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 l
– Deepwater wave
gT
C
, for D  0.5l
2
• If wavelength > depth
– Shallow water wave
C  gD, for D  0.05l
First (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
E
gH 2
8
• Wave power: rate at which work is done
– Shallow wavespeed (C) x energy
C  gD, for D  0.05l
– Mostly a function of H, amplitude (wave
height)
– Big waves do the most geomorphic work on
coasts
  EC
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 Currents
Two Types of Wave-Induced Currents:
Rip Current forms at low spots
Longshore
• Return flow from longshore currents
Wave Energy
Rip Current
Longshore
Current
Copyright © Rob Brander 2002
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
Littoral drift
Beach drift
Latin, litus
for “shore”
Longshore drift
• If wave angle is not normal
(90o), moves sand down
(parallel) the coast
• Water also moves parallel:
longshore current
Beach Drifting and Longshore
Currents
Littoral Drift
Littoral sediment flux=
Velocity, U x
Transport layer thickness, d x
Distance over which waves
influence 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, rivers
Sinks: suspension, submarine
canyons
Littoral sand cells (open)
http://walrus.wr.usgs.gov/outreach/mbay/mbay_map.gif
Rates of littoral drift
• dump truck
capacity =
10 cy
• Consider
ramifications
for coastal
engineering!
356
275
191
306
255
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
Wind
Storms
Waves
Currents
Tide
Range
Ridge
Runnel
Example
Runnel
Ridge
Bar
– Runnel
– Ridge
– Trough
– Bar
Trough
• Ocean Park,
WA quad
• Features
Beach Cusps
crescent-shaped scallops, parallel to shore
offshore horns bounding small bays
feedbacks between topography and fluid flow
Convergence of water by refraction =>
erosion of bays
Off-shore deposition in front of bay
Deceleration in front of horns
Flow “trips” on bar, accelerates near horn
Topography-wave-sediment transport loop
Spacing controlled by extent of swash
Bayhead beach
Sand
Rocks
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
“These capes are not deltas under present
conditions; however, at the beginning of a
glacial stage, the river gradients were
markedly increased by sea-level lowering
and deltas developed.
As the sea retreated, the deltas
formed farther seaward on the continental
shelf, resulting in the deposition of deltaic
ridges 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.
White, 1966
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, WA
Pacific Ocean
Puget Sound
Olympic Mountains
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
• Provincetown,
MA
• NOTE:
– Refraction
– Multiple
breakers
– Dunes
Cape Cod,MA.kmz
Spit
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