Performance of a dynamic cobble berm revetment for coastal protection, under increasing water level.

In a changing climate, sea level rise and projected regional–scale changes in storminess may increase the vulnerability of sandy coastlines to coastal erosion and flooding. As a result, there is increased interest in the development of adaptable, sustainable and effective coastal protection measures to protect these highly variable sandy coastlines. One such example is a dynamic cobble berm revetment; a “soft–engineering” solution (i.e., not fixed) consisting of a cobble berm constructed around the high tide wave runup limit, that has the potential to stabilise the upper beach, provide overtopping protection to the hinterland and translate with water level rise. However, there have been limited applications of dynamic cobble berm revetments to date, and there is a lack of understanding about the efficacy of this coastal protection to current and changing waves and water levels. This study details a prototype–scale experiment conducted to test the behaviour and performance of a dynamic cobble berm revetment as a form of coastal protection against erosive waves and water level increase. Results from the experiment showed that the revetment was “dynamically stable” under wave action as a consistent global shape was retained even though individual cobbles were mobilised under every swash event. Although the front slope and the crest responded to the incident wave condition, the net rate of change was always an order of magnitude lower than the gross rate of change. Tracking of individual cobbles using Radio Frequency Identification (RFID) technology showed that stability of the revetment was likely maintained by rollover transport of cobbles onto the crest, as the revetment moved upward and landward under water level rise. The presence of the revetment reduced the vertical and horizontal runup as well as the retreat of the upper beach. The experimental results presented suggest that a dynamic cobble berm revetment could be a cheap, efficient and low environmental impact engineering solution for protecting sandy coastlines in a changing climate. Some preliminary design guidelines for coastal engineers are also drawn from this experiment.