T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 1 Name: Section: Beaches Unit (4.5 pts) Beaches and Shorelines Are Always Changing Waves are slowly and inexorably altering the shoreline, eroding material from some places and carrying it to other places. Each wave has a small effect, but they keep coming hour after hour, day after day, year after year. Significant changes in the shoreline (10 s of feet) can occur within a human lifetime. For example, old maps of Encinitas located along the coastal cliffs by Interstate 5 on the way down to San Diego show that it has lost about a city block of land to the sea in the last century. In some places along the coast of Alaska, the shoreline has eroded 900 meters in 50 years, an average of 18 meters (60 feet) per year! Beach Sediments Beach sediments are composed of whatever sediments are available locally: sand, cobbles, gravel, coral fragments, shell fragments, etc. (In fact, tiny plastic sediments are making up a larger and larger component of our beaches.) Beach sediments are characterized by the kind of material that they are made out of, their size, their shape, and their sorting (well sorted: about the same size and shape, poorly sorted: many different sizes and shapes). Lithogenous sediments ( rock sediments ) are produced from the weathering of the rock of the land: rocks are broken down into pieces (sediments) by the physical impact of water, wind, and other rocks; chemicals diluted in water; and repeated heating and cooling (e.g., some parts of the rock expand more or less than other parts). Sediments are then eroded (carried, transported): carried to the shoreline by running water (rivers), and pushed along the shoreline by waves. Sediments are also produced at the shoreline itself by weathering and erosion of the rock of the shoreline. 1. How are well-sorted sediments different from poorly-sorted sediments? 2. What is weathering? 3. What is erosion? 4. What carry sediments from farther inland to the shoreline? 5. What weather the coast (push the shoreline back) and also push sediments down the coast?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 2 Sediment Transportation (Erosion) High energy water (fast-flowing rivers and strong waves) can lift and carry more sediments and larger, heavier sediments than low energy water. Once the water calms, the larger, heavier sediments are dropped (deposited), but smaller, lighter sediment continue their journey, which separates ( sorts ) the sediments. The smallest sediments (mud, clay, silt) sink very slowly, so they are easily carried and only settle in very calm water. Sand can be carried by stronger flows; even though it drops quickly to the bottom, it is picked up again and again. As sediments travel, they bump into one another and the bottom, chipping away at their surfaces. Typically this makes their jagged (angular, sharp) edges more rounded, though impacts can split a sediment, creating sharp surfaces as well. Large rocks at the bottom of rivers are smoothed by sand washing over them again and again (like using sand paper to round the edge of a piece of wood). The more time rocks spend in high energy conditions and the farther they travel, the more rounded they tend to become. Some minerals in rocks (e.g., quartz) are very resistant to being broken down; rivers and waves are just not strong enough to have much of an effect. Thus, beach sand is made up by a lot of quartz. (It has the same chemical formula as glass and is used to make glass.) The same is true of man-made materials like plastic: once they reach the size of sand, waves have little ability to break them down further. 6. What carry more sediments, big waves or small waves? 7. Which sediments are carried more easily by waves, large sediments or smaller ones? 8. Which is larger, sand or mud? 9. Which is carried more easily and in larger quantities by rivers, sand or mud? 10. As sediments travel, how does their (i) size and (ii) shape change? 11. True or false? Given enough time, sand grains exposed to waves will get smaller and smaller until they become mud.
