Læsø Trindel just 12 km north-east of the Danish island of Læsø used to be a large cavernous boulder reef formed during the ice-age. Over the years the reef has gradually vanished as the easy to get at boulders have been excavated for constructional purposes, leaving a much depleted reef. This has severely impaired the habitat. Shallow water boulder reefs have a high biodiversity and are important habitats for a wide range of species.
The original boulders were transported by the ice 10.000 years ago from Norway and Sweden. Most of them are now to be found in various breakwaters protecting Danish ports and harbours. Today boulders have arrived again from Norway and Sweden but this time transported by tugs and huge barges. Almost 95.000 tons of boulders have been blasted out at quarries and carefully dumped where the reef used to be. The hope is that the reef again will provide the biodiversity and habitat it used to do.
On a national level it is estimated that most of the natural reefs have been excavated with only few remaining. BlueReef is the first large marine nature restoration project in Denmark and is expected to provide a wealth of knowledge on how marine habitats can be improved.
A wide range of pilot studies have been carried out as part of the planning work. One important issue was if the seabed would be able to carry the weight of the boulders. Even if there used to be a reef there 50 years ago, the seabed could have changed by the current leaving layers of sand. Therefore the bearing capacity was studied by drilling 10 m down into the seabed and extracting samples. The samples showed that this would not pose a problem.
The next issue was to determine how big the boulders should be for the reef to be stable when exposed to storm situations and if the reef would significantly influence the current in the proximity. Also it was important to investigate if nearby subsea structures formed by leaking gases from the underground could be affected by increased currents or sediment transport due to the reconstruction of the reef.
The necessary size of the boulders was calculated by determining the forces from waves and currents the reef will be exposed to. The forces were determined by carrying out computer simulations of 10 years of current and wave conditions in the area of Læsø Trindel. ‘To find the right size of boulders is an optimisation process. On the one hand you do not want boulders which are not heavy enough to withstand a storm and on the other hand you do not want too big boulders as this becomes too expensive.’ says Sanne Lina Niemann, Coastal Engineer at DHI.
Once the size of the boulders had been determined a series of storm simulations were carried out to see if and how the reef would influence the surroundings. ‘As far as we can see from our computer simulations the impact on the surroundings is very local and minimal’ says Sanne Lina Niemann.
A marine study of the area was also conducted before the first boulder was placed. Marine biologists have carefully mapped the species in the area. Fish have been dissected to establish what they feed upon and how much of it comes from the remains of the reef. Small radio transmitters have been fitted to fish and lobsters to determine how they move around. The results showed that the cod used the remains of reef as a feeding area, while the lobsters which normally live in the cracks and crevices did not use it.
Marine biologists expect that the new reef quickly will be populated and attract many spices. Some may take a couple of years but most are expected within quite a short time span, thereby strengthening the food chain considerably. The way life develops at the reef will be studied carefully the coming years and it is expected that much of the experience gained from this project can be used elsewhere.
Læsø Trindel is designated as a Natura 2000 Site according to the EU Habitats Directive. The project cost is Euro 4.8 million of which the EU commission has funded 50%.