The Beach is Moving, Where is it going? Name: ______________________________ North Carolina’s sandy coast is one of the state’s greatest tourist attractions, and also one of its frequent sources of controversy. The 301 miles of coastline in the east have been the site of an ongoing struggle between stable structures created by humans and the dynamic forces of nature, between property rights and thousands of years of geology. Some of the factors that draw people to the beach — namely, the wind and waves — contribute to a constantly changing coastline that washes away in one area as it builds up in another. These natural processes made the North Carolina coast the way it is, and they continue to shape it. Part of this process of erosion and deposition is easy to observe on a typical day at the beach: When waves crash on the shore, they don’t break in straight lines parallel to the beach. Instead, they break diagonally. The wind and current patterns that influence the diagonal movement of the waves ensure non-stop sand movement along the coast. Before a wave breaks, it picks up sand and other sediment from the ocean floor. When the wave breaks on the shore, the underwater sediment is washed up onto the beach diagonally, at the angle in which the wave is moving. As the wave washes back out to the ocean, gravity draws it straight down the beach perpendicular to the shoreline, carrying the sediment with it. This means that ocean water, and the sediment it carries, moves down the beach in a repetitive zig-zag pattern. Over and over, sand and sediment is picked up in one location and deposited downstream on the beach. In this way, sand gradually erodes away in one place and accumulates further down the beach. Coastal erosion is a natural process even in pristine environments. However, in areas where human activity negatively impacts the shoreline, coastal erosion can become a serious problem. Beach sand originates mainly from rivers and streams which carry it directly to the ocean. Sand also comes from the gradual weathering of exposed rock formations and cliffs along the shore, and from the deterioration of shell, coral, and other skeletal fragments. Wave action, wind, and currents move sand up and down the coast. This movement is called longshore transport. Sand is also moved onshore and offshore by waves, tides, and currents. Beaches can change in the short term with the seasons. Waves in summer and winter are different in size and strength. In summer, waves tend to be low, gentle, and farther apart. These waves deposit sand on shore creating wide, gently sloping beaches. As winter approaches, waves become choppier, more forceful, and more frequent. Beaches are often eroded to become narrow and steep. Winter storms can cause large, powerful waves that crash onto the beach and pull sand offshore. Erosion and accretion (gradual accumulation) of sediment on coasts are natural processes influenced by the beach slope, sediment size and shape, wave energy, tides, storm surge, and nearshore circulation, among other things. Human activities such as dredging, river modification, removal of backshore vegetation, and installation of protective structures such as breakwaters can profoundly alter shorelines, mainly by affecting the sediment supply. (dredging – cleaning out of a harbor or river by scooping out mud, weeds, and/or rubbish) WAVES When a wave reaches shallow water, the lower part of the wave is slowed by the ocean floor, but the upper part continues at the same speed until it topples over. This is when it “breaks” on the beach. Breaking waves stir up sand and carry it on or off the beach. Changing winds affect the direction, velocity, and the size of waves. The stronger and longer the duration of the wind, and the farther it blows over open water, the larger and more forceful the waves. LONGSHORE CURRENTS AND LONGSHORE DRIFT The sand you observed on the beach comes from eroded sediments that are transported by the rivers that drain into the ocean. However, sand appears on beaches that are far from the river mouth. The transport mechanism is called “Longshore Drift.” Longshore currents are one of the mechanisms that transport sediment along shorelines and cause some of the changes we see in the sediment distribution at the beach. They travel parallel to the shoreline between the breaker zone and the shore. Conditions may vary locally. Longshore currents are formed and maintained when the incoming waves approach the shore at an angle. The breakers' energy has components both perpendicular and parallel to the shoreline (see Figure 1). Longshore currents are formed by the wave energy component that travels parallel to the shoreline which causes part of the water mass to be transported along the shoreline. If the breakers are approaching perpendicular to the shoreline, longshore currents will not form because there is no horizontal component of wave energy parallel to the shoreline. Sand is transported along the beach by longshore currents. Figure below shows how the waves come in an angle and the sand is pulled straightback into the water and the result is the longshore current and beach drift. Longshore drift has a very powerful influence on the shape and composition of the coastline. It changes the slopes of beaches and creates long, narrow sandbars of land called spits,that extend out from shore. Longshore drift may also create or destroy entire “barrier islands” along a shoreline. A barrier island is a long offshore deposit of sand situated parallel to the coast. As longshore drifts deposit, remove, and redeposit sand, barrier islands constantly change. Google image of Long Island with multiple barrier islands Responses to Erosion Seawalls When coastal buildings or roads are threatened, usually the first suggestion is to "harden" the coast with a seawall. Seawalls are structures built of concrete, wood, steel or boulders that run parallel to the beach at the land/water interface. They are designed to protect structures by stopping the natural movement of sand by the waves. If the walls are maintained they may hold back the ocean temporarily. The construction of a seawall usually displaces the open beach that it is built upon. They also prevent the natural landward migration of an eroding beach. When waves hit a smooth, solid seawall, the wave is reflected back towards the ocean. This can make matters worse. The reflected wave (the backwash) takes beach sand with it. Both the beach and the surf may disappear. Pre-Lab Questions 1. Where does the sand at the beach come from? 2. In what season is erosion of beaches more prevalent (happens more)? Why? 3. In what season is deposition of sand at beaches more prevalent? Why? 4. What causes the waves to “break”? 