St. Petersburg is an autonomous subject of the Russian Federation, while being a part of the Northwestern Federal District. The territory of the city is divided into 18 administrative districts, within the borders of which 111 intracity municipality units, 9 towns (Zelenogorsk, Kolpino, Krasnoye Selo, Kronshtadt, Lomonosov, Pavlovsk, Peterhof, Pushkin, Sestroretsk) and 21 village are located. St. Petersburg is one of the biggest Russian cities according to the population size (as of June 01, 2013 – 5047.9 thousand people), as well as on the basis of its square area (1436.2 sq.km).
Geographic location of St. Petersburg: immediate proximity to the Northern and Eastern Europe due to common borders with Finland and Estonia, with the access to the Baltic Sea – all these factors influence the city economic development rather favorably. The city is an important industrial and transport center, as well as the marine capital of the country. St. Petersburg is widely recognized as a cultural and scientific center of a global scale.
St. Petersburg is located on the Eastern coast of the Gulf of Finland, in the Neva entry, on its 42 estuary islands. There are 385 water courses 788 km long within the city boundaries, the principal of which is Neva river, and 632 water basins with a total area of 33 km2.
Almost all St. Petersburg area lies on a flat low plane, with plenty of ancient sea-shore terraces. Central regions are located 1 to 5 meters A.S.L., while the mean relief altitude in Sothern and Northern areas reaches 50-60m.
The territory of the city is situated at the junction of two large tectonic structures, in geological terms: the Baltic Crystalline Shield and the Russian Platform. Shield corrugated metamorphosed rocks (granite, gneiss) are deposited 200m deep. They are covered with a sedimentary mantle from above, in the section of which two strata are distinguished: the lower one being represented by compressed and practically anhydrous clays and Cambrian and Vendian sandstones, and the upper one consisting of sand-clay Quarternary-aged soils. Quarternary deposits were created at the result of multiple glacial and interglacial epochs interchanges, which explains complex geological and hydrogeological structural conditions of the area.
The city climate is transitional: from the marine to the continental one, with moderately cold winter and moderately warm summer. According to the long-term observations data, the average annual temperature is 5.6°С. According to own geographic position, St. Petersburg is located in a humid region. The annual average precipitation amount is 653mm, with uneven breakdown of fallouts within a year: about 70% fallouts precipitate during the warm period (April – October). Winds of Westerly and South-Westerly directions usually prevail on the territory. High cloud amount together with high humidity are typical of the region within the whole year. However significant changes of principal climatic variables are observed during last decades on the territory of St. Petersburg.
The natural and climate patterns of the territory in the aggregate predetermine dangerous geological processes occurrence.
Eleven risk factors for the territory of St.Petersburg were studied within the framework of CLIPLIVE project: reliable layer depth, biogenic gas formation, coastal erosion, surface water flooding, ground water and head water flooding, karst processes, neotectonic areas, paleovalleys, the level of radon impact hazard and daylight surface slope ratio.
Among risk factors typical for the territory of St.Petersburg studied within the framework of the project, only three factors, particularly coastal erosion, surface water flooding and ground water flooding, can be categorized as climate-dependent. Simulation of the process variability depending on different scenarios of climatic situation change was performed for these factors. The first stage of simulation was performed for three climatic scenarios – optimistic (В1), pessimistic (А2) and balanced (А1В). Later on in course of risks assessment the simulation results for the balanced (intermediate) scenario were considered to be insufficiently informative, and further investigations were performed only for the optimistic and pessimistic scenarios.
The maps were prepared for each of the hazardous nature processes. For stable risk factors that do not depend on the climate changes the maps were drawn up for the actual climatic situation, and for the climate-dependent ones – the maps of the process were prepared both for the actual climatic situation and for optimistic / pessimistic climatic change scenarios.
Land use types for St.Petersburg were designated considering the urban development zoning principles defined by the Law of St.Petersburg dated June 28, 2010 No. 396-88 "On standards of land use and development of St.Petersburg" (map).
Reliable layer depth:
One of the most important factors which determine geological conditions applicability for ground construction is the reliable layer depth. The upper geological unit on the territory of the city treated as a reliable base for ground construction is represented by the ostashkovsk moraine layer. This layer is of a great practical importance, while being a rather reliable natural base for buildings and structures foundations of all types, including the pile ones. Ostashkovsk moraine deposits may be found almost everywhere on St. Petersburg territory, with mean accumulation from 20 to 30 m and more. This strata occurrence depth changes from meters to dozens of meters, with ostashkovsk moraine formations coming out to the daylight surface from place to place.
The area of the city is divided into 4 classes: reliable layer depth <2 m, 2-7 m, 7-17 m and >17 m.
Biogenic Gas Formation:
Biogenic Gas (marsh gas) is a mixture of gases occurring during plant residues decomposition in air tight natural conditions. The gas is flammable, with 20-95% methane content. Small amounts of CO2 and N2 may also be traced in the biogenic gas.
