Simulating the rubble mound underlying armour units protecting a breakwater

Abstract

A variety of concrete armour units laid on top of rubble mounds are used to protect breakwaters and other harbour infrastructure. Coastal engineers build three-dimensional physical scale models to understand the dynamic processes caused by seas on such infrastructure. We are developing analytical techniques for understanding breakwater structural stability. We are modelling the infrastructure using a physics engine, which handles the rigid body mechanics. We report here on our attempts to model the rubble mounds underlying the armour units. In its most primitive form, we model the rubble as a static structure with flat surfaces and then pack the selected armour units on top. This reduces the complexity, but the porosity of the packing close to the rubble is much higher than it would be in practice, because of the lack of inter-penetration between the rubble and the armour layer. Further, the armour units slide easily on the flat surface, making it difficult to simulate a realistic packing. The two approaches to solve this problem that are discussed here are the height field and modelling individual rubble units. The height field is a rigid, square mesh with random heights distributed uniformly. It is computationally cheap to implement and largely solves the porosity and related problems, but it is rigid, so it cannot change shape to respond to movements by the armour units or the water. Modelling individual rubble units is computationally expensive, because of the number of objects required and their potential complexity. Currently, we have modelled the rubble using a simple sphere-like polyhedron, and have been able to model a very large packing of them