A study of wave-induced forcing and damage of rock armour on rubble-mound breakwaters
Creators: name: Cornett, Andrew Malcolm
Publisher: name: University of British Columbia
Year: 2009

description: This study investigates the relationship between the wave-induced forcing and the resulting damage of rock armour on rubble-mound breakwaters, and the manner in which these processes are influenced by changes in wave height, wave period, core permeability and slope angle. Experiments with physical models of various rubble-mound breakwaters have been carried out at the National Research Council in Ottawa, and at B.C. Research in Vancouver. Some of these featured simultaneous measurement of incident wave conditions, velocities on the surface of the test structure, wave-induced forces acting on sections of the armour layer, and the growth of damage to the armour. Different structures were investigated which allow for an assessment of the effects of core permeability and slope angle on the processes leading to damage. The influences of wave height and wave period are investigated using measurements obtained in a range of long-crested regular waves and irregular sea states. Results on the regular and irregular wave conditions required to initiate damage are compared to predictions from the design equations of Hudson and of van der Meer. Different estimates of the wave-induced forcing required to initiate damage are derived by considering the balance of driving and resisting forces on a single armour stone at the threshold of motion in five different failure modes. The relative stability of the armour in each failure mode is quantified in terms of a dimensionless failure index. These expressions of relative stability are used to compare measurements of wave-induced forcing to observations of damage in regular waves. Initiation of damage is found to be closely linked to the peak shear stress acting on the armour layer in the down-slope direction. Furthermore, the magnitude of the shear stresses the armour can be related to the slope-parallel velocity using the same wave friction factor developed for oscillatory flow over rough seabeds. Several different aspects of the wave-induced forcing of the armour layer are investigated in detail. Analysis of the vertical distribution of the peak horizontal forces indicates that the most dangerous forces occur below the still waterline. The temporal variations of the forcing at this critical elevation depend strongly on the type of wave breaking that occurs on the slope. Under plunging breakers, the strongest forces result from the sudden flow reversal that occurs under the steep wave crest. Under surging breakers, the largest forces result from seepage flows that occur towards the end of the downrush phase of the surface flow cycle. Collapsing breakers are particularly damaging to the armour layer because these two forcing mechanisms tend to occur simultaneously. Armour stones tend to be more stable on structures with greater permeability because of a reduction in the shear stresses acting on the surface of the armour layer. The forces generated by the individual waves in an irregular wave train are considered and found to be highly varied. While much of this variability can be attributed to differences in the height and period of each wave, some of it is due to additional factors including differences in the shape of each wave and the sequencing of successive waves. An analysis of the contributions due to these additional factors is presented which indicates that on milder slopes, the most dangerous irregular waves will likely feature a large upcrossing wave height and a deep trough and will likely follow a wave with a much shallower trough. From the perspective of a coastal engineer faced with the challenge of designing a rubblemound structure, the most important conclusion of this study is that the results and analysis presented herein support the design equations recently proposed by van der Meer. These equations include the effects of wave height, wave period, slope angle, permeability and storm duration in a manner that is generally consistent with the findings of this study. This endorsement is based on a combined assessment of the wave-induced forcing of the armour and the damage response. However, this endorsement must be supplemented by the recommendation that van der Meer's equations should not be applied at very low damage levels.