15.2.1.3. Flood Risks
Flooding poses risks to human life and property. Vulnerability to flood damages
is highly location-specific, and there is considerable variability across watersheds
in the value of developed property and population located within 500-year floodplains
as defined by current understanding of current climate (Hurd et al.,
1999). Flood events also can have significant impacts on ecosystems. For example,
heavy precipitation events may leach nitrogen and other nonpoint-source pollutants
from agricultural lands, and the resulting nutrient pulse may severely stress
coastal and estuarine ecosystems (Justic et al., 1996; Rabalais et
al., 1996). In addition, the influx of freshwater during floods may affect
estuary-dependent species. For example, oyster populations suffer severe declines
when floods reduce the salinity of Galveston Bay (Hofmann and Powell, 1998).
Coupling of natural disasters, such as extreme storm events, with large-scale
human disturbance of the landscape can cause extreme disturbance to freshwater
and marine resources that would not be predicted by considering these effects
independently. The consequences of Hurricanes Dennis and Floyd in eastern North
Carolina in September 1999 provide a recent example of the importance of this
coupling (see Section 15.3.2.7). Rainfall of almost 1
m generated highly polluted, organic-rich floodwaters as containment ponds for
poultry and hog waste were breached and raw sewage, fertilizers, decaying vegetation,
and other organic sources were entrained by the flood. One serious effect was
contamination of shallow groundwater sources with fecal coliforms and organic
pollutants, which may jeopardize local water supplies long after the floodwaters
subsided. Of even greater economic impact for this region, the surge of floodwater
caused the waters of the biologically rich estuary between the Carolina mainland
and the Outer Banks to be the color of weak coffee and deposited large amounts
of organic material in coastal sediments, especially in the estuaries and westernmost
Pamlico Sound. The Albemarle-Pamlico Estuarine System provides fully half of
the area used as nursery grounds for commercially important fish from Maine
to Florida. These waters are a vitally important feeding area for small sport
fish and menhaden and an important nursery for flounder, weakfish, shrimp, and
crabs. At the time, there was considerable concern that the release of nutrients
and consumption of oxygen as deposited organic material decomposed would cause
physical stresses, disrupting the coastal food web and commercial fisheries
for a significant period (Paerl et al., 2000). As it turned out, the
mesohaline estuaries west of the Pamlico Sound sustained the greatest damage
from pollution that was washed in and deposited to the bottom muds (Burkholder
et al., 2000). Pamlico Sound was protected from high impacts because
much of the pollution settled out in the estuaries and because of its high flushing
exchange with the ocean relative to the estuaries. The high dilution provided
by the extreme runoff associated with Hurricane Floyd was a "saving grace" that
appeared to buffer the pollution effects, so no fish kills were reported throughout
the system (Burkholder et al., 2000). However, concerns remain about
chronic, more long-term impacts from the pollution that remained behind in the
estuaries.
Possible changes in runoff patterns, coupled with apparent recent trends in
societal vulnerability to floods in parts of North America, suggest that flood
risks may increase as a result of anthropogenic climate change (see Section
15.2.5). Changes in snowpack accumulation and the timing of melt-off are
likely to affect the seasonal distribution and characteristics of flood events
in some areas. For example, in mountainous western watersheds, winter and early
spring flood events may become more frequent (Melack et al., 1997; Lettenmaier
et al., 1999). In southeastern Canadian and northeastern U.S. watersheds,
reductions in winter snowpacks and river ice will tend to reduce winter and
spring flood risks (Bruce et al., 2000), where at present "rain-on-snow
and snowmelt floods can be the largest and most destructive stormflow events
in the region" (Platt et al., 2000). However, Canadian rivers in northern
areas may begin to experience winter ice break-ups and associated flooding (Bruce
et al., 2000).
In inflation-adjusted terms, average annual flood damage has increased in the
United States over the past few decades. This increasing trend in damages appears
to be related to increases in population and the value of developed property
in floodplains, as well as changes in precipitation characteristics, with perhaps
as much as 80% of the trend attributable to population and wealth changes (Pielke
and Downton, 2000). Measured as a proportion of real tangible wealth, average
annual flood damages have been roughly constant over time (Pielke and Downton,
2000). This ongoing vulnerability comes despite the fact that various federal,
state, and local governments and private entities have built approximately 40,000
km of levees along the rivers and streams of the United States—a combined
total distance that is long enough to encircle the Earth at the equator (Pielke,
1999).
Recent severe flood events—particularly the 1993 Mississippi River floods,
the 1996 Saguenay flood, the 1997 Red River flood, and winter flooding in California
in 1997—have led to reexaminations of traditional approaches to flood management.
For example, a U.S. federal interagency task force was formed in the wake of
the 1993 floods, and its recommendations have contributed to altered federal
practices (IFMRC, 1994). In an assessment of the 1993 floods, which caused on
the order of US$18 billion in damages, Changnon (1996) notes that the extreme
and prolonged flooding had significant and unexpected impacts that defied previous
experience and design extremes. Changnon further concludes that many systems
for monitoring and predicting flood conditions were inadequate; that incomplete
or incorrect information was released during the flood; and that many previous
approaches to mitigate flood losses failed. He also identifies benefits, including
benefits to the natural ecosystem of the Mississippi floodplain.
It has been demonstrated that the efforts of one community to protect itself
from floods (for example, through levee construction) may affect the likelihood
of flood damages in other communities (Mount, 1995). Therefore, coordinated
regional planning and management may allow more efficient adaptation to changing
flood risks than uncoordinated efforts by individual communities. However, there
are many things that individual communities can do to rationally adapt to flood
risks and reduce the likelihood of serious damages (City of Tulsa, 1994). Many
entities that are responsible for floodplain management are rethinking the design
of levee systems and other flood management policies (City of Tulsa, 1994; IFMRC,
1994; Mount, 1995; Tobin, 1995; Pielke, 1996; Wright, 1996). These developments
may improve resilience to future flooding events, but it is not yet clear if
recent policy discussions will lead to substantial and effective changes in
floodplain management or flood response practices.
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