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 18.104.22.168). 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 Statesa combined total distance that is long enough to encircle the Earth at the equator (Pielke, 1999).
Recent severe flood eventsparticularly the 1993 Mississippi River floods,
the 1996 Saguenay flood, the 1997 Red River flood, and winter flooding in California
in 1997have 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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