The Talvera river has a length of 46 km and a catchment area of 429 km²; it drains the Sarentino Valley and it is the 2nd largest tributary basin of the Isarco River. The highest point within the Talvera catchment is at 2,781 m a.s.l. (Cervina Peak), while the lowest one corresponds to the confluence with the Isarco River (260 m a.s.l.). Just upstream Bolzano, the Talvera enters a deep gorge, named Sill, flanked by rocky walls, then downstream it flows over its fan. Due to the lack of glaciers on the upper part of the catchment, it shows a nivo-pluvial hydrological regime.
Talvera river reach (left) and basin (right)
Along its course, the Talvera is diverted several times, thus presenting a reduced flow rate until the diverted water is returned to the river in the city of Bolzano (St. Antonio hydropower plant). In addition, many tributaries of the Talvera are also highly impacted by diversions for hydroelectric purposes. The Talvera has been channelized and confined during past years. The channelization of the Talvera occurred between the 60s and the 80s, when the cross section, up to 180 m wide, was confined to a roughly constant width of 20 to 50 m. In addition, due to the torrent high solid transport capacity, several check dams were built in the past in order to foster sediment and woody debris deposition and retention and thus to protect Bolzano from flood risk.
The channel slope ranges between 1.8 and 2.4% with a prevalence of cobble-size sediment. The widespread presence of reinforced embankments reduces the river erosion and mobility contributing to a static equilibrium of the watercourse. The reduction of the stream velocity was achieved by building a series of 43 transversal structures. Riparian vegetation is also affected by anthropogenic pressures, especially in terms of reduced width of the perifluvial zone with consequent lack of woody material in the riverbed.
The hydropower plants along the Talvera in Bolzano relies on three storage reservoirs, two are located in the Community of Sarentino and the third in Renon, that induces to sharp and abrupt fluctuations in discharge downstream the S. Antonio power plant (hydropeaking effect). The fluctuation between high and low discharge values brings an additional burden to the aquatic life, especially in the spawning period and during the juveniles development. The overall morphological state of the described stretches is between Poor and Very Poor.
Aerial view of the Talvera river in 1954 (left) and 2014 (right)
The restoration project
The Talvera in the area of the S. Antonio power plant has a huge ecological potential providing spawning areas and good habitat for the juveniles of like grayling (Thymallus thymallus), common barbel (Barbus barbus) and marble trout (Salmo marmoratus). The main goals of the restoration project are the improvement of the longitudinal connectivity for the fish population, the increase of the number of habitats, the enhancement of stream physical heterogeneity and of flow variability.
The restoration works started in 2014 and went on up to spring 2019. They includes:
- the removal or partial opening of the weirs all along the stretch, in order to allow longitudinal connectivity;
- the establishment of a medium water profile to solve problems due to low water level (hydropeaking);
- sediment replenishment to restore the natural sediment size composition of the riverbed.
- dead wood introduction and recreation of natural macroforms (e.g. flow deflectors, bars, pools),
- the removal of 2 huge check dams in the upper reach, those were substituted by an innovative check dam, where sediment flux and fish migration are ensured;
- the enhancement of the recreational value of the Talvera by improving the longitudinal connectivity (kayaking and fishing).
Additional temporal measures involves the hydropower plants management, which should limit discharge fluctuations during the most sensitive periods of the year (e.g fish spawning period).