Technical Note About the Monitoring of Hydromorphological Management of the Avisio River (Autonomous Province of Trento, Italy)
Total Page：16
File Type：pdf, Size：1020Kb
Technical note about the monitoring of hydromorphological management of the Avisio River (Autonomous Province of Trento, Italy) PAT - Autonomous Province of Trento 1 1. General presentation of the study sites 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 1000 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. Pilot Sites Avisio River – Stramentizzo Avisio River – Pezzè reservoir reservoir Drainage area (km²)* 765 267 Location Upstream Stramentizzo Upstream Pezzè reservoir reservoir and downstream and downstream to the 2 to the confluence with Rio confluence with Rio San delle Seghe Pellegrino Length of the study reach (km) ~ 4-5 ~ 2-3 Active channel width (m) ~ 20-30 ~ 15-25 Channel slope (m/m) 0,013 0,032 Planform morphology single-thread (upstream the single-thread dam) and braided (downstream the dam) Lateral confinement Artificial and natural Artificial confined confined Dominant substrate gravel-bed gravel-bed Main sediment sources Active tributaries torrents Active tributaries torrents Table 1. Main physical features of the pilot sites Fig. 1: Basin for two case studies (Stramentizzo case study in orange, Pezzè case study in purple) 2. Hydromorphological restoration/management From an hydromorphological point of view, basin scale this artificial lakes induce 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. 3 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 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. Avisio river is affected from several pressures that affected hydromophology with consequence physical and ecological responses: • Damming of the river is the most important, dam presence induce: ◦ strong hydrological alteration that affected river dynamics from several point of view (morphological, biological, hydraulic,..), the alteration are even worse in presence of a hydroelectric plant 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; ◦ a barrier to fish migration that affect local fauna; ◦ clogging of the dam discharges and storage capacity reduction; • Artificial embankments, presents upstream respect Stramentizzo dam, induce an inhibition of the natural processes of stream morphological evolution, with a reduction of natural riparial areas; • Massive presence of river weir, especially in lateral tributaries, induce similar consequence of river damming in terms of sediment supply reduction; • Periodic sediment flushes, performed on river dams (for example Pezzé di Moena), induce several ecological problems, most of them are consequence of the high water turbidity that decrease dissolved oxygen and destruct habitat for breeding fish and small invertebrates organism. 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 objectives is to include in the current reservoirs management methodologies and actions in order to preserve and improve existing ES. The improvement in reservoirs management could include more environmental sustainable sediment flushes and studies, and if it's possible test, methodologies for artificial sediment reintroduction downstream respect to Stramentizzo dams. 4 Fig. 2: Avisio River 1973 (Stramentizzo dam up on the right) Fig. 3: Avisio River 2016 (Stramentizzo dam up on the right) 5 Fig. 4: Soraga reservoir during sediment flushes 3. Monitoring activities 3.1. General objectives of the monitoring program The main objectives of the monitoring activities are the evaluation of sediment disconnection consequence and the identification of the main factor where it's possible to operate in order to obtain an improvement in reservoir and river management. In particular the activities aims are: • estimate the upstream sediment supply respect the pilot case reservoirs; • estimate the sediment amount present in the reservoirs; • check the sediment composition present in the reservoirs; • estimate the consequence of sediment transport disconnection, especially downstream of the dams, in terms of fauna, flora, stability of flood defense structures and more in general in terms of ecosystem services; • estimate the general river evolution affected from sediment transport disconnection; • estimate the ecological response of reservoir management (sediment flushes,…); • estimate the effects, consequences, possibilities, and if it's poss to reintroduce sediments after Stramentizzo dam; ◦ after preliminary analysis could be possible to artificially introduce sediments and monitoring sediment transport