Coasts

1. Coasts An N. Hardy powerpoint.

2. What we will learn. • Coastal zones • Types of coasts • Waves – how they define coast • Beach Currents • Coastal Features • Erosional Coast Features • Depositional Coast Features • Coast Linearization • Emerging/Submerging Coastlines • Human Intervention

3. Zones (Description) • Backshore – Further inland from the shore • Shore – Sand and low depth areas. Between shorelines. Consists of the littoral/intertidal zone. • Nearshore – Beginning of the Continental shelf, where wave shoaling begins. • Offshore – Begins at the continental slope, continues out from shore.

4. Zones (Pictures)

5. Types of coasts • Major types: • Depositional Coast • Deposit sediments • Usually sandy • Erosional Coast • Erodes away. • Usually rocky Question for understanding: Which type of coast is the Huntington Beach Pier area?

6. Waves – How they define coasts • High-energy coasts • Heavy wave activity, many storms • Usually erosional • Low-energy coasts • Light wave activity, fewer storms • Usually depostitional Question for Understanding: Which type of coast is Huntington Beach?

7. Waves – how they define coasts • Usually seasonal changes in wave activity • A summertime beach • Occurs more often in summer • Low-energy weather, builds up berm • A wintertime beach • Occurs more often in winter • High-energy weather, destroys berm Before removing this bar, guess which diagram is in summer and winter! Did you guess right?

8. Waves – Cool table w/ effects

9. Beach currents • Wave movement linear to shore • Called a longshore current • Longshore drift/transport • Movement of sediment by longshore current • Create “upstreams” and “downstreams” of sediment movement on depositional beach • Why does longshore drift not occur further away from the shore? • Because waves only effect the ocean beneath them to their wave base, which is ½ of the wavelength

10. Beach currents • What is the mechanism for longshore currents? • Water that rushes onto the beach: • Swash is water that soaks into the beach • Backwash flows back into the ocean • Most water is backwash • Backwash flows linearly down the shore

11. Beach Currents • Rip Currents • Flow seaward • Caused when longshore currents meet and the backwash is greater than the incoming wave mass • REALLY COOL ANIMATION • DANGER • They kill people • In order to escape, one must swim parallel to shore.

12. Coastal Features • There are a lot. • They are boring.

13. Coastal Features (Erosional Coasts) • Headlands • Rocky seaward outcroppings • Wave Cut Cliffs • Waves cause water only up to a certain height on erosional coasts, when the rock above the notches cut into by the waves collapse, wave cut cliffs are formed • Sea Caves -> Sea Arches -> Sea stacks • Sea Caves can change into arches and stacks over time • Marine terrace • Blow Hole • Outlet for sea caves

14. Coastal Features (Erosional Coasts) • Picture!

15. Coastal Features (Erosional Coasts) • What is the cause? • Hydraulic Action • Air is compressed in cracks by water moving in, may cause cracks to widen in rocks • Wave Pounding • Sheer energy of the wave hits the rock • Abrasion/Corrasion • Waves launch seaload at the cliffs • Most effective/rapid form of coastal erosion • Attrition is when seaload is worn down by itself or a shoreline

16. Coastal Features (Erosional Coasts) • Erosion causes (continued) • Dissolution • Dissolving of rocks on shore by sea water • Can lead to caves • Corrosion is the dissolving of rock by carbonic acid in seawater • Limestone cliffs are particularly vulnerable to corrosion

17. Coastal Features (Erosional Coasts) • What happens to all of the eroded material? • Eroded Material is broken down into terrigenous sediment, then one of two things can occur: • Transported by longshore drift to a depositional beach • Stays at the home coast, and when enough sediment builds up a beach is formed. • When will it stop? • Erosion continues until there is a beach large enough to stop waves before they hit the rocks

18. Coastal Features (Depositional Coasts) • Mostly consist of build-ups of sediment • Spit • Extend in the direction of longshore drift. • Spits that continue to grow to the other side of the bay and completely cut off the bay from the open ocean are called bay-mouth bars, or bay barriers • Stupid people build structures on bay barriers, which are not stable or permanent structures. (1m) • Tombolo • connect islands to either shore or another island.

19. Costal Features (Depositional coasts) • Picture!

20. Coastal Features (Depositional Coasts) • Barrier Islands. Very Important, and in need of a sophomore to do a power point on them. • Here’s the basic idea: • They occur along depositional coastlines • Are long offshore deposits of sand that are parallel to shore • Develop into their own ecosystems • My opinion: Great learning powerpoint

21. Coastal Features (depositional Coasts) • Beach Compartments • Best comparison is describe a beach compartment as a cycle of sediment throughout an area. • Three main contributing factors: • Rivers supply sediment to coast • Sediment moving due to longshore transport • Sediment drained down into the ocean through submarine canyons • Beach Starvation • Human structures, such as a dam, limit rivers, slowing depositing of sediment

22. Coastal Features (Depositional Coasts) • Picture!

23. Coast Linearization • Coast Linearization tells of the age of a coastline and how it has developed due to erosional and depositional factors. • Not boring like coastal features, actually pretty fun to learn about. • Is a convergence of all of the stuff you just learned about

24. Coast Linearization! • The best example of coast linearization is the Mediterranean Sea. • For Sophomores: The Mediterranean Sea dried up a long time ago, but then was refilled from the Atlantic. • When the Mediterranean first filled up with water, it had a lot of high-energy waves from the waterfall hitting it • Basicly, a new coastline. • We will follow what would ideally happen to this new coastline in the next few slides.

