The hydraulic stability of rock armour layers has been extensively discussed in the literature, with numerous formulae proposed for design purposes. However, limited attention has been given to armour stability under shallow water conditions, largely due to the scarcity of experimental data. This research aims to address this gap by providing new insights into the stability of rock armour layers with rubble mound breakwaters in shallow water. Hydraulic stability was determined for four different structure slopes and various hydrodynamic conditions, spanning from deep to extremely shallow water in presence of a 1V:30H foreshore. Newly experimental data were compared with existing stability formulae valid in shallow water, specifically those by van Gent et al. (2003, VG), Eldrup and Andersen (2019, EA), and Etemad-Shahidi et al. (2020, ES). Initially, the data were used to evaluate the accuracy of the original formulae. Following this, the formulae were recalibrated to account for the influence of shallow water, with data grouped according to water levels. Finally, modified versions of VG and ES formulae were developed to fit the experimental data, incorporating the effects of wave steepness to better capture shallow water dynamics.

The hydraulic stability of the armour layer of rubble mound breakwaters has been widely discussed in literature and several empirical formulas have been proposed for design purposes. Nevertheless, few studies focused on the armour stability in case of water depth limited water conditions. The aim of the present research is to provide more insights into the stability of the armour layer including extremely shallow water conditions. New 2D physical experiments have been carried out at the EUropean Maritime and Environmental Research (EUMER) laboratory of the University of Salento (Lecce), in presence of a 1:30 foreshore. Tests aimed to investigate both the nearshore hydrodynamics and the hydraulic armour stability under different irregular wave loading. In the present paper the experimental set-up is described together with the main preliminary results. Specifically, the measured damage S after each test in different shallowness conditions is reported and discussed. Experimental results are also compared with the damage evaluated from the empirical formula proposed by Van Gent et al. (2003), in order to investigate the predicted damage level in the shallowness range analysed.