shores and coastal processes!

Transcription

1 shores and coastal processes!

2 one of the most dynamic and popular environments on Earth! > 50% of US population lives < 80 kms from ocean or Great Lake! coast: entire region bordering body of water! shoreline: precise boundary of water and dry land! shore: strip of coast closest to sea/lake, may have a beach!

3 consequence of living near the coast: coastal erosion! prediction of future shorelines! to 2050 in Long Beach, CA!

4 we have examined numerous processes! of erosion, transportation and deposition! mass wasting! streams! ground water! glaciers! wind! what is responsible for coastal erosion?!

5 waves! ordinary ocean waves are produced by wind! wave hitting coastline transfers wind energy to coast! eroding rocks and transporting sediments! storms increase wave energy and increase erosion!

6 water waves! wave height: vertical distance between crest and trough! crest: highest point on wave "trough: Lowest point on wave! ocean waves range from 0.3 to 5 m high; can be much higher! wavelength: horizontal distance between 2 crests or troughs! ocean waves range from 40 to 400 m! wave period: time for one wavelength to pass! ocean wave velocities range from 25 to 90 km/hr!

7 wave height, length, period and velocity are controlled by! speed and duration of wind! constancy of wind direction! distance wind travels across water surface (fetch)! southern ocean has unlimited fetch: no land masses! routinely has waves exceeding 15 m (50 feet)! rogue wave! off! South Africa! from Esso! supertanker! m high --! largest wind-generated wave ever observed was in 1933 by captain! of USS Ramapo in northern Pacific---34 meters high (115 feet)---!

8 waves from different winds may interfere! constructive interference:! "sum together to generate bigger wave! destructive interference:! "cancel to produce smaller wave! blue + red interfere to produce green! big waves in Malibu may start from Antarctic storms and! grow from interference from Hawaiian rain squalls!

9 water waves! motion of water in waves is nearly circular--orbit! energy advances with the wave but water does not! orbital motion decreases with depth and is gone at! depth of 1/2 the wavelength!

10 water waves! wave base: depth where orbital motion is zero! where water depth < 1/2 wavelength, waves feel bottom! bottom of wave slows down (friction from sea bottom)! top of wave moves faster, water piles up and topples over! "( breaks in surf zone)! in deep water, waves have no effect on sea floor! in shallow water, waves may disturb sediments on sea floor!

11 water waves! energy moves water from wave up beach as swash! "and deposits sediments! gravity moves water down beach as back swash and! "removes sediments!

12 water waves in the nearshore! what happens when waves approach shore at an angle?! refraction--waves bend and change direction! waves slow down in shallow water! waves move closer together! waves in deeper water! travel faster and! catch up to wave! in shallower water! effect is for waves! to bend as they approach! the shore to be close to! parallel to the shore! (the waves still are at! a small angle to shore)!

13 wave refraction!

14 waves approach shore at an angle! wind and wave direction! component perpendicular! "to beach! component parallel! "to beach!

15 beach parallel component produces longshore current! beach! swash moves! sediment up beach! at an angle! back swash moves! sediment straight! down beach in! response to! gravity! net effect is to! move sediment! parallel to beach! in direction of! longshore current!

16 longshore drift: movement of sand parallel to shore! sand grains move suspended in surf zone!

17 longshore drift: leads to depositional land forms! spit: ridge of sand that extends from land into! open water! longshore current meets! deep, quiet water of bay! and drops sediment load! -- forms spit --!

18 longshore drift: leads to depositional land forms! baymouth bar: ridges of sand, transported by longshore currents, that cut off bays from the open ocean -- starts as spit that grows across opening!

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20 bay mouth bar!

21 longshore drift: leads to depositional land forms! tombolo: bar of sediment connecting a former island (typically composed of bedrock) to the shore (deposition behind sea stack as refracted waves slow down)!

22 tombolo!

23 water waves in the nearshore! rip currents: -- fed by water in surf zone! " " -- form from wave interference! flow straight out to sea from surf zone! travel at the surface and die out at depth! interfere with incoming waves! carry fine grained particles (and surfers) to deeper water! form narrow currents perpendicular to shore, allowing swimmers! "to swim parallel to shore to escape them!

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25 tides! twice daily rise and fall of ocean surfaces! gravitational pull of Moon and Sun and rotation of Earth! Sun is bigger but farther away--exerts only 40% of pull of Moon! high tide: face of Earth toward Moon! strong gravitational pull of Moon! high tide: face of Earth away from Moon! weak gravitational pull of Moon! spinning force from Earth rotation is at! "maximum and causes bulge of water! 2 high tides per day at every location (Earth rotates)! 2 low tides at points midway between high tides!

26 flood tide: rising, elevates water, advances shore landward! tidal bore: flood tide whose upstream motion is turbulent! " "Bay of Fundy: tidal bore is km/hr! ebb tide: falling, lowers water surface, shore recedes! amount of change in water level depends on coastline shape! and size of body of water! narrow inlet forces water to pile into small area--puget Sound: 3-4 m! straight coastline has little effect--open Pacific at Hawaii: m!

27 turbulent! Bay of Fundy!

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29 beach! strip of sediment from low-water line inland to a cliff or! " " " "zone of permanent vegetation! beach face:! steepest part of beach,! water sloshes up as! waves are breaking! berm:! flat or gently sloping! platform! --narrowed during storms! --rebuilt during calms! marine terrace:! broad, gently sloping platform or rock or sediment just offshore!

30 beach features, CA! backshore! foreshore! berm!

