Known as the Venice of the north, Russia’s St Petersburg is one of the most beautiful cities on earth – and also one of the most prone to flooding.
In 1999, the city experienced one of its worst floods in years when the Neva River rose to +2.71m Baltic Chart Datum (with a normal level of +0.0m BCD) , over twice the level at which a flood is recorded. The bill for damages came to around £14 million, and that’s without counting the human and cultural costs.
St Petersburg has experienced floods on average once a year since its founding in 1703 due to storm surges originating in the Gulf of Finland. In the past 30 years, flooding has increased to become a twice-yearly event and a severe flood could occur at any moment.
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Video produced 2009
The area lies where the delta from the Neva River meets the Gulf of Finland – land that was originally little more than marsh. Today, most of the city’s downtown territory is located just a few metres above sea level. A future flood to a level of +3.0m BCD is projected to flood 94sqkm of the city and cause $490million in damage, and a flood level of +5.0m BCD (just over the current design flood) would flood 150 sq km of the city and is projected to cause $2.7billion in damage.
Halcrow is the lead consultant on the project for the stage 2 detailed design of a 25.4km long flood protection barrier in St Petersburg. The stage 1 design allowed the initial partial construction of the barrier by the mid 1980s. The stage 2 design completes the design works, including updating the environmental data and designing to modern national and international codes. As well as undertaking design and taking the overall design responsibility, the Halcrow-led team also secured the necessary government approval, supported the international tendering for the stage 2 works including preparation of the technical documentation, and have provided support during the construction phase. The structure aims to protect the city from flooding, with a return period of one in 1,000 years, caused by fast-flowing storm surges originating in the Gulf of Finland.
The barrier is located 25km away from the city and is 25.4km long. It includes 11 rock and earth embankment dams separated by channel openings and six sluice complexes, each with up to 12 steel radial gates 24m wide (a total of 64 gates) which allow water flow during normal conditions but can be closed in times of flood.
There are also two navigation channels to accommodate marine traffic. The first of these is 200m wide and will have the capacity to allow ships of up to 100,000 tonnes through. In order to close this channel, two of the largest hydraulic structures in the world are being built, floating radial steel gates with steel arms to a radius of 130m each. The second navigation channel is 110m wide and will be closed by a single vertical lifting steel gate.
The project also includes structures for a highway across the main hydraulic structures. A tunnel has been constructed as an integral part of the works at the 200m wide opening, and the road crossing at the 110m opening incorporates approach viaducts and a steel bridge, which can be vertically lifted to give additional headroom to ships as required. As part of a road constructed on the embankment dams, this highway will form part of the future western section of a six-lane ring road around St Petersburg.
Benefits
In 2010, work will be effectively completed on the £4.5 billion barrier that is designed to save one of Europe’s greatest cities from sinking back into the marsh on which it was built.
The barrier is already making a significant difference. It lowered the water level by 300mm in the 1999 flood making a small but significant contribution to keeping the city functional. The barrier protects the world-famous Hermitage museum as well as numerous buildings of national importance. It will protect St Petersburg for between 200 to 500 years.
Innovations
The design of the gates that close the main shipping channel, 200m wide, was a world first. The large radius sector gates allow a clear shipping channel and minimises the velocity of flows, easing navigation. The gates are parked in permanent dry docks that allow access for maintenance, always a critical concern for operators.