Running beneath the fields and forests of the west is a pipeline on which nearly half the country depends. Built in the 1960s, it has lost up to 80 per cent of its wall thickness in some places. Replacing it now means digging a trench up to 35 metres wide over a distance of nearly 18 kilometres.
On Friday 23 October, the Government Council ruled that the Sebes project met the criteria of ‘compelling reasons of major public interest’. This decision was essential: even the route deemed to cause the least damage will have significant impacts on the Mamer and Eisch Valley Natura 2000 site. The government is thus allowing the project to proceed through the authorisation process despite these impacts, in return for compensatory measures. The structure will link the sluice chambers at Schankegriecht, in the north-west of the country, and at Nospelt. The new pipeline will be made of steel, protected against corrosion and have a nominal diameter of 1.10 metres. It will operate mainly by gravity.
Nearly 700 litres per second
However, the term ‘customers’ does not fully convey its significance. Sebes does not supply every household directly: it supplies, in particular, the City of Luxembourg and the distribution networks, which then supply consumers. According to the environmental study, around 375,000 residents depend on this supply network, representing nearly half the national population.
The pipeline currently carries an average of 60,000 cubic metres per day. This represents 60 million litres, nearly 700 litres per second, or the equivalent of the volume of 24 Olympic-sized swimming pools every day. Around 21,000 cubic metres are supplied to the City of Luxembourg and 35,500 to the Syndicat des eaux du Sud. The remaining 3,500 cubic metres supply other reservoirs around the capital. This pipeline also serves as a backup supply. In the event of a failure of the DN700 pipeline connected to Schankegriecht, it can help supply the areas served by the Sec and Sidere water authorities, which together account for an additional 11,300 cubic metres per day.
Sebes is not simply building to meet current demand. The Water Management Authority estimates that national demand could eventually reach between 130,000 and 150,000 cubic metres per day. With its daily capacity of 60,000 cubic metres, the Schankegriecht–Nospelt section would still account for 40 per cent to 45 per cent of this future demand. The urgency stems from the condition of the existing pipeline. A rupture in Nospelt in January 2020 led Sebes to inspect the 14.5-kilometre section upstream. The inspection revealed 872 defects spread across 342 pipes. Urgent repairs were subsequently carried out on 27 pipes, at a cost estimated at the time at 1.35 million euros.
These works are merely a temporary solution. The DN900 ductile iron pipeline dates largely from 1966. In 2021, Sebes had already estimated the cost of replacing it at between 37.5 and 42.5 million euros. This figure, which predates the rise in construction costs and the completion of environmental studies, can no longer be regarded as an up-to-date budget. No new overall figure has been made public in the documents consulted.
Two pipes in parallel
The chosen strategy involves, first of all, laying the new DN1100 pipeline without taking the old one out of service. Once the new pipeline is operational, the DN900 pipeline can then be renovated. Ultimately, the two pipelines will operate in parallel. Sebes will then be able to isolate one of them for maintenance or repairs without interrupting the water supply to Luxembourg City and the south of the country.
Three main route options, ranging from 15.5 to 20.8 kilometres in length, were compared. The shortest option followed the current route more closely, but passed through more woodland. The longest option affected more protected areas. Sebes and the environmental authority have opted for a third solution, comprising two sub-routes of 17 and 18 kilometres, which runs mainly through farmland.
This decision should enable the land to be restored more quickly once the works are complete. Of the route, which is approximately 18 kilometres long, 3.25 kilometres would still pass through woodland, representing 18 per cent of the route. Deforestation affecting protected forest habitats is estimated at 11,845 square metres. Around 4,000 square metres will need to be offset under the Natura 2000 scheme.
Work will proceed from north to south, starting from Schankegriecht, and is expected to take around two years. The works area will comprise the trench, a track for machinery, an area where the pipework components will be laid out, and separate areas for topsoil and excavated material. It may be up to 35 metres wide. In sensitive areas, this width will be reduced to 25 or 20 metres, or even to 10 metres in certain places for a maximum distance of 100 metres. Topsoil will be removed and stored separately before being replaced. Excavated materials must be reused on site as far as possible. Once the works are complete, the fields will be returned to their original use. However, a 10-metre-wide strip must remain permanently free of trees and shrubs in order to protect the pipeline and allow access to it.
Two new valve chambers are planned. Each will occupy a plot measuring approximately 30 metres by 30 metres, but most of its structure will be underground. Additional structures will be built at the highest and lowest points along the route. At ground level, these will generally consist of a concrete cover and an access hatch.
The route must also cross the Attert, the Eisch, the Aeschbech, the Wollefsbaach and several smaller watercourses. The size of the tunnel and the topography of the valleys have led the engineers to rule out directional drilling and microtunnelling. These methods would have required start and end shafts over 90 metres deep – the height of a block of flats. The watercourses will therefore be crossed using open-cut construction. Depending on the location, the water will be temporarily channelled into a pipe laid in the riverbed or diverted to a temporary channel. The pipe will then be buried beneath the watercourse, before the riverbed and banks are rebuilt. The works must be carried out quickly and will be subject to environmental monitoring.
More than 200 metres from the villages
The pipeline will pass through three Natura 2000 sites. At Wollefsbaach, it will come close to Luxembourg’s only known population of the Mercury damselfly, a protected species. The works must be carried out outside the species’ breeding season, without causing any lasting changes to the hydrological conditions. Other measures are aimed at protecting bats, breeding birds, amphibians, beavers, sculpin and Planer’s lamprey.
The project avoids villages as far as possible. The chosen route generally runs around 200 metres from residential areas. Local adjustments are required, in particular to move the construction site further away from Tuntange, Roodt and the north-west of Nospelt. The main anticipated disturbances relate to noise, vibrations, heavy goods vehicle traffic and temporary disruption to access to certain paths and cycle routes.
Before it is brought into service, the pipeline will undergo a pressure test, after which it will be cleaned and disinfected. The flushing water will be discharged via several low points, after its composition has been checked and its dilution in watercourses has been calculated. During normal operation, no periodic disinfection or regular draining is planned.
