Oceanography 10, T. James Noyes, El Camino College

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Beaches and Hard Stabilization Beaches and Shorelines Are Always Changing When people buy a home or other property by the seashore, they may not realize that nature may radically alter the value of their investment. In addition to dangers from storms (e.g., Hurricane Katrina) and other disasters (e.g., tsunami), 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! Picture of the end of Point Arena from the top of the Lighthouse. The point has eroded 40+ yards (120+ feet) over the last 150 years. The house had to be moved back away from the end of the point which has now collapsed.

Oceanography 10, T. James Noyes, El Camino College

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Beach Sediments: Basic Concepts Beach sediments are composed of whatever sediments are available locally: sand, cobbles, gravel, coral fragments, shell fragments, etc. Beach sediments are characterized by the kind of material that they are made out of, their size, their shape, and 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 1). Sediments are then “eroded” (carried, transported): carried to the shoreline by running water (rivers 2), and pushed along the shoreline by waves (longshore transport). Sediments are also produced at the shoreline itself by weathering and erosion of the rock of the shoreline.

Photographs courtesy of the National Oceanic and Atmospheric Administration, Depart. of Commerce

Black Sand Beach, Hawaii (sand made from volcanic rocks)

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Ancient peoples used to break rocks by heating them in a fire, and then throwing water on them to cool them down. The outside of the rock will try to contract, but the warm inside stays expanded, so the only way that the rock on the outside can contract is to break into smaller pieces which contract individually. 2 And, to a lesser extent, winds

Oceanography 10, T. James Noyes, El Camino College

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“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) River-bottom sediments (center of the river). The quarter was placed to give you a sense of the size of the sediments. Notice the are very resistant to being broken absence of sand. It tends to get carried away, unlike the larger, heavier down; rivers and waves are just not sediments shown here. 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 Different of man-made materials like plastic: once they reach the size of sand, Kinds of waves have little ability to break them down further. Tiny plastic Quartz sediments are making up a larger and larger component of our 3 beaches .

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Plastics can be particularly dangerous to the food chain that we are a part of, because they absorb a number of dangerous man-made chemicals like PCBs.

Oceanography 10, T. James Noyes, El Camino College

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Example of Beach Erosion: Beach Shape and Rip Currents 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. During summer, waves tend to be smaller and have a longer period (and wavelength), 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, waves push sand up the beach, and then it 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.

Winter berm

high-tide level

berm low-tide terrace

low-tide level

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.

sand sand bar

End of Ladder Beach Winter Beach, North County San Diego. Notice that the ladder from the lifeguard tower is far above the beach! A lot of sand is (temporarily) removed by waves during the winter.

Oceanography 10, T. James Noyes, El Camino College

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Rip currents (also called “rip tides”) are fast-flowing streams 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 and muddy. They also disrupt the incoming waves, making them break differently than the Sand Bar crest to either side. If a rip current is dragging you out into He lp! 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 Beach 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.”) 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.

National Oceanic and Atmospheric Administration

Oceanography 10, T. James Noyes, El Camino College

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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 4. 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. Before Erosion

After Erosion

After Erosion

Will Fall

Cave

Courtesy of Dr. Douglas Neves Rain water carves channels in tops of cliffs.

Landslide. Rainwater soaked into the top of the cliff. Notice the dangling fence posts: someone lost part of their backyard.

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. 4

Wind is a smaller, less important factor except in deserts where water plays less of a role in erosion. The science journalist Robert Krulwich once said “The Grand Canyon bears witness to the magnificent violence hidden in a torrent of old raindrops.”

Oceanography 10, T. James Noyes, El Camino College

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fell from above

Erosion of the Bottom of Coastal Cliffs by waves at high tide. Notice the debris in front of the cave in the picture on the right. You can also see where it fell from above when the cave became too deep.

The sandy beach in front Cliff erosion tends to produce sediments with a variety of sizes of a cliff or buildings and shapes (“poorly-sorted sediments”). Both large and small protects them from erosion sediments fall out of the cliff. Over time, the larger ones are by waves at high tide. 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.

Beach Sediments

Recently Eroded from a Cliff

Oceanography 10, T. James Noyes, El Camino College

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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 Black Ovals = Large Rocks do not move (much). Yellow Circles = Sand hopping down the river. weathering and erosion tends to take place in the Small Brown Circles = Mud suspended in the water. 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 Sand is dropped on the suggests that cliffs may provide sides of the river where the more of the sand than previously water slows down due to rubbing against the land. 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 bestsupported by the available evidence): that under natural conditions, most beach sand appears to come from rivers.

NOAA

Oceanography 10, T. James Noyes, El Camino College Removing Sediments from Beaches: Submarine Canyons

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USGS

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.

Mountains & Rivers

Beach Compartments / Littoral Cells

Falls into In southern California, oceanographers have found that specific rivers feed sand to the beaches south of them underwater (waves typically come from the northwest, pushing canyon. sand to the south) and that specific submarine canyons remove sand, keeping beaches south of them from Redondo Canyon 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 Bluff Bay (e.g., Malibu) are pushed south down the coast by Cove waves 5, 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 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.)

Santa Monica

Redondo Beach

Cabrillo Beach

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 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.

