Laboratory Manual for Introductory Geology 4e

17.3.2 Coastal Erosion and Deposition
A few basic principles explain how waves erode and deposit materials along shorelines
and move sediment directly, forming landforms such as barrier islands and
spits.
■ Shorelines are in a constant state of conflict between destructive coastal erosion,
which removes material, and the constructional processes of wave deposition,
lava flows, and coral reef development, which add material.
■ Waves are generated by the interaction between wind and the surface waters of
oceans and large lakes.
■ The kinetic energy of waves causes erosion and re deposition of unconsolidated
sediment.
■ Like streams and glaciers, waves carry loose sediment that abrades the bases of
solid bedrock cliffs. Waves move sediment back and forth across the tidal zone, abrading
a flat surface and undercutting the cliff. When support of its base is undermined,
the cliff collapses. Waves then erode the resulting rubble, exposing the base of a new
cliff, and the cycle repeats. In this way, shoreline cliffs gradually retreat inland.
■ Wind itself also moves sediment in shoreline environments, forming coastal
sand dunes.
■ Shoreline currents redistribute sediment to produce barrier islands, spits, and
other landforms.
17.3.3 Longshore Drift
In areas where there is an abundant supply of unconsolidated sediment, longshore
currents move sand and silt parallel to the shoreline in a process called longshore
drift. Each time a wave strikes a shoreline at an oblique angle, it carries sediment
with it along that angled path. As the wave recedes, it follows a different path that
is perpendicular to the shoreline. Over many such zigzag cycles, sand grains gradually
move along the beach—in what looks like a straight-line path to someone who
hasn’t been watching closely (FIG. 17.10).
FIGURE 17.10 Mechanics of longshore drift.
Grain A eventually moves to Point B by a complex zigzag path shown by the dashed lines. A–1: A wave
drives the grain up the beach in the direction that the wave is moving. 1–2: Water is pulled downslope
to the shoreline, carrying the grain with it. As these processes are repeated (2–3–4–5–B), the grain is
transported parallel to the shoreline.
Beach
Gravity pulls water and grain back to shore
5 3 1
Wave drives grain
onto beach
Shoreline
B
4 2
A
Sand grain
Net movement me of particles along shoreline
Longshore drift
Wave direction
Wave fronts
17.3 SHORELINE EROSION AND DEPOSITION
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