Complex sentence: A breakwater structure is designed to absorb the energy of the waves that hit it either by using mass (e.g., with caissons) or by using a revetment slope (e.g. with rock or concrete armour units).
Split into simpler sentences: A breakwater structure is designed to absorb the energy of the waves that hit it. This is done either by using mass (e.g. with caissons) or by using a revetment slope (e.g. with rock or concrete armour units).