GEOGRAPHY WAVES PG3-5

Revision (03/09/26)

Main cause of wave formation:

Friction created as wind blows over the ocean surface

Three factors determining wave size and energy

1. wind speed

2. wind duration

3. fetch length (the distance over which the wind blows over the water surface)

Fetch:

The distance of open water which the wind blows over the surface

Potential energy in a wave

• (gravitational) potential energy: stored energy caused by an objects position/height.

• potential energy in a wave: energy resulting from the height of the water above the wave trough

Kintetic energy in a wave:

• Kinetic energy: object in movement possessed KE

• Kinetic energy in a wave: Energy produced by the motion of water particles within a wave

Wave crest

Highest point of a wave

Wave trough

lowest surface of a wave (between two wave crests)

Wave height (H)

Distance between wave trough and crest

Wavelength (λ)

Horizontal distance between two wave crests

Wave period (T)

The time in seconds for two wave crests to pass a fixed point

Wave frequency

Number of waves breaking on a shore per minute

Wave power formula:

P(kw/m) = H²(wave height [m]) x T(wave period [s])

Impact of doubling wave height on wave energy

Wave energy quadruples because wave height is squared in the power formula

Water particle motion in deep water

They move in circular orbits as the wave passes

(as they have more space for movement; as you go deeper down the orbits get smaller and smaller because the energy fades with depth; there’s enough space for them to fade before reaching the seabed hence the perfect circularity)

Effect of shallow water on water particle orbits

Seabed friction flattens circular orbits into squished elliptical (oval) shapes

(even at the top of shallow water since the lower orbits pressure the orbits above due to lack of space for movement hence cause the orbits above to squish even if they’re not close to the seabed, they’re just wider than the ones below due to less and less pressure)

Depth at which wave friction begins

When water depth falls below half of wavelength (Depth < 1/2 λ)

Condition required for wave to break

When water depth drops below [1.3 x wave height]

it means there’s more friction between sea floor and deepest orbiting water molecules slow the base down (they drag against seabed) since its more shallow now so orbits now reach seabase.

Swash

Surge of water rushing up TO the beach as the wave breaks

Backwash

Gravity pulls the water downhill at shore, and due to slope of the land the water is then sent back into the sea

Primary coastal role of constructive waves

Building up beaches by depositing sediment onto the shore

Swash vs Backwash balance in constructive waves:

Strong swash and weak backwash

Constructive wave frequency:

Low frequency (6-8 waves per minute)

Primary coastal role of destructive waves

Eroding coastline and removing sediment from the beach

Swash vs Backwash balance in destructive waves

Strong backwash and weak swash

Destructive wave frequency

High frequency (10-14 waves per minute)

Constructive wave physical features (shape) and why

• Low height, long wavelength

• This is because they form from gentle wind energy over long distances, causing the water percolate (soak) into the sand, which kills the backwash energy and allows swash to deposit material. (Usually during summer)

Destructive wave physical features (shape) and why

• High height, steep, short wavelength

• This is because they are generated by powerful local storms, The powerful backwash rushes rapidly down steep slopes which collides with and blocks the incoming swash which is why swash is weak. (Usually during winter)

How do beach percolation and beach slope impact swash and backwash energy?

• Percolation: Water soaking into porous sand weakens the backwash, allowing the swash to dominate and build up the beach.

• Steep Slope: Gravity accelerates the water, creating a rapid, high-energy backwash that drags material down into the sea and impedes incoming swash.