Caniçada Flood Spillway
The spillway construction works comprised:
- A WES spillway crest equipped with two surface radial gates, located on the left bank, upstream of the dam abutment area;
- A variable-geometry tunnel, located beneath the left abutment of the dam, approximately 200 m in length;
- A ski-jump structure;
- An environmental flow release structure.
The complexity of the works was mainly due to the construction of the intake cofferdam, which had to be maintained during the period in which EDP (Portuguese electric utility) ensured the lowering of the reservoir level. This construction process may be divided into 5 phases:
Intake Structure
The spillway intake structure is of the frontal type and comprises two equal bays, each 8.75 m wide, separated by a pier with a hydrodynamic shape in plan.
For both bays, the spillway crest has a WES profile, with an upstream face inclined at 1:1.5 (H:V), characterised by a design head of 12.50 m and a crest elevation of 138.50. Radial gates operated by hydraulic cylinders were adopted as the bay closure devices, with a working stroke of approximately 6 m.
Tunnel
The intake structure is followed by the concrete-lined tunnel, with a variable cross-section, whose invert extends approximately from elevation 134.30 to elevation 98.35, over a length of approximately 200 m. In profile, the tunnel features two straight sections with gradients of 77% and 10.3%, connected by a circular curve with a radius of 50 m. The tunnel features a straight alignment in plan.
Throughout its entire length, the tunnel cross-section is divided into two parts by a central dividing wall aligned with the pier separating the two bays of the intake structure.
Along the initial 60 m-long section, the cross-section converges from approximately 2 x 105 m² to a constant section of approximately 2 x 55 m². The final open-air section of the spillway consists of a chute channel approximately 56 m long, terminating in a flip bucket that is also divided by a splitter wall.
Outlet Structure and Tailwater Conditions
To dissipate the energy of the discharged flow, a solution based on airborne energy dissipation and, fundamentally, impact on the riverbed was chosen, given the good geotechnical characteristics of the ground in the jet impact zone. Consequently, a ski-jump terminal structure was adopted, which, like the tunnel, is divided by a splitter wall.
To direct the jets towards the riverbed, the final 26 m of the spillway describes a curve in plan, terminating at a 15° angle relative to the tunnel axis.
Cofferdam
This phase involved the construction of a plain concrete gravity wall along the entire length of the cofferdam.
In order to provide the gravity wall with foundation conditions compatible with the loads resulting from the water pressure acting on its rear face, a double curtain of jet grouting columns with a diameter of 1.00 m was constructed. These columns were arranged in a quincunx pattern, with a longitudinal spacing of 0.80 m between columns and a spacing of 0.70 m between the two rows of columns.
On the rear face of the wall, in order to increase its resistance to sliding and overturning, GEWI-type ϕ50 mm steel bar micropiles, or equivalent, were installed at 2.40 m spacing and inclined at 30° to the vertical.
Environmental Flow Release Structure
The environmental flow release structure consists of two independent hydraulic circuits, embedded within the right-wing wall of the supplementary flood spillway intake structure.
Each circuit comprises a horizontal, circular-section steel pipe, equipped from upstream to downstream with a trashrack, a cofferdam, an isolation valve and a flow control valve.
In Circuit 1, the first pipeline section has a nominal diameter of DN 1200mm, a centreline at elevation 134.00 and is approximately 7 m long. An isolation valve (butterfly type, DN 1200 mm) and a flow control valve (ring type, DN 1200 mm) are located at the end of this section.
Downstream of this valve is a second pipe section, approximately 15 m long with a DN 1400 mm diameter. This second section forms an angle of about 45° with the first, oriented towards the interior of the supplementary spillway tunnel, into which the environmental flow is discharged and subsequently returned to the Cávado River.
Circuit 2 differs from Circuit 1 in the dimensions of the pipelines and valves; its first pipeline section has a DN 800 mm diameter and is approximately 7 m long. Located at the end of this section are the isolation valve (butterfly type) and the flow control valve (ring type), both DN 800 mm.
Downstream of these valves is the second pipeline section, approximately 13 m long and DN 1000 mm diameter, which is properly vented. As in Circuit 1, this section forms a 45° angle with the first and is oriented towards the interior of the supplementary spillway tunnel, into which the environmental flow is discharged and subsequently returned to the Cávado River.
The environmental flow release structure was designed so that the combined capacity of both circuits can discharge a flow range between 0.24 and 8.90 m³/s, for reservoir levels between the Full Supply Level (FSL) at elevation 152.50 and the Minimum Operating Level (MOL) implemented in this reservoir at elevation 144.00.
