Anatomy of Coastal Regions

Transcription

1 The Coast

2 I. BEACH ANATOMY

3 Anatomy of Coastal Regions Terms for different parts of beaches and coastal regions Are all about ENERGY- ie, where the ocean s energy Mostly through tides and waves, and shape the land.

4 Some Definitions COAST: coastline to inland as far as ocean related features found (< 1 km - 10s km) Coastline - where influence of storm waves ends SHORE: coastline to low-tide shorelineand forth shoreline migrates back OFFSHORE - beyond breakers, where waves do not influence bottom WAVE CUT BENCH: Common overall feature of coastal zone is wave cut bench - flat rock over which sand and sediments constantly change. BERM: The sand piled up on the wave-but bench beneathespecially, the dry sand above the typical high water mark that we typically think of as the beach.

5 Coasts are ephemeral: always changing! These changes happen on both short and long time scales. Daily: Swash and backwash reshape the berm, and change the beach texture. Seasonally: Winter vs. Summer beaches- Decadal and longer: Erosion pulls down cliffs, moves shorelines, reshapes coves..

6 Anatomy of Coastal Regions Offshore Nearshore Shore Coast Backshore Foreshore low-tide breakerline low-tide shoreline high-tide shoreline

7 What we typically think of as the beach is technically Called a berm - ie, where dry sand (most of the time) accumulates Point ReyesCapistrano Beach, Ca Let s go to the BERM!

8 Beach Anatomy Offshore Shore Coast low-tide breakerline low-tide shoreline high-tide shoreline longshore bar wave-cut bench longshore trough berm BERM = dry sand at foot of cliffs/dunes Wet only during storms or large waves

9 Example of a Wave cut bench exposed during low tides Wave cut bench is rocky underlayer, cut flat by waves Over which the sand migrates with storms and seasons

10 Point ReyesCapistrano Beach, Ca Let s go to the BERM!

11 Wave cut bench exposed during low tide

12 II. MAIN SAND TRANSPORT MECHANISM

13 Movement of Sand QUESTION: How is it that WAVES can both erode a beach - and also deposit a beach? IE, how can what might seem like the exact same process ( a wave striking a beach) sometimes take sand away yet other times deposit sand?

14 Swash / soak / backwash Backwash SWASH (+soak)

15 Swash - Soak dominates Beaches with light waves - sand added to berm Reduced longshore bars Sandy wide berm Steep beachface Classic SUMMERTIME

16 Backwash

17 BackWash Dominates Energetic beaches - sand eroded from berm Longshore bars built up Rocky, narrow berm Flattened beachface Classic WINTER TIME

18 Summer Beach: Swash/Soak dominates Mitchell's cove series

19 Winter Large waves, Backwash dominates, beaches Mitchell's eroded away! cove series

20 Mitchell s cover, summer

21 BAXTER RETURNS

22 Mid- Winter

23 Spring

24 Quantitative? Spring vs. Winter 2*B 3.5*B

25 rrrrrrrr r-woof! But when did this happen to my beach..?..

26 Mitchell's cove series

27 Mitchell's cove series

28 Mitchell's cove series

29 Mitchell's cove series

30 Mitchell's cove series Mitchel s cove: Summer vs. Winter

31 A dramatic storm-cut cliff Mitchell's cove series

32 Summer vs winter beach Result: classic seasonal Beach differences! Summertime beach Wintertime beach

33 Winter Pocket cove: Extreme Armor

34 Another form of Armored beach: due to high wave energy all year

35 III. Sand Migration The river of sand

36 Longshore drift = movement of sand down down coast-lines, in general direction of most swell

37 Longshore Drift Path of sand particles - zig zag downstream Waves approach beach at an angle

38 Aside: Rip currents Backwash is concentrated in narrow area Can be 4-5 mi/hour

39 Huge Rip (hopefully those are surfers..) Backwash is concentrated in narrow area Can be 4-5 mi/hour

40 IV. Beach texture and Composition

41 Beach Texture tells you about: 1) Sand Sources and 2) ENERGY of Local Environment

42 Aside: Maturity of a sediment is based on hydrodynamic sorting

43 LARGE Sizes = HIGH Energy environment ( all small stuff is moved away..)

44 kelp beach - why does this kelp accumulate here Backwash is concentrated in narrow area Can be 4-5 mi/hour

45 So what are Sand Sources?

46 Do Grains MATCH local rock types?

47 Do Grains MATCH local rock types?

48 What are Sand Sources? SOURCES: On our coast, mostly rivers bringing sediments and sands from inland, also some erosion of coastal cliffs.

49 Sand Sinks = Submarine Canyons!

50 Sand sources + sinks : Beach compartments.

51 Features of Beach Compartments River (or erosian) to supply sand Beach with longshore transport Submarine canyons

52 Constant sand migration ( river of sand ) via long-shore drift sets up: Beach Compartments = defined by sand sources and sand sinks within a given region of coast.

53 Finally: HUMANS AND BEACH EROSIAN

54 ATTEMPTS TO STOP COASTAL EROSION-

55 Groin Barrier to longshore sediment transport Built perpendicular to coastline

56 Series of Groins, Santa Monica, California

57 Jetty Barrier to wave action at harbor entrance Built perpendicular to coastline

58 Breakwater Barrier to wave action Built parallel to coastline

59 Mission Bay, California

60 Santa Cruz Harbor

61 What its supposed to look like

62 Every few winters..what it really looks like

63 But makes a great..(if illegal) wave..

64 Beach Armoring/Hardening: attempts to save human-built on coast

65 Waves hit seawall- Bounce Energy Back- and carry sediment back offshore

66

67 Beach loss due to a SEAWALL at Waikiki

68 END