The huge curved floating gates that will close off the deep water navigation channel through the St Petersburg flood barrier are soon to be closed for the first time. Jackie Whitelaw reports.
St Petersburg’s main floodgates give the impression of straining at their seams, desperate to get to sea. But though they will float on water they do not have the look of ships.
The slender curve of their prows with ballast tanks above are more akin to submarines and their conning towers.
“That shouldn’t be a surprise,” says lead designer Halcrow’s project manager for the flood barrier David Edwards. “Because the gates were designed by submarine designers.”
And when they do set off to do their duty protecting St Petersburg they have to behave like submarines. The 122m long, 23.5m high and 4.7m thick steel structures will be floated in their dry docks and will then set sail to meet in the middle of the 200m wide, 16m deep navigation channel.
“The gates will take 45 minutes to close the navigation channel and will only take 25 minutes to land on the cill.”
Ivan Seryogin, Atomstroyexport
Chambers within the gates will then be flooded with sea water helping the structures to gently sink to the sea bed where they can use the weight of their steel and the weight of the sea water to hold back threatening surge waves heading ashore from the Gulf of Finland.
“The gates will take 45 minutes to close the navigation channel and will only take 25 minutes to land on the cill,” says main contractor Atomstroyexport contracts manager for the gates, civil engineer Ivan Seryogin.
“And we should get eight hours notice of a flood risk.” Last month they were each individually successfully launched and sunk.
“They weigh 3,000t each but they glided out so silently and powerfully, it was awesome,” Edwards says. Their biggest test will be when they are required to do their dance together for the first time − a date for which has yet to be fixed.
The gates and their method of propulsion are marvellous examples of really muscular engineering − good, solid design with nothing overcomplicated and the result undoubtedly of the design being originally conceived in the mid 1980s.
They are the embodiment of the old joke about the Americans spending millions of dollars inventing a ballpoint pen that will write in space, while the Russians took a pencil.
Each gate is controlled by two enormous 130m long horizontal A-shaped steel arms which are connected at their apex to a rather beautiful 1.5m diameter solid steel ball hinge encased in a bronze bushing with a pressurised lubrication system.
This is the pivot that the 1,500t arm and thus its gate swings around. Each gate is propelled by a chunky tractor that pushes it out from the rear via a steel cattle-prod type connecting arm.
The tractors run along cog and wheel tracks − designed, perhaps not surprisingly by a former army tank commander. At the moment each tractor is a flat bed affair with 28 sets of wheels. “But we are going to add a futuristic housing on top to make them look exciting,” says Seryogin.
Although the main gates can be moved out and closed quickly they will take longer to drain, refloat and return to the docking chambers.
“Our Dutch colleagues started later but opened earlier! But we are not arguing about copyright of the idea, just co-operating.”
Oleg Panchuk, Ministry of Regional Development
It is critical that the main gates (and their sister gate at Navigation S1 further north) are opened within 48 hours “because that is the amount of time it would take the waters of the River Neva pouring down from Lake Ladoga only 74km upstream to back up and start causing their own flooding problems in the city”, explains Ministry of Regional Development head of environment and flood warning Rosa Mikhailenko.
Anyone in the flood protection world who feels they have seen a similar type of flood protection gate at Meislantkering near Rotterdam would be right, but the Russians feel they came up with the original concept, just took a little longer to build it.
“The Dutch barrier does have practically the same gate design,” says the Ministry’s deputy general director of security and PR Oleg Panchuk. “Our Dutch colleagues started later but opened earlier! But we are not arguing about copyright of the idea, just co-operating.”
The St Petersburg team takes trips to see Maeslantkering in operation but Edwards believes that the Russian gate will be the more impressive.
When the team started on site in 2006 it had to unravel the jigsaw puzzle of the 40t to 80t pieces of steel that had been fabricated for each gate over 15 years before and start putting them together.
“Many of the 68 sections needed for each gate had been fabricated at the end of the 1980s,” explains Seryogin. “We had to repair them and manufacture the new ones that were needed.
“We did have the drawings and the sections were in the hands of experienced assembly workers and welders.”
The biggest challenge
Seryogin’s company Atomstroyexport is well known for its nuclear power station construction work and the quality of welding work carried out by its workforce.
The biggest challenge it encountered was appending some new bits of steel when it was discovered the gates needed some redesign work.
Edwards explains: “Part of my job was to go back to basics and we retested the hydraulics of the gates. We discovered that, when you started to get a difference in head as the gates closed, and the water level on the inside was lower than that on the outside, the gates became unstable.
- Client Ministry of Regional Development.Director, Department of Capital Investments − Vladimir Kogan; General Director, North West Directorate of Rosstroy − Vladimir Shchekachikhin.
