A jetty is a long, narrow structure that protects a coastline from the currents and tides. Jetties are usually made of wood, earth, stone, or concrete. They stretch from the shore into the water. Currents and tides of an ocean can gradually wash away a beachor other features along the coastline. This is called erosion. Strong river currents or waves from a lake can also erode a coastline. Jetties protect the shoreline of a body of water by acting as a barrier against erosion from currents, tides, and waves. -National Geographic
Entrance jetties are built at bay inlets, at entrances to harbours of the lagoon type, and at the mouths of rivers. They are also referred to as training jetties, because they “train,” or confine and direct, the currents so as to maintain the channel by promoting scour and extend farther into deep water where currents slacken and deposit silt and other transported material; they also protect such entrances from the disturbing effects of littoral currents, sand drift, and wave action. Entrance jetties are usually constructed in pairs, either parallel or converging toward the seaward ends, and are of several structural types. -Britannica
Jetties are used for the stabilization of navigation channels at river mouths and tidal inlets. Jetties are shore-connected structures generally built on either one or both sides of the navigation channel, perpendicular to the shore and extending into the ocean. By confining the stream or tidal flow, it is possible to reduce channel shoaling and decrease dredging requirements. Moreover, on coastlines with strong longshore currents and longshore sediment transport, jetties also function to arrest the crosscurrent and direct it across the entrance in deeper water where it represents less hazard to navigation. When extended offshore of the breaker zone, jetties improve the maneuvering of ships by providing shelter against storm waves. Jetties are constructed using methods and materials similar to breakwaters. -Pennsylvania State University
The cracks and crevices between the stones on man-made jetties create habitat for sea life and fishing opportunities for anglers. Animals and plants that live on jetties are battered by waves, left high and dry by tides, and buried by sand. But those same waves and tides bring moisture and food to the animals attached to jetty rocks. Periwinkles, limpets, barnacles, and other animals that live above the tide line trap seawater in their shells to survive exposure to air. Sea anemones high on the jetties close in on themselves to preserve the moisture they need until the tide rises again. Red, green, and brown seaweeds carpet the jetty rocks below the lowest tide line. Sea urchins, limpets, sea hares and a number of other animals graze on the algae, while hiding from predators among its fronds. Fish come to the jetties, too, drawn by the food that lives in its shelter. Some eat the seaweeds but most feed on crabs, snails, barnacles and even the small fish that live in crevices between the jetty rocks. -Texas Parks & Wildlife
1. Take a picture of yourself or your GPS receiver on the jetty.
2. How does the jetty impact the direction of the currents here, as well as the stability of the beaches? What do the currents and sand composition look like on the "left" side of the jetty (the side facing downtown Oceanside and the pier) versus the "right" side (the side facing Camp Pendleton)? Hint: the main current of the open ocean here flows northward.
3. Based on what the sand erosion on the left-side beach looks like, what do you think would happen if the jetty wasn't here?
Explore around the jetty. Feel free to look in all sorts of places around it; the GZ coordinates are just a general guideline for where the jetty begins to cut into the open ocean waters. As you explore, try to see how many sorts of living things you can find. You may see...
-crustaceans (crabs, sea slaters, etc)
-arthropods, mollusks, and other mostly-stationary shelled creatures (oysters, barnacles, etc.)
-seaweed and algae
Let's consider how the impact of jetties on surrounding natural processes creates a specialized environment for living things.
4. What heights do you see each of these animals at? (considerably above sea level, slightly above sea level, at sea level, below sea level) How do you think the currents factor into their well-being?
Now let's think about the specific geography of Oceanside and why this jetty matters to it.
5. Out at sea, the Southern California Eddy/Countercurrent flows northward up the coast, towards the large boat harbor that's close to here. How do you think the jetty plays a role in keeping the water in the harbor calm and protected?
A fun fact to get you thinking: There is a second, smaller jetty north of Tower 18 to also intercept the sand and then the Del Mar Basin Mound jetty actually protects the harbor and boat basin from direct pounding by the waves.