Helsinki Metropolitan Area
The Helsinki Metropolitan Area
The Helsinki Metropolitan Area consists of the cities of Helsinki, Espoo, Vantaa and Kauniainen. The housing density of the Helsinki Metropolitan Area is high by Finnish standards. The population density in certain parts of Helsinki's inner city area is over 16 000 inhabitants per km2. However, the average population density of about 3 000 inhabitants per km2 ranks the city as quite sparsely populated in comparison to other European capital cities. From a national perspective, the Helsinki Metropolitan Area is a significant centre of population, jobs, public and private investment, infrastructure construction, administration and research. More than one third of the GDP of Finland is generated in the metropolitan region.
The Helsinki Metropolitan area is characterized by clay deposits that cover low-lying areas and by bedrock outcrops that break through the clay plains. The fine-grained sediments were deposited in an aquatic environment during the different phases of the Baltic Sea. This has influenced the physical and chemical properties of the deposits due to variations in humus, clay and water content, as well as mineralogy. Isolated sea bays filled with gyttja and other organic sediments are common in the fragmented coastal zone. Peatlands and wetlands are scattered throughout the region mainly formed by isolated sea bays. Till covers the bedrock and evens out the bedrock relief and ground surface morphology. Glaciofluvial deposits are scattered and small.
Helsinki_Metropolitan_Area_Generalised_Quaternary_Geological_Map.pdf (includes outer islands)
Identifying hazards and estimating potential risks
Storm water flooding or flash floods occur during heavy, long lasting rains or short but very intense rain events. Factors contributing to storm water flooding include soil sealing with impervious pavement types, such as asphalt, bricks, stones or concrete. Especially growing cities with a large amount of new construction are vulnerable to storm water flooding due to the increase in paved areas. Geology has similar effect to paving in connection with intense rain events. Very fine grained or tightly packed sediments, such as thick clay layers or tightly packed moraine, as well as bedrock are not able to infiltrate rain water fast enough to the ground, leading to flash floods. To estimate how prone the study area is to storm water flooding, the hydraulic conductivity of soil types, the percentage of soil sealing and slope steepness were combined on 20 x 20 m raster maps. Then maps were classified into eight different classes according to the combination of qualities they possessed. '0' is the class for the highest infiltration capacity / lowest storm water flooding potential (green colour on the map) and '7' for the lowest infiltration capacity / highest storm water flooding potential (blue colour on the map). Bedrock areas and watercourses were excluded from the classification at this point. Sewerage, vegetation type or existing flood management structures were neither taken into account in this estimation.
In the Helsinki Metropolitan Area, the areas with lowest infiltration capacity are found in urban centres of the cities. This is especially due to high percentage of soil sealing. The surroundings of the River Vantaanjoki and its tributaries are characterized with fine-grained sediments with low infiltration capacity. These areas can be well distinguished from the infiltration capacity map. The northern part of Espoo is sparsely populated and soil sealing percentage is very low. There exist extensive bedrock areas that are surrounded by clayey valleys with low infiltration capacity or moraine layers with higher infiltration capacity.
In the Helsinki Metropolitan Area, over 12% of the area designated with a land use in the regional plan has low risk potential for storm water flooding, 21% has a medium risk potential, 22% a high risk potential and less than 1% has a very high risk potential. It should be noted that land use is based on regional plan and not on local master plan.
Helsinki_Metropolitan_Area_Generalised_infiltration_capasity_of_soil.pdf (includes outer islands)
Helsinki_Metropolitan_Area_Generalised_Storm_Water_Flooding_Risk.pdf (includes outer islands, based on DEM25m)
Helsinki_Metropolitan_Area_Generalised_Slope_Steepness_for_Runoff_Potential.pdf (based on DEM25m)
Surface water flooding and especially river floods usually occur during spring when snow accumulated during the winter months melts. Spring floods will decrease in southern Finland in the future due to climate change, but floods during the other seasons might increase. In southern Finland, the change in precipitation is the main factor affecting flood patterns. Warmer winters may also increase frazil ice phenomena in rivers. Rivers in southern Finland are of varying sizes and the smallest ones have only local significance. Large lakes and rivers may also be regulated with dams or their level may be kept within certain limits to protect agricultural areas around the watercourse. Although the most significant river flood areas have been defined, this does not mean that the other rivers are not prone to flooding.
In the Helsinki Metropolitan Area, the surface water floods mainly occur in the River Vantaa, which flows through the cities of Vantaa and Helsinki, and the River Kerava, which discharges to the River Vantaa. For the River Espoonjoki in Espoo, the flood depths are not available, but flooded area estimates exist.