Gas production processes on the territory of the city are developing in natural (swamp) landscape conditions, as well as on the areas with anthropogenically changed landscapes. Water courses and disposal dumps are often buried during the areas under construction development, with further surface soil layer compression with concrete slabs, construction debris, solid domestic waste, asphalt coating etc., which reduces the original soil permeability significantly. At that, favorable conditions for intensive gas production are preserved on separate parts of the covered area (paleorivers and lakes, swamps, channels, landfills and others) with primary organic substance abundance and increased soils moisture.
The biogenic gas production risk factor is based on the phenomenon, that if the biogenic gas may penetrate into the atmosphere through upper loose formations and soils layers relatively freely in the natural conditions, while moving away from the gas-generation area permanently, then the same gas, being actively produced and accumulated in soils, has no possibility of unhampered release into the atmosphere through solid man-made constructions in the conditions of the anthropogenically changed environment. At a definite moment of time, the biogenic gas may either break through the formation located above at the nearest weak point under the pressure, being released as a gas-and-mud blow-out, or it also may accumulate in neighboring underground structures including basements. Methane accumulation is especially dangerous, since it may create flammable or even explosive mixtures in the atmosphere when certain concentrations of this gas are achieved.
Following 4 classes were identified based on available data: areas, where conditions for biogas generation are absent, areas with potential gas generation (buried hydrology), areas with elevated and anomalous biogas in soil, and areas of environmentally hazardous level of biogas concentration.
Head water impact:
Head water of the top intermorainal (Polustrovo) water-bearing level distributed locally in the city constitute a potential hazard of flooding in the areas with high piezometric level (the areas above the daylight surface) and low layer thickness of overlying water-resistant sediments of Ostashkov moraine.
Head ground water flooding can be caused by the "hydrogeological windows" in the upper part of the section, within which the hydraulic junction between Polustrovo water-bearing level and the upper ground water level.
Besides, flooding of the territories can be caused by the man-induced factors, including breakthroughs of Polustrovo head water at the bores of poorly plugged junked exploratory wells. In course of construction such areas are hazardous for water breakthroughs into the pits, upcoming wellsprings, basements flooding and buildings deformation.
As to the complex of parameters that cause the city territory flooding, the following gradations were defined: the depth of the layer is less than 3 m with the thickness of overlying sediments less than 5 m; the depth of the layer is 3-6 m with the thickness of overlying sediments less than 5 m; and the depth of the layer above 6 m with the thickness of overlying sediments less than 5 m.
Karst is a complex of water (surface and subsoil) activity-related processes connected with rock formations dissolution and desalination leading to cavities of various shapes and sizes occurrence. Specific karst relief forms (bell pits, basins, blind creeks) are created on the areas of relatively readily soluble carbonate rocks development (chalkstone and dolomite rocks).
Karst processes development impedes surface-based construction activities. Karst processes were discovered in the South districts of St. Petersburg (Krasnoselsky and Pushkinsky), where Ordovician carbonates are abundant.
The city area in terms of this feature is classified in two classes: areas of potential karst formation and areas free of carbonate rocks (yes/no – values). Maps of karst processes and karst risk.
Neotectonic activity on the territory of St.Petersburg manifests itself through earth surface oscillations in different directions, amplitude, rate, frequency and scalability, all of which lead to significant changes in bearing soils and rocks mechanical properties, even up to drifting sands creation.
That is why the current neotectonic activity constitutes a significant threat for engineering structures, and should be assessed separately, especially for the underground space development, high-rise construction and erection of objects of a high ecological risk.
Neotectonic activity is recorded in recent tectonic zones mapped from logging or from geophysical data. Three classes are recorded in the city area. The highest potential for neotectonic danger is estimated for areas, where tectonic zones are overlapping. Areas within one tectonic zone indicate lower potential for neotectonic danger. The lowest potential is determined for areas beyond the influence of recent tectonic zones.
Palaeovalleys are represented predominantly by buried ancient rivers and their feeders’ shut-ins. From the point of view of their geological structure, one may subdivide palaeovalleys into 2 types: palaeovalleys of the first-type are filled with clayey and sandy-loam formations mostly, while those of the second-type – with sand and gravel-sand sediments. Existence of palaeovalleys filled with loose moist formations is an extremely negative factor, especially for underground structures construction, therefore the ranging of this factor assumes only two classes (yes – existence; no - absence).
Radon is a radioactive gas created in the course of natural radioactive decay of uraniferous formations. Radon is not prone to accumulation on the ground surface, as a rule, since it is 7.5 times heavier than the air, but it is capable to concentrate in closed building basements, lowlands etc. in quantities dozens times exceeding the threshold limit values. Uraniferous dictyonema oil shale Ordovician may be met at shallow depths in Southern districts of St.Petersburg, while coming out to the day in some places. This explains increased radon hazard in Krasnoselsky and Pushkinsky districts areas. Zoning of these districts areas is performed on the basis of radon volume activity in soils measurements.
