GEOL100
4-14-10
Shoreline processes

Shorelines:

Again we return to water as an agent of transport. Water behaves differently in large bodies than in streams.

Energy: Whereas streams derived their energy from gravity, water at shorelines largely derive it from the action of waves and tides.

Waves - Generated by winds blowing across the sea.

• Origin:
  • Sun heats atmosphere causing weather, movement of air.
  • Where air and water come together, energy of air motion is imparted to surface of water. Parcels of water pile up on one another to form waves
  • Waves receive their energy from the wind, so they are strictly a surface phenomenon.
  • Swells: As wave energy propagates outward, disorganized waves coalesce into long coherent waves.
    
  
• Fetch: A wave may grow cumulatively over time; however its life span is constrained by the size of the body of water across which it travels. The more distance it has to travel the larger it can become. Thus, bodies of water with long stretches parallel to prevailing wind generate large waves. This distance is called fetch. Best surfing waves, for instance, are in regions with long fetch like the Pacific coast of Australia and the northern Hawaiian Islands.
  Not much wave action in the Chesapeake.
  
• Wave motion: The wave form is what moves. Each individual water molecule simply moves in a circle as the wave form passes. The circles become smaller with increasing depth.

• Wave descriptors:
  • Wave length: The distance between two successive wave crests.
  • Amplitude (height): The vertical distance that a particle at the surface moves when a wave passes.
  • Period: The time taken for successive waves to pass a given point
  • Wave base: With increasing depth, the effects of surface waves are felt less and less. Their influence is not felt past a depth of half the wave length. This depth is called wave base.
  • Wave velocity: Is a function of wave length.
  
  
  C = L/T where

  C = velocity L = wave length T = wave period.

• Wave interactions with shoreline:
  

  In deep water, wave is unconstrained. As wave enters shallower water than wave base, its develops friction with sea floor. It becomes shorter and steeper. Additionally, its bottom "drags" causing it to spill forward or "break".

  At this point only, we get net transport of water in wave direction. Two types of motion:

  • Swash: movement up beachface
  • Backwash: movement down beachface into water

• Three common patterns of shoreline-wave interaction: wave refraction, rip currents, longshore currents.

  

  • Wave refraction: Recall, wave velocity varies with wave length, so when sea waves pass an obstacle above wave base that they can "feel" they are shortened.
    Indeed, it is rare that a wave would approach the shore in an absolutely perpendicular path. Because of refraction, as waves enter shallow water, they become shorter, hence slower. Thus waves "turn to face shore".
    
  • Longshore currents: Wave energy moves water onto beaches, but gravity moves it back. situation analogous to creep. Thus, with every wave that sweeps onto shore, water and sediment are moved slightly along the shore. The result is a longshore current.
    
  • Rip currents: Recall that breaking waves actually transport water up onto the shore. This water must return to its source, and generally follows the path of least resistance to do so. Any relatively deep area will have lower amplitude waves and afford a larger gap through which water may flow. Thus, water flowing off beaches tend to accumulate here, forming rapidly moving rip currents.
    

Tides:

• Basics:
  
  • Daily rise and fall in sea level caused by gravity of the Moon and Sun.
  • Interaction of gravity and centrifugal force causes equal bulges on both sides of the Earth. The bulges maintain the same position relative to the Moon (or Sun). Yields twice daily
    • Flood tide
    • Ebb tide
  
  
• Types of tide: The Lunar tide predominates. The solar tide is slightly less than half its magnitude. Thus, the tide we experience is a composite of the two.

  

  • Spring tides: when Moon and Sun align during full and new moons and solar and lunar tides are superimposed to reinforce one another positively.
    
  • Neap Tides: When Moon and Sun are at right angles (waxing and waning half moon). Lunar and solar tides interfere destructively.
  
