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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.
I have seen people using the jetty near my beach home to fish. I always thought it was because it allowed them to walk out into deeper water, so it would let them have access to more fish than simply fishing from the shore would.
I didn't think about fish actually hanging out and making homes inside the gaps in the rocks. It's kind of sad that the place where they feel safest is actually one of the most dangerous for them.
It almost seems like the jetty is serving as fish bait. The fishermen take advantage of the fact that the fish have their guard down.
So that's what that long projection of boulders into the ocean was! I saw a jetty last year on my beach vacation, and I didn't get its purpose at the time.
I was vacationing right inside the Alabama/Florida border on the Alabama side, and I thought that the rock projection might be the dividing line between the states. I thought it a little odd that the two states would want to separate their ocean water like that, though!
I walked out on the jetty, and it was pretty cool to feel the splash of the waves as they crashed into the rocks. It probably wasn't the brightest of ideas to walk out onto wet, slippery rocks beside the ocean, but it seemed fun at the time.