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A jetty is a device constructed out into the water rising to just above the surface to help control currents and provide protection for a channel at an inlet. While jetties usually cannot stop a current, they may help stop some beach erosion. This means that they can help areas along the beach keep their sand in the general area.
Usually, a jetty is made of large rocks, boulders, or cement slabs that are simply dumped into place. While this will allow some water to get through, unlike a solid seawall, it will also do the intended job of breaking up the current. The stone will also absorb some of the impact of the waves by having a more porous area.
In the case where a current is moving parallel to the shore, high tide can come in and move some of the sand in the direction of the current. Over time, it may affect some areas more than others, constantly moving more sand from one area and depositing it in another. This type of erosion is very harmful to those businesses depending on having a full beach for visitors to enjoy.
Building a beach jetty will not only break up the current, it will also help to keep any sand that is caught from going too far down the beach. Once the structure is built, however, others will soon have to follow in the direction the current usually travels. This is because, without subsequent jetties, the areas immediately downstream will find their erosion will often happen at an accelerated pace. This is because sand upstream is no longer being deposited on their beaches.
A jetty harbor entrance is similar in its design and function, as it also prevents currents from passing through and depositing sand. Where most jetties are built to keep sand in, however, these are built to keep sand out. The deep water channels are necessary so that larger watercraft can move in and out, so any sand that makes the channel shallower than it is supposed to be could severely destruct shipping.
One of the secondary benefits of these structures is that they are often used by anglers to get out into the water a little more. Jetty fishing may be some of the most productive near-shore fishing there is. The rocks provide plenty of hiding places for fish species, almost making it like a near-shore coral reef. In many cases, anglers will climb onto the rocks themselves, while in other locations, fishing piers are built over the stones.