25. Coast Linearization • When the waves first hit the rocks of the Mediterranean, an erosional coast was formed. • It was erosional in most areas • Irregularities in the rocks caused headlands and coves to form. • At first, the coast becomes non-linear

26. Coast Linearization • Important: Wave Refraction • Caused by uneven wave shoaling • Wave Refraction is the diversion of waves from coves, and the convergence of waves onto headlands. • Effectively Creates a High-energy coast on the headlands • Effectively creates a low-energy coast in coves • Concept recall: what do low energy coasts do that high-energy coasts do not do? • ANSWER: they build a berm, or a beach Did you guess right? Guess before you click!

27. Coast Linearization

28. Coast Linearization • What’s going on now? • A beach is formed in coves that protect the rock their from erosion • Headlands have no protection and have waves being directed towards them because of wave refraction. • Headland decline is much much faster than the decline of coves at this point • COOL ANIMATION: CLICK HERE

29. Coast Linearization • Headlands retreat • Leave behind sea stacks and archs • The coast is straightened • Sediment builds up, and the coast becomes depositional

30. Coast Linearization • Overall concept: • Most of the time, • A younger coast is erosional • An older coast is depositional

31. Emerging/Submerging Coastlines • Two things can move coastlines. • Surprise! Sea level changes. • Land moves up and down as well. • Coasts that are rising above sea level are called emerging shorelines • Coasts that are sinking below sea level are called submerging shorelines

32. Emerging Coastlines • Evidence is shown in: • Marine terraces • Stranded beach deposits • Evidence of sea life found above sea level • Sea caves above waterlevel

33. Submerging Coastlines • Drowned beaches • Submerged dune topography • Drowned river valleys

34. Emerging/Submerging Coastlines • Mechanisms on land: • Tectonic movements • Passive margins • Moving away from a spreading center • Usually submergent • Active margins • Tectonically active • Usually emergent

35. Emerging/Submerging Coastlines • Isostatic Adjustment • Earth’s crust, especially continental crust, is floating. • Crust sinks when • Crust sinks under accumulation of heavy loads of ice, vast piles of sediment, outpourings of lava • Crust rises when • Heavy loads are removed • A very slow process

36. Emerging/Submerging Coastlines • Eustatic changes in sea level • More ice = sea level lower • Less ice = sea level higher • Other Causes? • Formation/destruction of inland lakes • Changes in sea floor spreading rates • Faster spreading = raise sea level • Slower spreading = lower sea level • The slowest • Thermal expansion of water • For every 1°C, sea level changes about 2 meters

37. Emerging/Submerging coastlines • Link to ice ages • According to isostatic adjustment, continents sink under ice • However water level also raises when water melts from ice packs • Water also expands under heat • Which wins? • Global warming raises sea level because thermal expansion and melting ice win.

38. Emerging/Submerging Coastlines • Important word: Pleistocene • Estimated Sea level change to be 190 meters • 120 below • 70 above • Most Coastal Areas have evidence of both emergence and submergence • Most changes due to ice sheets forming/melting and taking/releasing water • Thermal = 10m/5C

39. Emerging/Submerging Coastlines

40. Emerging/Submerging Coastlines • Global warming • Increase in average ocean temperature of .6°C over the past 140 years • Eustatic sea level change of 4-10 in. up. • Global warmingraises sea level.

41. Human Intervention • Due to Eustatic sea level rise, 70% of sandy beaches are eroding • 60% of humans live near the coast • Rising sea level threatens human structures • we try to stop it

42. Human Intervention • Hard Stabilization • Structures built by humans to stop beach erosion. • Doesn’t work very well. • Groins and Jetties • Built perpendicular to shore • Groins create groin field • Protect harbor entrances • Both composed of large blocky material called rip-rap

43. Human Intervention

44. Human Intervention • Breakwater • Built to create harbors free of waves • create many problems, such as erosion downshore and unwanted tombolos. • Dredging solves some of these issues • Picture is very explanatory:

45. Human Intervention

46. Human Intervention • Seawalls • Constructed parallel to shore • Designed to stop erosion • Very expensive

47. Human Intervention • Alternatives to hard stabilization • Construction Restrictions • Stop building of structures close to coastlines • Beach Replenishment/Beach Nourishment • Sediment is added to a beach to stop erosion • Relocation • Moving of structures

48. Human Intervention