Waves and Tides Flashcards
What causes tides
Gravitational pull from the earths moon and the sun pull the ocean surface closer
Neap Tide
Twice a month
Sun and Moon are at right angles to the earth
A tide where the difference between high and low tide is the least
Strong spring tide
Occur few times a year - Full moon
Gravitational pull of the sun and moon upon the earth is strongest when moon is closest to the earth in its monthly orbit producing extreme high tides and erosion on beaches.
Weak spring tide
Moon is furtherest from the earth in its monthly orbit making a lower spring tide.
High and low tides
Two high tides and two low tides occur each day as part of the normal gravitational pull on the earth.
What makes waves?
Winds blow over fetch zones at sea and form small ripples which join together to form waves in a rising sea which then move out of the fetch zone and become ocean swells
What factors determine size of waves?
Size of fetch
Wind speed
Wind duration
Depth of water
What happens to water particles during a wave?
Waves transport energy not water.
As a wave crest passes, the water particles move in circular paths (orbital motion)
Fetch
Fetch, area of ocean or lake surface over which the wind blows in an essentially constant direction, thus generating waves.
Wavelength and Waveheight
Wave length (distance from crest to crest) Wave height (vertical distance between the top of the crest and the bottom of the trough)
Wave Period
The wave period is the time for two consecutive crests to pass a fixed point. The wave speed, C, can be calculated by dividing the wavelength by the wave period (C=L/T) since a wave travels one wave length each wave period
Orbit field
As the energy of a wave passes through water, the energy sets water particles into orbital motion
Tsunami
Produced by large, sudden movements in the Earths crust (earthquakes, volcanoes). Produce high speed waves that travel in all directions from the source.
Swell
Formed in fetch zones and transported through the worlds oceans without breaking. Rolling series of crests and troughs and can reach large heights and long wavelengths.
Breakers
Occur in a huge number of different sizes and shapes depending on the size of the swell, the position of the tide, the slope and structure of the seabed and wind direction.
Backwash
The motion after a wave breaks causing receding waves. The water runs back down the beach
Plunging wave
Throw from the top where the bottom rises sharply.
A plunging wave occurs when the ocean floor is steep or has sudden depth changes, such as from a reef or sandbar. The crest of the wave becomes much steeper than a spilling wave, becomes vertical, then curls over and drops onto the trough of the wave, releasing most of its energy at once in a relatively violent impact.
Surging wave
Break from the full face as deep water meets a cliff or rock.
Surging waves are produced when long period swells arrive at coastlines with steep beach profiles. The base of the wave moves fast and does not allow the crest to develop. As a result, the wave almost doesn’t break, and there is little whitewater.
Spilling wave
Crumble from the top where the bottom slopes up gently to the shore.
Spilling waves are waves that are produced when the ocean floor has a gentle slope. As the wave approaches the shore, it slowly releases energy, and the crest gradually spills forward down its face until it is all whitewater. These waves take more time to break than any other wave.
Swash
Turbulent water that washes up on beach after a wave has broken. Swash action can move beach materials up and down the beach.
Construction waves
Low energy, gentle slope, high water absorption
High energy in swash - accretes sand onto the beach
Accreting beach - Swash is stronger than the backwash
Destruction waves
Storm conditions create waves where energy is high and waves cause backwash that erodes sand off the beach.
Eroding beach - The backwash is stronger than the swash.
Erosion
Removal of sediment (sand/rock) from the beach caused by destruction waves.
Accretion
Accumulation of sediment (sand) on the beach caused by construction waves. Causes the beach to become wider.
Microridge
Waves leave ripple marks of sand deposits on the beach known as microridges. Visible on accreting beaches
3 types of waves
Swell
Breakers
Tsunami
Wave Calculations
Wave period (Time it takes a wave crest to travel one wavelength = secs ) Wave speed (Distance between crests = distance per second) Wave steepness (Wave height / wavelength)
Speed of a wave depends on
Gravity, wavelength, water depth
Waves with longer wavelength move faster than waves with shorter wavelength in deep water.
Waves travel slower in shallower water regardless of wave length.
As a wave moves from deep water to shallow water the speed decrease as the orbital field interacts with the sea floor
Ocean Currents
Ocean currents circle the globe, transferring heat, nutrients, and life within the water column and across ocean basins.
How are ocean currents driven
Temperature
Wind
Gravity
Wave formation processes
Consider: Fetch Wave height and type Water depth Wave celerity (speed at which a wave advances or propagates)
What happens to waves as they enter shallow water?
Speed decreases, wavelength decreases, height increases. The orbits of water molecules flatten because of the interference of the ocean bottom
The swell becomes higher and steeper and forms a crest wave shape that then breaks
What causes a wave to break?
Usually shallow-water waves begin to break when the ratio of wave height to wavelength is 1 to 7 (H/L = 1/7), when the wave’s crest peak is steep (less than 120˚), or when the wave height is three-fourths of the water depth (H = > 3/4 D).
Bar
Elongated sand body created by tidal currents or by waves. Can often be seen above water level at low tide.
Rip
Current that moves away from the shore bringing surplus water carried over breaker bars back into deeper water. Result of interaction between breaking waves and the nearshore bathymetry
How does water depth affect wave height
As waves enter shallow water, the orbit field causes interaction with the bottom and alters the waves and they decrease speed, wavelength shortens and wave height increases causing them to break.
How does wind speed affect waves
If wind speed is slow only small waves result, regardless of wind duration or fetch.
What affects wave height
Wind speed (stronger = bigger waves) Wind duration (longer = bigger waves) Size of fetch (larger = bigger waves) Depth of water/roughness of sea bed Direction and speed of tide
Shoaling
As waves travel from deep to shallow water, their shape alters (wave height increases, speed decreases, and length decreases as wave orbits become asymmetrical).
Wave Celerity
Speed of propagation of the wave crest or trough
In deep water the wave celerity may be calculated by the equation: c = (gλ/2π) 2 = 1.25√λ, where λ is the wavelength in metres and g is the acceleration due to gravity (9.81 m/s). The speed of shallow-water waves may be calculated by the equation: c = (gd) 1/2 = 3.13√d, where d is the depth of water in metres.
Visible signs of a rip
Waves not breaking
Swash reaching further up the beach
FORMULA Wave Period
= wavelength/speed(velocity) OR
= 1/frequency OR
= time/no. of waves during time
unit of measure = seconds
FORMULA Wave Speed
= wavelength x frequency
= wavelength / wave period
unit of measure = m/s (metres per second)
FORMULA Wave Steepness
= wave height / wave length
unit of measure = no units just a fraction