31 beach: seasonal development! winter: frequent storms; high wave energy! " sand eroded from beach anddeposited as bar offshore! summer: rare storms; low wave energy! " sand eroded from bar and redeposited on beach!

32 beach: depositional feature (sources of sand)! deposition from rivers (> 90%)! erosion of local cliffs! transport from regions seaward of surf zone! longshore currents (longshore drift)! bioclastic (shells of organisms--carbonates)! leads us to!

33 types of coasts:! erosional, depositional, drowned, emergent! waves: most significant factor shaping coasts! ~ 14,000 waves strike exposed rocks and beaches at coast each day! --erode by forcing water and air under high pressure into cracks! process is repeated every few seconds! orientation of coastline is critical--! " " "directly face waves or not?! coasts that directly face incoming waves! --erosion in California is cm/yr! --erosion in North Carolina has been 20 m in last decade! --erosion in Chambers County, TX was 3 m in 1 year!

34 erosional coast: irregular; headlands; bays! focusing of wave energy causes:! "erosion of headlands and deposition in bays! continued evolution results in coastal straightening!

35 straightening of coast through time! erosion of headlands and deposition in bays!

36 yields sea caves and sea arches! wave-cut notch! at cliff base! expands to! wave-cut bench! over time!

37 protect against erosion! seawall! rip rap: boulders piled seaward of shoreline!

38 depositional coast: gently sloping plains! deposition controlled! by longshore drift! barrier islands: ridges of sand that parallel coast! lagoons separate barrier islands from mainland! dynamic system, with rapid erosion/deposition! heavy population has led to property loss from! "rapid, localized erosion!

39 drowned coast: sea level rise of 130 m! fjords! during ice ages,! rivers flowed over! continental shelves! (now under water)!

40 emergent coast: elevated by tectonic forces! uplift is more rapid! than sea level rise! older marine terrace! (remember from beach)! is exposed! as it is uplifted! over time!

41 other types of coasts: organic (biological)! biological processes act together with deposition/erosion! mangroves stand and grow in tidal water on tropical coasts! "--roots dissipate waves and trap sediment--! reefs require warm, clear water and depths from m! "--sea level rises, coral grow vertically to keep pace--! fringing: built directly! against landmass! barrier: built offshore! and separated by lagoon! from mainland! atolls: built on volcanic! islands that have subsided! below sea level!

42 mangroves

43 underwater! mangroves

44 fringing reef

45 Great Barrier Reef

46 barrier reef

47 formation of an atoll subsidence of volcanic island!

48 atoll!

49 atoll!

50 atoll!

51 humans and coasts! impact of building near coasts! impact of interfering with coastal processes!

52 what happens during coastal erosion?! sea cliffs retreat and wave cut platforms widen! sea stacks, sea caves, and sea arches form!

53 wave cut platform - low tide! beach cliff retreats! wave cut platform widens!

54 stacks and arches! shoreline was here once!

55 coastal erosion eventually leads to! wave cut cliff retreat! seawall (preventative)!

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57 building on depositional coasts! barrier islands: low-lying, narrow, sand features! " " "in the near off-shore that have! " " " beaches and inland dunes! most have been narrowing since 1800 s! most common! coastal feature! of! North America! islands! from! New York! (eastern Long Island)! to! Florida! to! Gulf Coast! (Padre Island)!

58 formation of barrier islands not clear! longshore drift important once island is built! " but how was island built?! one idea! glaciation! beach ridge forms! (sea level lower)! sea level begins! to rise and! forms lagoon! barrier island forms! sea level at! present! island migrates! from:

59 sea level rise! along coast! through! time! rivers to coasts! barrier islands!

60 3D view of barrier island! note percent of beach/marsh and overwash fan! dynamic! environment! land! ocean! from:

61 barrier island! waves wash over island, remove sand from beach! and deposit the sand on landward side! barrier islands migrate landward through time! mainland also retreats, thus islands remain! Atlantic barrier islands migrate m/yr! from:

62 barrier island in its pristine state! from:

63 barrier island development! dunes destroyed for views! dunes prevent eroson! result is massive erosion! and intervention to! protect beach! i.e. importing sand! property is desirable! population of Miami Beach! ! ,925!

64 replenishment is not very effective!

65 effect of rising sea level on barrier island! note what happens to building!

66 interfering with coastal processes! jetties: typically in pairs! extend channel to stop sedimentation and keep channel open!

67 groins: singular features perpendicular to shoreline! interrupt longshore drift and cause erosion and deposition!

68 breakwaters: offshore features to absorb waves! allow build up of sand behind them onshore!

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70 effects of extreme events! hurricanes! and! associated! storm surge! anti-clockwise! rotation in! the northen! hemisphere!

71 Hurricane Katrina!

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73 changes to! barrier island! pre-ivan (2004)! to! post-katrina (2005)! green is deposition! red is erosion!

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76 some historical Texas storm surges! YEAR SURGE LOCATION COMMENTS FT GALVESTON FT GALVESTON FT INDIANOLA FT SABINE FT SABINE FT GALVESTON FT GALVESTON FT VELASCO FT GALVESTON FT GALVESTON FT CORPUS CHRISTI FT PORT ARANSAS FT MATAGORDA FT HIGH ISLAND FT MATAGORDA FT PORT LAVACA FT HOUSTON SHIP CHANNEL FT GALVESTON FT PORT ISABEL BEULAH FT PORT ARANSAS CELIA from:

77 reaction of Galveston, TX -- seawall! 17 ft concrete barrier! note lack of sand! seaward of barrier! everything has a consequence!