Chapter 11: The Coast: Beaches
and Shoreline Processes
Coastal region


Shore between low tide and highest
elevation affected by storm waves
Coast from shore to farthest inland
ocean features
Fig. 11-1
Beach



Actively changing
Shore: backshore, foreshore, nearshore,
offshore
Berm, beach face, longshore bar, longshore
trough
Fig. 11-1
Beach composition and shape


Locally available material
Coarser sediment


Beach steeper
Finer sediment

Beach more gentle
Sand movement

Swash and backwash
Sediments moved up and down beach face
 Smaller, low energy waves sand moved up
beach face



Summertime beach
Larger, high energy waves sand moved off
shore

Wintertime beach
Sand movement

Longshore current
 Longshore drift
or transport
 Parallel to shore
 Increasing
strength
More wave
energy
 Higher wave
frequency
 Steeper beach
 Greater angle


Longshore current main way
sediments moved along coasts

Southward along both U.S. coasts
most of the time
Erosional-type shore

Headlands eroded
Sea arches
 Sea stacks


Wave-cut cliffs


Sea caves
Tectonically active coasts

Example Pacific coast U.S.
Depositional-type shores
Sediments re-distributed by ocean
processes
 Typical in tectonically passive areas

Depositional features



Fig. 11-7
Spit
Tombolo
Bay barrier or
baymouth bar
Barrier islands







Fig. 11-10
Ocean beach
Dune
Barrier flat
High salt marsh
Low salt marsh
Lagoon
Rising sea level
pushes barrier
island landward
FIG. 11.11b
Deltas


Rivers deposit
sediment at coast
If more fluvial
influence than
ocean influence


Bird’s foot delta
If more ocean
influence than
river influence

Arcuate delta
Shepard
classification of
coasts

Primary
 Geologically
youthful
 Controlled by
non-marine
processes
 Examples
 “Drowned”
river or glacial
valleys
 Volcanic coasts
 Coasts shaped
by recent
tectonic
movements
Secondary coasts



Geologically mature
Controlled by
marine processes
Tectonically passive
 Examples
 Straightened
coasts
 Coral reefs
 Marsh grass
Fig. 11-13f
Emergent coasts
Uplift or sea level drops
 Once marine is now land

Marine terraces
 Stranded beach deposits

Submergent coasts


Subsidence or sea level rises
Once land (or shallower) is now
underwater (or deeper)
Drowned beach deposits
 Drowned river/glacial valleys
 Submerged dunes

Tectonic and isostatic changes

Tectonic uplift and subsidence
Larger scale: major parts of continents
 Smaller scale: local deformation


Isostatic adjustments to loading
Sediments or ice
 Regional and local effects


Affect sea level
Eustatic changes in sea level




Fig. 11-15
Global
Changes in spreading
rates of ocean floor
Changes in ice
volumes
Changes in ocean
temperature
 Global warming
 Sea level rises
 Ice melts, ocean
warmer
Atlantic coast of U.S.
Mainly submergent
 Barrier islands
 Glaciers shaped some
shorelines
 Eroding coasts
 Subsidence in most areas

Gulf coast of U.S.
Mainly submergent
 Barrier islands
 Mississippi delta
 High rates of erosion
 Tectonic subsidence

Pacific coast of U.S.
Mainly emergent
 Less erosion
 Mainly rocky shoreline
 Dams on rivers beach starvation
 Tectonically active

U.S. coastal erosion, deposition
Fig. 11-17
Hard stabilization

Groin and groin field
Perpendicular to shore
 Traps sand between groins
 Interrupts longshore current


Jetty similar
Built to protect harbor entrance
 Sand on upcurrent side


Breakwater
Parallel to shore
 Offshore
 Deposition behind breakwater





Seawall
 Built on beach
 Parallel to beach
Erosion enhanced
seaward of wall
Seawall destroyed
Beach narrowed
Fig. 11-25
Alternatives to hard
stabilization
Limit construction near shore
 Relocate businesses/homes
destroyed by wave erosion

End of Chapter 11: The Coast:
Beaches and Shoreline Processes