Geology Reference
In-Depth Information
188 II. Introduction to Geologic Hazards
losses are not new as revealed by two cases: Hog
Island, Virginia, and Isles Dernieres, Louisiana.
In the late 1800s 10-km long Hog Island had a
population of about 300, mainly in the town of Broad-
water, where there was a "lavish hunting and fishing
club ... and 50 houses, a lighthouse, a school, and a
church and cemetery." Much of the town was
destroyed by a hurricane in 1933, which flooded the
island and killed "the protective pine forest." All
inhabitants had left by the early 1940s and the town
site is now about 400 meters offshore under several
meters of water (Williams et al., 1990).
About 120 km southwest of New Orleans, Isles
Dernieres is a 32-km long barrier island chain that is
"one of the most rapidly eroding shorelines in the
world" (Williams et al., 1990). About 500 years ago this
area was part of the now abandoned Lafourche delta
complex of the Mississippi River. By the mid-1800s the
Isles Dernieres were a single wide barrier island, also
with a resort community and mature forest. A hurri-
cane destroyed the resort and killed hundreds of peo-
ple in 1856. Since then the single barrier island was cut
by tidal inlets into five islands, which have continued
to erode and retreat because of storm erosion and a rise
of >1 m in relative sea level (some subsidence occurs).
With low relief, some islands are now overwashed by
storms and high tides six to eight times per year. Thus
they now offer less protection to the wetlands and
estuaries behind them than a healthy barrier island
would. Between 1887 and 1988, the average erosion
rate was 11.1 m/year. Today, the islands continue to
decrease in area and migrate landward. These changes
help to support the suggestion that Louisiana is losing
an acre of coastal land every 24 minutes (O'Malley,
1999). Research continues on this gulfside coastal
retreat, which in 1992, the year of Hurricane Andrew,
reached 59 m/year.
In the questions for Part D we explore the nature
and impacts of hurricanes in the Gulf of Mexico, with
examples from the coasts of Texas and New Orleans
(Questions 1 and 2) and offshore Louisiana (Questions
3 to 11). At Isles Dernieres, a chain of barrier islands,
the focus is on changes to Raccoon Island located at
29° 03' N, 90° 56' W, between 1935 and 1994.
QUESTIONS 11, PART D
1. Use Figure 11.13 below, and the data shown in Figure 11.12
and Table 11.3, to analyze the likely impacts of a hurricane
on this area.
a. while a category 5 hurricane is still offshore
FIGURE 11.13
Hypothetical map of a community along the coast.
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