Final answer:
Jetties are the coastal structures that prevent tidal and harbor inlets from shifting or becoming filled with sand, helping to maintain navigable waterways.
Step-by-step explanation:
Jetties are coastal structures designed to keep tidal and harbor inlets from shifting location or filling with sand. These structures are built extending from the shore into the water, alongside water channels, to regulate water currents and sediment flow, hence maintaining stable and navigable channels for shipping. This is crucial, as natural sediment movement and deposition can lead to the shoaling of channels and the displacement of navigational routes, which would require costly and potentially environmentally damaging dredging to correct.
It's important to understand that while breakwaters are also structures designed to protect coastlines, they typically serve as protective barriers to reduce the impact of waves on the shore by absorbing and reflecting wave energy. Groins are usually intended to trap sand to build up a beach, while seawalls serve to protect the land from the encroachment of the ocean, especially during storms and higher tide conditions. Each type of coastal structure serves a specific purpose, which might include preventing erosion, protecting infrastructure, or maintaining navigable waterways.