8. The influence of armour stone gradation on dynamically stable breakwaters

1. Dynamically stable or berm breakwaters are structures in which the armour layer is comprised of a wide range of stone sizes and undergoes reshaping in response to wave action. As a result of this wave action, a stable profile is achieved.

2. The effect of the gradation of armour stones and the amount of rounded stones in the armour on dynamically stable breakwaters was assessed in a two dimensional wave flume. A total of 52 series of tests were undertaken at the Coastal Engineering Research Laboratory of Queen's University, Kingston, Canada using irregular waves. Profiles of the structure during the various stages of reshaping were measured using a profiler developed for this study. Four gradations of armour stones were used giving a range in uniformity coefficient (D₈₅/D₁₅) of 1.35 to 5.4. The core of the breakwaters was relatively permeable.

3. The volume of stones and the initial berm width required for development of a stable profile along with the extent to which the toe of the structure progressed seaward were chosen as representative parameters of the reshaped breakwater. The results indicated that the toe width formed as a result of reshaping and the volume of stones required for reshaping were dependent on the gradation of the armour stone. The initial berm width required for reshaping was found to be dependent on both the gradation and the percentage of rounded stones in the armour.

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