The influence of structure permeability on wave overtopping at rock armoured slopes along EurOtop

The EurOtop Manual account for the structure permeability of rock-armoured slopes, through the roughness factor γ f , with values of 0.40 for permeable cores and 0.55 for impermeable cores. These values, however, have not been validated against wave overtopping tests in a consistent study with various layer compositions. The present study investigates this white spot through 591 new small-scale physical model tests covering five structural permeability compositions and two typical slope angles. Results show that core permeability affects overtopping only for low steepness waves on steep front slopes ( s m-1,0 < 0.02 for a 1:1.5 front slope), with no significant influence observed for steeper waves or flatter structures. Consequently, the roughness factor for impermeable cores is revised to γ f = 0.40, matching that of permeable cores. Additionally, the influence of the wave steepness is revised based on the new tests. Verification with existing datasets demonstrates an improvement in prediction accuracy, enhancing the reliability of the method across a wider range of conditions. The revised influence factors in the EurOtop Manual formulation reduce the prediction error by a factor of 2 for the current data and by up to a factor of 3 for some of the existing data.