- Lead designer Halcrow with DHV and Norplan
- Designers Lenhydroproject, LenMornilproject, Transmost, VNIIG, Rubin, Constructor, LenHydrostahl, LenMetroGyroTrans, SPbGyproShacht, VAMI Automatics, Techno SPb
- Project implementation FISP
- Project management Geningconsult, Royal Haskoning, Stroyproject
- Contractors Morteknika/ Boskalis/Hochtief; Metrostroy; Atomstroyexport; Transtroy
“The problems started at about 500mm. When the gates are shut in the design flood there is a 3.5m difference and with instability, lowering would be fraught with problems.
“It was a big technical challenge because our ideas were constrained by the fact that the docking chambers were already built and the most of the gate sections were already cast. “The solution was to add a hydrofoil section to the lower edge of the gate.”
The contractor opted to weld this on insitu within the docking chambers once the gates were finished, allowing it to keep working while the detail for the hydrofoil was refined.
“It was a tight space but not a problem for experienced workers,” Seryogin says.
“It was a big technical challenge because our ideas were constrained by the fact that the docking chambers were already built and most of the gate sections were already cast.”
David Edwards, Halcrow
Over the last year the huge hole that was the legacy of the first stage of construction of the barrier has disappeared. The tunnel section beneath the main navigation channel was jointed up and had its original steel plate waterproofing removed and a double skinned PVC waterproofing membrane added.
In summer last year the water was allowed to flood in, separating the two gates by sea and stranding the one to the south on its own island, only accessible by the tunnel. This new navigation channel was formally opened last October.
Since then a 1km long, 26m deep cofferdam has joined the southern gate to the embankment on the south blocking off the original navigation channel.
Within the cofferdam the final 700m of tunnel and ramp works are underway to allow completion of the barrier and its motorway. Workers on the project can look up to see huge tankers sailing above them.
Boskalis carried out dredging and earthworks and Hochtief is creating the 39m by 7m high concrete box for the tunnel.
“The idea is to finish the concrete works by mid 2010 and in parallel the tunnel will be backfilled and the utilities will go in,” says Halcrow project engineer Alexey Kislenko.
The desired, politically accelerated completion date of the end of next year is looking achievable.
The secondary 110m wide navigation channel through the barrier is designed to allow the passage of river-sea vessels.
The bridge over the channel can lift from its normal 16m height above the channel to 25m in three to four minutes.
The huge steel gate that will close off the channel in the event of a flood sits in a concrete slot below cill level and is lifted by four hydraulic cylinders. When work on site restarted the 2,500t of steelwork that had been fabricated in the 1990s was sitting in a yard in Gorskaya in 230 sections.
Finished measurements of the gate put it at 117m by 12m deep and 9m thick. The 11.3m stroke hydraulic cylinders will help the gate punch through a predicted 600mm of ice in the channel in the winter.
The six sluice gates that are spaced along the length of the barrier allow water into and out of Neva Bay and also double as flood defences. Each of the sluices has 10 or 12, 24m wide, steel radial gates and there are 64 sluice gates in total.
When they have to be closed they are pushed down into up to 5m of water in as little as a minute by pairs of hydraulic pistons. Concrete infill in the gates gives them some extra clout when it comes to forcing their way through the up to 600mm of ice they are designed to penetrate.
Even when partially built the embankment and sluices have been providing some protection to St Petersburg in flood scares over the last few years. Recent floods would have been much worse without them, according to Mikhailenko.
She keeps a wary eye out throughout autumn and winter as these are the critical seasons, when 85% of the city’s floods have occurred and water has been known to rise by 1m an hour.
“Since 1998 we have had a sophisticated flood warning system based on GIS that gives us good early warning and from that we have developed a useful flood modelling programme,” she adds.
“It is used by many agencies in the city − civil protection, planning, city development − to forward plan evacuation in the case of civil protection and how and where to develop the city in the case of the others.”
The sluices have also been proving their worth by improving water quality and environment in Neva Bay, she says. By closing some gates and keeping others open during normal weather it has been possible to change currents in the bay to help increase aeration and natural cleaning. “And we can use the flows to dissipate pollution − from oil spills from shipping for instance,” Mikhailenko says.
“St Petersburg is still the northern capital of Russia and still the country’s window to Europe. And the significance of the city as a transport node is growing.”
Oleg Panchuk, Ministry of Regional Development
As St Petersburg and the Leningrad region develop in the coming years the amount of shipping is expected to increase dramatically so the sluices’ cleaning facility will be particularly useful. The new, deep navigation channel that has been created as part of the barrier works will allow much bigger ships to ply into and out of the bay. There is good reason for this.
“St Petersburg is still the northern capital of Russia and still the country’s window to Europe,” says Panchuk. “And the significance of the city as a transport node is growing because new ports are being constructed nearby for liquefied gas export and for container cargos.
“Previously we had ports in Latvia, Lithuania and Estonia. Now we can only count on our own resources, so for several years we have been building in the Leningrad region and St Petersburg.”
St Petersburg’s flood barrier is significant for more than just the city’s protection, it seems.
St Petersburg flood barrier: Swing through the sea