A jetty is a structure built nearly perpendicular to the shoreline, extending into a body of water like a sea, lake, or estuary. Its primary purpose is to maintain the navigability and safety of a harbor entrance by influencing the flow of water and sediment. Unlike a breakwater, which is often a detached offshore structure built solely to absorb wave energy, a jetty is typically connected to the shore and often built in pairs to stabilize the harbor mouth.
The Natural Forces Jetties Counteract
Jetties counteract the two persistent natural threats to a coastal navigation channel: sediment transport and wave turbulence. The primary source of channel infilling is littoral drift, where waves cause sand to move parallel to the coastline. This relentless movement of sediment would naturally “silt up” the mouth of a harbor, making it too shallow for large vessels to pass safely.
Oceanic waves and strong currents also pose a hazard to maritime traffic. Unmitigated wave energy creates high turbulence and dangerous cross-currents at the inlet, increasing the risk of vessel damage or grounding. By physically interrupting these natural processes, a jetty ensures the engineered entrance remains deep and calm for safe transit.
Protecting Navigational Depth by Managing Sediment
The primary mechanism by which a jetty protects navigational depth is by blocking littoral drift. As sand is transported along the coast, it encounters the jetty, forcing it to accumulate on the updrift side of the barrier. This buildup of sand is known as accretion, and it prevents sediment from entering and shoaling the navigation channel.
Trapping the migrating sand starves the harbor entrance of sediment that would otherwise require constant dredging. However, this accumulation is not limitless; sand eventually begins to move around the tip of the structure as the updrift beach extends seaward. To manage this, periodic maintenance, often involving mechanical dredging or sand bypassing, moves the trapped sediment to the beach on the opposite side of the inlet.
Calming the Waters Through Wave Attenuation
Beyond sediment management, a jetty reduces the destructive force of incoming waves, a process known as wave attenuation. The solid structure of the jetty causes incoming wave energy to be reflected back toward the ocean, preventing it from penetrating the harbor entrance. Waves that pass the seaward tip are subjected to diffraction, causing their energy to spread out laterally and dramatically reducing their height and force within the channel.
This combined action creates a protected zone of relatively calm water, often called a wave shadow zone, providing safer passage for ships. Furthermore, the confined nature of the inlet, especially when two jetties are present, helps to direct tidal currents. This current management stabilizes the flow within the channel, reducing the formation of dangerous cross-currents that impede navigation.
The Necessary Trade-Offs: Erosion and Accretion
While jetties are effective at protecting harbor entrances, their interference with natural processes creates trade-offs, primarily affecting the adjacent coastline. The trapping of sand on the updrift side simultaneously starves the beaches on the downdrift side of their natural sediment supply. This sand starvation results in severe downdrift erosion, leading to the recession of the shoreline and the loss of coastal land.
These changes in sediment distribution impact local coastal ecosystems, altering habitat for marine life and birds. To mitigate these negative impacts, coastal managers often employ solutions like mandatory sand bypassing, where sediment is mechanically pumped from the accretion side to the erosion side. Another common effort is beach nourishment, which involves importing sand to replenish the starved downdrift beaches.