Laboratory Manual for Introductory Geology 4e

FIGURE 17.27 Flooding caused by hurricanes.
(a) Large areas of Houston, Texas, were flooded by more than
50 inches of rain from Hurricane Harvey.
(b) Approximately 80% of New Orleans, Louisiana, was flooded
by water from Lake Pontchartrain when levees that protected the
city failed during Hurricane Katrina.
relatively straight shorelines. Second, the effective height of storm surge depends
on the stage of the tide (high or low) when the storm makes landfall. Storm surge
is measured in height above normal levels. Storm surge from a hurricane that comes
ashore at low tide will therefore not reach as high as if it had struck at high tide, nor
travel as far inland from the coast.
■ Flooding: Hurricanes produce torrential rain, locally overwhelming the ability
of soil to absorb the water and sewer systems to drain it away. Hurricane Katrina
produced a state record 15 inches of rain and caused widespread flooding in Louisiana,
but Harvey set a historic U.S. record when it stalled over Texas for several days
and deluged the region with an incredible 52 inches of rain (FIG. 17.27a). Extreme
amounts of rain may also overload dams and levees, as when Katrina’s rainfall broke
some of the levees that protected New Orleans (FIG. 17.27b).
Problems related to a major storm last much longer than the storm itself, as
demonstrated by the difficulties in restoring basic services to residents of Puerto
Rico following the double strike of Irma and Maria. Storm surge and flooding damage
buildings, which must be inspected to guarantee their safety. Saltwater damage
must be repaired, bacteria and mold disinfected, and sand deposited by storm surge
removed. Downed trees block traffic, making it difficult to rescue isolated families
and deliver emergency food and medicine. Water treatment plants overwhelmed
by storm surge dump millions of gallons of sewage into flooded areas. Exposure to
toxic materials in floodwaters, such as gasoline from damaged gas stations and cars,
is a long-term health problem. Today, 12 years after Hurricane Katrina struck New
Orleans and 5 years after Sandy devastated New York, neither city has completed
repairs to its infrastructure, and many residents have not been able to return to
their homes.
Exercise 17.12 examines how the path of a hurricane and coastal geography control
the amount of damage it causes.
17.5.3 Tsunamis
Tsunamis are enormous shock waves generated by earthquakes or volcanic explosions
in ocean basins. Harmless and barely detectable in the middle of the ocean,
they become walls of water tens of feet high when funneled into narrow coastal
embayments. Tsunamis were discussed in detail in Chapter 11; here, we’ll look at
their effects on a single coastal city.
17.5 WHEN SHORELINES BECOME DANGEROUS
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