Composite Beaches & Dynamic Revetments: Coastal Protection Inspired by Nature

: (Alternative Format Thesis)

Abstract

The coastal zone is vital to both the local and national economy of many countries, therefore, protecting them is important to the prosperity of many communities. The coastal zone is threatened by both erosion and flooding, this risk is growing with increased sea levels due to climate change. Protecting these regions will require improved management practises including new coastal protection structures. This work focusses on composite beaches and dynamic cobble berm revetments. Composite beaches are a naturally occurring beach configuration in which the foreshore is sand and the backshore is a ridge of gravel sediments, they are a remarkably stable beach type common on the coastlines of the UK, Australia and the USA amongst others. Dynamic cobble berm revetments are a nature inspired coastal protection structure based on composite beaches. They consist of a berm of artificial gravel sediments placed at the high-tide limit of a sandy beach, the structure prevents erosion of the upper beach and limits wave excursion reducing inundation of the hinterland. The uptake of these structures has been limited due to the limited studies into their performance and behaviour under wave attack. Further, no valid morphodynamic model has been developed for these structures. In this work several findings are presented as part of a combined study. First, a comparison of two dynamic cobble berm revetments constructed from differing gravels; one well-sorted, rounded and another poorly-sorted, angular. Both revetments were constructed at prototype scale in the Großer Wellenkanal (GWK) large wave flume under identical hydrodynamic forcing. Both revetments can be considered effective erosion control for the upper beach and limit wave excursion primarily to the structure. A key feature of these structures is the ability rapidly reshape under wave action to geometries that excel at limiting wave excursion. However, during the experiment the well-sorted, rounded revetment suffered continuous sinking under the front face and maintained a flat crest. Conversely, the poorly-sorted angular revetment developed a peaked crest which further limited wave excursion. Secondly, an investigation into the dynamic stability of the structures, dynamic stability is the concept where the gravel berms surface sediment is being constantly rearranged under swash action but the overall geometry remains consistent. For both revetments it was shown that the volume of sediment being rearranged by a swash event can be up to a factor of 10 times bigger than new material being introduced to the location. Finally, the Xbeach-X morphodynamic model is used to simulate the morphodynamic response of a dynamic cobble berm revetment constructed from poorly sorted, angular gravel. A full sensitivity test of the model shows that is can be optimised to achieve excellent results according to the Brier skill score, a metric of model performance. However, this optimum set up performs poorly as the hydrodynamic conditions change suggesting that the model is not suitable for application by coastal practitioners in this
scenario.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Blenkinsopp (Supervisor) & Jun Zang (Supervisor)