<P> Revetments are slanted or upright blockades, built parallel to the coast, usually towards the back of the beach to protect the area beyond . The most basic revetments consist of timber slants with a possible rock infill . Waves break against the revetments, which dissipate and absorb the energy . The shoreline is protected by the beach material held behind the barriers, as the revetments trap some of the material . They may be watertight, covering the slope completely, or porous, to allow water to filter through after the wave energy has been dissipated . Most revetments do not significantly interfere with transport of longshore drift . Since the wall absorbs energy instead of reflecting, the surf progressively erodes and destroys the revetment; therefore, maintenance is ongoing, as determined by the structural material and product quality . </P> <P> Rock armour is large rocks placed at the sea edge using local material . This is generally used to absorb wave energy and hold beach material . Although effective, this solution is unpopular for aesthetic reasons . Longshore drift is not hindered . Rock armour has a limited lifespan, is not effective in storm conditions and reduces recreational values . </P> <P> Boulders and rocks are wired into mesh cages and placed in front of areas vulnerable to erosion: sometimes at cliffs edges or at right angles to the beach . When the ocean lands on the gabion, the water drains through leaving sediment, while the structure absorbs a moderate amount of wave energy . </P> <P> Gabions need to be securely tied to protect the structure . </P>

Which of the following shoreline management structures are parallel to the shoreline