Preliminary Study of Marginal Retention with Riprap for the Protection of the Final Reach of the Algés Stream

bluemetrics carried out the preliminary study of a riprap revetment solution for the protection of the end reach of the Algés Stream, at its confluence with the Tagus Estuary, next to the retaining wall of the Pedrouços Dock.

The main objective was to define a technical solution capable of:

• mitigate localised erosion induced by river flow jets in the discharge zone;
• protect the foundation of the existing wall;
• reduce the incident wave height in the area designated for the installation of a hydraulic gate.

The proposed solution consists of a marginal riprap revetment approximately 25 m in length, covering an area of about 150 m². The structure comprises an armour layer (10–30 kN) placed over an underlayer (1–3 kN), installed on a levelled foundation following dredging to a depth of –6.20 mZH.

The study included:

• characterisation of the intervention area and associated hydraulic and maritime constraints
• definition of design actions for a 100-year return period;
• analysis of water levels, wave conditions, tidal currents, and fluvial discharge;
• design of the riprap protection based on formulations from the Rock Manual;
• stability verification under design wave conditions;
• definition of the structural geometry (slopes, layer thicknesses, crest width, and crest elevation);
• proposal of a phased construction methodology (preparatory dredging and layer-by-layer placement);
• technical specifications for laboratory testing, numerical modelling, and geological–geotechnical investigations to be carried out during the detailed design phase.

The study establishes the technical framework for advancing to the detailed design phase, including recommendations for comprehensive hydrodynamic modelling and an associated geotechnical investigation programme.