A jetty is a substantial, human-made structure that extends outward from the land into a body of water, such as a sea, lake, or river. Jetties are most frequently encountered at the mouth of a river or the entrance to a harbor or bay. They act as a permanent barrier interacting directly with the dynamic forces of waves, currents, and moving sediment. The construction of a jetty is designed to modify the natural coastal environment for a specific functional outcome.
Stabilizing Inlets for Navigation
The primary purpose of jetties is to stabilize a navigable channel, most often a tidal inlet or river mouth, which naturally tends to shift or fill with sand. Jetties are typically built in opposing pairs on either side of the waterway entrance. The structures interrupt the littoral drift, which is the natural movement of sediment parallel to the coastline caused by waves approaching at an angle.
By confining the water flow between the two structures, jetties increase the velocity of the ebb and flood currents within the inlet. This focused, faster flow creates a hydraulic effect that scours the seabed, effectively deepening the channel. The scouring action removes sand and silt that would otherwise deposit and create a shallow sandbar, which is a hazard to commercial and recreational vessels. This process maintains a reliable, fixed-position channel depth for safe passage, reducing the need for constant mechanical dredging.
Distinguishing Jetties from Similar Coastal Structures
Jetties are often confused with other coastal structures, but their function and placement are distinct from groynes and breakwaters. A groyne is a smaller structure built perpendicular to the shore, often in a series, intended to capture sand to widen an adjacent beach. Groynes trap sediment carried by the longshore current on the updrift side, which is their primary design goal.
A jetty, in contrast, is fundamentally an inlet-stabilization structure, typically larger and extending farther offshore than a groyne. It is designed to maintain an open channel rather than accrete sand for beach nourishment. Breakwaters have a different purpose, acting as barriers built parallel or sometimes perpendicular to the shore in deeper water.
Breakwaters function to dissipate the energy of incoming waves, creating a tranquil area of water behind them for a harbor or anchorage. Unlike jetties, which focus on current and sediment control for navigation, breakwaters provide physical shelter from wave action. While both jetties and breakwaters are constructed from similar robust materials, their differing orientation and primary objective define their respective roles in coastal engineering.
Typical Construction Materials and Design
Jetties must be built using durable and heavy-duty materials capable of resisting constant wave attack and corrosive saltwater. Large, angular stone—often called armorstone or riprap—is a common material used to form the core and outer layer. These stones are precisely placed to absorb and dissipate wave energy, preventing the structure from being undermined.
For jetties supporting heavy loads or providing a smooth docking surface, reinforced concrete and steel sheet piling are frequently incorporated. Treated timber may be used for lighter-duty or older structures, though its lifespan is shorter in the marine environment. The foundation must be robust, often involving the driving of deep piles or the installation of caissons into the seabed to ensure stability against powerful lateral and vertical forces exerted by tides and storms. Design considerations include the jetty’s height, which prevents wave overtopping, and its length, which stabilizes the inlet channel.
Environmental and Sedimentation Effects
While jetties successfully maintain navigable waterways, their presence significantly alters the natural coastal process of longshore sediment transport, leading to predictable environmental consequences. The structure acts as a nearly complete barrier to the flow of sand, causing sediment to accumulate on the updrift side, which faces the incoming longshore current. This results in accretion, or sand buildup, creating wider beaches near the jetty.
Conversely, the downdrift side of the coastline is deprived of its natural sand supply, leading to sand starvation. This interruption results in chronic erosion and shoreline retreat on the downdrift side. To mitigate these unintended effects, coastal managers implement strategies such as sand bypassing. This involves mechanically dredging accumulated sand from the updrift side and moving it to the eroding downdrift side, attempting to restore the natural sediment flow and maintain the coastal equilibrium.