In the Helsinki Metropolitan Area, many of the identified surface flood-prone areas are located in built up areas. Urban land use is considered to be more vulnerable to floods than other land use classes. The potential risk due to surface water flooding is mainly low or medium (values '1' and '2') but over 30% of the flood-prone land area has a high risk potential (value '3') in both 1/100a and 1/250a floods. The flooded area for 1/100a floods is approximately 8.8 km2 and 9.2 km2 for 1/250a floods. This might imply that flooded areas will not increase significantly in the future, but the flooded areas will suffer form deeper floods instead.
Storm surges in Finland are mainly caused by rapid sea level rise. Causes include low air pressure in the area, internal oscillation caused by a long-lasting heavy wind towards the north or north-east from the Baltic Proper, and heavy winds through the Danish straits. One important factor affecting storm surges is post-glacial land uplift. Until now, the land uplift rate has been higher than sea level rise, increasing the land surface in coastal areas. According to recent estimates of sea level rise, the future land uplift rate will not be able to fully compensate for sea level rise, as it will accelerate in the coming decades. Therefore, storm surges will become higher in the future, even if the land uplift rate remains stable.
Storm surges are expected to increase in the future along with sea level rise. According to the estimates, the flood levels will slowly rise from current situation to 2050, but the rate of rise will increase from 2050 towards 2100. In 2100, the 1/100a floods are expected to be similar to the 1/1000a floods of today. In addition to sea level rise, extreme weather events (especially strong, long-lasting westerly winds) are estimated to be more severe in the future.
In the Helsinki Metropolitan Area storm surges with 100 years return period cover approximately 22 km2 of the coastal zone. The flooded areas in Espoo in 2011 are found mainly in the west in the Saunalahti bay and in the easternmost parts of Espoo in the Laajalahti bay. In Helsinki, the highest flood levels are found in the Vanhankaupunginlahti bay and in the eastern parts of the city in the Granöfjärden area. In 2050, the areas of Suomenoja and Vermo in Espoo become more prone to storm surges. In Helsinki, the Santahamina island, the Töölönlahti bay and the Vermo-Tali region will experience higher flood levels. Towards 2100 the storm surge inundated areas are mainly the same, but flood levels will be higher. It is notable, that the flooded area will not grow significantly towards 2050 (flooded area covers approximately 23 km2 in 1/100a flood in 2050), but from 2011 to 2100 the flooded area in 1/100a floods decreases by 43 % being approximately 32 km2.
The areas belonging to the two highest risk classes ('3' and '4') will increase from 5.7 km2 in 2011 to 11.4 km2 in 2100. This means 100% increase in the size of areas with highest risk potential of storm surges.
Integrated flood risk map was made by combining single flood risk maps of surface water flooding, storm surges and storm water flooding. The integrated geological risk results from the highest risk level given to one of the single risk, i.e. if value "4" is given to any raster cell it will result "potentially very high geological risk" within the integrated geological risk map. In this evaluation, all factors are equally important and no weighting is implemented.
In the Helsinki Metropolitan Area, the integrated risk map is dominated by the storm water flooding risk as it covers the largest area of the flood types presented in the area. In the city of Espoo, the areas with high risk potential are located in the southern part of the city. This is mostly due to intense land use. The northern part of the city is largely rural and forest area including large national park. In the city of Vantaa, the risk potential is high in the middle part of the city, as most of the urban functions are located there. This area is storm water flooding prone and it is located between two rivers with potential flood hazard. In Helsinki, the areas with potential high risk are located in the northern part of the city in the junction of rivers Vantaa and Kerava, in the city centre and along the main roads and railroads. Eastern Helsinki urban centers can also identify with potentially high risk as local daily services, schools and health centers are located there.
The construction conditions of soils may face unfavourable impacts due to climate change. In particular, changes in the soil frost period may cause unexpected challenges to existing foundation techniques. Heavy rains as well as changes in precipitation patterns, in snow cover and in groundwater conditions alter soil properties such as the bearing capacity and frost susceptibility. They may also contribute to landslides and erosion. In addition to changes in soil characteristics, changes in flood patterns will narrow the areas that are most suitable for construction purposes.
The geological characteristics affecting construction conditions were studied based on existing geological maps and data at the regional level. This evaluation was based on the general geotechnical characteristics of different soil types. The fine-grained component directly affects the soil permeability. Frost susceptibility properties and organic matter weaken the geotechnical suitability. The data were amended with information on slope steepness that has been calculated according to the airborne 3D laser scanning data of the National Land Survey of Finland (or alternatively the 25 x 25 m pixel-size digital elevation model).
In the Helsinki Metropolitan Area, about 30% of the land area is classified as a bedrock area. The characteristics of these areas for construction purposes have not been considered. The most favourable construction conditions with sandy soil and low topography cover less than 2% of the land area. The favourable construction conditions with moraine and low topography cover less than 15% of the land area. Demanding construction conditions with clay areas or areas with varying topography are over 35% of the land area. The most demanding construction conditions with soft organic soils such as peat and gyttja are about 5%.