LITTORAL PROCESSES
Definitions
Littoral processes result from the
interaction of winds, waves, currents,
tides, sediments, and other phenomena in
the littoral zone.
Definitions
LITTORAL ZONE: In beach terminology, an indefinite
zone extending seaward from the shoreline to just
beyond the breaker zone.
LITTORAL DRIFT: The sedimentary material moved in
the littoral zone under the influence of waves and
currents.
LITTORAL TRANSPORT: The movement of littoral drift
in the littoral zone by waves and currents. Includes
movement parallel (longshore transport) and
perpendicular (on-offshore transport) to the shore.
LITTORAL TRANSPORT RATE: Rate of transport of
sedimentary material parallel or perpendicular to the
shore in the littoral zone. Usually expressed in cubic
meters per year.
Sediments (littoral drift)
Sediments are various particles such
assand, mud, stone, shells, voids etc.
found in the coastal zone. Sediments are
transported within the coastal zone by
various means such as waves, currents,
wind, rivers
Accretion : More sediment enters a
bounded region than leaves it
Erosion : More sediment leaves a bounded
region than enters it
Causes
Waves (good at mobilizing and stirring
sediments)
• Currents (tide or wind-induced - good at carrying
• sediments)
• Winds - can transport dry sand in large
quantities
• Human interventions - “mining”, dumping waste
materials
Transport types
• Parallel to beach contours - longshore transport
• Onshore-offshore or cross-shore transport
• Vertical movements “suspension” or wind-borne
•
Longshore transport of sand by wind
Generation of Longshore Currents
• Longshore currents caused by waves or
wave set-up
• By waves
–Angle of wave approach is the acute
angle (less than 90o)
–This angle usually less than 10o
–The greater the angle of approach, the
stronger the longshore current
Longshore Currents
Generation of Rip Currents
• Where two opposing longshore currents
collide (convergence), they form a swift,
narrow seaward rip current
• Rip current drains excess water from the
surf zone
• Rip currents flow perpendicular to the
beach.
Rip Currents
Wind (and wave) direction
Wa
ve
Beach
Fro
nt
Wa
ve
Fro
nt
Beach
When water depth < 1/2 wavelength, the wave begins to break.
It also begins to refract or bend toward the shore as the part of
wave in shallow water slows down. As the wave breaks, water
moves forward toward the beach.
Wa
ve
Fro
nts
Beach
As successive waves reach shallow water, more water actually
begins to move forward and be squeezed along the shore.
Wa
ve
Beach
Fro
nts
Wa
ve
Fro
nts
This is called Longshore Drift. As waves bend to meet the
shore and begin to break, water is pushed along the shore
parallel to the beach. Longshore drift can be a significant
contributor to the movement of material along a beach.
Swimmers, too, can be carried along the shore.
Waves approach a bay.
Wave direction
Beach
As waves approach the bay the “ends” begin to refract
and the wave bends.
This refraction and the resulting breakers set up
longshore currents toward the back of the bay.
When the longshore currents meet at the back of the bay,
they can create a rip current (also inaccurately called a rip
tide) that moves off shore.
In deeper water the rip current dissipates into what is
called the rip head.
Swimmers being carried offshore by a rip current should
escape by swimming sideways to the current and not
straight back toward the beach. This may exhaust them.
Wa
ve
Fro
nts
Backwash
Swash
When waves break on shore at an angle, the water rushes up
the beach on an angle but gravity tends to draw it straight
back down.
The flood of water up the beach is called swash and the flow
back down is called backwash.
Wa
ve
Fro
nts
Net direction
of beach drift
Any material (sand, etc.) in the breaking wave is, therefore,
moved along the beach. This is called beach drift and,
coupled with longshore drift, is responsible for the transport of
materials along shorelines.
Littoral drift
How do sediments move?
Accretion and erosion
Littoral materials
• Littoral materials are the solid materials
(mainly sedimentary) in the littoral zone on
which the waves, wind, and currents act.
Classification :
• The characteristics of the littoral materials
are usually primary input to any coastal
engineering design. Median grain size is
the most frequently used descriptive
characteristic.
Classification of Sediment by Size
a. Particle diameter
b. Sediment size classifications
c. Units of sediment size
d. Median and mean grain sizes
e. Higher order moments
Characteristics of sediments
• Compositional Properties
a. Minerals
b. Density
c. Specific weight and specific gravity
d. Strength
e. Grain shape and abrasion
• Fall Velocity
• Bulk Properties
a. Porosity
b. Bulk density
c. Permeability
d. Angle of repose
Littoral Transport
• Littoral transport is the movement of sedimentary material
in the littoral zone by waves and currents.
• Littoral transport is classified as onshore-offshore
transport or as longshore transport.
• Onshore-offshore transport has an average net direction
perpendicular to the shoreline
• Longshore transport has an average net direction parallel
to the shoreline. The instantaneous motion of
sedimentary particles typically has both an onshore
offshore and a longshore component.
• Onshore-offshore transport is usually the most significant
type of transport in the offshore zone, except in regions
of strong tidal currents.
• Both longshore and onshore-offshore transport are
significant in the surf zone.
Sediment transport
Sediment Transport Types
Cross-shore Transport
Factors affecting sediment transport
Sediment transport rates
NET RATE, Qn : Is the difference between transport rates
and is defined as positive
Qn = | QL – QR |
Where
QL : sediment moving left
QR : sediment moving right
GROSS RATE, Qg : Is the sum of the two rates
Qg = QL + QR
Net Rate vs Gross Rate
Beach cusps by longshore transport
Measurement of longshore sediment transport
1. Long-term measurements
• Surveys at structures (impoundment, erosion)
• Shoreline change (aerial surveys, profile surveys
• Dredging records at channels
• Bypassing records
• Beach nourishment records
2. Short-term measurements
• Artificial and natural tracers
• Traps
• Electronic instrumentation
• Temporary structures
Methods for predicting longshore transport rate
1. Adopt the best known rate from a nearby
site
2. Historical changes in the topography of
the littoral zone
3. Energy flux method
4. Empirical method
Coastal sediment budget
Is the difference between the volume of
sediment entering a bounded region and
that going out of it in a certain time interval.
OR
The total volume of beach topography/
bathymetry change in a given time interval
Coastal sediment budget
Principal components of a sand budget
Coastal sediment budget
Beach profile
• A beach profile is a cross section of the
beach along a line that is perpendicular to
the shoreline.
• A swell profile is concave upward with a
wide, broad berm (relatively flat
backshore) and steep intertidal beach
face.
• A storm profile displays erosion of the
berm and a broad flat intertidal beach
face.
Beach profiles
Coastal erosion
Happisburgh - July 2002
Happisburgh - August 2002
Happisburgh - October 2002
Happisburgh - December 2003
Damage to property - by erosion
Damage to property - by erosion