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 3 Winter and Summertime Beaches The shape of a beach is primarily affected by wave conditions and tide levels. Waves can both push sand onto the beach from the ocean and drag sand back into the ocean. Typically, a mound or hill of sand (the berm ) builds up along the shoreline. At high tide, this is the only part of the beach that is above water, so the beach slope appears quite steep. As waves push into the shoreline, they erode a flat area in front of the berm called the low tide terrace. At low tide, this flat area is exposed, and the beach appears less steeply sloped. berm low-tide terrace high-tide level low-tide level berm Winter sand sand bar During summer, waves tend to be smaller and have a longer period, because there are fewer, weaker nearby storms and the stronger storms are farther away (in the other hemisphere where it is winter). Since wave crests arrive at the shoreline less frequently, the water of the breaking waves has time to soak into the beach sand and can work its way back to the ocean through the sand. Thus, during summer waves push sand from nearby ocean floor up onto the beach. The sand then remains on the beach. During winter, wave crests crash against the beach so frequently that the sand becomes saturated with water, and more water flows back into the ocean over the sand (and under the incoming waves, hence the term undertow ), often dragging more sand back into the ocean than they push up the beach. Thus, wintertime beaches often have less sand, which sometimes exposes the larger rocks beneath the sand. The sand, though, is not permanently removed from the beach: it will be pushed up and out of the ocean again during the spring and summer, only to be removed again during the fall and winter. The sand removed from the beach piles up offshore, sometimes forming underwater hills called sand bars. By causing the water to get shallow very quickly, sand bars can cause waves to break quickly, producing the plunging breakers beloved by surfers. However, sand bars can also help create one of the greatest dangers at recreational beaches: rip currents. 12. When is there more sand on beaches, during the summer or the winter? 13. What pushes sand onto the beaches during this season? Where does the sand come from? 14. When is there less sand on beaches, during the summer or the winter? 15. What pulls sand off beaches during this season? Where does the sand go?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 4 Rip Currents Rip currents (also called rip tides ) are fast-flowing streams Sand Bar of water rushing away from the shoreline. They may be nearly invisible, particularly when they are starting to form, but they typically pick up sediments, making the water brown Beach and muddy. They also disrupt the incoming waves, making them break differently than the crest to either side. If a rip current is dragging you out into the ocean, do not try and fight it; it is too strong, so you will only waste your energy, increasing the likelihood that you will drown. Instead, swim out of the rip current by swimming up or down the coast ( parallel to the shoreline ). Once you are out of the rip current, you can safely return to the beach. (There is a great brochure on rip currents on the course website. Go to the websites section, and page down to Beaches. ) Help! Rip currents can form in several ways, some of which are not well understood. All involve waves breaking more strongly in some places than others along the shoreline. The extra water rushing up the beach at these locations has to flow back into the ocean, and it finds it easier to flow back into the ocean where the waves are breaking less fiercely, resulting in a stronger offshore flow at these locations. The best documented cases of rip currents involve holes in the ridge of a sand bar, because these rip currents persist longer and occur in the same places again and again, allowing them to be studied. Waves break over the sand bar on either side of the hole, causing water to surge over the sand bar. The easiest place for it to flow back into the ocean is where the water is deepest: the hole in the sand bar. 16. If a swimmer tries to swim against a rip current, who will win, the swimmer or the rip current? Beach Swim this way? 17. Sketch arrows into the picture on the right to show a path that the swimmer should take to escape from the rip current. Help!!! Rip Current
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 5 Sources of Beach Sediments: Coastal Cliffs Coastal cliffs are one source of beach sediments along the coast of California. Rain water flows over the top of the cliffs, carving out channels at the top and carrying sediments onto the beach below. Rain water can also soak into the sediments on top of a cliff, adding weight and lubricating them enough for gravity to pull them down onto the beach (a landslide ). Waves pound the bottom of the cliffs, eroding a notch or sea cave at the bottom. As they approach the cliff, they pick up sediments and fling them at the bottom of the cliff, enhancing waves ability to erode the cliff. As the cave becomes larger and larger, the rock above becomes too heavy for the rock below to hold up, and the entire cliff faces collapse. Rain water carves channels in tops of cliffs. Rainwater can carve canyons all the way down to the shoreline given enough time. Vertical cliffs are created by waves eroding too much rock from the bottom of the cliffs, so all the rock above tumbles down into the sea. Cliff erosion tends to produce sediments with a variety of sizes and shapes ( poorly-sorted sediments ). Both large and small sediments fall out of the cliff. Over time, the larger ones are ground down into smaller ones, but the beach stays rocky, because additional landslides add more large sediments to the beach while smaller sediments are more easily carried away by the waves. On beaches with large waves ( high energy ), few small sediments may be left behind, leaving the beach comprised of larger rocks. Initially, when sediments fall out of the cliff, they are typically angular (jagged, sharp), but as they roll and grind against one another owing to the waves, they begin to become more rounded (smooth). Thus, sediments that have been on the beach for a long time are rounded, while those that recently fell from the cliff have sharper edges. 