Construction project and site management for restoring connectivity in the Ter river near La Pilastra weir

Many fish species in river ecosystems undergo migrations at some point in their life cycle. These migrations are crucial for accessing specific habitats that offer favorable environmental conditions, such as water temperature, dissolved oxygen levels, and hydraulic features (depth, velocity, and substrate) essential for reproduction, feeding, and refuge. Without properly designed passage devices, transversal river structures like dams and weirs can create impenetrable barriers, disrupting fish populations and river habitats.

In the La Pilastra area, two weirs were constructed to divert water. The Reial Weir diverts part of the Ter’s flow to the secondary channel, which is currently the most active, while the Pilastra Weir channels water to the Monar Canal. However, the Pilastra Weir faces sedimentation and clogging issues, as well as suboptimal hydromorphological conditions. This impedes native species, including eel, barbel, mountain barbel, river blenny, and stickleback, from crossing this barrier.

The project’s goal is to restore uninterrupted connectivity in this river section naturally, effectively, and permanently, preserving the high ecological value of the Ter riverbanks. Fish ramps are one of the preferred solutions for overcoming low-height obstacles, given their natural design, versatility, and the ease with which they enable fish passage.

The proposed fish ramp is 54 meters long and 4 meters wide, with 5 cm high steps on a rockfill base of 1,200 to 2,000 kg blocks. The surface is finished with river pebbles (150-300 mm) embedded in concrete during setting. At the upper end, there is a 3-meter long, 39 cm deep notch, and at the lower end, an attraction pool formed with 1,200-2,000 kg rockfill blocks.

The ramp is designed to release part of the maintenance flow through a secondary notch. In accordance with the Catalan Water Agency’s specifications, the notches have been designed to optimize hydraulic conditions (depth and velocity) for the swimming abilities of local fish species. This design allows functionality across a wide flow range, from the minimum maintenance flow (1.968 l/s) to the Q5% of the Ter River (63.73 m³/s).