December 15th, 2019

Dike restoration on the island of Borkum

Installation of theVerkalit®-Cover

In October 2019, the city of Borkum acquired the flood protection system in the area of the former Borkum naval facility from the Bundesanstalt für Immobilienaufgaben (BImA). The entire flood protection system consists of a 1,100 m long flood defence wall, five dike swells and 1,400 m dike, the so-called "black dike".

A prerequisite for the takeover was the over-planning and refurbishment of the flood protection system. Tractebel Hydroprojekt GmbH was commissioned with the planning of the refurbishment at the end of 2016 (see also report No. 69). Construction for the actual dye refurbishment began in May 2019. The scope includes the refurbishment of the approximately 40,000 m2 deck of the dike, the repair of the five screens in the dike, including the extension structures, and the demolition and extension of the flood defence walls that are integrated at the dike ends. The special challenge here is the specification of the realization of the project within the storm-free period from 15 April to 15 October 2019.

The Verkalit-system was chosen for the new cover fastening. The decking stones have a full-surface composite effect through a circumferential "ball-bearing" groove-spring system. Settlements or elevations of the dye core can accommodate the cover through the system and cavities are detected faster than with rigid constructions. Due to the high weight of 30 to 50 kg per stone and the area to be managed in the short period of time, machine laying was absolutely necessary. At the beginning of the construction work, the contractor proposed to mill through the asphalt top layer, which had to be broken off and disposed of as planned, and to use the material as a support layer for the cover stone. The biggest advantage of this approach is the time saving, as the exposed dike does not have to be additionally protected from erosion. In addition, the asphalt demolition does not have to be transported to the mainland at a cost-intensive cost for disposal. The purchase of material for the filling of the defects in the dike support body is also omitted.

The work on the siels and the flood protection walls went largely without interference. The main work is expected to be completed by the end of November 2019. The start of the third construction phase, the replacement of the five dike sections, is planned for April 2020.

Jennifer Robinson, Hamburg