Beach morphodynamics under sequences of high and low energy wave conditions

Abstract

This thesis studies the morphodynamic evolution of beach profiles under high and low energy wave conditions using large-scale laboratory and long-term field measurements. This work presents an important contribution towards a better understanding of beach profile evolution under varying wave conditions, including storm sequences and the relevance of the antecedent beach morphology. First, beach profile evolution is studied using several large-scale data sets comprising one high energy (storm) followed by one low energy wave condition. These data support the traditional understanding of high and low energy wave conditions leading to beach erosion and recovery, respectively. However, existing literature has postulated that several storms in quick succession (storm sequences) have a greater potential for erosion, as the beach has less time to recover. Consequently, the RESIST data set is studied to explore beach profile evolution under storm sequence conditions. This data set comprises three storm sequences, which each consist of two storms followed by subsequent low energy conditions. Overall, in contrast to previous findings, no enhanced beach erosion due to storm sequencing is observed. The beach evolves towards an equilibrium configuration that is specific to each wave condition. This implies that the same wave condition can generate bulk onshore or offshore sediment transport, depending on the antecedent beach morphology. The antecedent morphology is shown to be very important for the hydrodynamic-morphological coupling that leads to an equilibrium configuration: while a large breaker bar promotes wave energy dissipation, sheltering the beach and limiting shoreline erosion, a beach profile with a small bar allows waves to propagate further onshore, resulting in erosion of the large swash berm. Finally, the occurrence and influence of storm sequencing on beach profile evolution are studied using long-term field measurements from Hasaki Beach, Japan. Hasaki Beach is subjected to frequent storms that often cluster in sequences. Despite a tendency for storms and storm sequences with larger power to cause more erosion, the present data set does not demonstrate increased beach erosion by storm sequences. In fact, it is shown that some storms and storm sequences can also result in recovery of the beach. Following these findings, the tendency of the beach to evolve towards equilibrium and the importance of the antecedent beach morphology are demonstrated.