COASTS

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COASTS

• Learning objectives
• Explain wave and tide processes
• Understand linkages between wave/tide processes
  and sediment dynamics
• Describe coastal landforms and key processes of
  coastal landform development
• Explain coastal hazards and risk
• Describe and evaluate a range of coastal
  management strategies

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Introduction

• Approx 50 of world population lives within 2km
  of the coastline
• Many countries are dominated by coastal cities
• The coastal zone a dynamic geomorphological
  environment
• Exhibiting change over a range of timescales
• Adjusting to wave, tide and current processes
• Important buffer between the marine and
  terrestrial environment
• The coastal zone acts as an..
• Important ecological reserve
• Economic resource
• Communication corridor
• Recreational playground

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The Coastal Zone

• The coast can be split into zones
• Shore
• Foreshore, backshore
• Nearshore
• wave are forced to break in the surf zone (the
  swash and backwash occurring at the shoreline)
• Offshore
• Coastal plain
• Extends to the continental shelf

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• COASTAL
• PROCESSES

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Waves

• The movement of energy through a fluid
• Sea waves - produced localised storm activity at
  sea
• Swell waves - once the waves have left the
  generation area they lose height and energy to
  become swell waves
• Wave form
• Sinusoidal form with a number of definable
  components - wave crest, trough, height,
  steepness, period, frequency

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Figure 17.2
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Deep water waves

• Wave height increases with wind speed, duration
  and fetch distance
• Largest ever recorded wave 34m (February 1933
  in the Pacific)
• Relationships between wave variables for water
  depths gt ¼ of wavelength (L)
• L 1.56 T2
• indicates that small increases in wave period
  (T) cause large increases in L
• Long waves move fast and lose little energy.
  Short waves move more slowly and dissipate a lot
  of their energy along their journey
• Coasts facing open ocean receive long waves that
  have overwhelmed short waves
• Deep water waves are deflected by the Corriolis
  effect.

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Figure 17.3 Source After Short, 1999
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Deep water wave development

• Evolve from small ripple into a full sea wave due
  to wind duration and frictional drag on the sea
  surface
• On a calm sea, there is a small amount of
  frictional drag causing a ripple
• The ripple increases sea surface area and
  therefore frictional drag
• Air mass then has more frictional pull on the
  surface increases wave amplitude
• Ascending limb is pulled up by the push of the
  air mass
• Descending limb is pulled down by the force of
  gravity
• Height of the wave is directly proportional to
  the strength and duration of the wind passing
  over the surface
• Continues to propagate long after the wind has
  ceased until energy is dissipated

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Wave fields

• Waves produced at different times and in
  different places and which vary in magnitude,
  direction and speed meet together
• Become superimposed on each other
• Produce complex wave fields
• The patterns are cyclic surf-beat
• Short fetch coastlines
• different waves arrive at the same time as choppy
  conditions since they have insufficient time to
  separate
• Long fetch coastlines
• long waves dominate, surf beat develops

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Figure 17.4a
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Figure 17.4b
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Waves in shallow water

• Waves change as they approach the coastline
• A wave perturbs the water depth equal to ½
  wavelength (to the wave base)
• Shoaling occurs where the wave depth is greater
  than the water depth
• Frictional drag of bed wave slows
• Wave length decreases but wave height increases
• Steepens until unstable and breaks
• Critical ratio of water depth to wave height
  between 0.6-1.2 (low waves travel further
  coastward before breaking)

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cont

• Shoaling causes the orbital wave motion to become
  distorted
• Angle of shore is important
• Steep waves break close to shoreline
• Flat waves break further offshore
• Wave will spill, plunge, surge or collapse
• Interaction of the wave with nearshore topography
  causes
• Refraction
• Reflection
• Diffraction

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Figure 17.5
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Figure 17. 7
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Tides as waves

• Tides waves generated by the gravitational pull
  of astronomical bodies (esp. the moon)
• Gravitational force of the moon causes reduction
  in Earths centrifugal force effecting the
  oceanic surface mass
• Predictable diurnal and monthly lunar cycle
• Spring tides
• Greater magnitude tides during new and full moon
• Sun and moon pull along the same vector
• Neap tides
• Less magnitude tides during half moon phases
• Sun and moon pull in opposite directions

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Waves and sediment

• Swash and backwash mechanism for movement of
  sediment up and down the beach
• Longshore drift sediment movement along the
  beach in swash and backwash at an angle
• Longshore drift with no sediment supply from up
  the coast net erosion
• IMPORTANT FOR COASTAL MANAGEMENT

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Figure 17.9
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CURRENTS

• Longshore currents
• 10-20 cms-1 to 100cms-1 if wind direction is
  along shore
• water moving along the beach trapped between
  breaking waves and beach slope
• Rip currents
• Water forced up the beach forces its way back
  down slope against breaking waves along line of
  least resistance
• Strong circulation cell develops dangerous
• Tidal currents
• due to rise and fall of the tide

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Figure 17.10
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• COASTAL LANDFORMS

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Wave dominated coasts erosional landforms

