Coastal Processes and Landforms
Erosional and depositional landforms of coastal areas are
the result of the action of ocean waves.
Erosional Landforms
Sea Cliffs
Wave-cut Notches
Caves
Sea stacks
Sea arches
Depositional landforms
Beaches
Barrier Spit
Baymouth Bar
Lagoon
Tombolo
Wave Properties
The energy source for both coastal erosion and sediment
transport are the ocean waves generated by the frictional
effect of the winds incident on the ocean surface
(1) Kinetic Energy:
The motion of the water within the wave.
(2) Potential Energy:
Due to the position of water above the wave trough.
Wave energy increases with wind speed and fetch
Wavelength
Distance from one wave crest to the next
Wave height
The distance between trough and crest
Wave period
The time taken for two crests to pass a given point (remains
almost constant)
=V*P
The wavelength, , is the product of its velocity and period.
Wave motion
(a) Ocean depth > ½ the wavelength
- waves not affected by ocean floor
(b) Ocean depth < ½ the wavelength
- wave height increases and wavelength decreases
The wave becomes more peaked
“Breakers” form
Breaking of waves: conversion
of potential to kinetic energy
Work done on the shoreline
Wave Refraction
Straight shoreline
- drag exerted by the ocean floor causes waves to break
parallel with the shoreline.
The direction of travel of a wave varies as it approaches an
indented coast.
Crests approaching the headlands experience the drag of
the ocean floor first, which causes:
1. Increase in wave height
2. Decrease in wavelength
3. Decrease in velocity
The crest of the wave approaching the bay continues
without change until it gets closer to shore, bending the
wave somewhat toward the overall shape of the shoreline
*** Headlands are sites of intense erosion
*** Embayments are often sites of deposition
(protected from currents, gradual transfer of energy)
Mechanical and chemical processes affect rocky
shorelines leading to a number of erosional and
depositional landforms. Such landforms are affected by:
Geology, nature of wave action, tidal ranges, and
sea level change
Transport of Sediments by Wave Action
Rock particles are eroded from one area and deposited
elsewhere. Wave refraction affects this process.
Beach Drift:
Swash and backwash rarely occur in exactly opposite
directions
Upward movement occurs at some oblique angle
Backward movement occurs at right angles to the beach.
This creates lateral movement of particles (beach drift)
Rip Currents:
Rip currents form when
waves are pushed over
sandbars.
The weight of excess water
near the shore can ‘rip’ an
opening in the sandbar,
causing water to rush seaward.
Source: NOAA web site
Longshore Currents:
Backwash and rip currents often cannot remove enough
water to compensate for the buildup of water in the shore
zone. A "longshore current" results, which parallels the
shore in the overall direction of the breaking waves. The
longshore current moves sediment-laden water parallel
to the shoreline (longshore drift - together with beach drift,
we have littoral drift).
Tides and Tidal Currents:
Tides are produced by the gravitational pull of both the
Moon and the Sun on the Earth, which stretches solid and
liquid surfaces (as well as the atmosphere)
Tides are strongest when the sun and moon are wellaligned (spring tides)
The Earth’s surface rotates into and out of the moon’s tidal
bulges twice daily (Moon’s effect is twice that of the Sun)
Incoming and outgoing tides produce currents in opposite
directions, but the current in one direction is sometimes
stronger than in the other, resulting in sediment transport
Spring Tide
Tides enhanced
during full Moon
and new Moon
Sun-Moon-Earth
closely aligned
Neap Tide
Tidal effects of
Moon and Sun
not additive
Influence of Perigee, Apogee, Perihelion and
Aphelion on the Earth’s Tides
Stronger for perigee and perihelion
Erosional Coastal Landforms
Along rugged, high-relief, tectonically-active
coastlines
Sea cliffs
A tall, steep rock face,
formed by the undercutting
action of the sea
Wave-cut notches
A rock recess at the foot of a sea cliff where the energy
of waves is concentrated
Sea Caves
Caves form in more erosive sediment when the rock does not
fully collapse in a deeply-notched environment
Sea Arches and Sea Stacks
Sea Arches form when waves erode from two sides through
a thin headland, creating an arch-shaped hole beneath part
of the headland
Sea stacks, steep
pillars of rock
not far offshore,
occur when
sea arches
eventually collapse
http://www.geog.ouc.bc.ca/physgeog/contents/11m.html
Wave-cut platform
Horizontal benches in the tidal zone extending from the
sea cliff out into the sea
If the sea level relative to the land changes over time
(becoming lower with respect to the land), multiple wave
cut platforms (terraces) result
1
2
3
4
5
6
http://www.rgs.edu.sg/events/geotrip/cliff.html
Depositional Coastal Landforms
Located along land of gentle relief
Beaches
Beach Formation Process
1.
Erosion of bottom of steep slope by waves causes instability
2.
Upper portion of cliff collapses
3.
Waves crash against reworked material. Deposition of finer
material leads to offshore terrace while waves continue to
erode and steepen cliff.
4.
Offshore terrace gets progressively shallower due to deposition
5.
Gentle slope favours constructive waves, transporting material
from terrace toward land
6.
Deposition at base of cliff forms a beach
Barrier Spit
Material transported by
littoral drift deposited along
ridge, extending outward
from a coast in an area with
weak offshore currents
If the spit grows to completely
block an embayment, it is
called a bay barrier or
baymouth bar
A lagoon is a body of water
behind the barrier
Bay Barrier
A tombolo occurs when sediment deposits connect the
shoreline with an offshore sea stack or island
http://www.geog.ouc.bc.ca/physgeog/contents/11m.html