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Factors That Determine Wave Height (H) - Keynote pdf
Wave Interference - Keynote pdf
Wave Dispersion - Keynote pdf
Surf - Keynote pdf
Wave Refraction - Keynote pdf
Longshore Drift - Keynote pdf
Longshore Currents - Keynote pdf
Wave Reflection and Seawalls - Keynote pdf
Tsunamis - Keynote pdf

Wave-Shoreline Interaction

Factors That Determine Wave Height (H)

Large wind waves are generated in a sea - a storm region.
The height of the waves generated by the strong winds depends primarily on three factors:

  1. Wind speed
  2. Duration
  3. Fetch

Duration - the time that the wind blows in a single direction.
Fetch - the distance that the wind blows in a single direction.

Wave heights increase as all three factors increase:

For a given wind speed, once the maximum fetch and duration are achieved, the sea is fully-developed.

Fully developed sea - the theoretical maximum height attainable by ocean waves given wind of a specific strength, duration, and fetch. Longer exposure to wind will not increase the size of the waves.

Wave Interference

Oceanographers use the concept of wave interference to explain the chaotic nature of the surface of the ocean.
Deep water waves are energy propagating across the surface of the ocean.
Non-breaking waves will pass through each other, but, as they do, they interfere with one another to determine the elevation of the ocean surface.

Two types of interference:

  1. Constructive interference
  2. Destructive interference

Constructive Interference

Two types of constructive interference:

  1. Crest-to-crest
  2. Trough-to-trough

Crest-to-crest is when two wave crests come together and the elevation of the ocean surface is the sum of both wave heights above sea level.
An example is when a 1 m crest passes through a 2 m crest and the resulting ocean surface elevation in 3 m above sea level.

Trough-to-trough is when two wave troughs come together and the elevation of the ocean surface is the sum of both wave heights below sea level.
An example is when a 3 m trough passes through a 2 m trough and the resulting ocean surface elevation is 5 m below sea level.

In each instance, the ocean surface is higher or lower than either wave.

Destructive Interference

Destructive interference occurs when a wave crest coincides with a wave trough.
The crest and the trough totally or partially cancel each other.
An example is when a 2 m crest comes together with a 1 m trough and the resulting ocean surface elevation is 1 m above sea level.
In the instance of destructive interference, the ocean surface is neither as higher or as low as the largest wave, because it is partially canceled by the smaller wave.

The elevation of the an individual point on the surface of the ocean, at any given instant of time, is the sum of all of the waves are passing that point.
Because the elevation of the surface of the ocean is constantly changing as different waves pass through each other, the term mixed interference pattern is used.

Some rogue waves form by constructive interference when several wave crests coincide.

Wave Dispersion

The speed of a deep-water wave is a function of its wavelength - the longer the wavelength, the faster the wave speed.

In a sea, where large wind waves are generated, the gusting wind creates a mixture of waves of different wavelengths.
Once the waves leave the fetch of the storm and become free waves, the pack of waves undergoes wave dispersion.
In wave dispersion, the faster, longer wavelength waves outrun the slower, shorter wavelength waves.
In other words, waves sort themselves based on speed (or wavelength).

If a storm region is sufficiently far away, so that the waves undergo complete dispersion, the largest waves should arrive first and the size of the waves should decrease with time.

Wave trains are packs of swell traveling at the same speed.
Wave forms form at the pack of the wave train.
Wave forms travel at twice the speed of a wave train.
Wave forms move to the from of the wave train, whcre the wave forms disappear.

Surf

A deep-water wave is described as energy propagating across the surface of the ocean, with no significant transport of mass.
A deep-water wave, also, is a wave in water deeper than L/2 (a wave stirs the ocean to one half its wavelength).

After a wave enters water that is shallower than L/2, it interacts with the seafloor and the characteristics of the wave change:

Wave steepness is defined as wave height divided by wavelength (H/L).
As a wave approaches a coastline, its wave height increases as it wave length decreases, which results in increasing wave steepness.
Once H/L >1/7, a wave breaks.
After a wave breaks, forming surf, water is moving in the direction of wave propagation, which is generally towards the shore.

Breaking waves can be classified into three types of surf:

  1. Spilling breakers
  2. Plunging breakers
  3. Surging breakers

The type of surf that forms at a given location is a function of the gradient of the seafloor:

  1. Spilling breakers form when the seafloor gradient is low.
  2. Plunging breakers form when the seafloor gradient is steep.
  3. Surging breakers form when the seafloor gradient is very steep.

