Ferradosa Dam and Pumping System

It is a conventional concrete gravity dam, although with a slight curvature at its abutments. The dam has a maximum height above foundation level of 33.4 m and the reservoir has a flooded area of 8.11 ha and a total storage capacity of 714.6 × 10³ m³ at Full Supply Level (FSL) height 528.50.

The project covered the following works:

- Dam;

- Temporary diversion of the river;

- Access roads with a total length of approximately 1.4 km with a 7 m wide platform including longitudinal drainage (cast in-situ side ditch) and cross drainage (six culverts);

- Pumping system between the dam reservoir and the water treatment plant (WTP), comprising:

• - Water inlet tower in the reservoir;

• - Pumping station installed inside the intake tower;

• - Rising main between the pumping station and the WTP.

The body of the dam consists of a central block with a straight axis, incorporating the spillway and two curved lateral abutments, both with a plan curvature radius of 200 m.

Along the crest, there is a 7.5 m wide and 121 m long road deck with a horizontal grade at elevation 531.50. There are two galleries in the body of the dam: the Main Drainage Gallery, which houses the foundation drains and pumping wells, and the Inspection Gallery, which together with the previous one, accommodates the monitoring instrumentation – extensometers, three-dimensional survey bases, direct and inverted plumb lines, piezometers and totalising flow measurement devices.

The hydrological structures incorporated into the dam include the flood spillway, the bottom outlet and the ecological flow outlet. The flood spillway, designed to discharge a maximum flow of 50.1 m³/s, is configured as follows from upstream to downstream:

- Creager-type free-overflow spillway  crest at elevation 528.50, separated into two 6.25 m wide bays, separated by a 1 m thick pier supporting the roadway deck;

- Stepped spillway chute on the downstream face of the dam, 13.5 m wide with a slope of 1V:0.8H;

- Bureau of Reclamation Type III energy dissipation basin, 13.5 m wide and 11.9 m long;

- Tailrace channel lined with gabions and Reno mattresses.

The Freixo stream was diverted through two pipes of 1.2 and 1.5 m, supported on steel towers and reinforced concrete towers, respectively. The single site access connects to the Freixo de Espada à Cinta WTP along 1 km adjacent to the reservoir up to the dam crest and then continues for 220 m beyond the dam.

Further information about the project:

- The water supply pipeline has a length of 1.20 km and a nominal diameter of DN 160 mm;

- The pumping station has a flow rate of 20 l/s and a pumping head of 44.3 m;

- The pump sets are GRUNDFOS SB77-4-B.