Zeebrugge outer harbor: the evolution of scouring over the past 20 years

At the design stage of the breakwaters of Zeebrugge harbor 30 years ago, erosion around the western breakwater was expected. Hence, the design was modified in order to account for the long term scouring nearby the toe of the breakwater. By extending the willow mattresses and setting up a monitoring program with alarm- and limit lines, the scouring was expected to stabilize at an acceptable level, thus guaranteeing the geotechnical stability of the breakwater. Bathymetric soundings of the last 20 years were digitized in order to examine the evolution of the scouring. As expected, steep slopes were detected at the willow mattresses, but the limit line was not exceeded. It is concluded that the extensions of the willow mattresses was and still is a very effective and straightforward methodology to keep the scouring at an acceptable level, and hence guaranteeing the geotechnical stability of the breakwater.