Spilling breakers form as the breaking waves release energy slowly and the crest of the wave slowly spills down the face of the wave.
Plunging breakers form as the breaking waves release energy fast - the top of the wave outruns the bottom of the wave and plunges over.
Surging breakers form as the breaking waves release energy all at once - the wave breaks on top of the beach.

Spilling breakers commonly form along Waikiki Beach because the seafloor is relatively flat.
Plunging breakers commonly form along North Shore beaches because the seafloor is moderately steep.
Surging breakers commonly form along Yokohama Beach because the seafloor is very steep.

Wave Refraction

Wave refraction is the bending of waves which causes waves to change direction.
Wave refraction occurs when different parts of a wave travel at different speeds.

Wave refraction generally occurs because waves initially approach a beach at an angle.
Waves refract when one part of a wave enters shallow water and slows, whereas the rest of the wave still is in deeper water and traveling faster.

Waves tend to refract parallel to shore.

Longshore Drift

Longshore drift is the transport of sand along a beach face.
Sand generally flows from one end of a beach to the other, which explains why many beaches are thinner at one end.

Remember that when waves break water is moving in the direction of wave propagation.

Waves that approach a beach at an angle run up the beach face at an angle.
The water moves sand up the beach face at an angle, too.
After the wave stops, gravity pulls the water and the sand straight back into the ocean.

This pattern is repeated with each subsequent wave.
The result is that sand moves in a zigzag pattern along the beach, moving in the direction opposite to the direction that the waves approach from.

Longshore Drift and Shoreline Structures

If a structure along the shoreline, such as a groin, impedes the transport of sand, sand will collect (deposition) on one side of the structure.
On the opposite side of the structure sand starvation (erosion) occurs.
The direction of the waves determines which side undergoes deposition or erosion.
Sand builds up on the side of the structure that waves approach from.

Longshore Current

Longshore current is a current that flows parallel to shore in the surf zone.
Once waves break, water is moving in the direction of wave propagation.
Commonly water cannot return to the ocean directly through the surf zone.
In this instance water piles up along the shoreline and begins to flow parallel to shore.

This flow intensifies as wave heights increase.

The longshore current flows parallel to the shoreline until it reaches a location where the water can flow back into the open ocean.
Often this offshore current forms in a confined area where large amounts of water are moved rapidly in a narrow region.
This current is called a rip current.

Wave Reflection and Seawalls

Beaches absorb the energy of waves.
In other words, this is where waves slow to a stop before gravity pulls the water back into the ocean.

Most human structures originally were constructed away from the coastline.
The builders were not concerned with the small, everyday waves.
They alloted space for large storm waves that can cross a beach and flood the nearshore region.

However, as our islands erode and sink, the beaches eventually will move inland.
Once a beach nears a structure, humans commonly erect seawalls to protect structures from waves.
The seawalls nearly always lead to the destruction of adjacent beaches.

If a wave strikes a relatively vertical structure along the shoreline, such as a seawall, it can reflect back into the ocean.

Seawalls tend to destroy beaches, which is why the construction of most seawalls is illegal.

Once the shoreline erodes to the point where the small, Trade Wind waves hit a seawall, generally sand will no longer deposit there.
The waves hit the seawall, reflect, and go back fast in to the ocean.
This fast moving wave eroded sand from the shore.

Tsunamis

Most tsunamis are seismic sea waves that are generated by earthquakes waves (seismic waves).
However, most earthquakes do not cause tsunamis.
To generate a tsunamis, the seafloor must fault (fracture and move), which moves seawater.
The movement of water by the faulting seafloor is the disturbing force.

The wavelength of tsunamis waves can be as long as 100 km.
However, in the open ocean, the wave heights of most tsunami waves are 1 m or less.
The wave period of tsunami waves average 12-15 minutes.
Tsunami wave speeds can be as high as 500-700 km/hr.

Most tsunamis waves are like a flood coming ashore.
Commonly a tsunami bore forms where the top of the wave breaks free, forming a wall of water separating two different levels of seawater.

Historically, large tsunamis strike Hawaii about every 25 years.
The last two large tsunamis hit Hawaii in 1946 and 1960, so Hawaii residents should be prepared for a tsunami in the near future.

 

 

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