Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 36

Longshore drift

Beach

Sand
movement

Waves

Surf zone

FIGURE 2.14
Processes of beach erosion by longshore current or drift.

Crown

Main scarp

Earthen
material

Toe
Eroding and
slumping
Earthen material

(a) (b)

FIGURE 2.15
(a) Coastal erosion near San Francisco, CA. (USGS 1998) U.S. Department of the Interior, U.S. Geological Survey,
Center for Coastal Geology http://coastal.er.usgs.gov/cgi-bin/response.pl?site=cc&loc=9. (b) Homes in coastal
California in jeopardy of falling into the Pacific Ocean. (Photo by Richard McCarthy.)

The sand along a coastline is much easier for the waves to erode than rock. Along some
beaches, there is an annual cycle where sand is removed from a beach one season, only to
be returned by waves during the successive season. In addition, a steady or seasonal wind
direction at an oblique angle toward the shoreline produces waves and crash along the
beach at an angle. The release of energy produces a longshore current . Longshore currents
are responsible for erosion and migration of beach sediments along the beach front from
the action of waves. When a longshore current is present, sediments in the surf zone move
parallel to the beach in the direction of the wind.
Figure 2.14 shows the processes of beach erosion. Seasonal winds drive the longshore
current and transport large quantities of sand along a beach.
In the United States, coastal erosion is significant since roughly 15% of the land area in
the United States is coastal and 40% of the population lives near the coastline. This dis-
tribution of population places many of our urban environments under significantly more
stress than other less urbanized locations and is illustrated in Figure 2.15a and b.
Within coastal zones, some anthropogenic activities influence—and in some cases—
make coastal erosion more severe (NOAA 2009):