Waves, Beaches and Coastal Erosion

Tidal forces associated with Moon

1

Tidal forces associated with Sun and Moon

High tide staysaligned with moon but Earthrotates, yielding2 high and 2 lowtides each day.

Tides are cyclical floods

Tidal forces associated with Sun and Moon

Neap tides: Sun and Moon tides are out of phase

Spring tides: Two tides are in phase--High tide will be higher than usual

Tidal Ranges Are Highly Variable: Bay of Fundy--17 m!

Tidal Ranges influenced by local conditions and bathymetry

Bays tend to focus the water and create larger ranges

Organisms are stratified basedon tidal zone

2) Sea Level: What and When?

Mean sea level is the average height of the sea…

Relative to what?

When? High or low tide?

Measured with tide gauges

Sea Level: Changes through time

Eustatic Sea Level Changes

“Eustatic” refers to changes related to amount of water in the oceans

3) Tidal Flats: exposed mud at low tide

4) Waves and Sediment Transport

Wave Velocity-

Deep Water

Wave Energy

Doubling wave height quadruples energy:

Ew = 0.125rgH2Lwhere:

Ew = energy of waver = water density (g/cm3; close to 1)g = acceleration of gravity (9.8 m/sec2)H = wave height (m)L = wavelength (m)

Passive continental margin

Submarine canyons

Coastal plain

Continental shelf

Continental slope

Continental rise

Continentalcrust

Oceanic crust

Mantle

Deep-sea fan

Abyssal plain

Path ofwaterparticles

Longshorecurrent

Ripcurrent

Path ofsandparticles

5) Waves approaching the shore at an angle cause a longshore current.

Longshore drift results from movement of sand particles by swash and backwash.

Sand will move in this direction

The beach is a “river of sand”

6)Headlands receive

strong wave energy-leads toerosion

7) Rip Currents

7) Rip Currents

8) Sand Supply/Loss

8) Sand Supply/Loss

8) Sand Supply/Loss

Offshore sand bars migrate to beach during summer

Winter storms remove beach sand to deeper water

Hurricanes result in summer beach sand loss

8) Sand Supply/Loss

Engineered Beach Protection Structures

9) Erosion of Gently Sloping Coasts and Barrier Islands

Beach migrated 400 m in 60 years

Barrier Islands are Geologically Temporary

Chandeleur Islands

Before (above) and after (below)

Hurricane

Katrina

10) Sea Cliff Erosion

Headlands erode from wave actionCreates caves, arches, sea stacks

Sea Cave

Arches

Sea Stacks

Sea Stacks

Sea Stacks

Sea Cliff Erosion

Coastal Erosion