News from the work-sites
Dredging - the last bucketful in three months
Dredging in progress
beside the Risban footbridge
Interview with Thibault Van Craen, project manager, and José Dewulf,
dredging pontoon skipper with the contractor Ghent Dredging
DEM The dredging works, by far the biggest operation for restoration of the canal (more than €7 million), are nearly completed in France. What remains to be done?
TVC In a few weeks we will have finished dredging to 1.90m on the canalised river Marque up to Marcq Lock. But we’re not stopping there; most of the 22-strong team remains to dredge a further 22,000 m3 between Marcq Lock and the river Deûle. At the same time, a second dredging pontoon will be brought up to the canal on the Leers side, to dredge the section which we couldn’t deal with it to date because of closure of the lift bridge at Grimonpont.
DEM Wasn’t dredging of this section down to the river Deûle planned in the original Blue Links project?
TVC Yes, but various problems arising during the works have led to a situation where this section now has to be financed 100% by VNF. This additional dredging campaign is programmed to start in July 2008 for an approximate duration of three months. This section of the canal is still used by loaded barges which serve the Lessaffre factory; this is why it is still part of VNF’s main network.
DEM How do you go about dredging the canal?
JD We proceed in sections of a few dozen metres, working backwards. I first place the pontoon on the left half of the canal, then lower the two hydraulic piles which are incorporated in the pontoon, to make it stand fast in the bed of the canal. The barge is placed on the right-hand side of the pontoon. The reach of the arm with its large bucket (0.8 m3 capacity) enables me to handle an area of about 25 m2, always working backwards. Then I lift the piles to move the pontoon back a few metres, in position to handle the next area. When the barge has been filled, the push tug has normally returned from Wasquehal with an empty barge. I then move the pontoon back up to the right-hand side of the canal so as to deal rapidly with the other half of the section just dealt with.
DEM How do you know when you’ve reached the required depth?
JD With experience, you can tell by the behaviour of the arm and the bucket, but there is a gauge on the arm which gives an immediate reading of the depth.
DEM What is the quality of the sludge removed from the canal and the river Marque?
TVC The prefectural decree which authorises us to deposit the sediments on the PCUK site at Wattrelos, future public park of the metropolitan area, imposes physical and chemical analyses of one sample every 3000m3, i.e. one barge in 40. We were afraid of finding serious pollutants such as PCB, as are found in many rivers in Europe, but luckily the area between the Deûle and Scheldt is exempt. The rate was systematically less than 1mg/kg, which is insignificant.
DEM And what about the dreaded chrome? PCUK was after all a factory which produced exclusively the chrome which was the pride of our cars, motorbikes and bicycles since the 1940s!
TVC There again, we were pleasantly surprised. The maximum rate authorised for depositing sediment was 20 mg/kg, but no sample showed more than 2 mg/kg. In fact in most samples the chrome was even undetectable (less than 0.05 mg/kg).
The hydraulic piles in raised position,
allowing the pontoon to move to a new station;
they are then lowered to fix the pontoon
DEM How do you explain this phenomenon?
TVC Only a laboratory analysis would provide a definitive answer, but this does seem to be exogenous material, probably tipped into the canal from worksites in the 1970s or 1980s. This is not dry soil, but it is much more compact, which explains the difference in cohesiveness and the much greater difficulty of extracting it. The difference in colour is explained by the completely different proportion of organic matter (compact soils are free of organic matter).
JD The sediments are amazingly clean, both on the River Marque and on the Roubaix Canal, although we found many more objects on the latter, including a car. In the old days, on a site like this, people would have been coming to collect the sediment from the spoil disposal area to spread it on their fields or their gardens!
DEM How do you see the canal today?
TVC It’s looking good! Our only disappointment is that we won’t have been able to complete the job through to the Scheldt: we have to stop at the border, just 10m before the lock at Leers*.
*It remains to determine the date when dredging works can start on the Espierres Canal, under the authority of the Walloon waterways. Today there are two blackspots where the depth is limited to about 1m: the junction of the canal with the river Scheldt and the approaches to Leers lock at Estaimpuis. On the rest of the canal, the available depth is about 1.60m. The objective is eventually to provide the same depth as on the French side, i.e. 1.90m, but it is hoped to offer at least 1.60m for the reopening of the link in 2009.
DEM The sediments you’re taking out of the Marque seem to be dry in part. Is this not surprising in a river bed?
TVC We were the first to be surprised to find this material, which could not be part of the sediment brought by the flow of the river Marque.
JD This was a problem for us, because this matter is much more compact than normal sediment and is much more difficult to scoop up. We are supposed to dredge 480 m3 per day, but some days we were not able to do more than 400 or 410 m3.
Pumping stations ready to perform
Acceptance tests have just been completed for the 10 pumping stations (see Newsletter No. 8), and VNF has taken delivery of the facilities. Inspections started on 29 May at the most difficult site, Trieste Lock at Wasquehal, where it was necessary first of all to integrate operation of the two pumps into the lock’s programmable controller. Because it is out of the question to pump water from the River Marque into the canal, the lock has to be emptied here exclusively by pumping within the chamber and discharging into the upstream pound, so that the recycling of the lockage water takes place in real time. This programming task was carried out by the contractor Rouby (who had built the lock) on Tuesday 27 May, just two days before the tests! Rouby knew its locks, of course, but programming pump controls was a completely new challenge for its technicians. Fortunately, all went well and the pumps were activated without difficulty by the controller. Final adjustments were done in minutes, to the satisfaction of all.
The results were excellent also from the point of view of performance: complete emptying of the chamber took only 16 minutes, whereas the studies of canal operation had taken into account a much less favourable situation (30 minutes).
On the other nine locks in France, the two pumps are activated as soon as the level has dropped by 10cm, and stop when the pound is just 1cm short of getting back to this target level. This margin is necessary to take into account the wave of propagation of the flow discharged into the pound.
Conclusion: the canal’s water supply system is shipshape! On the Espierres Canal in Belgium, the works will also be completed within a few weeks.
For further explanations on the canal’s water supply system
The “Maison du Canal“ soon to serve its first pint!
At the “Maison du Canal” it remains only to lay out the terrace, which will soon be done by the local council, manager of the facility on behalf of the owner, the Walloon Region (MET)
The “Maison du Canal” at Leers lock will be inaugurated on the weekend of 12/13 July by the commune of Estaimpuis. Acceptance of the restoration works took place last Thursday 5 June. The exterior has been repainted in a ceramic beige colour, with a grey base. The interior is also spanking new: attractive imitation stone tiles and grey walls. The exhibition and projection room has a pearwood floor. The bar is also in pearwood and stainless steel. The furniture is expected to arrive in late June. In the meantime, the bar was used for the first time for a meeting of the Blue Links technical committee on 6 June. The house will soon be alive again, with locals and visitors enjoying drinks and snacks inside or on the lockside terrace. The mayor hopes to welcome the future managers in July. The reception facility will then become an attractive complement to the “Guinguette” at Grimonpont bridge in France, just 1.3 km away! It is surely only a matter of time before a cross-border shuttle boat service becomes feasible!
The bar, fitted out as a meeting room for the Blue Links technical committee meeting on 6 June :