Transcript wave cut notch - Cal State LA

Beaches
What is a beach?
• Area along coast where sediment accumulates
• Coastal zones begin when waves interact with seafloor
Beaches
Shoreline Divisions
• Littoral zone
– Part of coast that stretches from land plants to where waves
start to break
o Shoreface
– Permanently covered by water
o Foreshore
– Exposed at low tide and covered at high tide
o Backshore
– above mean high tide – only affected by storms
Beaches
Shoreline Divisions
Divided by wave action:
• Breaker zone
– Where waves become unstable and break
• Surf zone
– Shallower waves move up beach face
• Swash zone
– alternately covered by “swash” and uncovered by “backwash”
Beaches
Sediment Profile
• Berm
– Flat-top ridge of backshore
• Berm crest
– High point on a beach
• Beach face
– Sloping portion of the beach
• Longshore bars
– In winter, sediment from berm is transported
below breaker zone
Beaches
Types
• Shape and structure
• Composition
wide
Quartz
Pink
sand
Cobble
• Size
• Color
narrow
BlackShell
sand
sand
Beaches
Summer and Winter
winter
summer
• Summer
• Winter
– Gentle waves
– Erosive storm waves
– Carry sand to shore but too
weak to carry back to sea
– Sand carried seaward to
offshore sandbars
– Wide, sandy beach
– Narrow, rocky beach
Beach Processes
Refraction
•
•
•
•
Wave crests approach shore at an angle
Slows over shallow water
Wave bends
Wave rays travel perpendicular to wave crest
• Energy focused on headlands  erosion
Beach Processes
Rip Currents
• Incoming waves create underwater
sandbar close to shore
• Waves cause sandbar to collapse
• Excess water is pushed through gap
• Creates a strong, narrow current
away from shore
Beach Processes
Longshore Current and Transport
• Longshore current moves parallel to shore
• Longshore transport moves sediment along shore by LS current
• Zig-zag motion has an overall net transport
– Swash moves sediment onto beach at an angle
– Backwash moves sediment down perpendicular to shore
Beach Dynamics
Deposition and Erosion
• Deposition
– Accumulation of sediment
– Coasts build out by
waves and currents
Sandy shore
• Erosion
– Little incoming sand,
usually no major river
– Sediment is removed
– Reshaping of coastline
by wave action
Rocky shore
Beaches
Depositional Features
• Sand spits
– Longshore current deposits
sand in a line downcurrent
• Bay mouth bars
– When a sand spit closes
off a bay
• Barrier island
– Long sand island
enclosing a bay or lagoon
Beaches
Erosional Features
• Arches
– Weaker rocks erode
Stack
Arch
• Stacks
– Offshore column formed
from collapsed arch
• Tombolo
– Sand bar connecting to a
rock or offshore outcrop
Tombolo
Beaches
Erosional Features
Cliff
• Cliff
– Cliff bottom attacked by waves
– Waves undercut cliff to form wave cut notch
– Overhanging rock collapse
• Wave cut bench
– As cliff retreats, area of flat rock exposed
Wave cut bench
Beaches
Erosional Features
• Marine terrace
– If sea level lowers or
land uplifted, wave
cut bench becomes
marine terrace
Beaches
• Sea walls
Human Impacts
– Parallel concrete structures
– Absorbs wave energy
– Actually promotes erosion
• Groins
– Wall perpendicular to shore
– Restore erosion
– Erodes downcurrent side
• Jetties
– Designed to maintain
navigation routes
– Erosion and deposition
• Breakwaters
– Wall parallel to shore
– Reduce wave energy
– Interrupts beach drift
Beaches
Human intervention
Miami Beach
before
after
• Beach nourishment
– Import sand into coastal areas
• Dredging
– Underwater excavation to clear waterways
• Primary
Shepard
– Geologically
classification of
youthful
– Controlled by
coasts
non-marine
processes
– Examples
• “Drowned”
river or glacial
valleys
• Volcanic coasts
• Coasts shaped
by recent
tectonic
movements
Secondary coasts
• Geologically mature
• Controlled by
marine processes
• Tectonically passive
– Examples
•Straightened
coasts
•Coral reefs
•Marsh grass
Fig. 11-13f
Emergent coasts
• Uplift or sea level drops
• Once marine is now land
–Marine terraces
–Stranded beach deposits
Submergent coasts
• Subsidence or sea level rises
• Once land (or shallower) is now
underwater (or deeper)
– Drowned beach deposits
– Drowned river/glacial valleys
– Submerged dunes
Tectonic and isostatic changes
• Tectonic uplift and subsidence
– Larger scale: major parts of continents
– Smaller scale: local deformation
• Isostatic adjustments to loading
– Sediments or ice
– Regional and local effects
• Affect sea level
Eustatic changes in sea level
• Global
• Changes in spreading
Fig. 11-15
rates of ocean floor
• Changes in ice
volumes
• Changes in ocean
temperature
– Global warming
•Sea level rises
•Ice melts, ocean
warmer
Atlantic coast of U.S.
• Mainly submergent
• Barrier islands
• Glaciers shaped some shorelines
• Eroding coasts
• Subsidence in most areas
Gulf coast of U.S.
• Mainly submergent
• Barrier islands
• Mississippi delta
• High rates of erosion
• Tectonic subsidence
Pacific coast of U.S.
• Mainly emergent
• Less erosion
• Mainly rocky shoreline
• Dams on rivers beach starvation
• Tectonically active
U.S. coastal erosion, deposition
Fig. 11-17
Hard stabilization
• Groin and groin field
– Perpendicular to shore
– Traps sand between groins
– Interrupts longshore current
• Jetty similar
– Built to protect harbor entrance
– Sand on upcurrent side
• Breakwater
–Parallel to shore
–Offshore
–Deposition behind breakwater
• Seawall
– Built on beach
– Parallel to beach
• Erosion enhanced
seaward of wall
• Seawall destroyed
• Beach narrowed
Fig. 11-25
Alternatives to hard stabilization
• Limit construction near shore
• Relocate businesses/homes
destroyed by wave erosion
End of Chapter 11: The Coast:
Beaches and Shoreline Processes
Deltas
• Rivers deposit
sediment at coast
• If more fluvial
influence than
ocean influence
– Bird’s foot delta
• If more ocean
influence than
river influence
– Arcuate delta
FIG. 11.11b