Laboratory Manual for Introductory Geology 4e

FIGURE 17.21 Aerial view showing how jetties are designed to trap sediment moved by longshore drift.
Longshore drift direction
Jetty built
to protect
this area
Sediment is
trapped behind
jetty
to downdrift areas (FIG. 17.21). These structures, called jetties (or groynes), are effective
in preserving the targeted beach, but often have unintended consequences that
cause problems for other beaches.
17.4.4 Unintended Consequences of Human Shoreline
Management Efforts
Beach management practices sometimes backfire because the results of building
a seawall or jetty were not thought through fully. The most common problem
is that while the shoreline is protected in one area, the seawall or jetty concentrates
erosion in different areas, creating problems where there were none previously.
For example, waves crashing against a seawall may remove the sand that
otherwise would have accumulated naturally along the shoreline, doing exactly
the opposite of what was intended. Or, as shown in FIGURE 17.22, sand trapped
on the updrift side of a jetty is no longer available to replenish the beach naturally
on the downdrift side. The beach is preserved in one place, as intended,
but eroded in another. Exercise 17.10 gives you some experience in recognizing
these common problems.
FIGURE 17.22 Aerial view of shoreline showing potential negative effect of jetty
construction.
Updrift side
Beach broadened
Drift
Jetty
Downdrift side
Beach narrowed
Former beach line
Drift
17.4 HUMAN INTERACTIONS WITH SHORELINE PROCESSES
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