The Talvera Torrent has a length of 46 km and a catchment area of 429 km²; it drains the Sarentino Valley and it is the second 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 enter a deep gorge, named Sill, flanked by rocky walls, then downstream of 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 Torrent reach (left) and Talvera Torrent 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 mainly for hydroelectric production, heavily impacting its natural ecological conditions.
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 stream reach target of the restoration within the HyMoCARES project includes the last 5 kilometres of the Talvera before the confluence with the Isarco River. Before the restoration, the average stream width along the studied reach ranges between 20 m to 50 m. However, formerly the Talvera river sections were twice their actual size 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 presence of check dams determines a significant alteration of morphological forms and inhibits river active processes. The restoration projectaims at promoting sediment continuity by the removal of man-made cross section structures and at enhancing longitudinal connectivity for fish migration. In addition, the check dams removal enhances fish movements upstream the torrent, and therefore its ecological connectivity. 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 overall morphological state of the described stretches is between Poor and Very Poor.
Hydro-morphological restoration project
The main human alterations affecting the restored river reach of the Talvera are river channelization, hydropeaking due to the hydropower plan activities and hydro-morphological alteration due to a series of check dams and bank protection. 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, a stronger river bed incision due to higher flow velocity occurred. The reduction of the stream velocity was achieved by building a series of 43 transversal structures. These check dams cut the longitudinal connectivity of aquatic populations along the river reach and a loss of habitat occurred. Moreover, a selection on fish species occurred according to the check dams height preventing the movement of some of them upstream: as an example, the common barbel (Barbus barbus), the European bullhead (Cottus gobio) and in general all the juveniles individuals cannot overcome obstacles higher than 80 cm.
Since the construction of those check dams, sediment transport and supply have been deeply reduced; this to a loss of gravel areas, which provided habitats for several fish species.
The hydropower plant 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. In addition the hydropeaking, due to shear stress along the river bed has an effect also on bed armouring and vegetation removal.
Talvera Torrent in 1954 (left) and 2014 (right)
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 trutta 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, aiming at:
- 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 introduction to restore the natural sediment size composition of the riverbed. Bulk of gravel material of the embankment areas limit allochtonous woody species, such as tree of heaven (Alianthus altissima) or black locust (Robinia pseudoacacia), more difficult.
- Dead wood introduction and recreation of natural macroforms (e.g. flow deflectors, bars, pools),
- The identification of the indicator species, such as the European bullhead (Cottus gobio), whose spread can be used to evaluate the efficiency of the restoration measures.
- The removal of a consolidation check dams in the upper reach, which was substitute with a retention check dam , where fish migration is ensured also in low water regime;
- The enhancement of recreational value of the Talvera. The longitudinal connectivity of the stretch allow recreational activities such as kayaking and fishing.
Due to the natural redesign of the riverbed and the removal of anthropic obstacles, the good state of the water body should be achieved from a water-morphological point of view, according to the EU-WFD. 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).
Upper Talvera river-reach pre-restoration. Series of weirs and a consolidation check dam impeding fish passage. On the left bank, the S.Antonio hydropower plant.
Additional info can be found at the following link: