Jetties are large, fixed structures constructed by coastal engineers that extend outward from the shoreline into a body of water. Typically built at the mouth of a river or a tidal inlet, their primary purpose is to stabilize the shoreline and alter the flow of water and sediment. Understanding jetties requires exploring their specific design, their fundamental function in maritime commerce, and the significant changes they impose on the coastal environment.
Defining the Structure and Function
A jetty is a fixed structure, usually built perpendicular to the shore, and it is most often installed in pairs flanking a navigable channel, harbor entrance, or river mouth. This orientation is specifically engineered to interact with the natural water currents and sediment movement that occur along a coast. The primary engineering goal is to maintain and stabilize the depth of a waterway, ensuring a channel remains open for commercial and recreational vessel traffic.
Jetties achieve this goal by effectively constricting the flow of water and preventing the deposition of sand and silt within the channel itself. By narrowing the passage, the structures create a scouring action, which is an increase in the velocity of the water flowing out to sea. This accelerated flow naturally carries away the sediment that would otherwise accumulate and clog the entrance. Jetties also act as physical barriers that block the longshore current, keeping that sediment out of the stabilized inlet.
Distinguishing Jetties from Other Coastal Structures
Jetties are often confused with other types of coastal structures, but they are distinct in their purpose and scale. Unlike groynes, which are smaller structures built in a series along an open beach, a jetty is significantly larger and is designed to protect a navigational opening. Groynes are primarily intended to trap sand and widen the beach on the updrift side, focusing on shore protection rather than channel maintenance.
Breakwaters also differ from jetties because they are typically constructed parallel to the shore, often offshore, and their main function is to dissipate the energy of incoming waves. By creating a calm zone behind them, breakwaters shelter anchorages and harbors from rough seas. While jetties can offer some degree of wave protection, their defining role is to stabilize the inlet and keep the waterway clear.
How Jetties Impact Coastal Environments
The construction of a jetty fundamentally alters the process of littoral drift, which is the natural, continuous movement of sand along the coastline driven by waves and currents. Because the jetty acts as a dam to this flow of sand, sediment is forced to accumulate on the side facing the direction of the prevailing current, known as the updrift side. This process results in a noticeable buildup of beach material, causing the shoreline to advance seaward through accretion.
Conversely, the sand supply to the downdrift side is cut off, leading to a phenomenon known as beach starvation. The natural currents and waves on this side continue to remove sand from the shoreline, but no new material is supplied to replenish it, causing an accelerated rate of erosion. This imbalance can lead to changes in beach morphology, with the updrift beach growing wider while the downdrift beach rapidly recedes.
To mitigate these erosional effects, engineers sometimes implement a sand bypassing system. This involves mechanically dredging the accumulated sand from the updrift side and depositing it on the downdrift side.
Construction and Material Choices
The construction of a jetty is a large-scale civil engineering project that begins with extensive site surveys and hydrodynamic analysis to determine the optimal length and alignment. Before the main structure can be built, the seabed is often excavated or dredged to create a stable foundation and ensure the required water depth. Piles are then driven deep into the seabed to support the structure, particularly for jetties that include a deck or trestle for access.
The most common material for constructing robust jetties is large, heavy rock known as armorstone or riprap, which is highly durable and effective at absorbing wave energy. These rubble-mound structures are built by carefully placing massive stones to withstand the forces of the marine environment. Other materials include precast concrete units, which offer high strength and resistance to corrosion, and sometimes steel sheet piles or timber. Material selection is determined by factors such as the local wave climate, the required lifespan, and resistance to the corrosive environment of saltwater.