• Coastal cliffs and shore platforms
• Erosion function of wave environment, local
  geology and coastal morphology
• Marine and subaerial processes
• Wave quarrying, abrasion, corrosion,
  wetting-drying, salt crystallisation, thermal
  expansion and contraction, biological activity
• Landslides, rotational slumps, mudflows
• Coastal retreat
• Cyclical removal of fallen sediment from cliff
  base
• May be a sediment source for downstream
  longshore drift
• Development of wave cut notches, caves, arches,
  stacks and blowholes
• Complex feedback between cliff erosion, platform
  width and wave energy potential

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Figure 17.11a Source After Sunamura, 1992
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Figure 17.11b Source After Sunamura, 1992
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Figure 17.11c Source After Sunamura, 1992
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Figure 17.12a Source Ken Hamblin
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Figure 17.12b Source Dorling Kindersley
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Wave dominated coasts depositional landforms

• Beach accumulation of unconsolidated sediment
• Dynamic equilibrium maintained because sediment
  is highly mobile
• Beach profile
• Function of coastal processes and sediment type
  e.g. wave type
• Sediment size sorting
• Topographical features cusps, berms and ripples
• Often coastal dune system above high tide
• Beach planform
• Curved features due to refraction
• Spits, barriers islands and beaches, tombolos,
  lagoons

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Figure 17.14 Source After Goudie 1995
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Figure 17.15
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Figure 17.16 Source After Bird, 2000
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Figure 17.17 Source After Waugh, 1995
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Barrier formation hypotheses

• 1. Emerged-transgressive model
• Offshore bars formed during last glacial low SL
  period
• Bars have develop vertically and accumulated
  sediment as sea levels have risen
• 2. Submerged-transgressive model
• Coastal dunes during lower sea level
• Become isolated from mainland upon submergence
• 3. Emerged-standstill model
• Barrier islands formed since post-glacial
  sea-level stabilised (4000 yrs ago)
• However many barrier island deposits are older
  than 4000 years ago

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Wave dominated coasts coral reefs

• High energy wave environments
• Delicate balance of erosion and biological
  construction
• Corals are animals with plant like properties
• Produce calcium carbonate build-up (thousand of
  yrs)
• Zonation of coral forms across the reef
• Forereef, reef flat, backreef
• Different types
• Barrier reefs, coral atolls (both with a lagoon),
  fringing reef
• 2 main settings
• (1) Continental shelf (2) Edges of volcanic
  islands (hot spots)
• Threats
• pollution, tourism, exploitation, climate change
• Can vertical reef growth keep pace with rising
  sea levels?

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Figure 17.19
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Tide dominated coasts estuaries

• Coastal embayments from which rivers flow
• Intertidal tidal currents shift inlet channels
• Receive sediment from sea and river
• Form when net sediment movement is landwards
  (opposite to deltas)
• Local sea level rise
• Sub-dived into spatial facets
• Upper estuary fluvially dominated
• Lower estuary tidally dominated
• Middle estuary well mixed
• Salt-fresh water mixing through diffusion and
  advection
• Stratified estuaries
• Partially-mixed estuaries
• Lateral salinity gradients
• Estuaries form becomes more similar over time

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River dominated coasts Deltas

• Form at mouth of sediment rich fluvial channels
• Deposition from the river
• River velocity reduction causes carrying capacity
  loss
• 3 main groups
• Cuspate
• Elongate
• Estuarine
• Delta morphology
• Delta plain, topset beds, foreset beds, delta
  fronts, bottomset beds, pro-delta

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Sea level change

• Short term changes - Tidal, meteorological
• Longer term changes - Isostacy, eustacy
• Evidence for sea level change
• Erosional and depositional landforms (e.g. wave
  cut notches, tidal flats)
• Biological indicators (e.g. fossils)
• Archaeological remains and historical records
• Coastal response to sea level depends on nature
  of the coastline
• Falling sea level
• cause abandonment of coastal features
• Rising sea levels
• Causes drowning of coastal features, migration of
  mobile features (e.g. beaches), increased erosion
• IPCC (1995) estimate by 2100 sea levels will rise
  by 50cm

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Coastal management

• Management problems
• Sediment fluxes from natural and human
  activities, resource exploitation, pollution,
  coastal hazards
• Coastal systems are very dynamic and have inputs,
  outputs and stores of sediment
• If input of sediment is reduced erosion results
• Hard engineering
• Sea walls - ignore natural beach profile changes
• Artificial beaches and land reclamation - reduce
  incoming wave energy
• Shore normal structures - groynes, jetties
• Soft engineering
• Problems encountered with hard engineering
• Restoring natural protection (e.g. damaged dunes)
• Managed retreat where defences are not
  economically viable
• Difficult decision making process science,
  politics, economics

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Figure 17.26 Source Photo courtesy of Joseph
Holden
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Figure 17.27a Source US Army Corps of Engineers
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Figure 17.27b Source US Army Corps of Engineers
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Figure 17.28a Source Photos courtesy of Joseph
Holden
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Figure 17.28b Source Photos courtesy of Joseph
Holden
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Summary

• Coastal areas are highly populated and constantly
  under threat from natural and human hazard
• Wave forms and processes of refraction,
  reflection and diffraction are fundamental
• Dynamic environment rapid sediment flux
• Wide range of coastal landforms depending whether
  wave, tidal or fluvially dominated
• Coastal management strategies vary from hard
  engineering structures to doing nothing site
  specific
• Many engineering structures fail or cause
  problems elsewhere if the full range of coastal
  processes is not taken into account