Name: LAB: WHERE S THE BEACH Introduction When you build a sandcastle on the beach, you don't expect it to last forever. You spread out your towel to sunbathe, but you know you can't stay in the same spot all day without getting wet. Venturing into the ocean to swim, you are cautious - wondering what the currents are doing that day. Subconsciously, you are attuned to the fact that the coastal environment is constantly changing. 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. Fig. 1: 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. Seawalls Google image of Long Island with multiple barrier islands Responses to Erosion 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 so-called "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. Updrift, sand is piling up on windward side of groin Erosion is occurring on the downdrift 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. Questions 1. Where does the sand at the beach come from? 2. In what season is erosion of beaches more prevalent (happens more)? and why? 3. In what season is deposition of sand at beaches more prevalent? and 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. 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 around the jetty? 20. What eventually happens to a beach if a breakwater is built? 21. Explain how humans are negatively impacting beaches?
REGENTS QUESTIONS 1. Winds transport sand and other material by (1) water and rolling (2) suspension and bouncing (3) ice and snow (4) ocean currents and water 2. Water particles near the surface of a deep water wave (1) move with the wave (2) move against the wave (3) do not move (4) move in a circle 3. When waves from the southeast strike a beach that runs east and west, the direction the longshore current is likely to move toward is the (1) west (3) northwest (2) east (4) southeast 4. Add an arrow to the diagram below to indicate the direction of the longshore current.
Base your answers to questions 5 through 10 on the diagram below, which shows ocean waves approaching a shoreline. A short wall of rocks perpendicular to the shoreline and an offshore structure have been recently constructed along the beach. Letters A, B, C, D, and E represent locations in the area. 5. What is the most common cause of the approaching waves? (1) underwater earthquakes (2) variations in ocean-water density (3) the gravitational effect of the moon (4) winds at the ocean surface 8. At which location will the beach first begin to widen due to sand deposition? (1) A (3) C (2) B (4) E 6. The size of the bulge in the beach at position D will (1) decrease (2) increase (3) remain the same 7. How was this beach directly formed? (1) uplifting from an earthquake (2) sinking of land during a landslide (3) deposition of sediments by ocean waves and currents (4) erosion of bedrock by ocean waves and currents 9. Water particles near the surface of a deep water wave (1) move with the wave (2) move against the wave (3) do not move (4) move in a circle 10. When waves from the southeast strike a beach that runs east and west, the direction the longshore current is likely to move toward is the (1) west (3) northwest (2) east (4) southeast