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Due to the sea-level rise, induced by climate change, more severe coastal storms and a high risk of flooding are expected in the future. Hence, the protection of densely populated coastal areas has become a concern for coastal engineers and managers. Coastal-defence structures such as breakwaters and seawalls are commonly built to protect harbours, people, properties, and coastal infrastructures against wave attack and coastal flooding. For a safe design of a coastal structure, the accurate prediction of wave overtopping responses is crucial, which should not exceed the specified tolerable values in the design manuals (e. g. EurOtop, 2018). Historically, the mean overtopping discharge q (m3/s/m), as a key parameter, is used to assess the crest level of coastal structures. However, field observations and laboratory modellings show that the most severe damages are also related to large individual overtopping events, which are not necessarily characterised by the mean overtopping discharge. Hence, besides q, it is recommended to consider the maximum individual wave overtopping volume during a storm (Vmax) as an additional design parameter. Due to the complicated nature of the overtopping process and the contribution of several uncertainties in terms of nearshore wave conditions, the prediction of wave overtopping responses is challenging. Empirical formulae are commonly used to estimate the overtopping parameters (e. g. q and Vmax) which have been derived from laboratory measurements, based on structural and wave parameters. Rubble mound seawalls consist of armour layer(s) with slopes in the range of 1:1.5 to 1:3. Unlike breakwaters, as the most common rubble mound structures, seawalls have an impermeable core which may lead to a higher wave overtopping risk. In the case of rubble mound seawalls, the existing knowledge and data are limited, and empirical formulae are associated with large estimation errors, especially in terms of individual overtopping responses. Hence, the wave-by-wave study of overtopping as well as statistical analysis of the distribution of individual wave overtopping volumes are required. In light of the above, the present research work aims to study wave overtopping at rubble mound seawalls and to provide improved formulae to estimate wave overtopping responses used for the crest level assessment of these structures. For this purpose, a comprehensive literature review was conducted to identify the gaps in the existing data related to rubble mound seawalls and the limitation of prediction formulae. Then, according to the identified gaps in terms of key influential parameters, 2D small-scale physical model tests were conducted on a two-layer rock armoured rubble mound seawall with the slopes of 1:1.5 and 1:2.

Conventionally, allowable mean overtopping discharge is used as a design criterion for coastal structures. The mean overtopping discharge needs to be limited to ensure structural stability as well as the safety of people, vehicles, and properties behind the structure. Nowadays, limits for the maximum individual overtopping volumes are also specified in the design manuals, which requires the study of wave overtopping in wave-by-wave form. In some cases, in order to achieve more reliable safety for pedestrians and vehicles or to assess the stability of the inner slope, the maximum velocity and thickness of the overtopping flow need to be considered as well. The present paper aims to review the basic concepts of the individual wave overtopping such as the estimation of the probability of overtopping and the statistical methods to study the distribution of individual overtopping volumes. The temporal evolutions of the overtopping flow thickness / velocity along with the calculation of the overtopping discharge are discussed. Moreover, since the continuous recording of parameters is required for the study of individual wave overtopping, the most common experimental measurement and data analysis techniques with an emphasis on their advantages and limitations are discussed.