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Lecture 11

Earth Sciences 1022A/B Lecture Notes - Lecture 11: Longshore Drift, Refraction, Beach Nourishment

Earth Sciences
Course Code
Stephen R Hicock

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Week 11-Shorelines November 24th, 2015
Waves: from wind blowing over the ocean; a surface waveform of
energy moves through the water whose molecules move side to side
and up and down
As a wave approaches the shore it “feels bottom” and breaks into
surf that moves up a beach as swash then water 'ows back
down as backwash
Storm waves erode headlands by wave impact compressing air
in fractures, by abrasion, undercutting
Sand Movement on the Beach: waves bend as they approach an
irregular shoreline (wave refraction) so that erosion focuses on
headlands, deposition occurs in bays
Wave Refraction
oWaves bend toward irregular shoreline to erode headlands,
and deposit in bays
Sawtooth pattern of swash and backwash results in:
oBeach drift, longshore current that transport sediment
along the shore
Most waves hit the shore at an angle and move sediment along
beaches by the zigzag pattern of swash and backwash called
beach drift
Also, oblique waves in the surf zone produce longshore currents
that 'ow parallel to the shore
Move most of the sediment in transport that is supplied mainly
by rivers
Shoreline Features: erosional include, depending on geology and
wave activity:
Wave-cut cli/s
oBy surf erosion
oBy cutting action of surf against the base of cli/s, causing
them to collapse and retreat, leaving a wave-cut
platform as debris is swept out to sea
oWhere waves(surf) cut caves into the sides of headlands
until the caves join
oEventually arch collapses and leaves a stack sitting alone
on the wave-cut platform
Depositional features depend on sediment supply and current
oBeach drift forms sand ridges into mouths of bays; may
become a bay-mouth bar
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