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prior to the coastal development of the last century

Oceanography 10, T. James Noyes, El Camino College

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Small sandy There can be small, sandy Clearly, beach along beaches in the middle of a erosion of a rocky shore. rocky shoreline, typically cliffs is not in the back of coves. All producing the sand that is made by Courtesy of much sand. eroding the larger sediPDPhoto.org ments 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.

Santa Monica

Courtesy of Faris

Mountains & Rivers

Santa Monica

Redondo Canyon

Redondo Beach

Redondo Beach (South) Bluff Cove

Cabrillo Beach

Bluff Cove

Oceanography 10, T. James Noyes, El Camino College The East and Gulf Coasts 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. Barrier Island. National Oceanic & Atmospheric Administration, Dept. of Commerce

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Oceanography 10, T. James Noyes, El Camino College

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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 6. Below, I discuss 4 examples of hard stabilization: groins, jetties, breakwaters, and seawalls. Groins

I kid thee not. They are really called “groins.”

LST

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: it is “starved” of sand because the lost sand is not replaced 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, not to add more sand: it merely makes one person’s beach wider at the expense of other people’s beaches down the coast.

Groin

Beach Groin

LST

sand piling up

sand being removed Groin. Longshore transport is represented by the red arrows. Sand is being pushed down the coast by waves to the left in the picture. The sand runs into the right side of the groin and cannot get past it, so the sand stops and piles up. On the left side of the groin, waves push sand farther down the coast, so there is less and less sand on the beach. Groin field. A series of groins along a coast. Notice how sand builds up on the right side of each groin, and there is less sand on the left side of each groin. LST is going to the left in the picture (red arrow). Courtesy of North Caroline Division of Coastal Management.

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Typically expensive – hundreds of thousands or millions of dollars – and ultimately fruitless.

Oceanography 10, T. James Noyes, El Camino College

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Jetties LST Jetties resemble groins, but are typically longer and come in pairs. They are built at the entrances of harbors to keep Beach sand from being pushed into the mouth of the harbor by waves, making the mouth of the harbor shallower (ships Jetty could run aground) and eventually blocking it. (If the entrance to the harbor is narrow, jetties also block waves, Entrance to Harbor keeping the water in the entrance calmer.) Like groins, Jetty 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 LST 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. In spite of jetties, sand still builds up in the entrance of harbors. About 15 years passed between the last two “dredges” (removing sand) of the entrance to King Harbor in Redondo Beach (the removed sand was put on shrinking beaches south of the harbor which do not get sand anymore owing to the jetties). The cost for the last dredge was $560,000. So even with the jetties, it costs about $37,000 a year to keep the entrance to King Harbor safe for boats.

Jetties

Jetties. Left: Courtesy of the National Park Service. Notice the sand built up one side, and the lack of sand on the other side. Right: Courtesy of Rick Crawford, NOAA. The jetties keep the entrance to a harbor (an estuary) from being blocked by sand.

Oceanography 10, T. James Noyes, El Camino College

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Breakwaters LST Beach Breakwater

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 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 7 can still be seen.

Edge of a Breakwater at the entrance to a harbor. Ex-Breakwaters. Waves refracted around the breakwaters and pushed sand behind them from both sides until the sand reached all the way out to the breakwater. This is how a natural feature called a “tomobolo” forms (see reading 6B).

Breakwater. Notice the small lighthouse at the end to mark the entrance to the harbor. 7

See the pictures on page 314 of your textbook. It was torn down once the problem was realized: a lot of wasted money…

Oceanography 10, T. James Noyes, El Camino College

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Seawalls

Seawall

Seawall

Wave Crests

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 Before Erosion 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, Land 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 After Erosion seawall, causing they seawall to collapse.

Seawall Seawall

Seawall at low tide (upper left) and high tide (lower right).

Land

Oceanography 10, T. James Noyes, El Camino College

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More erosion near the end of a seawall.

Seawall

After Erosion

Beach What can we do about beach erosion? As we have seen, hard stabilization can be used to maintain the shoreline, but this is both expensive and can have negative consequences (often unforeseen). Dams (which block the flow of sediments to the shoreline) can be removed, but the dams themselves provide a variety of benefits that we would have to forego. Millions of dollars are spent by cities on a regular basis to replace lost sand. Sand for “beach renourishment” can be trucked in from deserts or behind dams, or dredged from the ocean floor. It may only stay on the beach for as little as a year, so this is an ongoing expense. Careful research is needed to estimate how long it will remain. When possible, the best strategy is to help the sand “bypass” obstacles like long jetties: remove the sand from one side and carry it to the other side.

Places that regularly replenish their beaches using dredged sand are finding that they must go farther and farther out into the ocean to find sand, because we have already dredged the sand closest to shore. This make dredging more and more expensive over time.

Another option is to do nothing, let nature takes its course, and plan for the changes (to adapt). For example, shoreline buildings can be built so that they can be moved when necessary. People who live on shifting barrier islands are building these kinds of homes more and more often.

Perhaps we could grind up re-cycled glass bottles to make sand?

All and all, I would argue that there is no one right answer (certainly one right answer that applies in all circumstances). Each choice involves expensive trade-offs; some people will “win” (benefit), and some people will “lose” (suffer). Dredging Sediments. NOAA, Dept. of Commerce.