18. What is the main cause of erosion at the top of coastal cliffs? In other words, what breaks down most of the rock at the top? 19. What is the main cause of erosion at the bottom of coastal cliffs? In other words, what breaks down most of the rock at the bottom? 20. Describe the size and shape of the sediments that fall down from cliffs. In other words, are they large, small (sand and mud), large, or a mixture of large and small? Are they angular (sharp), rounded (smooth), or a mixture of angular and smooth? 21. Describe how the size and shape of sediments changes as they spend time on a beach. In other words, do they get larger or smaller? Do they get sharper or smoother?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 6 Sources of Beach Sediments: Rivers If you look at the bottom of a natural river, you will see that it is covered by sediments. These sediments are being carried down to the shoreline from the mountains. More weathering and erosion tends to take place in the mountains, because their steep slopes lead to faster-flowing ( high energy ) water. As the rivers leave the mountains, their slope becomes gentler, the water slows down, and the heaviest sediments are dropped and left behind. Smaller sediments (mud and sand) are carried down to the shoreline, where the river runs into the ocean, greatly reducing its speed. The mud particles are very small, so they sink very slowly and are easily picked up by waves; they stay suspended long enough to drift out into the ocean. The heavier sand, on the other hand, falls along the shoreline, and is pushed up and down the coast by waves (longshore transport); the wave effectively spread sand along the coast, covering up bigger rocks from local erosion and thus creating sandy beaches. Sources of Beach Sediments: Rivers vs. Coastal Cliffs The dominant view among oceanographers is that rivers provide most of the sand to the shoreline. However, a recent study of cliff erosion along the coast of southern California suggests that cliffs may provide more of the sand than previously thought (perhaps as much as half or more). These ideas are not necessarily contradictory. Humans have dammed California s rivers over the last century, and dams hold back sand as well as water, keeping sand from reaching the coast (a major problem for dam operators as sand begins to fill up the reservoir). In the past, sand from rivers was pushed down the coast by waves and protected our coastal cliffs, but as our beaches narrow, cliff erosion is producing more and more of the sand on our beaches. Another complicating factor is that there has been a lot of coastal development over the last century; excess sediments from construction were dumped on the beach and significantly increased the size of our beaches, so we may have gotten used to unnaturally wide beaches. I want you to know the dominant view (and the view best-supported by the available evidence): that under natural conditions, most beach sand appears to come from rivers. 22. Where does most sand on naturally sandy beaches come from? In other words, is it brought to the coast by rivers, or does it come from waves breaking down the land along the coast (e.g., cliff erosion)? 23. What take sand away from the ends of rivers and spread the sand along the coast, creating sandy beaches? 24. How will damming a river affect beaches? Will the beaches at the coast have more sand or less sand if dams are built in rivers?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 7 Removing Sediments from Beaches: Submarine Canyons Many underwater valleys ( submarine canyons ) get close to the shoreline along the coast of California. When waves push sand down the shoreline, some of it falls into these canyons, and underwater landslides carry the sand down the canyons onto the deep sea floor (once enough sand had piled up). (You can actually see this at the end of Redondo Canyon on contour maps of ocean depth.) Beaches farther down the coast get less sand, so they are narrower and more rocky: rocks from local erosion do not get covered by sand. Beach Compartments / Littoral Cells In southern California, oceanographers have found that specific rivers feed sand to the beaches south of them (waves typically come from the northwest, pushing sand to the south) and that specific submarine canyons remove sand, keeping beaches south of them from receiving sand. The route that sand flows down rivers, to particular beaches, and into the ocean via a submarine canyon is called a beach compartment or littoral cell. For example, sediments washed down the coastal streams on the north side of Santa Monica Bay (e.g., Malibu) are pushed south down the coast by waves, adding sand to the beaches of Santa Monica. The sand continues down the coast until it falls into Redondo Canyon, which keeps it from reaching the Mountains & Rivers Falls into underwater canyon. Redondo Canyon Bluff Cove Santa Monica Redondo Beach Cabrillo Beach beaches of Palos Verdes, so Palos Verdes has rocky beaches. (The shape of the shoreline and human construction are also important factors in this case.) 25. What typically causes sand to be permanently lost from the shoreline? In other words, how and why does sand leave the shoreline and never return? 26. What direction does sand typically move along the coast of California, towards the north or towards the south?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 8 Key Concepts Sandy beaches are sandy, because lots of sediments are eroded up in the mountains and carried down to the shoreline by rivers. Waves push sand down the coast to our sandy beaches. Rocky beaches are rocky, because waves take sand away from the beach, and something keeps sand from reaching the rocky beach (e.g., underwater canyons) and covering up the larger rocks from local erosion by rain and waves. Note: Waves push sand down rocky shorelines too (longshore transport does occur), but there is very little sand to push. Rocky Shores and Beaches in Coves There can be small, sandy beaches in the middle of a rocky shoreline, typically in the back of coves. All the sand that is made by eroding the larger sediments is pushed into the back of the coves by waves, making the beach there sandy. I do not consider these small patches of sand to be good examples of sandy shorelines. They are the exception, not the rule, for these coastlines. The East and Gulf Coasts of the United States As you will see in topic 6B ( Shorelines ), the east and Gulf coasts of the United States (and some parts of the Pacific Northwest) are quite different from southern California. Among other things, they have barrier islands long, thin piles of sand along the coast owing to their flatter continental shelves and more sediments leftover from previous ice ages (when sea levels where lower, sand traveled farther out into the ocean). Along the east coast, sand typically does not leave the coast at submarine canyons. Instead, it piles up at the ends of barrier islands where the water is deep. 27. Rocky beaches are rocky because the sand and mud that are eroded from cliffs are taken away by the waves and are not replaced by sand from other places. (a) Why does the sand and mud get taken away but the larger rocks remain by the cliff that they came from? (b) What might be preventing lots of sand from reaching the beach and covering up the rocks? 