Radon and products of its decay pose a hazard to health only when they are concentrated in the indoor air, usually in basements or in ground floors.
In general, in St.Petersburg the radiation dose caused by geological factors does not exceed permissible standards, but accounting of the geological features of the area during the construction of new buildings, as well as the implementation of surveys and radon-preventive measures in existing ones are main components in the set of actions to reduce the exposure to natural sources of ionizing radiation.
Three classes were identified in this area in terms of radon hazard: no danger, moderately dangerous zone and dangerous zone.
Slope means the steepness of a slope; the elevated to flat horizontal surface rate within the area where it is observed. This factor determines potential hazard of rock formations gravitational shift. At slopes exceeding 10% the danger of soil slip development occurs. On the territory of St.Petersburg the surface slope risk is not that widely-spread. But at the same time soil slip processes developing on river-valleys’ and canals’ slopes in St.Petersburg may affect stable and proper functioning of embankments, utilities, as well as building and structures placed along water courses.
The area of the city is divided into three main classes in terms of the surface slope steepness: <5%, 5% - 15% and >15%.
Coast erosion is a process of coast destruction under the sea waves, streams and ice impact.
The main causes of coast erosion are: coastal zone geological structure, current tectonic structure, coasts and underwater shoreface relief peculiarities, as well as a complex of hydrometeorological factors. Extreme coastal washouts occur under the storm influence in surge wave conditions without an ice coating. Industrial processes (underwater mining of sand material, hydraulic structures construction, absence of science-based coast protection strategy, recreational objects construction, taking of emergent coast protection measures on coastal areas, etc.) influence coastal stability in a negative way either.
The total coastal length of the Gulf of Finland within St. Petersburg boundaries is 190 km. Today the Gulf of Finland coasts within the Resort region are being washed-out and withdrawing significantly. Destructions in the coastal area cause its degradation with irrecoverable losses of highly valuable coastal territories, as well as destabilize engineering and geological conditions of adjacent territories.
The maximum coast washout rate (up to 1.8 m/year) was fixed in the Eastern part of the Gulf of Finland, to the West of protective constructions complex (PCC) of St. Petersburg. However the intensive washout may also produce a negative impact on a shoreland part of the gulf protected with the PCC (the Neva Bay), while reaching 1.5 m/year.
Surface water flooding map:
The main risk factor for St. Petersburg area flooding with surface waters is a surge phenomenon. The principle of the Neva surges occurrence mechanism is that cyclones crossing the Baltic Sea from the South-West to the North-East create a specific-type wave, forcing it towards the Neva entry, where it joins with the natural river flow. Water raise is intensified due to shallow waters and flatness of the river floor in the Neva Bay, as well as due to the Neva estuary converging towards the Gulf of Finland. The wave height ranges from 30 to 50 cm in the beginning, and the wave crest speeds up to 40-60 km per hour.
Nowadays the Neva Bay embankment and islands of the Neva estuary are considered as protected after the PCC commissioning, while the PCC closed section lines lead to water level increase at floods occurrence in the Resort area by approx. 5-10% at the result of the wave action turned aside from the dam.
The current tendency of water level raise in the Baltic Sea and the Gulf of Finland, as well as precipitation rate and amount increase, may lead to the increase of the number and frequency of surge phenomena occurrence on St. Petersburg territory.
In order to assess the risk of the city territory flooding with surface water, simulation of the water rising was performed for the Gulf of Finland and the Neva Bay as a result of surges for the actual climate situation and under different scenarios of the climate change.
Land flooded areas for three scenarios (the actual average sea level, the water rise by 0.4 m and 1.0 m) with probability of once in 100 years and once in 10 years were calculated on the basis of combination of a digital terrain model of Saint Petersburg and the appropriate matrix of the maximum water rise level.
Ground water underflooding:
The risk factor of St. Petersburg area waterlogging with ground waters is connected with phreatic water level occurring close to the surface, first of all. This water bearing stratum is spread practically everywhere on the territory of St. Petersburg, and is marked by a high ground water level, which may lead to embedded structures (buildings basements, foundations, underground walkways and parking spaces, etc.) flooding under certain circumstances. To assess the risk of subsoil waterlogging occurrence, the city was divided into zones according to the maximum expected ground waters level occurring depth.
This risk factor should be treated with due consideration in the course of civil and industrial construction planning on newly-build areas, as well as during repair or reconstruction works conduction on residential and non-residential areas. The predicted growth of precipitation amounts in future may cause ground water rise with further territory flooding as a consequence.
To assess the probability of ground water underflooding under current climate conditions, the city area is divided into the following gradation: ground water level less than 0.5 m, 0.5-1.0 m, 1.0-1.5 m, 1.5-3.0 m and more than 3 m.
Integrated natural risk assessment:
The integrated risk map combines all developed risk maps. Separate risks factors are combined in such a way, to allow for a maximum value choice among eleven specific parameters within each cell. Green color marks the lowest risk potential, red color the highest risk potential.