AbstractsEarth & Environmental Science

Seaports are often protected against waves and currents by rubble mound breakwaters. At the interface between outer breakwater slope and seabed a toe structure is often build, which provides stability to the outer slope. The toe consists of a relatively small heap of rock. Since 1977 dedicated studies are made to the stability of these rock elements under wave attack. A large number of stability methods is available, but prediction accuracy is low and validity ranges are too small for use in practice. Clarity on applicability of these methods is desired by designers. In Baart (2008) a new approach towards toe stability is defended. The ‘decoupled model approach’ determines stability with a two-step model. In the first step local hydraulic conditions right above the toe bed are calculated. The second step uses these conditions in a general formula for stone motion to predict motion. In this thesis the decoupled model approach is implemented and tested by means of the computational fluid dynamics model IH-2VOF. Prediction capacity of existing toe stability methods is reviewed against numerical results. The approach predicts motion rather than an amount of damage. To achieve this, critical values for stability and damage were imposed where necessary. The IH-2VOF model was reviewed first. Convergence tests gave recommendations for the computational grid layout. During testing it was found that position of the partially standing wave, produced by breakwater reflection under regular waves, is of major importance when reviewing different tests. It was discovered also that turbulence modelling in IH-2VOF did not function properly. The Nammuni-Krohn (2009) cases were modelled and numerical results were compared with physical measurements by Nammuni-Krohn. Little correspondence was found, likely caused by differences between numerical and physical model. High sensitivity to stone properties (diameter, porosity and Forchheimer coefficients) was encountered. Analytical solutions for flow velocity either over- or underestimated the numerical results. Work by Peters (2014a) increased confidence in the utility of IH-2VOF for breakwater modelling. Under the assumption that turbulence is not of large importance, the Ebbens (2009) cases were modelled. By literature study the formulae by Izbash (1930), Rance and Warren (1968), Dessens (2004), Steenstra (2014) and Peters (2014b) were selected to predict stone motion. Calibration of these formulae was necessary; Rance and Warren (1968) and Peters (2014b) produced most reliable results. They probably do not need any calibration, making them more universally applicable. Prediction of motion by toe stability methods and decoupled model approach were compared. The formulae by Van der Meer (1991), Gerding (1993) and Van der Meer (1998) give good agreement when validity limits are respected. If neglected, prediction capacity did not decrease much. Van Gent and Van der Werf (2014) and Muttray et al. (2014) then perform good as well. Low sensitivity to the critical values for stability and…