5. What three aspects of waves will change if the winds change? 6. What are the factors that affect the size of the wave? 7. What are longshore currents? With respect to the beach, which direction do longshore currents travel? 8. For a longshore current to form, do the waves need to come in at an angle? 9. How is longshore current different than longshore drift? 10. What kind of erosion can longshore drift create? 11. What kind of deposition can longshore drift create? 12. What is a barrier island? 13. In your own words, explain why barrier islands are always changing. Lab: Today you will be looking at beach erosion. You will create a beach then measure erosion rates based on changes in wave motion and materials. Part 1: Modelling basic wave movement 1. Move the sand to one side of your “ocean” bin 2. Fill the bin with water, to the middle of the sand pile. 3. Place a ruler in the middle of the sand 4. Complete wave making and answer questions Small (summer) wave movement: 1. Using the plastic drink bottle, create waves by pressing the block half way into the water parallel to the beach. 2. Make waves for 30 seconds 3. Change the direction of the block to 45 degrees to the shoreline repeat wave making for 30 seconds Parallel to shoreline Starting Measurement Ending measurement 45 degrees to shoreline Starting Measurement Ending measurement Move the sand back to the beginning location Larger (winter) wave movement: 1. Using the wooden block, create waves by lifting the block out of the water and pressing it almost to the bottom 2. Make waves for 30 seconds 3. Change the direction of the block to 45 degrees to the shoreline repeat wave making for 30 seconds Parallel to shoreline Starting Measurement Ending measurement 45 degrees to shoreline Starting Measurement Ending measurement Summarize the difference in the amount of erosion from the change in direction of wave and the size of the wave. What causes the increase in winter wave production? Part 2: Beach protection Seawalls When coastal buildings or roads are threatened, usually the first suggestion is to "harden" the coast with a seawall. Seawalls are structures built of concrete, wood, steel or boulders that run parallel to the beach at the land/water interface. They are designed to protect structures by stopping the natural movement of sand by the waves. If the walls are maintained they may hold back the ocean temporarily. The construction of a seawall usually displaces the open beach that it is built upon. They also prevent the natural landward migration of an eroding beach. When waves hit a smooth, solid seawall, the wave is reflected back towards the ocean. This can make matters worse. The reflected wave (the backwash) takes beach sand with it. Both the beach and the surf may disappear. Groins Groins are another example of a hard shoreline structure designed as socalled "permanent solution" to beach erosion. A groin is a shoreline structure that is perpendicular to the beach. It is usually made of large boulders, but it can be made of concrete, steel or wood. It is designed to interrupt and trap the longshore flow of sand. As sand accumulates on the updrift side of the groin, the beach at that location becomes wider. However, this is often accompanied by accelerated erosion of the downdrift beach, which receives little or no sand via longshore transport. It is important to realize that groins do not add any new sand to the beach, but merely retain some of the existing sand on the updrift side of the groin. Jetties Jetties are large, man-made piles of boulders or concrete that are built on either side of a coastal inlet. Whereas groins are built to change the effects of beach erosion, jetties are built so that a channel to the ocean will stay open for navigation purposes. They are also built to prevent rivermouths and streams from meandering naturally. Jetties completely interrupt or redirect the longshore current. Just as a groin accumulates sand on the updrift side, so do jetties. The major difference is that jetties are usually longer than groins and therefore create larger updrift beaches at the expense of the smaller downdrift beaches. On East Coast barrier islands, ocean tidal inlets migrate naturally with the longshore current. A jetty system will permanently disrupt the equilibrium of the beach. This may seriously affect the tidal circulation and the health of the wetlands between the barrier islands and the mainland. Breakwaters A breakwater is a large pile of rocks built parallel to the shore. It is designed to block the waves and the surf. Some breakwaters are below the water's surface (a submerged breakwater). Breakwaters are usually built to provide calm waters for harbors and artificial marinas. Submerged breakwaters are built to reduce beach erosion. These may also be referred to as artificial "reefs." A breakwater can be offshore, underwater or connected to the land. As with groins and jetties, when the longshore current is interrupted, a breakwater will dramatically change the profile of the beach. Over time, sand will accumulate towards a breakwater. Downdrift sand will erode. A breakwater can cause millions of dollars in beach erosion in the decades after it is built. Lab: You will create three different types of beach protection to compare how they protect the beach. 1. Place gravel on the shore of the beach. Create a seawall. 2. Using the wooden block, create waves by pressing the block half way into the water parallel to the beach. 3. Make waves for 30 seconds 4. Use strainer to strain out the gravel from the beach and create a breakwater with the gravel 5. Using the wooden block, create waves by pressing the block half way into the water parallel to the beach. 6. Make waves for 30 seconds 7. Place a pile of gravel perpendicular to the beach. Create a groin/jetty Seawall Starting Measurement Ending measurement Breakwater Starting Ending Measurement measurement Jetty/Groin Starting Ending measurement measurement Draw a picture of the end result of each type of erosion: Seawall Breakwater Jetty/groin Summarize the ending results of the three types of protection. Which worked better? Explain. Questions: 14. What is a seawall and how are they designed to protect beach front property? 15. Is a sea wall a positive or negative fix and why? 16. What is a groin (Earth Science)? How are they designed to protect the beach? 17. If there is an easterly wind with a groin running north to south, where will deposition and erosion take place. Make a sketch below. 18. What is a jetty? How are they designed to protect the beach? 19. How is a jetty different than a groin in reference to the erosion and deposition of the beach? 20. Explain how humans are negatively impacting beaches?
