Fill it up, please!
In less than a month’s time the Deûle-Scheldt canal will be equipped with a brand new water supply system that should be sufficient to handle canal traffic for decades to come. Each of the 13 locks between Wasquehal (Trieste lock) and Pecq (Warcoing lock, in Belgium) is equipped with a pumping station. Two pumps will recycle the water used each time the lock is emptied into the downstream reach. This water volume amounts to between 400 and 700 m3.
As mentioned in an earlier Newsletter, simply replacing the lockage water is not sufficient to keep the reaches full, because water is also lost through evapotranspiration and leakages. The supplementary water resources from Grimonpont sewage treatment plant are used to compensate for losses considered “permanent”.
The pumps at the bottom of the Union recycling station,
and the pump outlet at the Noir Bonnet station
The civil engineering works have now been completed and the pumps installed at each of the 10 locks in France. The massive structure is impressive, especially at Trieste lock in Wasquehal, where the entire station with its concrete pit weighs 110 tonnes. The stations were built using the “cut and lower” method of shaft-sinking using caisson piles, a means of excavating successive sections that ensures optimum safety for workers. Section by section the formwork was built (see photo), the concrete was cast into it, the formwork was removed, and the excavations continued in the chamber thus created. The station descends by itself under its own weight.
Reinforcement bars within the formwork prior to casting in the pumping station pit at Trieste lock (Wasquehal).
The suction pipe is on the downstream side outside this pit, and the delivery pipe through which the water is pumped back to the upstream reach is on the upstream side.
Check valves and delivery pipe at the Union station outlet.
What a pity boaters will never see the extent of these works or the pipes and valves attractively painted in blue and red. Nor will they ever appreciate the complexity of this system, designed by Amodiag Environnement in Valenciennes.
The design engineers had to deal with the differing lock heights (between 2 m and 3.50 m) and strike the right balance between pumping costs (investment and operation) and the disruption to navigation caused by variations in reach levels. Working with VNF and MET engineers, they adopted the automation system principle illustrated in the diagram below. It would obviously have been absurd to attempt to pump the water drained from a lock chamber in 5 minutes back to the upstream reach instantaneously. That would require a pump capacity of 2000 litres per second! Pumping a tenth of this capacity, it will take 40 to 50 minutes for the upstream reach to return to its normal level. Hence the notion of “level fluctuation” illustrated on the diagram. In the end, canal pounds will be allowed to come close to their minimum levels at the end of the day, and the pumps will continue operating for a few hours during the evening to refill them.
An additional problem arose at Trieste lock, where all the lockage water must be retrieved in real time because it is impossible to pump water from the river Marque, whose water is of poorer quality. The pumping station will therefore operate as a closed circuit at this lock. The suction system is connected directly to the lock chamber rather than the downstream reach. Here again, the right balance between costs and performance had to be struck. The capacity of each of the two pumps at this lock is 360 litres per second. This means it will take about 20 minutes to empty this lock, which is acceptable.
Each station is equipped with a PLC that controls the pumping flow rate depending on the water levels in the upstream and downstream reaches, which are measured by sensors. This PLC already makes the station “intelligent”: as soon as the upstream level drops below the programmed level, the PLC starts up the pumps. In most cases a chain reaction is triggered, with the water being lifted from lock to lock to supply the summit level of the canal. This reach, at the canal’s “summit”, retains its historic purpose of acting as a reservoir, with an additional 30 cm water layer, making a volume of 25 000 m3.
Eventually all the PLCs will be linked with the Union management centre, set up in the former lockmaster’s house, and management will be centralised. In the meantime, each station has been equipped with a telephone line so it can be managed manually from a distance when necessary.
The system includes several safety features to ensure that it operates reliably. A first grid at the suction pipe inlet will trap bottles, beverage cans and other waste. A second grid with a finer mesh at the pump inlet itself will trap plants (notably water primrose). The maintenance teams will therefore have to clean these grids regularly. The delivery pipe, through which the pumped water flows into the upstream reach, is equipped with a valve, preventing backflow downstream should the pump be shut down.
The designers also intended the stations to be easy to maintain. Hooks are used to lift the pumps, close or open the pipe valves and lower or raise the safety grids.
In the future, the pumping stations will be controlled from the management centre.
In the meantime, a PLC makes each station “intelligent”.
Completion of works and testing
A few finishing works remain, and the electric cabinets and various safety devices have yet to be installed. On 22 March, VNF will begin restoring the land (replanting the banks) and start the test phase. The programming settings will be adjusted by analysing the behaviour of the pumping system. After having tested the system, the manager will have a number of means at his disposal to make up for shortfalls. He will notably be able to raise reach levels by adding beams to the lock gates. Three “overflow” beams will thus raise the upstream reach by 20 to 30 cm, constituting an additional reserve when required.
The principle of the Deûle-Scheldt Canal water supply system is not new. All canals crossing a watershed must be supplied with water at their “summit level”, from which it flows down through locks on both sides. In previous centuries, prior to the Industrial Revolution, water had to be brought to the summit by gravity flow from reservoirs created in valleys upstream. This is what Pierre-Paul Riquet achieved so brilliantly in the Montagne Noire starting in 1662, to build the Canal du Midi. In exceptional cases, hydraulic power alone was sufficient to lift water over a certain height, e.g. from the river Seine at Port-Marly to supply the pools at the Château de Versailles, or from the river Marne near Meaux, to supply the Canal de l'Ourcq. The invention of steam-powered machines made it possible to lift bigger flows. This was the case with the Canal de Roubaix pumping station in Lille, which will finally cease working once the new system has fully proved itself. Recycled water pumping at each lock was implemented in the 1960s on the Canal du Nord, and in the 1970s on the high-capacity Main-Danube Canal in Germany. In both cases only part of the water has to be lifted, since 60 to 80% of the lockage volume is stored in water-saving basins. The originality of the system described here lies rather in its sophisticated regulation and reduced operating costs, having kept the size of all the pumps to the minimum.