Regularisation for the Baixo Mondego

The project aimed to redefine the course of the river either along a new river bed or the existing one for the Mondego along a stretch of some 30 km between Coimbra and the mouth of the river at Figueira da Foz. The double trapezoidal transversal profile has a wider bed of between 112 and 380 m for operation with the maximum flow in winter and a narrower bed between 88 and 142 m in drought conditions with minimal flow.

The redefinition of the course and the corresponding profiles for the most part had recourse to the dredging process for the solid materials for the river bed, using the hacking/aspiration/spray method. The transversal profile was complemented by protective dykes with a height between 2.7 and 4.6 m and a trapezoidal channel for the supply of water for irrigation to the crops on the nearby agricultural land.

The bed of the river subject to regularisation was made in steps similar to the slopes for the dykes and protected by layers of rock laid on a clay core. The excavations and the backfill were carried out by hacking/aspiration/spray process dredging. The dredges used were modular comprising of interlinked floating pontoons.

The hacking tool used for a maximum depth hack of 14/15 m was fitted with a rotating device for breaking up and mixing the material excavated with water to form a suspension pumped and sprayed into floating pipes with a 500 mm diameter and those stretches of pipe on land. In circumstances where it was necessary to spray over longer distances where the target area was at some distance from the channel protection, it was necessary to use an intermediate booster pump in order to cover distances of up to some 2 km.

The dredges were also equipped with spud generators (metal pipes fixed on the ground that served as rotation pivots for movement of the dredge, winches for connection to the anchors (the movement of which, together with the spuds, controlled the rotation and movement of the dredge and a control cabin from where the dredging operations were supervised.

The work and the progress of the dredges was undertaken in the following manner:

1. One of the spuds was raised and dropped so that it lodged in the river bed;

2. The “spud” acted thus as a pivot for the rotation of the dredge;

3. The movement of the dredge was achieved by means of cables activated by the winches on each side of the dredge, the cables being fixed to the anchor’s’ edges;

4. The arm and the respective disaggregation tool then moved in an arc until the required depth of excavation was achieved;

5. The dredge then sought a new position for the lodging of a spud and for the anchors, the previous spud then being released.

An inverted filter was applied as protection against the effects of water erosion on the river bed and consisting of successive layers of sand and crushed aggregate of increasing sizes up to the final layer of limestone rockfill.

The material used for the rockfill was obtained from the various quarries in the area. For the slopes on the greater bed the protection had recourse solely to vegetable lining. The construction of a waterproof clay core was necessary on the structure for the dykes that prevented the seeping of water and its consequent destruction.

Various items of equipment were used in the construction as a complement to the dredges including:

- Bulldozers;                                                                    - Concrete mixing plant;

- Wheeled and tracked backhoes;                         - Concrete mixers;

- Wheeled and tracked loading shovels;               - Barges;

- Motor graders;                                                           - Boats;

- Plain Roller Cylindrical compactors;                     - Submergible pumps;

- Treaded Cylinders;                                                    - Electro pumps;

- Manual cylinders and vibrating plates;                - Motorised pumps;

- Cranes;                                                                         - Generators;

- Rock drilling vehicles;                                 - Compressors;

 - Crushing installations (feeder, primary jaws, rotating secondary and tertiary, sifting devices, transport belts, etc.).

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.

Accommodation was constructed for the housing of the labour force.