MONITORING OF ZEEBRUGGE BREAKWATERS

Abstract

From the moment they are constructed, rubble-mound breakwaters are due to be damaged someway. Accepting damage is one of the basic principles of mound breakwaters design. Damage can take place either gradually in time or catastrophically after a major storm. Gradual deterioration of the armour layers or foundation may be unnoticed without the aid of a monitoring program and may ultimately result in the failure of the armour layer, in slope instability or in unacceptable large settlements. By comparison of measurements of the state of the structure at a number of points in time, a monitoring program allows these changes to be identified at an early stage, thus enabling the appropriate maintenance action to be carried out. Several structural and environmental monitoring techniques are used in Zeebrugge. Three structural monitoring techniques are presented in more detail. The emerged armour units of the Zeebrugge breakwaters are monitored using aerial remote sensing. An observation flight is made once a year. Each time, the position of over 15,000 armour units is very accurately retrieved by stereometric digitization. The coordinates are stored in a computer database. Several types of data visualization have been developed for a fast and efficient evaluation of the survey results. For the underwater inspections of the breakwater two acoustic techniques are used. On the one hand, digital side-scan sonar recordings are used to produce highresolution and contrasting images of the breakwaters'underwater armour layer. Such images allowed the detection of structure modifications in the breakwaters'toe protection. On the other hand, high-frequent multibeam echosoundings, allow to exactly quantify the actual underwater armour unit movements in time.