MSCI LAB WAVES

Crest

top of the wave

Trough

Bottom of the wave

Wavelength

distance between corresponding points on two successive wave crest

Wave height

Vertical distance between crest and trough

Wave period

Time for 1 wave (crest to crest) to pass a fixed point

Waves

Energy in motion

When does motion die out?

Motion dies out at ½ of the wavelength (wave base)

Deep water waves

• Orbital motion does not reach the seafloor

• Defined as water depth greater than half the wavelength

• D > L/2

Shallow water waves

• Waves affects the seafloor and orbit becomes ellipitcal

• Defined as water depth less than 1/20 of the wave length D < L/20

Celerity of deep water waves

• Depends on wavelength

• the longer the wavelength, the faster the wave travels

Celerity of shallow water waves

• Depends on water depth

• As water depth decreases, celerity decreases

Celerity formula

• C - L/T

• C = celerity

• L = wavelength

• T= Wave period

As waves approach shore

• Wavelength decreases and wave height increases

• waves slow down

• Bottom of orbital motion drags along seafloor

Tsuanmi

• Generated by seismic activity

• Very long wavelength so always shallow water wave

Wave refraction

• A wave bends (refraction) as it approaches shore

• waves approach shore at an angle, and then align almost parallel to shore

Why large waves affect beach

• cause erosion

• beach gets smaller with steeper slope to water

• Eroded sand piles up offshore as offshore bars

Why small waves affect beach

• Rebuild the beach (net deposition)

• Push sand from offshore bar back onto beach

• Beach gets bigger with shallower slope to the water

Wintertime beaches

• Generally larger waves

• erosion and steep slope

Summertime beaches

• Smaller waves (except hurricanes)

• Deposition and shallow slope

Longshore transport

• Occurs when wave hits the coast at an angle, (due to wind)

• Results in zig zag motion of sediment along coast with net movement down the beach (parallel)

Groins

• Built perpendicular to coast to stop longshore transport

• Pro- Keeps sediment between groins

• Cons- Causes sediment build up upstream and erosion downstream

• Can result in a groin field (multipule groins)

Jetties

• Long, narrow structures built at mouth of inlet

• Pros- Protects inlet from waves

• Cons- Cause sediment build up in inlets, requiring dredging. Causes deposition upstream, erosion downstream

Sea Walls

Walls built on beaches to prevent wave action on property

• Pros- Protect property from storm waves

• Cons- Higher erosion on seaward side (reflect wave energy) hard to maintain

Breakwaters

• Barriers built offshore, parallel to coast

• Pros- Reduce wave energy, protecting property/beachfront behind them

• Cons- Cause sediment build up behind them, require dredging. Redirect wave energy to another area, causing erosion