Floods in Saint Petersburg

Floods in Saint Petersburg refer to a rise of water on the territory of St. Petersburg, a major city in Russia and its former capital. They are usually caused by the overflow of the delta of Neva River and surging water in the eastern part of Neva Bay but sometimes caused by melting snow. Floods are registered when the water rises above 160 cm with respect to a gauge at the Saint Petersburg Mining Institute. More than 300 floods have occurred since the city was founded in 1703.

The construction of Saint Petersburg Dam, started in 1978 and completed in 2011, is expected to protect the city from devastating floods. The dam is the last completed part of the Saint Petersburg Ring Road. Its first use to hold back the incoming Baltic water into Neva bay took place 28 November 2011 and had resulted in decrease of water rise to 1.3 MASL, that is below flood level equal to 1.6 masl, which prevented the 309th flood in the history of the city and saved some 1.3 billion roubles of possible damage.

Floods in St. Petersburg are caused by several factors. Cyclones, originating in the Baltic Sea with a prevalence of west winds, induce a "slow" matched Kelvin wave to rise and move towards the delta of Neva River where it meets the natural river flow moving in the opposite direction. The water level rises because of the shallowness of Neva Bay, flatness of its bottom and the narrowing of the Gulf of Finland near the delta. Seiches, onsets and another factors also contribute to the floods. Besides flooding as a result of storm surges, in 1903, 1921 and 1956 floods were caused by the melting of snow.

Prior to the founding of St. Petersburg in 1703, the largest flood occurred in 1691. Swedish annals report that the water covered the entire area of the present St. Petersburg by 25 feet (7.62 meters). Knowing about frequent floods from the locals, the Swedes laid the fortress Nyenschantz and the city of Nyen away from the delta upstream of the Neva River, at the confluence of the river Ohta to Neva.

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