  
• Tide - topography interactions:
  • Tide Range: In unrestricted areas like Hawaii, the difference between mean high and low tide is only 0.5 m. In embayments that restrict or focus water movement, tides can be higher. The Bay of Fundy has tide range of 16 m. (See Bay of Fundy at low and high tides.)
  • Tidal currents: In restricted areas, water flows in and out with tides.
  • Tidal surges: Combination of flood tide and storm surge
  • Tide flats: Regions that are:
    • exposed at low tide and submerged at high tide
    • in which tidal currents provide most of energy for transport.

Patterns of shoreline erosion:

• Wave cut platforms: In zone of maximum wave activity, hyrolic action, and abrasion conspire to accelerate erosion. Result is wave cut platform. This records the elevation of ancient shorelines as well as modern ones.
  
• Headlands and pocket beaches: Due to wave refraction, embayments in the shoreline tend to be the site of deposition of sediment - i.e. beaches, whereas erosion is most intense around headlands.
  
• Sea stacks: Erosional remnants of headlands completely surrounded by water.
  

Patterns of shoreline deposition:

Beaches: A deposit of unconsolidated sediment extending from low tide to a topographic break such as a line of dunes or cliffs.

• Requirements for beach formation:
  • loose sediment
  • Wave energy strong enough to remove clay and silt sized particles, sand moved by saltation, actually moves sand up onto shore, covering heavier particles
  • source of sediment - usually streams, sometimes cliffs onshore, sometimes from organisms offshore (atoll islands) gravel or sand
    • Middle Atlantic - weathering of rocks - quartz sand
    • Florida - CaCO₃ - sea shells break down into beautiful white beaches
    • Hawaii - black beaches from mechanical weathering of basalt
      
  
• Beach regions:
  
  
  • Foreshore: Between high and low tide lines.
    
  • Backshore: Between normal high tide and margin of beach. Characterized by berms, shelf-like storm deposits.
  
  
• Seasonal variation: Shoreline development represents the equilibrium of wave energy, sediment supply, and shoreline geometry. When one of these elements changes the others will re-equilibrate. This happens during seasonal change. We generally see characteristic change between summer and winter beach profiles:
  • Summer: Large berms, small offshore sand bars. Fine sediments in beach deposits.
    
  • Winter: Small berms, large offshore sand bars. Coarse sediments in beach deposits. Any large storm can produce a "winter beach" profile.

Other shoreline depositional features:

• Tombolo. Sometimes, a sea stack or island may form a wave shadow. When that happens, sediment can accumulate behind it, eventually connecting it to the mainland.
  
• Spits: Imagine that a beach ends at an embayment, and that a longshore current is carring sediment into the embayment. Voila! A spit! A projection of a beach into a body of water.

  
  

• Baymouth bar: If the spit continues to develop, it may completely enclose the embayment, forming a baymouth bar.
  
• Barrier islands: Of course, a baymouth bar does form a barrier between two bodies of water. If this is broken, tidal inlets or passes can form. The separated segments become barrier islands.
  

Beaches in the rock record:

Beach sediment can be cemented by precipitates from sea-water to form beachrock. Characteried by:

• low angle bedding
• Evidence of wave action (symmetrical ripples)
• Incorporation of marine skeletal remains i.e. seashells.

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Key concepts and vocabulary:

• Energy sources
  • Waves
  • Tides
• Wave facts
  • Swells
  • Fetch
  • Wave length
  • Amplitude
  • Period
  • Wave base
  • Breakers
  • Swash
  • Backwash
  • Wave refraction
  • Longshore current
  • Rip current
• Tide facts
  • Tidal bulge
  • High tide
  • Low tide
  • Ebb tide
  • Flood tide
  • Spring tide
  • Neap tide
  • Tide range
  • Tide currents
  • Tide surges
  • Tidal flats
• Shoreline erosion
  • Wave cut platforms
  • Headland
  • Pocket beach
  • Sea stack
• Shoreline deposition
  • Beach
  • Sediment sources
  • Foreshore
  • Backshore
  • Offshore
  • Sand bar
  • Berm
  • Summer beach characteristics
  • Winter beach characteristics
  • Tombolo
  • Spit
  • Baymouth bar
  • Tidal inlet
  • Barrier island
  • Beachrock