Coastal Processes

From GeoClasses

Contents 1 Natural Processes along the Coast 2 Waves 2.1 Parts and behavior of waves: 2.1.1 circular motion of waves at sea 2.2 How much energy do waves have? 2.3 Waves at the shore: 2.3.1 breaking waves have different slopes depending on the slope of the beach 3 Anatomy of a beach: 4 Coastal Erosion 4.1 1. beach erosion 4.2 2. seacliff erosion 5 Engineering "solutions" to coastal erosion

Natural Processes along the Coast

Waves

• formed by wind pushing water on the surface of the sea
• size of waves determined by:
  • 1. speed of wind (faster wind = higher waves)
  • 2. duration of wind (longer duration = larger waves)
  • 3. fetch = distance wind blows continuously over water (longer fetch = larger waves)
• wind is created by storms, even far out to sea, waves eventually reach coastline

Parts and behavior of waves:

• wave height (peak to trough)
• wavelength (peak to peak)
• wave period (time for wave to pass)

circular motion of waves at sea

• but when waves get close to shore - at depth ~1/2 wavelength they "feel bottom" and motion becomes elliptical
• makes forward and backward motion - breaking waves (fig. 9.2)
  • wave period stays same but wavelength and velocity decrease and wave height increases
  • waves change shape to peaks and velocity of peak is greater than at trough so wave falls

waves coming into shore: notice the shape changes as the closer waves "feel bottom"

wave height increases just before breaking

wave breaking

How much energy do waves have?

• energy expended on 250 mile stretch of coastline by 1 meter high waves = energy produced by average sized nuclear power plant in the same amount of time

Waves at the shore:

• as they approach, they bend (called refracting), to become parallel to coastline (fig. 9.3 of point)

• • creates convergence of waves at a point = greater erosion
  • creates divergence in bays = deposition
  • works to straighten out coastlines

breaking waves have different slopes depending on the slope of the beach

• if steep beach, makes plunging breaker (for surfing)
  • creates more erosion

• if shallow beach, makes spilling breakers (more gentle)
  • creates deposition

Anatomy of a beach:

• beach itself is made of loose material (sand, gravel, shell pieces, volcanic sand, quartz sand)

• berm - flat terrace created by deposits from last bit of wave energy (where people sunbathe)
• beach face - part that slopes toward water
• swash zone - wet part that gets wet from waves
• surf zone - turbulent water after wave breaks
• breaker zone - area where waves break
• longshore trough and bar - elongated depression and adjacent ridge of sand created by wave action (underwater)
• longshore drift - created by waves coming into shore at an angle
  • creates littoral transport (zigzag transport) which moves ~ 300,000 m3 of sand/year
• rip currents -
  • water in waves pushes to shore everywhere along coast, but does not go back out everywhere the same - is concentrated in zones (called rip currents or riptide or undertow)
  • up to 4 mph - cannot swim against it - must swim parallel to shore until out of it then swim back in

berm showing footprints and beach face of wet sand	swash zone - the end of the surf zone is visible at the top of the image	breaker zone, surf zone and swash zone	breaker zone, surf zone and swash zone on a rocky beach
barrier islands, an extreme case of longshore bar	diagram explaining rip currents

Coastal Erosion

• more continuous and predictable than other hazards (earthquakes etc)
• increased lately by global rise in sealevel (2-3 mm/yr due to thermal expansion of top water)

1. beach erosion

• supply of sand is from rivers carrying products of weathering
• humans have decreased that supply by building dams that block sediment flow
• beach erosion occurs naturally during storms (stronger storms like hurricanes erode more)

2. seacliff erosion

• occurs by wave action but also land erosion from running water and landslides

• coastal erosion is seasonal -
  • summer = long gentle spilling breakers deposit berms
  • winter = storms and plunging breakers erode more
• urbanization near coast increases runoff and adds weight to top of cliff/slope

Engineering "solutions" to coastal erosion

• 1. seawalls
  • built parallel to coastline to help slow down erosion
  • usually vertical and reflect waves away from shore so that beach erodes even more

• 2. groins
  • linear structures built perpendicular to coastline
  • usually need them in groups because deposition occurs updrift but greater erosion occurs downdrift (fig. 9.11)

• 3. beach nourishment
  • artificially depositing truck loads of sand on beach from somewhere else

• 4. breakwaters
  • structures built out away from shore to create a protected harbor for boat moorings
  • blocks natural transport of sediment, acts as sand trap and accumulates sand updrift that may eventually block entrance of harbor (dredging needed)

• 5. jetties (fig. 9.12)
  • pair of structures at mouth of river that stabilize channel for shipping
  • also block natural transport of sediment
  • beach on updrift side widens, beach downdrift erodes away

Hurricanes? Hurricane Katrina