4 Technical description
4.7 Artificial island
4.7.1 Pre-feasibility study for the location of the artificial island
The artificial island in project alternatives ALT1a and ALT1b will be located inside Finnish territorial waters, at a distance of some 15–20 kilometres from Helsinki, in the Hramtsow or Ulkomatala shoal. In alternative ALT2, the planned location of the artificial island is on the Uppoluoto island/shoal. The location of the artificial island pursuant to ALT2 was chosen dur-ing the pre-planndur-ing stage of the FinEst Link project.
The preliminary analysis stage that preceded the project’s EIA procedure in the spring of 2018 included the selection of an optimal location for the artificial is-land in relation to the ALT1a and ALT1b project alternatives. Alternatives ALT1a and ALT1b looked for shoals with an av-erage water depth of more than 10 me-tres. The preliminary analysis used exist-ing documentation to survey the natural values of open sea areas in Kirkkonum-mi, Espoo and Helsinki, and key fishing areas in the area limited according to the figure shown below (Figure 4-10). Sea bird fauna and important sea bird areas were also surveyed.
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Figure 4-10. The pre-feasibility study area of the Finest Bay Area project alterna-tive (ALT1a and ALT1b) marked with a red dashed line. The blue circle indicates the location of Uppoluoto, which is the service connection island location for the FinEst Link project alternative (ALT2).
The purpose of the pre-feasibility study was to minimise the environmental im-pacts of the project and its impact on fisheries; in other words, to find a lo-cation for the artificial island that has a
minimum of natural value, taking into ac-count fish spawning areas.
Considering the project’s technical limitations (track geometry that does not allow for sharp turns and the potential
methods for constructing an artificial is-land), the suitable locations for the pro-ject, in addition to Uppoluoto, are those open sea areas with a maximum depth of 20 metres that are located along the axis of the eastern parts of the Kirkkonummi sea area, the Espoo sea area and the Helsinki sea area, with the exception of its easternmost parts. As regards the Es-poo sea area, the Natura 2000 area (Kirk-konummi archipelago) is ruled out since an artificial island cannot, as a rule, be located in a Natura 2000 area.
On the southern coast of Finland and the sea areas of the coast, the number of species and, thereby, biodiversity are clearly reduced when water depth ex-ceeds 10 metres and, therefore, the im-pacts of the project on sea bird fauna and fish populations that feed on mus-sels (which mostly lives at a depth of be-low 10 metres) are much less substantial than in cases where the artificial island is located in water that is less than 10 me-tres deep or in the region of an islet that rises to the surface.
When the artificial island is built in deeper water instead of a shoal, which is vital for the sea ecosystem, a new artifi-cial reef is created instead of a reef being destroyed, which promotes biodiversity in the open sea area. Artificial reefs are technologically sustainable and excel-lent results regarding them have been achieved in Danish sea areas, for exam-ple.
71 The pre-feasibility study examined
ex-isting location data from the Finnish en-vironmental administration’s open loca-tion data services, SYKE databases, the Finnish Biodiversity Information Facility’s portals, the City of Helsinki, publications from BirdLife Finland and Helsinki Region Bird Scientific Association (Helsigin Seu-dun Lintutieteellinen Yhdistys), place-ment plans for professional fisheries and aquaculture and other studies carried out in the region (earlier EIA procedures and permit applications). Whenever neces-sary, the material was supplemented with expert interviews, among other things.
The location data studied included, for example, the data from the VELMU pro-gramme (Finnish Inventory Propro-gramme for the Underwater Marine Environment), the regional underwater biotype inven-tories, information on fish spawning and feeding areas and key data on fisheries, important seal shedding areas, shoals, Natura 2000 areas and other important natural protection areas as well as bird areas of regional, national and interna-tional importance.
As regards bird fauna, the most im-portant task was the identification of important feeding areas (shoals) for the long-tailed duck (Clangula hyemalis), ei-derduck (Somateria mollissima), black guillemot (Cepphus grylle) and other waterbirds relevant in terms of protec-tion, and to look for population data in the literature and other existing docu-mentation. For bird fauna, on-site
sur-veys in the form of bird counting were also carried out during the spring, sum-mer and autumn of 2018.
As regards fish populations, it was im-portant to determine the key spawning and feeding areas in the open sea as well as important areas for fisheries.
Based on the pre-feasibility study, two potential locations for the artificial island were selected for examination in the EIA procedure: the Ulkomatala and Hramt-sow shoals.
4.7.2 Construction of the artificial island
ALT1a and ALT1b
When the average water depth is ap-proximately 15 metres, the surface area of the island constructed from broken rock will be a minimum of approx. 1 km2 and a maximum of 2–3 km2. The system will consist of a main island and smaller shoals and islets that, when completed, will support the living conditions of the marine environment and bird fauna in the region. The island’s shore areas will be shaped in adherence with the principles of natural shore construction and the aim is to add diversity in an otherwise barren open sea region. In accordance with the principles of natural shore construction, the lining of the shore should mainly use natural rocks of varying sizes or material corresponding to natural rock that allows for colonisation by perennial species.
The surface of the rock lining should be
sufficiently smooth and steep vertical walls should be avoided. As regards the shore zone, it is essential that, in addi-tion to structures designed for boating and recreation (quays, shore boulevards, beaches), structures typical of the nature of the archipelago are also designed, such as capes and sheltered bays of var-ying shapes.
