Monitoring of rubble-mound breakwaters using UAV technology

Abstract

Breakwaters are protective and sheltering structures which aim to provide either protection from waves and currents for people and goods at the coast or sheltering conditions for ships and boats moored at the port, thus ensuring that port operations are performed in safe conditions. For that reason, occurrence of damages on such structures usually implies significant economic losses, such as harbour inoperability. In Portugal where severe sea states due occur, rubble mound breakwaters (RMB) are the most common type of these breakwater structures. The most conventional structure of a RMB consists in a core of a mix of fine and coarse material covered by one or two layers of natural rock or/and artificial concrete blocks that forms the so-called armour layer. In order to predict damages that may occur in RMB’s and evaluate its importance in terms of structural safety and functionality, it is of utmost importance to follow a monitoring program on such structures. The main goal of such monitoring is to detect changes, movements and instabilities in the position of the armour layer blocks, since those mechanisms may lead to a weakening or even a breakage of the structure. Results on the monitoring will enable authorities to plan and prioritize repairs and minimize future short and long-term costs. Traditionally, the monitoring of the RMB is made with visual, systematic, observations, but this technique is both time-consuming and depends heavily on the experience of the observer and, additionally, a limited amount of quantitative information is obtained, although a relevant qualitative evaluation is achieved. Photogrammetric techniques, using photos acquired by cameras mounted in UAV (drones) have already proved to be the most suitable technique to complement traditional monitoring, as they provide quantitative, with required accuracy, measurements on the surveyed area, they enable observation of specific areas and new perspectives difficult for human observers, and at present they are not too expensive to implement. This paper joins, as authors, two groups of experts: one with experience in flights with different UAV models and another one with experience in systematic monitoring surveys of breakwaters and maritime structures. This paper includes, as example, the results of studies conducted in three rubble mound breakwaters and presents conclusions according to the views of each group of experts.