First barrier removal in Luxembourg within the framework of the Pétrusse Valley restoration project


The Pétrusse Valley
The Pétrusse Valley is located in Luxembourg City (Figure 1) and is an intrinsic part of its identity. The valley is part of the World Heritage Site of Luxembourg City protected by the UNESCO. Its two main features are its rocks, which are a natural extension of the fortifications, and the green space which functions as a recreational area for locals and tourists alike.

Figure 1. (left) Location of Pétrusse Valley and (right) weir at Pétrusse River in Luxembourg City (indicated by the red pin), Luxembourg (photos by OpenStreetMap)

Through the valley runs the Pétrusse River, a 12.8 km long stream that springs from the “Aalheck” area of Dippach forest and flows through the Grund district before joining the Alzette near Rue St Ulric. It has two main tributaries, the Aalbaach and the Zéissengerbaach. Pétrusse River was straightened in the 1930s and given a concrete riverbed (Figure 2) while artificial barriers were constructed along its corridor (Figure 3). The rationale behind this project was to remove the refuse from the city flowing into the river as fast as possible to reduce the risk for public health. However, this radical modification of the natural course and characteristics of the river which turned it into an open sewer had detrimental effects on the ecosystems and biodiversity of the whole catchment area (4600 ha) and also increased the flooding risk for the adjacent areas. Pétrusse River has an atypical hydrology characterized by dry periods with low flow rates (a few liters / second) and long periods of moderate rainfall. However, in the event of heavy precipitation the water level can rise fast and substantially.

Figure 2. Pétrusse River in Luxembourg City, Luxembourg (photo by TR Engineering)
Figure 3. Weir at Pétrusse River in Luxembourg City, Luxembourg (photo by Ralph Moelter)

Pétrusse Valley and River restoration
The Pétrusse Valley restoration project aimed to revitalize the valley’s deteriorated and modified ecosystems and return them to a more natural state. The project was planned to be executed in two phases undertaken in separate locations and at different times. The initial phase focused on the part of the valley from the Bourbon Lock to the confluence of the Pétrusse and Alzette rivers, was initiated in 2020 and lasted till the spring of 2023. Due to the cultural heritage and historic value of the Pétrusse Valley, the public was involved from the very start of the project. To do so, innovative tools were utilized, including architectural drawings, visualizations, animations and interactive screens, while a new park was created along the newly formed river corridor aiming to improve the life quality of the local communities (Video 1).

Video 1. Animation of the redeveloped Pétrusse Valley

The restoration of the Pétrusse River was part of the phase 1 and mainly involved the removal of the artificial concrete bed (Figure 4; Video 2) and the re-meandering of its course (Figure 5; Video 2) that will allow the river to regain its natural course, will boost its biodiversity and improve its ecological status and water-retention levels. The new riverbed was covered with natural stone, and riverbanks were reprofiled. Indigenous plant species were planted along the banks.

Figure 4. Removal works of the concrete riverbed of Pétrusse River in Luxembourg City, Luxembourg (photos by Ralph Moelter)
Figure 5. Re-meandering of Pétrusse River in Luxembourg City, Luxembourg (photo by Ralph Moelter)
Video 2. Removal of the concrete riverbed and re-meandering of Pétrusse River in Luxembourg City, Luxembourg (video by Ralph Moelter)

Along with the concrete riverbed, a weir was also removed in September of 2022 as part of the project to enable fish migration and natural sedimentation processes (Figure 6). Phase 1 of the project included also preparatory works, like the construction of a temporary car park and the conduction of archaeological excavations at St Ulric church from May 2020 and on by the National Centre for Archaeological Research (Centre National de Recherche Archéologique – CNRA). It also included the construction of first-flush collection system at the confluence of the Pétrusse River with the City’s wastewater pipeline network near the Pont Adolphe. This system will block a significant amount of pollutants and city waste to enter the rainwater drainage system and eventually the Pétrusse River. This system is expected to improve the water quality of the river. The construction of paths alongside the river’s corridor, playgrounds, fitness areas, a minigolf course and a skate park are among the rest sub-projects included in the phase 1 of the project. Six new bridges were also planned to be built to replace existing structures. The 2 second phase of the project is planned for spring of 2024 and includes land restoration of the Pétrusse Valley and redevelopment of the adjacent park. All works were planned in a way to guarantee access to pedestrians and cyclists to the valley throughout the duration of the project.

Figure 6. The removal site at Pétrusse River, Luxembourg after the removal operations of a weir in September 2022 (photo by Ralph Moelter)

The budget for phase 1 of the Pétrusse Valley land restoration project is estimated at €25,909,385.84 including VAT. Funding is provided by the City of Luxembourg and the Water Management Fund (Fonds pour la gestion de l’eau). The Pétrusse Valley restoration project required cooperation between the City of Luxembourg and its external partners and compromises were needed from all parties, including the Grund and Hollerich local interest groups, the CNRA, the National Sites and Monuments Service (Service des sites et monuments nationaux), the Luxembourg Commission for Cooperation with UNESCO, and the Ministry of the Environment, Climate and Sustainable Development (Ministère de l’Environnement, du Climat et du Développement durable), the Water Management Authority (Administration de la Gestion de l’Eau), the Nature and Forest Agency (Administration de la Nature et des Forêts), UNESCO, and the National Roads Administration (Administration des Ponts et Chaussées). Several City departments were also involved in this project, like Service Canalisation (Sewer Department), Service Communication et relations publiques (Communications and Public Relations Department), Service Parcs (Parks Department), Luxembourg City Photothèque, and Service Technologies de l’information et communication (Information and Communications Technology Department).

The Pétrusse Valley restoration project was among the candidate projects for the European Dam Removal Award of 2022. This annual award is the first of its kind in Europe and is hosted by Dam Removal Europe, the World Fish Migration Foundation, the European Investment Bank, The Nature Conservancy and supported by the Dutch Postcode Lottery and Forest Peace Foundation. The most inspiring project was awarded 10,000 euro towards the team’s next removal project and handed out the Happy Fish Statue for a year.

Ecological and community benefits
The removal of the concrete riverbed and the re-meandering of its course was expected to lead to a much slower and natural flow, and thus reduce the risks of flooding. During the two years of construction, a new riverbed was created through natural sedimentation and erosion processes, vegetation has flourished and terrestrial and semiaquatic animals, like foxes and ducks, resettled in the area. Migrating fish haven’t been spotted yet at the restored part of the Pétrusse River, but this is expected to happen in the following years. The local community has now embraced the efforts to restore Pétrusse River and Valley despite their initial scepticism.

Before & After photo pairs

The removal site at Pétrusse River, Luxembourg (left) before and (right) after the removal operations of a weir in September 2022 (photos by Ralph Moelter)
Pétrusse River, Luxembourg (left) before and (right) after its restoration (photos by (left) TR Engineering and (right) Ralph Moelter)

Written by Foivos A. Mouchlianitis

  • Name: Weir at Pétrusse River
  • Location: Luxembourg City, Luxembourg
  • Type: weir
  • Dimensions: Height 1 m
  • Aim of removal: To revitalize the ecosystems of Pétrusse valley and return them to a more natural state
  • Year of removal: 2022