The Isarco River is the second largest river in South Tyrol and its length is about 95 km Its springs are located near the Brenner Pass, at an altitude of about 1,990 m a.s.l.. The catchment area is about 4,200 km² and it ranges between 3,509 m a.s.l. and 235 m a.s.l., at the confluence in the Adige River (in the city of Bolzano). The hydrological regime of Isarco river is classified as nivo-pluvial due to a glacier area contribution lower than 2% of the total (Koboltschnig and Schöner 2011). The mean annual precipitation in the main valley ranges from 690 mm (in Cardano) to about 1,100 mm in the upper portion of the basin.
Overview of the study area. The picture shows the region of South Tyrol, north of Italy, and the blue area identifies the Isarco catchment
The Isarco River is subject to heavy human impacts, being largely channelized with artificial banks and presenting strong hydroelectric exploitation. The target river reach within the HyMoCARES project lies between Cardano (just downstream of an important power plant) and the confluence with the Adige River. It flows across the city of Bolzano for a total length of about 10 km. It s course is characterized by alternate bars, typical morphological feature of channelized rivers having large cross section: alternate bars generally resettle downstream after flood events. The alternate bars shape in the Isarco is generally stable and as a consequence a strong channel-bed incision occurs. Moreover, during high flow events they might suddenly move downstream causing deep riverbanks incision. Other typical morphological features are islands in the middle of the river section, which in some cases present woody vegetation on top.
The target reach results mostly confined and altered both in the hydrological regime (hydropeaking) and in the morphological characteristics, being rectified, embanked and subject to frequent clearcuts of riparian vegetation for its entire length. Nonetheless it still hosts an important fish population.
The Civil Protection Agency of the Autonomous Province of Bolzano has been working on restoration works along this reach since 2013 to solve problems due to channel narrowing and straightening. The main objective was to avoid a further incision of the riverbed.As reported above, gravel bars in Isarco river are mainly static, due to the substantial absence of solid material inputs from the tributaries, and from the Isarco itself. This involves the progressive erosion of the channel bed and deep incision of the riverbank (up to 4-5 m) once the bars have moved downstream because of a flood event. Along the studied reach the embankments are walls supporting the cycling path and protecting the urban area from flooding. Therefore reaches erosion might end up in the collapse of the wall, with danger for human lives and important potential expenses for the Autonomous Province of Bolzano. Human interventions caused the loss of natural microhabitats for fish communities and the loss of the natural shape and dynamics of the river. Another important human alteration regards flow fluctuations. Many hydropower plants are located along the Isarco and one of them is placed on top of the study reach (in Cardano). The amount of water released downstream the plant is often insufficient influencing negatively the natural flow depth of the river. In particular the typical winter low water discharge is often lost.
Alternate bar structure of the Isarco River
The restoration project spans over a period of several years, starting in 2013.
The main goals of the restoration project are:
- Foot reinforcement of the bank wall: erosion processes might incise the terrain below the wall and cause the wall to collapse. For this reason, the bank wall has been footed with a reinforced concrete kerb on micropiles to ensure wall stability and flood protection;
- Re-designing the river reach to increase the morphological variability through flow deflectors, islands and single large boulders placed into the riverbed;
- Stabilization of riverbanks wit the positioning of large boulders: in some parts, they were covered with terrain, with the aim of enhancing the natural aspect of the reach.
- Channel widening and renaturalization of the riverbanks;
- Reshaping of stable bars to activate sediment transport, increasing morphological variability and reducing channel incision and degradation;
- Sediment replenishment to improve the morphological variability, to fill the missing diameter in the granulometric size distribution, altered by hydropower segregation effect, and to limit erosion;
- Promoting the connection between the city of Bolzano and the Isarco River through recreational areas along the river, thanks to smoother embankments that pedestrians and cyclists can access.