The Pétrusse Valley
The Pétrusse Valley, a fixture in panoramic images of Luxembourg City, dominates the capital's picturesque landscape. The valley has two main features: its rocks, which are a natural extension of the fortifications, and the greenery afforded by its trees, hedges, lawns and flowers.
In the centre of the valley is the Pétrusse, the stream that was channelled back in the 1930s. Its source can be found in the "Aalheck" area of Dippach forest, and its main tributaries are the Aalbaach and the Zéissengerbaach. At 12.8 km long, the Pétrusse flows through the Grund district before joining the Alzette near Rue St Ulric.
Project overview
Following a feasibility study and extensive consultation with the relevant government authorities, the project was approved as follows:
Ecological redevelopment of the Pétrusse
Objective:
- manage rising water levels;
- take into account the many infrastructural features located in the valley; and
- prevent any increase in terms of exposure to flooding for private land parcels located in the valley.
The concept provides for the following works:
- demolition of the existing concrete riverbed, with a moderate raising of the waterway;
- removal of one of the walls bordering the Pétrusse in some places to widen the waterway channel;
- development of a bed bottom that is more resistant to extreme stresses when the water level rises;
- construction of a fish ladder at the structure located on Rue St Ulric; and
- and construction of a new retaining wall downstream of the Bourbon Lock on the left bank (city side).
Development of the park along the Pétrusse
The ecological restoration and redevelopment of the Pétrusse Valley involves:
- creating a rest and recreational area near the park entrance (Rue St Ulric side);
- revamping play areas and sports facilities such as the outdoor fitness equipment and the minigolf course near the park entrance (rock-face side);
- incorporating the existing skate park into the new surroundings;
- installing terraced seating and relaxation areas at different locations along the banks of the Pétrusse;
- building a 4.5-metre-wide path (suitable for motor vehicles) alongside the Pétrusse over the entire length of the valley, and a path measuring 2 to 3 metres wide (not suitable for motor vehicles) on the opposite side of the main promenade, near the river;
- building six new bridges to replace existing structures (two open to vehicular traffic and four for pedestrians); and
- reintroducing species of fish by installing a fish ladder so that they can move between the Alzette and Pétrusse rivers.
Estimated budget
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).
Rehabilitation of the ecosystem
The main aim of this project is to revitalise the valley's deteriorated and modified ecosystems, and return them to a more natural state. With this in mind, the Pétrusse River needs to be restored. This mainly involves removing the concrete riverbed so that it flows along its natural path, which will support a much richer diversity of flora and fauna and improve nutrient and water-retention levels. To this end, indigenous plant species will need to be planted and some trees cut down. This will allow urban biotopes to flourish, providing a long-term habitat for a wide variety of plants and animals.
The first step is to ensure that no trees are left standing in the natural path the river will take after its banks have been redeveloped and widened, as unstable trees pose a safety risk. In addition, to prevent any further decline in water quality, excavation work must be carried out in different areas.
Next, a grassy area for recreational and leisure activities near the current playground and minigolf course will be created to improve the quality of life of residents and visitors and provide them with a place to relax.
Although some trees will need to be removed, new ones will be planted in prime locations, and veteran trees near to where the works are taking place will be protected.
Project execution phases
Ecological restoration of the Pétrusse River – a two-phase project
The ecological restoration of the Pétrusse River and the redevelopment of the adjacent park will be carried out in two phases undertaken in separate locations and at different times.
The overall project consists of several sub-projects. While some of these can be carried out separately, others are spatially linked and require complex construction processes. These works include replanting and redesigning the landscape of the riparian zone and the adjoining park, restoring the riverbed and the immediate aquatic environment to a more natural state, building new infrastructure and civil-engineering structures, building the first-flush system, implementing water-supply and sanitation measures, and performing archaeological excavations (St Ulric church). After the installation of temporary structures and planted areas in time for LUGA 2023, phase 1 will be complete.
Phase 1: works spanning multiple sites
This phase will cover the part of the valley from the Bourbon Lock to the confluence of the Pétrusse and Alzette rivers. Since the Pétrusse Valley is one of the main sites of the LUGA 2023 horticultural exhibition, ecological-restoration work began in June 2020 and will be completed in spring 2023.
- Preparatory work (temporary car park, archaeological excavations at Place St Ulric)
The National Centre for Archaeological Research (Centre National de Recherche Archéologique – CNRA) began conducting archaeological excavations at the site in May 2020. These will be carried out on a continuous basis.
- Restoration of the Pétrusse River
The concrete riverbed will be removed so that the river can once again flow along its natural path. The riverbed will then be covered with natural stone, and the newly created banks of the river turned into a natural habitat for flora and fauna. Strips of wooded area and plant life along the banks will create an ecological corridor. Restoring the river will make it flow much slower and more naturally, thereby reducing the risk of flooding.
- First-flush system
The first-flush system will be built separate from the other construction phases. This project, which will run throughout 2020, involves building a first-flush collection system where the Pétrusse connects to the City's wastewater pipeline network near the Pont Adolphe. This system will stop a significant amount of pollution (mainly emissions from vehicular traffic and atmospheric deposition) from flowing into the rainwater drainage system and then into the Pétrusse, and ultimately lead to a marked improvement in water quality, especially during dry periods followed by heavy rain.
- Playground / fitness area / minigolf course / Bourbon Lock / fish ladder / skate park
This phase involves dismantling the existing bank walls and bank protection, building a new bank wall on the left side of the river, land restoration and associated earthworks, making surface adjustments and planting. In order to blend in with the city's fortifications, which are classified as a UNESCO World Heritage Site, some of the new wall components will be built using natural stone.
Once these works are complete, a fish ladder will be installed, the "Liegewiese" recreational area will be created, and the skate park will be incorporated into the new surroundings (river side and rock-face side).
Phase 2: from the Bourbon Lock to Rue d'Anvers
This phase will start in spring 2024 (after LUGA 2023) and involve the land restoration of the Pétrusse Valley and the redevelopment of the adjacent park between the former Bourbon Lock (removed during phase 1) and Rue d'Anvers.
Progress on the site
Practical information regarding access
The course of the roads and pathways along this valley, which follow the river and cross over it at various points, will be partially redesigned. As a result, the existing bridges are to be demolished and new ones built over the restored stretch of the river. Existing bridges and crossings in the Pétrusse Valley will be kept for as long as possible so that visitors can get around. They will also be used to transport construction machinery, which will reach the site via Rue de Prague and Montée de la Pétrusse or Place St Ulric.
Access for pedestrians and cyclists will be guaranteed throughout the duration of the works, and appropriate signage will be erected to provide information about diversions.
An ongoing process with many parties involved
The land restoration project in the Pétrusse Valley is a challenging undertaking that requires close cooperation between the City of Luxembourg and its external partners, as well as compromise between all of the parties involved.
All aspects of the project (the river, history, urban planning, transport and biodiversity) were duly considered during the planning process thanks to the commitment of and ongoing communication between 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 especially 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).
The following City departments are involved in this project:
- Service Canalisation (Sewer Department)
- Service Communication et relations publiques (Communications and Public Relations Department)
- Service Parcs (Parks Department)
- Luxembourg City Photothèque
- Service Technologies de l'information et communication (Information and Communications Technology Department)
