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Floods and climate change

Annual average precipitation departures from mean (mm): Canada

In Canada (as in the United States), annual precipitation (running mean, solid line) has recently been above the 1951-80 mean

Source: EC 1998a

The disruption and intensification of the Earth's water cycle is believed to be one of the most fundamental effects of climate change (White House 2000). Changes may already be occurring in North America's hydrological conditions, as demonstrated by the increase in average annual precipitation over the past 30 years (see figure). In the United States, the average amount of moisture in the atmosphere increased by 5 per cent per decade between 1973 and 1993 (Trenberth 1999). Most of the increase has been due to heavier precipitation events resulting in floods and storms (O'Meara 1997, Easterling and others 2000).

During the 1960s and 1970s, more than 90 per cent of the natural disasters in the United States were the result of weather or climate extremes (Changnon and Easterling 2000). Flooding is natural and essential to the health of watersheds but floods can also be destructive and cause economic damage (see box bellow). In response to these events, the United States introduced the National Flood Insurance Act of 1968 and the 1974 Disaster Relief Act. Many of the separate and fragmented responsibilities of parallel state and local level disaster programmes were merged in 1979 under the Federal Emergency Management Agency (FEMA 1999). In 1975, Canada introduced the Flood Damage Reduction Program (FDRP) and, in 1988, it established Emergency Preparedness Canada (EPC) (EC 2000). These programmes provided better flood mitigation, preparation, response and recovery measures.

Evidence shows that deaths and damage from floods have increased sharply since the early 1970s (USGRP 2000). More people and their settlements are exposed tofloods because of population increase and concentration, and increasing affluence (Easterling and others 2000). A tendency to settle in flood-prone areas is also influenced by a perception that risk has been lowered by protective structures such as dams, dykes and diversions, and because of the availability of disaster relief (Brun and others 1997, Bruce and others 1999).

Structures that prevent rivers from flooding often provoke extremely damaging floods when water eventually overflows (see box below). In the 1990s, the United States, which is subject to more frequent and severe weather events than Canada, began to encourage non-structural approaches to flood prevention such as resettlement projects and wetland restoration. In Canada, settlement in flood-prone areas has been discouraged through mapping and the designation of more than 320 flood risk areas (EC 1998b). Canada established the Office of Critical Infrastructure and Emergency Preparedness (OCIPEP) in 2001 to develop and implement a more comprehensive approach to disaster prevention (OCIPEP 2001).

Major floods over the past 30 years

The 1993 Mississippi flood, which submerged 75 towns and killed 48 people, cost US$10-20 billion, surpassing all previous US floods in terms of economic losses, area, duration and amount of flooding (Dalgish 1998, USGCRP 2000). It was the result of record-breaking spring rains in the midwest, a larger than usual snow cover, and high soil moisture content - but levees and dykes also confined the river to its channel, helping increase the flood crest (Dalgish 1998). In 1996, Canada experienced its most destructive and costly flood in the Saguenay River valley in Quebec. Nearly 126 mm of rain fell in 48 hours, resulting in 10 deaths and about US$750 million in damages (EC 1998b, Francis and Hengeveld 1998, EC 2001). In 1997, the Red River, which flows north from the United States into Canada, experienced its worst flooding in 150 years, incurring costs of almost US$5 billion (IJC 2000).

Floods can have significant environmental consequences. The Mississippi flood, for example, damaged much of the midwest's fertile farmland and altered the natural ecosystems of the region's rivers and their floodplains (Dalgish 1998). Human modifications over the past century led to the loss of some 85 per cent of the river basin's wetlands, and changes in riparian and in-stream habitat. Wetlands and temporary lakes act as storage areas for excess water and their loss increases the vulnerability of the watershed to flooding (Searchinger and Tripp 1993).

According to some climate change models, the magnitude, frequency and cost of extreme hydrological events in some regions of North America are forecast to increase (USGCRP 2000). Projected effects of climate change include changes in the El Niņo. An uncommonly strong El Niņo in 1997-98 is believed to account for heavy floods in Florida, California, some midwest states and parts of New England (Trenberth 1999). Where rainstorms intensify and flooding increases, there is greater potential for damage to lowlying settlements and dock and port facilities as well as for problems with water distribution and sewage systems that can have health implications (EC 1999a).

The International Joint Commission assists both governments in managing their shared waters. In a report on the 1997 Red River flooding, it cautioned that, given impending increased flooding due to climate change, a comprehensive, binational strategy should be developed and implemented (IJC 2000).