Lower Mondego River Training Scheme

The works involved the redefinition of the horizontal and vertical alignment of the Mondego River, either along a new channel or by modifying the existing riverbed, over an approximately 40 km stretch between Coimbra and its mouth near the city of Figueira da Foz. The composite double-trapezoidal cross-section features a major channel (width between 112 m and 380 m), designed to convey maximum winter flows, and a minor channel (width between 88 m and 142 m), intended for low-flow conditions during the dry season.

This realignment and reprofiling were carried out mainly by dredging the riverbed sediments using the cutter suction dredging method. The cross-section was completed with protection embankments ranging from 2.7 m to 4.6 m in height and a trapezoidal irrigation canal designed to convey water to the adjacent agricultural land.

The bed of the regulated channel was constructed in steps and, together with the slopes of the adjacent embankments, was protected by rockfill revetments laid over clay cores. Excavation and hydraulic fill operations were carried out by dredging using cutter suction dredgers. These dredgers were modular units, consisting of interconnected floating pontoons.

The cutter ladder, designed to operate at a maximum excavation depth of 14–15 m, was equipped with a rotating cutter head that mixed the excavated material with water, forming a slurry that was pumped and discharged through floating pipelines of 500 mm diameter and through sections supported on land. Where longer pumping distances were required, with disposal areas located away from the channel protection embankments, an intermediate booster pump was installed to achieve pumping distances of up to approximately 2 km.

The dredgers were also equipped with generators, spuds (steel piles fixed into the ground and used as rotational pivots for dredger movement), anchor winches (whose operation, combined with the spuds, also controlled the dredger's rotation and movement) and a control cabin from which all dredging operations were managed.

The dredgers’ work and progress were carried out as follows:

1. One of the spuds was hoisted and abruptly dropped, driving into the riverbed;

2. The spud then acted as the centre of rotation for the dredger movement;

3. The dredger movement was achieved by means of cables operated by winches located on both sides of the dredger, the cables being anchored to the riverbanks;

4. The cutter ladder and the corresponding cutter head then moved in circular arcs, excavating to the required cutting depth;

5. The dredger was then repositioned to drive in another spud and to secure the anchors, after which the previous spud was released.

To counteract the erosive effect of the water on the minor channel, protection was provided in the form of an inverted filter, constructed from successive layers of sand and crushed aggregates of progressively increasing size, culminating in a final layer of limestone rockfill.

The material required for rock filling was sourced from various quarries in the region. The slopes of the major channel were protected solely by means of vegetation cover. Within the embankments, the construction of an impermeable clay core was required to prevent water infiltration and the resulting structural deterioration.

In addition to the dredging equipment, various items of heavy construction machinery were used, including:

• - Bulldozers;

• - Wheeled and tracked backhoe excavators;

• - Wheeled and tracked loaders;

• - Motor graders;

• - Smooth-drum rollers;

• - Padfoot rollers;

• - Walk-behind rollers and plate compactors;

• - Cranes;

• - Rock drilling rigs;

• - Crushing and screening plants (feeder, primary jaw crusher, secondary and tertiary gyratory crushers, screening sieves, conveyor belts, etc.);

• - Concrete batching plants;

• - Concrete mixers;

• - Barges;

• - Boats;

• - Submergible pumps;

• - Electric pumps;

• - Motor-driven pumps;

• - Generators;

• - Compressors.

Complementary works were carried out, including the laying of roads and accesses to ensure the completion of the project, to permit communications with the local population centres and to support the maintenance of the work for the project. Complementary works were also carried out, including roads and access routes, to facilitate execution of the main works, to ensure access to nearby communities and to support construction operations.

A residential complex was also constructed to accommodate the workforce during the execution of the project.