Moena), that induce several ecological problems; most of them are consequence of the induced high water turbidity that decrease dissolved oxygen and destroy breeding habitat for fish and small invertebrates; furthermore, high water turbidity during sediment flushes affect also other derivation located downstream in terms of water quality (fish farm) and derivation obstruction.
The main objective is to include in the current reservoirs management methodologies and actions in order to preserve and improve existing Ecosystem Services. The improvement in reservoirs management could include more environmental sustainable sediment flushes and studies and methodologies for artificial sediment reintroduction downstream the Stramentizzo dam.
Soraga reservoir during sediment flushes