Caniçada Flood Spillway
The spillway construction works included:
- A WES weir equipped with two surface gates, located on the left bank, upstream of the dam abutment;
- A tunnel of variable geometry, located under the left abutment of the dam, about 200 metres long;
- A flip bucket terminal structure;
- An ecological flow device.
The complexity of the work was mainly due to the construction of the cofferdam at the intake, which had to be executed when the client EDP could guarantee the lowering of the reservoir. This construction process can be divided into 5 phases:
Intake Structure
The intake structure of the spillway is of the frontal type and comprises two equal spans, each 8.75m wide, separated by a pier with hydrodynamic shape in plan.
In these two spans, the weir has a WES-type profile with the upstream face inclined at 1:1.5 (H:V), characterised by a design head of 12.50m and a crest at elevation 138.50. Segment gates operated by servomotors, with an effective stroke of approximately 6m, were adopted as the closure devices for the spans.
Tunnel
Following the intake structure is the concrete-lined tunnel with a variable cross-section, which extends from elevation 134.30 to elevation 98.35, over a length of approximately 200m. In profile, the tunnel has two straight sections with slopes of 77% and 10.3%, connected by a circular curve with a 50m radius. The tunnel has a straight alignment in plan.
Throughout the tunnel, the cross-section is divided in half by a septum located in the extension of the pier separating the two spans of the intake structure.
In the initial 60m-long section, the cross-section converges from approximately 2 x 105m² to a constant section of approximately 2 x 55m². The final open-channel section of the spillway is about 56m-long and terminates in a flip bucket structure, also divided by a septum.
Outlet Structure and Discharge Conditions
To dissipate the energy of the discharged flow, an air dissipation solution and, fundamentally, impact on the riverbed were chosen, given the good characteristics of the terrain in the jet impact area. Thus, a terminal flip bucket (aka ski jump) structure was adopted, which is divided by a septum, just like the tunnel.
To direct the water jets towards the riverbed, the last 26 m of the spillway follows a curved alignment, terminating at a 15º angle to the tunnel axis.
Cofferdam
This phase involved the construction of a gravity wall in plain concrete along the entire length of the cofferdam.
To provide the gravity wall with adequate foundation conditions to withstand the hydraulic thrust on its upstream face, a double row of jet grouting columns with a diameter of ϕ1.00m was planned. These were arranged in a staggered pattern, with a longitudinal spacing of 0.80m between columns and a spacing of 0.70m between the two rows of columns.
To enhance sliding and overturning safety, GEWI-type steel micropiles (ϕ50 mm) or equivalent were installed at the rear of the gravity wall. These were spaced at 2.40 m intervals and inclined at 30º from the vertical.
Ecological Flow Device
The ecological flow discharge device consists of two independent hydraulic circuits, embedded within the right-side wall of the complementary flood spillway intake structure.
Each circuit comprises a horizontal steel pipe with a circular cross-section, equipped, from upstream to downstream, with a protection grid, cofferdam, isolation valve and flow regulation valve.
In Circuit 1, the first pipe section has a DN of 1200mm, axis ate elevation 134.00 m, about 7 m long. At the end of this section are the isolation valve (butterfly type, DN 1200mm) and the flow regulation valve (ring type, DN 1200mm).
Downstream of this valve is a second pipe section, about 15m long and with a DN of 1400mm. This second section forms an angle of approximately 45º with the first section, oriented towards the interior of the complementary flood spillway tunnel, where the ecological flow is discharged into the Cávado River.
Circuit 2 differs from Circuit 1 in pipes and valves dimensions, with the first pipe section having a DN of 800mm and being about 7m long. At the end of this section are the isolation (butterfly type) and flow regulation (ring type) valves, both with a DN of 800mm.
Downstream of the valves is the second pipe section, about 13m long and with a DN of 1000mm, properly ventilated. As with Circuit 1, this section forms an angle of approximately 45º with the first section, oriented towards the interior of the complementary flood spillway tunnel, where the ecological flow is discharged into the Cávado River.
The ecological flow device was designed so that the two circuits together can discharge a range of flows 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 of this reservoir at elevation 144.00.
