OMB UNIT 4 WAVES+

Tides: 

Waves with extremely long wavelengths and periods caused by the gravitational attraction of the moon and sun for ocean water

Tides are created by the gravitational pull of the moon

Semidiurnal Tides

• High and Low tides are roughly six hours apart

• Each High tide is the same height 

• Each Low tide is the same 

Diurnal Tides

• High and Low tides are roughly 12 hours apart

Mixed Tides

• Varying intervals separate high and Low tides

• Two High tides of different heights

• Two Low tides of different heights

Spring Tides and Neap Tides

• As the moon and sun change relative positions over the course of a month, the tidal heights change

• Spring Tides:

  • Gravitational attractions of the moon and sun are in line and increase the range of tidal heights

• Neap Tides: 

  • Gravitational attractions of the moon and sun are at right angles to one another and decrease the range of tidal heights

Tsunamis

• Incorrect term “Tidal Wave”

• Caused by seismic disturbances of the sea floor

  • Plates rubbing, landslides into the ocean, landslides underwater, volcanic activity, meteorite impacts, etc.

• In the open ocean

  • Wavelength of 100-200km

  • Height of <2m

  • Speed of 450 mph (720km/hr)

  • Not dangerous to ships 

• As tsunamis approach the shore

  • Wavelength decreases

  • Height increases

  • Speed slows

  • Becomes much steeper

    • 10s of meters high as it moves inland

• Some coasts have protective sea walls/ warning systems

  • Example being Japan

• If you feel an earthquake on the coastline you should make your way to higher ground 

• The trough arrives before a crest, so water will be sucked into the sea a great distance

  • If you see this it means a tsunami is coming

Waves: 

• Waves represent energy that passes through open ocean water (essentially no net movement of water)

• In deeper water, water molecules move in a circle (circles become smaller as you go deeper below the surface)

Wave Creation

• Wind

  • Wave size/height increases when wind speed, wind duration and fetch(distance that wind blows over water) increases

• Impacts on ocean water due to seafloor movements (ex. earthquakes tsunamis, etc)

• Gravitational attraction of moon and sun (tides)

Basic Wave Terminology

• Crest = Point of highest elevation

• Trough = Point of lowest elevation

• Wavelength (L) =  horizontal distance between successive crests or troughs

• Wave height (H) = vertical distance between crest and trough

• Wave Amplitude (A) = Vertical distance between crest or trough and the mean water level (-A=H/2)

• Wave Steepness = ratio of wave height to wavelength (H/L)

• Wave Period (T) = time (in seconds) that elapses between the arrival of two successive crests or troughs

• Wave Frequency (f) = number of wave crests or troughs that pass a given point in one second (can be a fraction) (-T=1/f)

• Wave Speed (C)= speed of the crest or trough as it travels through the water 

Wave Interference

• Waves can travel through one another

• The waves at any given location are the result of constructive and destructive interference of waves with one another

• Crests or troughs of waves can combine to form higher crests or lower troughs

Wave Breaks in ocean

• When waves break in the open ocean, it's called a whitecap (causes net movement in open ocean)

• Occurs when height to length ratio reaches 1/7

  • Waves become unstable and tumble from the crest down to the water’s face

Waves and Shorelines

• As waves enter water that is less than ½ wavelength deep, they interact with the bottom

  • Wave speed decreases, wave period doesn’t change

  • Wave height increases rapidly in shallower depths

  • Wavelength decreases

  • Wave steepness (H/L) increases, wave eventually breaks

Wave refraction

• When waves enter shallow water their speed decreases

• This can result in wave fronts being bent (refracted) and travelling in different directions

• This can result in wave energy at certain locations being reduced or concentrated (Know how wave refraction works with the headlands)

Wave Breaks

• Waves typically break where the water depth is 1.2-1.3 times the wave height

• Wave increases in steepness until it becomes unstable

• Net forward movement of water occurs

Longshore Currents

• Water pushed by breakers arriving at an angle to the shoreline causes the formation of a current in the surf zone

• The current runs parallel to the shoreline

• Transports lots of sediments

Rip Currents

• Water deposited on beach by breakers has to return to the ocean

• Rip currents run perpendicular to the shoreline and carry water seaward through the surf zone

• Fed by longshore currents

• To escape a rip swim parallel to the shore

Erosion

• Erosion is highest where wave energy is highest

• Sediments are transported where wave action is high and currents are low

Longshore drift of sediment

• Sand is transported parallel to the shoreline by longshore current and longshore movement of sand on beach face

Longshore Drift Problems

• Erosion and transport of sand from some ares exposes property to damage from breakers and the eventual advance of the ocean

• Deposition of sand can block harbor entrances

Effects of erosion, transport and deposition on uneven coast of headlands separated by bays

• Refraction of waves causes wave energy to be concentrated on headlands and dissipated in bays

• Material is eroded from headlands and transported to bays where it is deposited

• Eroded headland becomes sea cliffs, sea caves, stacks and arches

• Coastal straightening eventually occurs

Seasonal Changes - Sandy Beach

• Summer

  • Waves smaller, breaker moving up the beach has enough speed to transport sand grains upwards, but returning water lacks speed to transport them back

• Winter

  • Waves larger, both breaker moving up the beach and returning water have enough speed to transport sand grains

  • Results in sand being removed from beach and deposited in offshore sand sand bar