The stability of rubble mounds beneath caissons

A boundary element model is developed for the hydrodynamic interaction of water waves with a caisson founded on a permeable rubble base. The calculated wave-induced velocities are used to determine the destabilizing forces on individual armor units. It is shown that there are two critical instability conditions. The first is for very long waves. For this case, there is a significant flow under the caisson (which is nearly steady) induced by the head differential across the caisson. This instability mechanism is unique to caissons and does not occur for typical rubble mound breakwaters or rubble toes. The second instability occurs for intermediate length waves. In general, smaller wave heights, deeper bench depth and wider bench all lead to increased stability. Computed results compare favorably with experimental measurements.