At present, the areas commonly have a water depth of 5–20 metres. The soil on the seabed currently consists of mostly clay and moraine. Prior to the construc-tion of the island, the area will be dredged to remove the soft seabed sediment and clay, some 2–5 million m3 in total, which will be piled elsewhere in Finnish terri-torial waters using existing piling areas where possible. Dredging and piling are subject to a permit pursuant to the Wa-ter Act. Structures built on the island may also require excavation, such as in the case of inner harbours. The maximum excavated volume will be around a few hundred thousand cubic metres.
Following the dredging, split hopper barges (SHBs), will be used to transport and pile the broken rock into the area from the tunnel excavation sites on the mainland and, via the service tunnel, from elsewhere along the route. By filling in the seabed in this way, a water depth of 3–5 metres can be achieved overall.
Using a trench dredger, the broken rock will be lifted above the water level to form an island where a worksite can be established. Working from the island,
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a dragline excavator can be used to ex-pand the island by digging the piles of broken rock up from the water. The typ-ical reach of such a machine is 40–60 metres.
In the first stage, most of the materi-al will be coarser blasted stone created from the drilling and blasting; its intend-ed use is in protecting the island against erosion and forming a filter layer that re-duces the puddling caused by the finer crushed stone from the TBM process.
For building a vertical tunnel shaft, a water-tight trench will be dug on the is-land and supported by a bored pile wall.
The piles are bored into bedrock using air flushing through backfill. All the bro-ken rock will be removed from the com-pleted water-tight trench and the vertical shaft will be excavate downwards from the bottom of the trench to its future lo-cation.
A full-profile boring machine (TBM) will be used for most of the tunnel excavation, and the rock material generated from this will be fine with a granularity similar to crushed stone. Fine rock material is not optimal for filling waterways, since it is efficiently consumed by wave erosion and the rock material will not embed into the ground as effectively as stone with a coarser structure. Therefore, the aim is to build the main part of the shoreline during construction from coarser blasted stone from the drilling and blasting method.
During construction, waves will erode the shoreline in a controlled manner
wherev-er finwherev-er blasted rock is used. Once the island has been filled to its final extent, the final erosion protection will be con-structed from crushed stone with a grain size of 0.6–1.2 metres. The erosion pro-tection against higher waves will likely be a riprap pitching of broken rock. Struc-tural solutions, such as shore walls made of steel reinforced concrete or steel, may be used in parts of the island.
The island’s elevations and shore structures take into account the antici-pated sea level rise and wave action. At the present time, the basic level for the sea filling is +3.0 (N2000). The island’s shores are steep which reduces the damping of the waves when they hit the shore.
On 15 November 2001, a storm from the south-west created a significant wave height of 5.2 metres on the Gulf of Finland, off the coast of Helsinki, and the highest individual wave was estimat-ed to have a height of approx. 9 metres.
Both eastern and western winds may cause very high waves in the Ulkomat-ala and Hramtsow shoal areas; however, they cannot really grow above the figures stated above due to the long and narrow shape of the Gulf of Finland. A significant wave height of 5.2 metres was reached again in 2012 during the storm “Antti”
with winds from the east.
Breakwaters will be designed around the artificial island, if necessary. In terms of profiles and materials, the outer linings of the shore and breakwaters will take
into account the fact that they need to be potential living environments for the Baltic Sea’s natural flora and fauna that attaches to hard surfaces.
If the puddling caused by dredging and filling could potentially have a sig-nificant negative impact, it will be limited by building a curtain structure around the worksite whenever possible. However, the silt curtain suitable for these condi-tions would need to be very substantial and it would nevertheless require con-stant maintenance. The alternative to the silt curtain would be a bubble curtain that would work even in rough conditions;
however, its operational reliability poses a challenge. Only parts of the island will have worksites at the same time, which means that using structures to limit pud-dling across the entire outer edge of the artificial island at the same time would not be purposeful.
If necessary, the bored pile wall could be built directly into the water system from a raft before the small worksite is-land is constructed. In this case, the water system would initially have a steel structure, from where the excavation of the vertical tunnel and the construction of an artificial island around the structure would begin.
The preparatory construction of the island enables its purposeful future use.
The preparatory construction activities are determined by the future use. Insofar as buildings will be constructed on the backfill, the structure can be compacted
73 using dynamic deep stabilisation and a
static preloading terrace. Following this, lower multi-storey buildings can be con-structed on natural foundation beds;
bored pile foundations are used for taller buildings.
ALT2
In project alternative ALT2, an artificial is-land will be built in the current Uppoluo-to region. The island will have a clearly smaller surface area than in project alter-natives ALT1a and ALT1b. In project al-ternative ALT2, the artificial island mainly works as a permanent service connec-tion and no residential buildings will be built on it. A small harbour is built on the island to facilitate construction and service during operation. Coarse bro-ken rock will be used for protecting the shore area against erosion, similarly to alternatives ALT1a and ALT1b. In project alternative ALT2, the rock material from the excavation of the tunnel cannot be fully utilised in the construction of the is-land; when necessary, the broken rock is transported elsewhere to be used in con-struction.