4.6 WAVES Flashcards
What are the characteristics of wave motion?
- Waves transfer a disturbance to one part of a material to another
- The disturbance is propagated without any substantial overall motion on the material itself
- The disturbance is propagated without any significant distortion of the wave motion
- The disturbance appears to be propagated with constant speed
What are the 2 types of waves here?
- Regular (monochromatic)
- Irregular (random)
What is the period of a wave?
The time interval between 2 successive peaks (or troughs) passing a fixed point, in seconds.
What is the frequency of a wave?
the number of peaks (or troughs) which pass a fixed point per second (1/T)
What are the statistical techniques for describing the wave conditions of irregular waves?
- Significant wave height (Hs)- average wave height of 1/3 of the highest waves in the record. Approximately corresponds to visual estimates of wave heights.
- Mean wave period (Tz) - mean period of all waves in the wave record (referred to as the zero-crossing wave period)
What is the process of wind-generated waves?
- wind blows over ocean surface: frictional stress between air and ocean surface -> transfer of energy
- this deforms ocean surface into small, rounded waves with short wavelengths -> capillary waves
- surface tension = dominant restoring force trying to move back to smooth ocean surface
- as capillary waves increase, sea surface gets ROUGHER in character, catches more wind - transferring more energy -> GRAVITY WAVES develop
- because they reach greater height, gravity replaces surface tension as they dominant restoring force
What are the factors increasing the amount of energy that waves obtain?
1- Wind speed
2- Duration of time that wind blows in one direction
3- Fetch; distance over which wind blows in one direction
What is a fully developed sea?
For a given wind speed, the maximum duration and fetch beyond which waves cannot grow.
When do whitecaps develop?
When steepness reaches a critical value of 1/7
What is fetch?
The distance of open water that the wind blows across before reaching the coast.
- longer fetch = bigger waves = more energy they have
What is the process of swell waves?
- Over a storm driven area, wind waves propagate energy away from storm
- Longer waves travel faster than short waves (short waves dissipated in storm) and reach the coast first
- Chaotic waves are sorted into regular waves (wave dispersion = sorting of waves by their wavelengths; wave groups = swell wave train)
- Swell moves with little energy loss over large stretches of ocean (100-10,000km)
What are deep water waves?
- Water depth is greater than half the wavelength and waves aren’t affected by the ocean floor.
- D > L/2
What are transitional waves?
- Water depth is smaller than half the wavelength, but greater than 1/20 of it
- C & L decrease, H increases. Rounded waves form peaks
- L/20 < D < L/2
What are shallow water waves?
- Water depth is less than 1/20 of wavelength. Waves are affected by ocean floor
- D < L/20
- Shallow water wave orbitals become smaller and flatter with depth, until they’re essentially flat at the bottom
What is the general equation for celerity (wave speed)?
L/T or k/f
where k is wave number (2 pi/L)
and f is wave frequency (2 pi/T)
What is the equation for celerity of deep water waves?
C = gD / 2pi C = square root (gL/2pi)
because g = 9.81 and 2pi ~= 6.28 …
C0 = 1.56T
L0 = 1.56^2
deep water waves are a function only of wave period
What is the equation for shallow water waves?
C = square root (gD)
i.e. celerity is only dependent on water depth in shallow waters, bc it dictates how much friction is being experienced.
What is diffraction?
- Energy transferred along wave CREST, rather than in the direction of wave propagation
- Occurs when wave encounters a feature e.g. island, breakwater/groundwater reef
- A WAVE SHADOW ZONE is created behind obstacle, but diffraction causes the energy to spread behind it
What is reflection in waves?
- Under certain conditions i.e. presence of wall/breakwater, waves don’t break in shallow water -> instead reflect on shore
- Interaction of reflected and incoming waves results in formation of STANDING WAVES
- Water particles move horizontally & vertically
What is wave shoaling & breaking?
Wave shoaling = effect by which waves entering shallow water experience a change in wave height
- As wave continues to shoal, it reaches a point where the water depth becomes too shallow for stable waves to exist - WAVE BREAKS
^ occurs when the horizontal velocities of the water particles in the wave crest exceed the velocity of the wave form - Breaking wave height Hb is a function of the local water depth hb at which waves break
Hb = 0.78 hb
What are spilling waves?
Large wave height relative to wavelength - shallow beach slope
What are plunging waves?
Shallow to intermediate beach slope; ideal for surfers (tubers)
What are collapsing waves?
Front faces collapses - intermediate to steep beach slope
What are surging waves?
Wave slides up beach without breaking - steep beach slope
