Scour protection structure
A comprehensive safety concept represents the foundations for successful operation of the Mittelplate Drilling and Production Island in the Wadden Sea tidelands on the coast of Schleswig-Holstein. Since construction of the island began in June 1985, the so-called scour protection – a structure made of riprap and mortar that fully encloses Mittelplate – has made a vital contribution in protecting the island from environmental impacts.
The shifting “Trischenflinge” tidal channel
Due to the high degree of natural dynamism of the Wadden Sea tidal flats, the path of the “Trischenflinge” tidal channel has been shifting northwards and has now reached Mittelplate. To protect the drilling and production island against erosion – the scouring away or withdrawal of sand around the island – an upgrade of the scour protection structure has become necessary. At the end of June 2010, RWE Dea filed the final application documents for a planning approval process regarding the construction measures with the State Department for Mining, Energy and Geology (LBEG). It may take up to twelve months before a ruling is made by the LBEG.
Objective: integrating the extension measures into the general operating plan
All scour protection extension measures – those carried out to date as well as those potentially needed in future – form part of the current application, so that they can be included in the general operating plan for Mittelplate. The scour protection structure can be extended downwards in stages, as required. The first stage of the structure extends to five metres below mean sea level, the second to eight metres, and the third down to eleven metres. Upgrade measures will only be implemented if the dynamic shift processes unfolding in the tidal mud flats make it necessary in order to protect Mittelplate.
With any construction measures necessary, nature is always a priority
The upgrade of the scour protection therefore represents an adaptation to the prevailing natural conditions. The measures required will be implemented in strict compliance with environmental directives and under the control of the supervisory authorities. When extending the outer reinforcement, a composite weave is put in place first and then weighed down with stone rubble. To increase the stability of the packing, the rocks in the upper layers are subsequently bound with a particularly water-resistant mortar.
Renaturation of salt marshes as a compensatory measure
In legal terms, the scour protection measures represent a substantial intrusion into the natural environment of the Wadden Sea tidelands since they impact on the natural dynamism of the mud flats, which is responsible, among other things, for the movement of the tidal channels. One of the stated objectives of the Wadden Sea National Park – part of Natura 2000, the European network of protection zones – is to maintain the dynamism so typical of these tidal mud flats. EU legislation therefore calls for compensation in kind in close proximity to the affected protection zone – referred to as a coherence-securing measure. This measure is required to be planned and implemented by the operator.
A one-to-one compensation in kind would be optimal, but it is simply impossible to create mud flats. So, after checking a great many alternatives in collaboration with the national park authorities and the Environmental Ministry in Kiel, RWE Dea decided that renaturation of 54 hectares of salt marshes would be a suitable measure. The salt marshes off the coast of Friedrichskoog have been so badly affected by anthropogenic influences that they are no longer anywhere near their natural state. Specifically, a chessboard-like system of channels was put in place to drain the salt marshes. The renaturation measure is aimed at increasing the effect of tidal movements as a means of re-establishing a natural system of tidal channels across the salt marshes. A planned information facility will also allow interested visitors to the National Park to experience this natural region.