Jetties are human-made structures that extend from the land into a body of water, often built in pairs at the entrances to harbors or tidal inlets. Their primary purpose is to stabilize navigation channels by preventing sediment from accumulating and to protect harbors from wave action. While constructed with these benefits in mind, jetties can introduce a range of unintended environmental and economic challenges to coastal areas.
Coastal Erosion and Deposition
Jetties significantly interfere with the natural movement of sand and sediment along coastlines, a process known as longshore drift. Longshore drift is the movement of sand along the beach, driven by waves approaching the shore at an angle. A jetty acts as a barrier, blocking this flow of sediment.
This interruption causes sand to accumulate on the updrift side of the jetty, leading to wider beaches or the formation of new land. Conversely, the downdrift side of the jetty becomes “starved” of sand, as its natural supply is cut off. This sand deficit results in accelerated and severe erosion on the downdrift beaches, causing them to narrow or even disappear. The longer a jetty extends, the greater its impact on adjacent areas.
Impact on Marine Ecosystems
The construction and presence of jetties can disrupt the natural dynamics of coastal ecosystems, leading to various biological consequences. Altered sediment patterns and water flow can degrade or destroy coastal habitats such as salt marshes, estuaries, and seagrass beds, which are vital as nurseries and feeding grounds for many marine species. These changes can affect the seafloor topography and impact benthic species, which are organisms living on the seabed.
Changes in water quality, including increased turbidity (cloudiness from suspended sediment), altered salinity, and temperature, can negatively affect marine life. The disruption of natural water circulation can also increase the risk of pollution and eutrophication. Fish, shellfish, and migratory birds are particularly vulnerable due to habitat loss, changes in food availability, and disruption of their migration routes or spawning grounds.
Altered Water Flow and Navigation
Jetties change the natural dynamics of water movement near the coast, which can create hazardous conditions. They can cause the formation of strong, dangerous rip currents, which are channeled currents of water flowing away from the shore. These currents can pull swimmers away from the shoreline, posing risks to recreational users.
Jetties also impact navigation by causing shoaling, which is the accumulation of sand in navigation channels adjacent to the structure. This makes it difficult for boats to pass and necessitates frequent dredging to keep the channels clear. Altered currents can also affect the transport of sediment into and out of harbors.
Maintenance and Economic Burdens
The construction and ongoing presence of jetties impose significant financial and logistical burdens. Building jetties is costly, with estimates varying widely depending on design and location, but can range into the millions of dollars for repair work alone. Once built, jetties require continuous maintenance, including repairs due to storm damage and regular dredging.
Dredging operations, which remove accumulated sand from navigation channels, can cost tens of thousands to millions of dollars annually, depending on the volume of sediment. Furthermore, the downdrift erosion caused by jetties often necessitates costly beach nourishment projects. These projects involve pumping sand back onto eroded beaches, creating a continuous cycle of expense to mitigate the unintended consequences of jetty construction.