Since the foundation of Saint Petersburg certain areas of the city have been subject to floods caused by seawater surges from the Eastern part of the Gulf of Finland under the action of cyclonic processes. These floods caused significant damage to the city's economy. The Saint Petersburg Flood Prevention Facility Complex was completed in 2011 to protect the city from flooding. Nevertheless, the lower coast areas of the city continue to be flooded. The amount of flooded areas is currently estimated by using the mathematical model of the flood and the digital elevation model of the territory of Saint Petersburg. On September 27, 2018, the water level elevated up to 176 cm above the standard benchmark in the Neva's delta. During the flood period the European satellite Sentinel-1 passed over the city and performed a radar survey of the Eastern part of the Gulf of Finland. This allowed for the first time to register the structure of sea swell during the flood and to map the flooded areas of marches (coastal wetland areas). A "change detection" and color coding methods were used to sense signal differences in the radar scenes received on September 15 and September 27, 2018. The results indicate the possibility to include satellite radar monitoring in the complex of post-project monitoring methods for «online» detection of the flooded zone boundaries along the entire coast of the Neva Bay and the Gulf of Finland within the Saint Petersburg area.