The Uusimaa Region
The Uusimaa region is located on the south coast of Finland and has a population of around 1.6 million, which is more than a quarter of Finland’s total population. The region is one of the fastest growing areas in Europe. The birth rate is high and people are continually moving to the region, both from within and from outside of Finland. In 2013, the population density in the Uusimaa region was 174 per/km2, being the most densely populated region in Finland. The Uusimaa region is also the most urbanized area in Finland. It consists of 26 municipalities, including the capital city Helsinki. The modern infrastructure and high living standards offer excellent opportunities for business development in the region. Outside the Helsinki Metropolitan Area and its neighbouring cities, there are small towns, villages and sparsely settled rural areas and islands.
The topography of the Uusimaa region comprises small barren hills of rounded rocks alternating with clay-bottomed valleys. Coastal bays and narrow inland lakes and rivers are often located in straight and steep fracture zones of the bedrock. The mid-southern part of the Uusimaa region 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 formed by overgrown small lakes, ponds and isolated sea bays. Glaciofluvial deposits are scattered, mainly eskers in most parts of the region. The most significant formations are the Salpausselkä ice-marginal deposits that are mainly glaciofluvial in origin. The green areas in the Uusimaa region include forests, parks, national parks and cultural landscapes. The archipelago is a central part of nature in the maritime cities of the Uusimaa region.
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, 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.
The Uusimaa region is largely covered by bedrock areas that either crop out or are covered with thin overburden. When estimating the infiltration capacity of soils, the bedrock areas are not considered. It can clearly be seen from the generated maps that all the largest city centres in the study area (Hanko, Raasepori, Helsinki Metropolitan Area, Porvoo and Loviisa) have a low infiltration capacity and thus a higher potential for flash floods. This also applies old river valleys that have thick clayey sediment layers with low water permeability, such as the surrounding areas of the River Vantaanjoki, the River Kerava and the River Kymijoki in the easternmost part of the Uusimaa region.
In the Uusimaa region, over 18% of the area designated with a land use in the regional plan has low risk potential for storm water flooding, 26% has a medium risk potential, 13% a high risk potential and only 0.3% has a very high risk potential.
In the future, the annual average precipitation is estimated to increase, and during warmer winters even more precipitation will be as water instead of snow. This would also mean a higher potential for winter storm water flooding. Since geological factors such as the soil type or the slope of the ground surface of our living environment cannot be mitigated, the areas that are prone to storm water flooding will most probably be the same in the future. As the areas highly prone to storm water flooding add up to almost 18% in study region, this is an issue that should be continuously taken into account when planning new areas.
Note! The separate map of soil infiltration capacity for the cities of Loviisa and Porvoo are also provided. Risk maps are based on regional land use plan.
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 rivers 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 Uusimaa region, the surface water floods mainly occur in the River Kymijoki in Loviisa, the River Vantaa, which flows through the cities of Hyvinkää, Nurmijärvi, Vantaa and Helsinki, and the River Kerava, which flows through Kerava to Vantaa and discharges to the River Vantaa. For the River Espoonjoki in Espoo, the flood depths are not available, but flooded area estimates exist. The changes expected in the size of future river floods are quite small (±10%). Flood-prone areas are also identified for Lake Lohjanjärvi.
There are no major changes from the 1/100a floods to the 1/250a floods in the Uusimaa region. Difference between 1/100a and 1/250a flood areas is only 5 km2, and the distribution between different flood depth classes is the same. This could be a direct outcome from the topographical features around watercourses: the rivers and lakes in this region have steep, steadily rising slopes, which keeps the distribution of flood depth classes the same, even though the absolute area covered by floods increases. The rivers mainly follow the topographical features of the bedrock, and the river channel shape is less likely to have changed during past millennia. Furthermore, the soil type in the surroundings of river valleys is mainly clay, so the infiltration capacity is very low. The amount of soil sealing next to rivers and lakes is very low, and cannot therefore be considered as being a contributing factor to flooding.
In the Uusimaa region, many of the flood-prone areas are built up. Urban land use is considered to be more vulnerable to floods than other land use classes. In the Uusimaa region, the potential risk due to surface water flooding is mainly low or medium (values '1' and '2') and nearly 10% of the flood-prone land area has a high risk potential (value '3'). The change in flood frequency from the 1/100a floods to the 1/250a floods does not significantly affect the risk potential.
Note that in the 1/100a surface water flood map for Uusimaa more watercourse models are based on a coarser elevation model than in the 1/250a map (several watercourses modelled with DEM 25 m for 1/100a, mostly laser scanned elevation based model for 1/250a floods).
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.
The topography influences the estimated flood depth. In the Uusimaa region, the coastal areas are quite low lying, which can also be seen from the distribution of the flood depth classes. From 2011 to 2050, there will be minor changes in the flood class distribution, but from 2050 onwards the change is expected to be stronger, and in 2100 the region will experience higher flood levels. Larger parts of the flooded area will be covered by deeper floods. In particular, the proportion of 2–3 meters deep floods will significantly increase, from 1 % in 2011 to 18 % of the total flooded area by 2100.
In the Uusimaa region, the low and medium risk potential (classes '1' and '2') is dominant, but towards the end of the century the risk potential will slightly increase towards classes '3' and '4', i.e. high or very risk potential. The increase will not, however, be remarked compared to the current situation. The most notable increase can be seen in classes '3' and '4', which is most likely due to urban land use on the coast. Many major cities in the Uusimaa region (e.g. Helsinki Metropolitan Area, Porvoo) are located on the coast.
Note! The separate maps of the storm surge in 2011, 2050 and 2100 for the Porvoo-Loviisa are also provided. Risk maps are based on regional land use plan.
Integrated flood risk map was made by combining single flood risk maps of surface water flooding (1/100a flood), storm surges (1/100a flood 2011) 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 Uusimaa region, 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. The two highest storm water flooding risk classes ('3' and '4') cover approximately 525 km2 of the Uusimaa region. This is approximately 5.5 % of the total land area of Uusimaa. Surface water flooding risk is mainly low (classes '1' or '2') and it does not increase the integrated risk potential significantly. The two highest risk classes of the 1/100a surface water floods cover approximately 4 km2 of the Uusimaa region. Storm surge has a visible impact on the integrated risk potential on the coastal areas, especially in the urban centres of Hanko, Tammisaari, Helsinki Metropolitan Area, Porvoo and Loviisa. The area of storm surge risk classes '3' and '4' is approximately 18 km2 in 2011 increasing to 31 km2 by 2100.
Note! The separate maps of the integrated flood risk for the cities of Porvoo and Loviisa are also provided. Risk maps are based on regional land use plan.
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 Uusimaa region, nearly 40% of the land area is classified as a bedrock area. The characteristics of these areas for construction have not been considered. The most favourable construction conditions with sandy soil and low topography cover about 5% of the land area. The favourable construction conditions with moraine and low topography cover less than15% 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%.
Note! The separate maps of construction conditions of soil for the cities of Loviisa and Porvoo are also provided. Risk maps are based on regional land use plan.