The Work of Waves and Wind

1. The Work of Waves and Wind • Objectives: • Explain the characteristics of ocean waves • Present coastline features of erosion, transport and deposition • Human influence on coastal processes

2. How are waves formed • When wind blows over the surface of the ocean, it causes the surface water to rise and fall. • When wind blows, energy is transferred to the waves. • Larger waves contain more energy than smaller waves.

3. Wave Energy • Wind speed – The faster the wind speed, the greater the wave energy. • The duration of the wind blowing – The longer the wind blows, the larger the waves. • The fetch – The greater the distance of the sea over which the wind blows, the more energy the waves have.

4. Land Sea X • Land C A B Land Land

5. COASTAL LANDFORMS Crest (highest point) wave length trough (lowest point) wave height particles in waves follow a circular pattern

6. At the shoreline • Water becomes shallow, friction on the surface of the sea floor slows down wave movement • Wave height increases because wave length decreases • Waves become steeper, then collapse (breakers) • A sequence of breakers is referred to as surf.

7. Swash - water sliding up beach (will carry materials up the shore) • Backwash - water flowing back down beach to sea (will carry materials back into the sea)

9. Types of waves • Constructive Waves - occur in calm weather on gentle sloping coasts - waves break gently with little energy - have a strong swash and weak backwash - gentle gradient allows waves to surge a greater distance up the coast and carry more sediments up - helps to build up the coast

10. Constructive Waves Spilling Breaker Powerful Swash Weak Backwash

11. Destructive Waves - occur on steep sloping coasts - waves break violently with high energy - have a weak swash and strong backwash - steep gradient causes the waves to break and plunge directly back down the coast - associated with storms - erodes coasts away

12. Destructive Waves Crashing breaker Weak swash Powerful Backwash

13. Longshore drift • Waves arrive at a coast at an angle (swash) • Backwash returns at 90degrees Sand is moved along the beach = longshore drift or longshore current

14. Coastal Processes Hydraulic Action Attrition Abrasion Solution

15. Hydraulic Action Refers to the direct impact of the waves against the coast The sheer force of breaking waves pounding against the sea cliff can exert great pressure on the cliff face (esp. by destructive waves) Water enters lines of weaknesses; air in the cracks may be compressed by water; pressure exerted may widen cracks When waves retreat, air in the cracks may expand and exert pressure on walls of cracks; cracks may enlarge and widen with repeated compression & expansion.

16. Hydraulic Action The force of the waves pushes air into cracks in the rock The compressed air exerts enormous pressure at the tip of the crack. As the water falls back the air pressure is released and rock is pulled out from the crack further enlarging the hole.

17. Solution (Corrosion) Waves react chemically with soluble minerals contained in the rocks and dissolve them (A coast comprising of limestone is most vulnerable to this process)

18. Abrasion (Corrasion) Refers to the impact of the materials carried by the waves scraping against the coast. Destructive waves are known to be capable of lifting up considerably large pieces of rock from the sea bed and hurling them against the coast.

19. Attrition When particles carried by the waves rub or hit against each other, they break down into smaller pieces They become smoother pver time to form rounded pebbles.

20. Factors Affecting Coastal Erosion Rate of Coastal Erosion is affected by Type of waves: Destructive waves have more energy than constructive waves. Structure & Composition of coastal rocks: Coastal rocks with numerous lines of weaknesses will erode and break down faster Coast consisting of soft rocks such as clay will erode faster.

21. Position of the Coast: Coasts that are protected or sheltered from prevailing winds and wave actions will experience less erosion than coasts that are open and unprotected.

22. Headlands & Bays

23. Resistant Sandstone Less Resistant Clay Waves approaching the coast Resistant Limestone • Coasts with alternate strips of hard & soft rocks • Arranged at right angles to the coast • Less resistant soft rocks will get eroded faster than the more resistant hard rocks • Forms indented coasts with headlands & bays Less Resistant Clay

24. Wave refraction • Close to coast, water gets more shallow • Waves are slowed down • If waves arrive at an angle, one part is slower than the rest • Causes waves to bend = wave refraction

25. Headland Wave refraction Bay

26. Headlands may be eroded back leaving a remnant (stack) stack

27. Wave erosion may cut through the caves and an arch is formed Arch collapses and leaves a rocky pillar called stack Stack eroded by waves and wind to form a stump Old cliff line Original Land surface stack stump Natural Arch Wave-cut platform Constant wave erosion along lines of weaknesses causes caves to develop on both sides of the narrow headland.

28. Erosional Landforms • Waves arriving at bays are slow (deposition) • At headlands, faster (erosion)

29. High Tide High Tide Low Tide Low Tide Notch Cave sea sea sediment Land Land Wave erosion produces a notch along the line of weakness on the coast The notch is further eroded to form a cave

30. Roof of cave collapses Cliff retreats inland High Tide High Tide Low Tide Low Tide Cliff Wave –cut platform sea sea Land Land Further erosion by the waves causes the roof of the cave to collapse into the sea and a cliff is formed Continuous erosion causes the cliff to retreat inland. Over time, a wave-cut platform is formed at the foot of the cliff.

31.  Tom Abbott, Biddulph High Cliff line Joints and beds allow waves to attack the cliff more easily HIGH TIDE LEVEL Waves can use rocks to erode the cliff Wave cut platform Wave cut notch

33. Transportation – Beach Drifting & Longshore Currents = Longshore Drift Zig-zag movement Backwash Swash Longshore currents flow parallel to a coast. They are formed by waves that approach the coast at an angle. Wave Direction

34. Waves approach coast at an angle, materials will be transported down the coast. • Swash carries materials up the beach at an oblique angle • Backwash carries materials perpendicularly down due to gravity • This zig-zag movement of the materials is known as beach drifting. • Wind affects the direction of the longshore drift. • If wind is blowing from south-east direction, the direction of the longshore drift will be east to west.