Investigation at Gilleleje Flak and Tragten in preparation for reef restoration. Geophysical, biological and hydrographic investtigations performed by GEUS, Orbicon and TT Hydraulics

Sara Skar, Zyad Al-Hamdani, Niels Nørgaard-Pedersen, Jørn Bo Jensen, Lars-Georg Rödel, Birgitte Nielsen, Jan F. Nicolaisen, Mikkel L. Schmedes, Thomas Ruby Bentzen, Torben Larsen

Publikation: Bog/rapportRapport (offentligt tilgængelig)


The aim of this project is to identify areas for restoring stone reefs in the Gilleleje Habitat area 195. Extensive geophysical, biological and hydrographic efforts have beenput together by GEUS, Orbicon and TT Hydraulics to achieve this goal.

Results of the geophysical survey show the spatial distribution of the stones on the seabed surface, the bathymetry, and the seismic stratigraphy of the surveyed area. Results indicate that a major part of the area is composed of till (containing stones of differentsizes) of glacial origin, with stretches of stratified glaciomarine clays (containing fewer stones) and some areas covered with sandy younger Holocene marine deposits.

Biological work was endeavoured to validate the geophysical interpretation of the seabed substrate, and to gather information of marine biological communities at selected locations and selected depths. Biological interpretation and classification of the dives (paravane and spot dives) show very well developed macro algae communities associated with the widelydistributed stones in the area. The macro algae communities dominate at all depth intervals but most species divers on depths above the thermocline (approx. 11-12 m). Different epi fauna species and fish were observed associated with the macro algae communities. The biological communities are typical for the southern part of Kattegat.

The hydrographic model for waves and wave generated currents show that the area is stable and the effect of the proposed reef on wave height is minimal. It also indicates that sediment transport is only affecting sand and small gravel sediments when the wind speed is high (18-25m/s).

Information about the type and size of stones to be used in thereef restoration project were obtained from consulting engineering companies that had previous experience with such endeavour. Other important information was deduced from the Blue Reef project guidelines and reports.

After contacting several offshore building companies and individuals who has long experience with this kind of worka project proposal has been completed. On the basis of this information two optimum positions were chosen for reef restoration, one at shallow waters 10m. The location of the proposed reefs will ensure the flourishing, abundance and diversity of the pelagic fish as well as the benthic fauna and flora. The southern reef PR1 location will ensure connectivity between two existing reef areas.

The deep northern proposed reef PR2 isa 600 m long dam of 3m height anda 15 m base. The shallow southern proposed reef PR1 is a series of 18 circularly truncated cones with 4m height and a 20 m base.
Rekvirerende organisationNaturstyrelsen
Antal sider58
StatusUdgivet - 31 dec. 2016


NavnDanmarks og Grønlands Geologiske Undersøgelse Rapport


  • Denmark


  • Programområde 5: Natur og klima


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