The Preliminary Study of the Final Channelled Section of the Ribeira de Algés, prepared for the Intermunicipal Water and Sanitation Services (SIMAS) of Oeiras and Amadora, provides for a set of interventions aimed at “minimising and controlling the risks of flooding, while also ensuring the hydraulic, structural and environmental resilience of the system”.

In the document, to which Lusa had access, structural interventions are estimated at around €63 million, divided between “urgent priority works, justified for structural reasons” and “hydraulic” (€26 million) and “urgent works” (€37 million) on the stream that rises in Amadora, crosses Oeiras and flows into Lisbon.

The urgent priority works include “the rehabilitation and improvement of the Algés canal discharge into the Tagus estuary, with a final section in an open ditch, as a linear development lagoon that runs downstream from the railway line” and the "complete structural rehabilitation of the section between Miraflores Urban Park and the linear retention basin (regularised ditch)".

The urgent works include the “reprofiling of the final section of the basin (section 8) and new entrance works in the downstream channelled section, with a much larger cross-section”, the duplication of the section under Av. Bombeiros Voluntários de Algés and “control of undue inflows”, and replacement of the section between the intersection of Rua Latino Coelho and the section contemporary with the CRIL (IC17) works.

This phase also includes the “reactivation of the left arm of the hydraulic passage under the railway line, currently plugged”, the “partial structural rehabilitation” between Largo Augusto Madureira and the Pedrouços dock and the “disconnection of the rainwater network that currently discharges into the Algés canal, with a redesign of the gravity network on the right bank” and a rainwater lifting system to discharge into the Tagus River.

“These structural solutions must be accompanied, over time, by non-structural, complementary measures (medium and long-term interventions) for monitoring and warning” and “raising awareness among the population and control at source” in the territories further upstream of the river basin, “which delay and minimise flood peaks”, the study points out.

In addition to the problem of flooding, which has a particular impact on Baixa de Algés, some of the canalised sections of the river “are in poor condition”, as evidenced by collapses such as those in December 2009, 2019 and November 2023.

A report by the National Civil Engineering Laboratory (LNEC) in September 2024 concluded that, over a length of 912 metres, 57% of the sections inspected are at risk of collapse in the near future.

According to the study by Hidra, Hidráulica e Ambiente, flooding is aggravated by a number of factors, namely “downstream hydraulic conditioning, due to the influence of the tides and silting of the discharge in the final section”, or the “topographical configuration of Baixa de Algés”, with poor energy availability for gravitational drainage.

“The risks of flooding will tend to worsen in the future if corrective measures are not taken, due, in particular, to the effects of land use and the effects of climate change,” in particular, the worsening of short-term precipitation intensities and sea level rise,” it notes.

Among the planned projects, the study refers to the Algés Tower, a 13-storey residential complex in the vicinity of the market; the “Ocean Campus”, dedicated to the blue economy and the sea, next to the Pedrouços dock; and urban redevelopment up to Cruz Quebrada.

Downstream of the Cascais railway line, two alternative solutions were evaluated, one with the final section in an open trench, on the current route, partly buried and partly uncovered, with a sluice gate to mitigate silting at the end, and the other providing for a new section fully channelled to the discharge point, maintaining the existing one, and a jetty to reduce sedimentation at the outlet.

Both solutions ensure “the proper hydraulic functioning of the area under study, even in the most critical tidal scenarios”. In terms of costs, they are very similar, but the open trench’s operating costs are more advantageous, reinforcing the city’s historical and symbolic connection to the Tagus River and earning the Portuguese Environment Agency’s (APA) preference.