Streambank Stabilization in River Póvoa, Loures, Portugal

This project is part of a wider EU-funded investment totalling 5,8 M€, that generally aimed for the sustainable flood control using bioengineering solutions in a total of 32 km. It represents the biggest investment ever made in Portugal regarding river restoration.

For this case, the objectives were to stabilize both river margins of a 250 m stretch of river Póvoa in Loures city and to restore the native vegetation.

The site is located in a highly populated area near Lisbon and is prone to recurring (and severe) floods.

This is due to several reasons: it falls within the floodplain of river Trancão; part of its basin drains water from steep hills located at a distance of less than 1 km; it suffers the influence of the Atlantic Sea tides via river Tagus.

The problems observed were both geotechnical and ecological. In the first case, several mass movements and cracks with more than 20 cm depth and 30 m long where present. The ecological problems were mainly related to the dominance of the giant reed (Arundo donax), an invasive species which the superficial roots do not contribute to in-depth slope stabilization.

The major aim of the latter was to develop a global solution for sustainable water management in the event of floods. During this process, the hydrology and hydraulics of river Loures basin was studied. The interventions included not only several works of civil engineering – culvert widening, installing passive tide gates, etc. – but also soil and water-bioengineering techniques (SWBT). These would promote erosion control, slope stabilization and consolidation as well as the ecological recovery of the site.

Regarding the Póvoa stretch, the hydrological and hydraulic model concluded that a maximum velocity of 1,8 m/s and a maximum shear stress of 17,068 N/m² was expected for the 100-year flood, this including a high-tide scenario. Taking this into consideration along with the site characteristics and a thorough review of SWBT literature, the solution adopted to stabilize the streambanks was a vegetated crib wall. This structure is able to withstand water velocities between 3 and 6 m/s and 200-300 N/m² of shear stress, thus suitable for the existing conditions.

The design included a retaining structure in two terraces in both riverbanks and a total height of 2,6 m. The bottom crib wall was to be filled with drain rock and the upper structure with soil and live willow stakes.

The works started in June 2022 and were at a good pace. The contractor was following the Project’s drawings as well as the Project team’s recommendation – a team with several years of experience in implementing SWBT in Portugal and abroad.

A few months later, the site was subject to heavy rainfall that caused 3 major floods, including the nearby area. Although the vegetated crib wall was not totally finished, the structure remained intact.

This Project and specifically the constructed vegetated crib wall showed that a careful conception and design of SWBT allied with a solid construction is vital for a successful outcome.