Geoscience Reference
In-Depth Information
Although this process results in beach widening on one
side of the groin, the portion of the beach immediately down-
current of the structure becomes a zone of erosion because the
longshore current is regenerated there due to unaltered waves
striking the shore obliquely. Thus, it is possible for one land-
owner, say, landowner A, to build a wide beach in front of her
property by installing a groin that, in turn, causes erosion and
beach loss at the adjacent property of landowner B down the
coast. To avoid this loss, landowner B might also install a groin
to intercept the sand eroded on his property caused by the groin
on the property immediately up the coast. Given this downshore
ripple effect, it is quite common to see a series of groins along
a coast in distinct
groin fields.
.
the channel safely. Like groins, jetties cause deposition on the
upcurrent side of the structure and erosion on the downcurrent
side. To offset beach loss on the downcurrent side of jetties,
beaches are often nourished either through direct deposition of
sand or with groins.
Global Climate Change and the Impact
on Coastlines
Recall from Chapter 9 that one of the major environmental issues
today is global climate change. As noted in that chapter, a strong
scientific consensus now exists that average global temperature will
likely increase between 2°C and 4°C (4°F to 7°F) by the end of this
century. Among other negative effects, this increased temperature
will cause further melting of polar ice caps and alpine glaciers, as
we discussed in Chapter 17. This meltwater will return to oceans,
causing sea level to rise. The rising sea level caused by melting
glaciers will be magnified by the thermal expansion of water due
to increased global temperature. According to some of the nation's
top Earth scientists, sea level appears to be rising faster than was
predicted just a few years ago. More importantly, current projec-
tions indicate that sea level will rise somewhere between 20 cm
and 2 m (8 in. and 6 ft) by 2100 (Figure 19.41a).
Assuming that the amount of sea level rise by 2100 falls
somewhere in the middle between the two extremes shown in
Figure 19.41a, it still means an increase of about 1 m (3 ft) this
century. The impact of such a rise in sea level on coastal zones
around the world will be significant. On a global scale, it is es-
timated that over 100 million people are at risk of coastal flood-
ing, with governments in countries like Bangladesh, Thailand,
Indonesia, China, and the Netherlands particularly concerned.
In the United States, significant regions are susceptible to rising
water levels, with up to 8 million people at risk. For example,
Figure 19.41b shows the areas vulnerable to these changes
along the coast of North Carolina. This map is color-coded:
areas in red are coastal zones less than 1.5 m (5 ft) above sea
level, whereas those in blue lie between 1.5 m (5 ft) and 3.5 m
(11.5 ft) above sea level. As you can see, much of the eastern
part of the state is very close to sea level. Of these regions, the
extensive zone less than 1.5 m (5 ft) will likely be impacted
somehow from coastal flooding associated with rising sea level.
The likely response of environmental planners and government
officials to this threat will be more funds devoted to coastal en-
gineering and beach stabilization.
Another reason rising sea level is a cause for concern is
that it will likely increase the amount of damage associated
with strong storms and hurricanes in the future. As discussed
in Chapter 8, one reason why Hurricane Sandy was such a
catastrophic storm was that it made landfall during the highest
tide of the month. In addition, sea level was a few cm higher
due to thermal expansion than during the winter months when
water temperature is cooler. Given these combined factors, the
storm surge in northern New Jersey and Staten Island, New
York, was particularly intense and the impact on coastal com-
munities disastrous (Figure 8.27). It is possible that this storm
was a harbinger of future events should the sea levels predicted
Jetties
Another purpose for protecting beaches and coast-
lines is to maintain a channel between the open water and inte-
rior water bodies such as lagoons, lakes, and rivers. This access
is maintained with
jetties
, which are long stone or concrete
structures used to create a permanent channel by reinforcing
both sides of the passageway (Figure 19.40). From the shore,
jetties project several hundred meters out into the ocean and
provide a zone of relatively quiet water for a boat to approach
Figure 19.40 A typical jetty.
This jetty on the Texas coast pro-
vides a reliable passageway into the ocean for boats. Given the
variable beach thickness on either side of the jetty, can you tell
which way the longshore current is flowing? Note the sequence
of foredunes on the updrift (arrow) side of the jetty. These dunes
formed due to high sand supply because sand was trapped.
Jetties
Long stone or concrete structures used to create a
permanent opening for a channel by reinforcing both sides of
the passageway.
