GEOG 140
Intro Physical Geography
Lecture Notes
The Oceans, Coastal Processes, and Landforms
-Major Concepts
-wind generated waves provide most of the energy for shoreline processes
-wave refraction concentrates energy on headlands and disperses it in bays
-longshore drift is one of the most important shoreline processes
-generated by waves advancing obliquely toward the shore
-erosion along a coast tends to develop sea cliffs by the undercutting action of waves and longshore
currents
-wave cut platform develops as a cliff recedes
-until equilibrium is established between wave energy and coastal configuration
-sediment transported by waves and longshore current is deposited in areas of low energy
-form beaches, spits, and barrier islands
-erosion and deposition along a coast tend to develop a straight or gently curving shoreline
-in equilibrium with energy expended upon coastline
-reefs grow in special environments
-form coasts that can evolve into atolls
-worldwide rise in sea level associated with the melting of Pleistocene glaciers drowned many coasts
-15,000 - 20,000 years ago
-tides are produced by the gravitational attraction of the moon (and Sun)
-exert major local influence on shorelines
Tides
-spring tides
-earth bulge (due to rotation) and alignment of moon and sun produce extremely high tides
-neep tides
-moon at right angle to Sun producing least extreme tides
-tidal range important in coastal geomorphology
-microtidal
-< 2 meters
-mesotidal
-2-4 meters
-macrotidal
->4 meters
Waves and Their Properties
-most waves generated by winds
-velocity (high)
-direction (persistent)
-duration (protracted)
-fetch (long)
-swells create orbital motion in open water as they pass a given point
-waves of oscillation
-depth equal to half the length
Waves Against the Shore
-wave interacts with bottom
-wave of translation (erosional)
-contact with bottom slows wave down (breakers)
-wave types
-spilling breakers
-water spills down wave face
-steep, deep water waves and onshore winds
-plunging breakers
-conventional waves
-face becomes vertical-plunges shoreward
-associated with long low deep water swells and offshore winds
-surging breakers
-base of wave surges up beach face
-crest collapses and disappears
-associated with steep beach slopes
-swash
-thin layer of water that moves up the beach after wave breaks
-carries sand and gravel
-backwash
-receeding swash
-also carries material
-wave refraction
-bending of wave as it approaches shoreline
-wave approaches at an oblique angle
-“front” of wave slows down before “back”
-reduces original angle by the time wave breaks
-in headland/bay topography
-waves approach roughly parallel
-slow down in shallows associated with headlands
-refracts wave so that energy is concentrated on the headlands and dissipated in bays
-net result is straightening of coastline
-longshore current and beach drift
-takes place by different processes within the different zones
-breaker zone-zone in which waves become unstable and break
-surf zone-zone between breaker and swash zones
-location of longshore current
-swash zone- sloping belt between upper limit of high tide swash and low tide backwash
-breaker zone transport
-most effective longshore sediment transport
-breaking waves put sand into suspension
-moved by longshore current as a "river of sand"
-belt landward of the breaker line
-can create barrier islands
-surf zone transport
-shoreward of longshore stream of transport
-movement determined by balance between wave velocity and velocity of longshore current
-swash zone transport (beach drift)
-swash hits beach at wave angle
-backwash moves back down beach perpendicular to beach profile
-net downshore movement
-longshore transport also affected by swash and backwash
-equal-material remains at same elevation on fore shore
-equilibrium of beach profile
-swash dominates
-material moves up beach profile (aggradation)
-backwash dominates
-seaward movement (erosion)
Degradation and Aggradation by Waves
-erosion by hydraulic action
-speed and weight of water against vertical bedrock in deep water
-erosion by corrasion
-like rivers and glaciers, pieces of rock abrade rock surfaces with help of wave action
-erosion by corrosion
-chemical action of seawater on bedrock
Coastal Landscapes
-deep water coasts
-primarily erosional landscapes
-emergent coasts
-areas of uplift
-cliffs and wave cut platforms raised above present sea level
-shallow water coasts
-depositional landscapes
-submergent coasts
-areas of drowned landscapes (due to rise of sea level)
Erosional Landforms
-Cliffs
-rate of cliff retreat (variable in time and space)
-rock lithology and structure
-resistant, impermeable rocks
-little change over long periods
-susceptibility to chemical weathering, mass wasting, erosional processes
-height of cliffs
-controls amount of basal debris which must be removed to initiate renewed cliff
face erosion
-orientation of the coast
-controls wave energy
-wave energy
-winter storms in N. Hemisphere produce the greatest rates of retreat
-height of sea level
-high tides, storm surges and tidal inundation of cliff base - rapid cliff retreat
-Wave Cut Platforms
-Sea Stacks, Arches, and Caves
Aggradational Landforms
-Beach Dynamics
-beach profile
-foreshore
-area between high and low tides (intertidal zone)
-nearshore
-area inundated even at low tide
-backshore
-area between high tide line and dune line
-berms-areas created during storm conditions
-summer profile (depositional)
-winter profile (erosional)
-changes in seasonal sand supply –equilibrium
-changes in long-term sand supply
-wave energy the same-net erosion
-Coastal Dunes
-created by wind action
-irregular mounds and stabilized by vegetation
-unlike desert dunes
-Sand Spits and Sandbars
-long narrow ridge of sand resulting from longshore drift
-building of spits tends to straighten shorelines
-block embayments
-become sediment traps and fill
-spits constructed by dominant wave action
-as they extend into deeper water - attacked by refracted wave action
-and waves generated by infrequent storm action
-barrier beach
-long narrow sandy ridge lying above high tide level
-parallel to coast - separated by lagoon
-barrier island similar but consisting of several ridges with dunes and vegetation
-barrier spit
-island connected at one end to the mainland
-Barrier Islands
-barrier coasts make up about 13% of present world coastlines
-characterized by major longshore bars (barrier beaches)
-lengths up to 100km
-favor depositional coasts
-low gradient
-abundant supply of loose sediment
-steeply plunging waves
-also occur with storm waves in places
-small tidal range (but not essential)
-barrier beaches frequently breached by wave attack
-form inlets