28. True or false? Rocky shorelines have small stretches of shoreline that are sandy. 29. What are barrier islands? 30. Where are barrier islands common along the coast of the United States: the eastern coast, the Gulf coast (the southern coast), and/or the western coast?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 9 Hard Stabilization Hard stabilization refers to large, heavy, and/or strong objects that humans build in an attempt to resist nature and keep the present shoreline from changing. Below, I discuss 4 examples of hard stabilization: groins, jetties, breakwaters, and seawalls. Groins are long, thin walls that extend out into the ocean. They are built to try to hold onto a sandy beach, to keep waves from carrying the sand away, and perhaps even build it up. In this, they tend to be successful. Waves push sand down the coast, but the groin blocks the flow of sand, so sand piles up on one side. However, waves continue to push sand down the coast on the other side of the groin, so the beach there erodes: sand is taken away and the lost sand is not replaced by sand from farther up the coast. Eventually, so much sand piles up on the one side that sand begins to leak around the edge and the shoreline stabilizes. Thus, the main effect of the groin is to change the shape of the shoreline. Jetties resemble groins, but are typically longer and come in pairs. They are built at the entrances of harbors to keep sand from being pushed into the mouth of the harbor by waves, making the mouth of the harbor shallower (ships could run aground) and eventually blocking it. (If the entrance to the harbor is narrow, jetties also block waves, keeping the water in the entrance Jetties. Courtesy of the Army Corp of Engineers. calmer.) Like groins, sand piles up along one of the jetties, and sand erodes from the opposite side of the other jetty. Because jetties are so long, sand cannot get around the jetty, because they reach out into deep water where waves cannot reach the bottom where the sand is; instead the sand keeps building up or slides off deeper into the ocean. As a result, the shoreline never stabilizes, because sand never reaches the beaches on the other side of the jetties. On the east coast of the United States, there are documented cases of the construction of jetties resulting in entire barrier islands shifting hundreds of feet in a few decades (say bye-bye to a bunch to people s homes and businesses), a process that the builders argued would take a thousand years.
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 10 Breakwaters are long, thin walls built along ( parallel to ), but separated from, the coast. Like jetties, breakwaters are also built for harbors, but their job is to stop waves from entering the harbor, making the harbor calmer for Breakwater working on your ship, loading/unloading cargo, and so on. (Sometimes a jetty and breakwater are combined into one structure.) Since breakwaters block the waves, they also block the flow of sand down the coast, so sand tends to pile up behind them, beginning to fill in the harbor and making it useless. A famous example comes from Santa Monica; the remains of the breakwater can still be seen. Seawalls are walls built along the coast to keep waves from eroding it, typically to protect a building. Waves eventually erode the seawall (just like they erode the land), so it needs constant maintenance. If funds run out, then debris from the seawall litters the beach, including rusty pieces of iron that were used to bind the seawall together. The shoreline on either side of the seawall continues eroding, of course. The seawall can actually help the shoreline on each side erode, because the seawall s ends reflect waves towards the land on either side. As the shoreline to the side of the seawall erodes, the seawall has to be extended (costs more $), because more of the building s property is exposed to the sea. This will never end, costing more and more money. Worse yet, seawalls can cause the beach in front of the seawall to erode. Wave energy is reflected from ( bounces off ) the seawall towards the sand in front of the seawall, pushing it out into the ocean. As the sand is removed from the base of the seawall, the land that the seawall is built on is exposed to the waves. The waves erode the land beneath the seawall, causing they seawall to collapse. Seawall
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 11 31. Label each kind of hard stabilization. In other words, write the name of each kind of hard stabilization next to it in the map below. 32. Where will the sand pile up in the picture below? In other words, where will the beach grow larger? Write Deposition at this location in the picture below. What pushes the sand up against the groin at this location? 33. Where will sand erode in the picture above? In other words, where will the beach get smaller? Write Erosion at this location in the picture above. What carries sand away from the beach at this location?
T. James Noyes, El Camino College Beaches Unit (Topic 6A) page 12 34. What is the purpose of each kind of hard stabilization? In other words, what is the primary reason each one is built? Built to block waves, making it easier to load and unload boats Built to capture sand and build up a beach in a specific location Build to keep sand from blocking a harbor Built to protect the shoreline from erosion by waves Possible Answers: breakwaters, groins, jetties, seawalls 35. Circle all the true statements about seawalls: Seawalls can cause the beach in front of the seawall to erode. Seawalls can cause the land on either side of the seawall to erode faster. Seawalls erode over time, and eventually collapse if not maintained. Seawalls protect the land behind them from eroding.