The Saint Petersburg Flood Prevention Facility Complex (Russian: Комплекс защитных сооружений Санкт-Петербурга от наводнений, tr. kompleks zashchitnykh sooruzheniy Sankt-Peterburga ot navodneniy), unofficially the Saint Petersburg Dam, is a 25 km (16 mi) complex of dams for flood control near Saint Petersburg, Russia. The dam extends from Lomonosov northward to Kotlin Island (and the city of Kronstadt), then turns east toward Cape Lisiy Nos near Sestroretsk.
The complex is intended to protect Saint Petersburg from storm surges by separating the Neva Bay from the rest of the Gulf of Finland. Historically, the storm surges from the gulf had caused over 300 floods in the city, several of which had a massive devastating effect. The dam has the capability to protect the city from water rising up to 5 m (16 ft). 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 damages.
The construction of the flood prevention complex started in 1978 and became one of the longest construction projects in Russia. After a protracted halt in the 1990s and early 2000s, construction was resumed in 2005 due to the intervention of Russia's President Vladimir Putin, a native of Saint Petersburg. Putin finally inaugurated the finished complex in 2011, when all the facilities at the southern part of the dam were completed, along with the 1.2 km (0.75 mi) long underwater roadway tunnel below the main southern lock, the longest undersea tunnel in Russia.
Over 30 water purification installations are placed around the dam, a part of a larger program to clean the water in the Neva Bay. The dam tunnel is also the last completed part of the Saint Petersburg Ring Road. The northern and southern parts of the dam act like two giant bridges and provide an easy access from mainland to Kotlin Island and Kronstadt.