Coastal Processes

Wavelength and Amplitude

• Wave height (amplitude) = f(vel of wind, duration of wind, and fetch)

• Wave speed (celerity) = f(wavelength and water depth)– Period = 1/frequency

• If depth >> wavelength – Deepwater wave

• If wavelength > depth– Shallow water wave

5.0for ,2

DgT

C

05.0for , DgDCFirst (storm) waves to hit shore are the fastest!e.g., c=30m/s, travel 2400km in one day!

Wave Energy

• Energy per unit length of wave– KE + PE

• Wave power: rate at which work is done– Shallow wavespeed (C) x energy

– Mostly a function of H, amplitude (wave height)

– Big waves do the most geomorphic work on coasts

8

2gHE

EC

05.0for , DgDC

Shallow waves moving onshore

• Shoaling: waves interact with bottom topography

• Edge of wave closest to shore encounters shallower water (vel = f(depth))– Slows down

• Wave front and wave rays bend

• Wave refracts, impacting shore at obtuse angle

Breeze Point (Yellowstone Lk.)

• Note refracted wave crests

3. Nearshore CurrentsTwo Types of Wave-Induced Currents:

Rip CurrentLongshore

• Return flow from longshore currents

Copyright © Rob Brander 2002

Wave EnergyLongshore

CurrentRip Current

forms at low spots

Rip Current

• Forms at low spot or break in sand bars

• Water looks smoother (or choppier) in rip current zone

• Zone extends from shoreline, through surf zone, past breaking waves

• Flows parallel and normal to shoreline• Shape topography on beach and nearshore

zones• Move sediment (apart from waves and tides)• On-beach (“beach drift”) and off-shore

(“longshore drift”) movement is huge– X00,000 m3/year

Beach drift

Longshore drift

Littoral drift

• If wave angle is not normal (90o), moves sand down (parallel) the coast• Water also moves parallel: longshore current

Latin, litusfor “shore”

Beach Drifting and Longshore Currents

Littoral DriftLittoral sediment flux=Velocity, U xTransport layer thickness, xDistance over which wavesinfluence bed, L

Drift rate = f(wave angle,and wave height)

Littoral Cell

• “A self-contained unit of coastline within which sand sources and sinks are contained” (Anderson, 2008)

• At steady state, volume of sand in each cell is constant– Inputs = outputs

Sources: cliff retreat, riversSinks: suspension, submarinecanyons

Littoral sand cells (open)

http://walrus.wr.usgs.gov/outreach/mbay/mbay_map.gif

• dump truck capacity = 10 cy

• Consider ramifications for coastal engineering!

356

275

191

306

255

Rates of littoral drift

Coastal Landforms

Types of Coastlines

• Depositional

• Erosional

Seasonal beach morphology

• Late summer

• Winter storms

• Early spring

• Early summer

• Late summer

Seasonal Beaches

• Example: Australia (sand stored off-shore)

Beach Morphology

Ridge

Runnel

CurrentsWavesStormsWind

TideRange

Example

• Ocean Park, WA quad

• Features– Runnel– Ridge– Trough– Bar

Run

nel

Rid

ge

Tro

ugh

Bar

Beach Cuspscrescent-shaped scallops, parallel to shore

offshore horns bounding small bays

feedbacks between topography and fluid flow

Convergence of water by refraction => erosion of baysOff-shore deposition in front of bayDeceleration in front of hornsFlow “trips” on bar, accelerates near hornTopography-wave-sediment transport loopSpacing controlled by extent of swash

Bayhead beach

Rocks

Sand

Baymouth bar

Capes and Spits• Created by longshore sediment

transport– = f(angle of wave approach)– 0o incidence angle: no onshore

momentum– 90o: no momentum in alongshore

direction– Max. transport at ~45o

– <45o: enhances transport => erosion

– >45o: decreases transport => deposition

White, 1966

“These capes are not deltas under presentconditions; however, at the beginning of aglacial stage, the river gradients were markedly increased by sea-level lowering and deltas developed.

As the sea retreated, the deltasformed farther seaward on the continentalshelf, resulting in the deposition of deltaicridges of sediment perpendicular to the coast.

During the subsequent submergence which accompanied glacial melting, the deltas became the loci of barrier islands and prominent capes.

Submergence was accompanied by erosion and retreat of the barrier capes resulting in the present capes, shoals, and embayments.”

Depositional Coastal Landforms

Spits and Bars: Dungeness, WAOlympic Mountains

Pug

et S

ound

Pacific O

cean

Schwartz et al., 1987

Dungeness Spit

• Primary driver: littoral drift powered by long fetch and large sediment source

• Modified by – tidal ebb/flow– river delta deposition– near shore current

Spit• Provincetown,

MA• NOTE:

– Refraction– Multiple

breakers– Dunes

N

Cape Cod,MA.kmz

Geologic map of Cape Cod (generalized from detailed mapping by K. F. Mather, R. P. Goldthwait, L. R. Theismeyer, J. H. Hartshorn, Carl Koteff, and R. N. Oldale).

Tombolo (wave shadow zone)

Copyright © Ann Dittmer 2002

Incipient tombolo