Coastal Processes. Wavelength and Amplitude Wave height (amplitude) = f(vel of wind, duration of...
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Transcript of Coastal Processes. Wavelength and Amplitude Wave height (amplitude) = f(vel of wind, duration of...
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