Coastlines are extremely valuable assets for many countries as they play a major role in the national and local economy. In this context, sandy coastlines are particularly challenging as they are one of the most developed types of coast but also one of the most vulnerable in terms of erosion risk. The threat of erosion is mainly driven by storms, including storm surge and waves, and long term sea level rise which is expected to increase in rate with the on-going climate change. In response to this forcing, sandy beaches have been and will be protected in many areas. However, most of the coastal defences, whether hard or soft, are not designed to face the combination of these threats, with storms acting on a higher water level. Therefore, there is a need to develop a sustainable coastal protection which would take these threats into account to provide a reliable protection level. In this context, dynamic cobble berm revetments which mimic natural composite beaches that consist of a lower sandy beach and a back-shore gravel ridge, seem to be a promising solution. The potential of this structure has not been studied in depth, and therefore their performance, behaviour and dynamics remain unclear. To study these aspects and get a better understanding of their capacity to protect the coast and improve their design, one laboratory and one field experiment were undertaken on two different types of dynamic revetment. Under energetic waves and increasing water level conditions, dynamic revetments are able to protect the hinterland by reducing the wave runup and excursion. They also protect the underlying beach by armouring the sand and protecting it from backwash erosion. While protecting the coast, dynamic revetments show a dynamic stability as their particles are constantly mobilised by swashes but the overall structure maintains a coherent shape. Revetments are also able to quickly reach a stable state at both the intra- and inter-tidal timescale, but the overall stable state is only attained when the internal sand becomes stable. The volume of dynamic cobble berm revetments can vary through time, mainly caused by sand volume variations driven by water level and offshore wave conditions, and is able to rapidly recover from periods of erosion. In summary, dynamic cobble berm revetments are relatively inexpensive, easy to install and in many cases the material can be locally sourced. Further improvements in our understanding of dynamic revetments can be achieved through additional laboratory experiments, and a new scaling method is developed to compare distorted experiments. The scaling method is valid for both fixed and increasing water level experiments, and is used to better understand the beach profile evolution under rapid sea level rise.
Coastal protection and nearshore evolution under sea level rise
Bayle, P. (Author). 14 Oct 2020
Student thesis: Doctoral Thesis › Doctor of Engineering (EngD)