Hydrologic hazards are severe events caused by either excess or lack of water: floods and drought respectively. Floods are the most common natural disaster, and floods are a serious environmental hazard in North American in terms of lives lost, people affected, property damaged and frequency. On the other hand, the economic and social costs of droughts can also be large, and exceed those due to floods. Some floods develop over a period of several days or weeks, while others can occur in just a few minutes. Much of Canada is at risk of flooding, with certain areas at higher risk, these are known as "flood plains".
Floods
Floods are caused when water overflows its normal range because of an excessive rise in the water level. The rise in water level can results from heavy rains, the rapid thawing of snow or ice, when ocean waves come on shore, or when dams
or levees break. There are several types of flooding, with riverine flooding being the most common phenomena. River
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flooding can be divided into slow rising, resulting from heavy rainfall or rapid snowmelt, and the more abrupt flashfloods, caused primarily by intense thunderstorms. Flashfloods are extreme and short-lived, and can result in heavy damage. Given the proper conditions, every river has the potential to flood. Across Canada the conditions needed for flooding are met regularly. Flooding can occur at anytime of the year, but is most common in the late spring when snow melting combines with rain. The rainstorm that caused the 1996 Saguenay flood, for example, dropped an average of 126mm of rain over a 100,000 square kilometre area in 48 hours. In Canada, federal payments for this hazard have exceeded any other type of weather disaster.
Although people generally think of floods in terms of damage to property and loss of life, floods can benefit the natural environment and sustain many ecosystems. However, the increased risk of flooding will require effective adaptation measures that minimize losses while preserving the environment. Flood mitigation measures can be expensive initially, but can prevent much higher costs when heavy flooding occurs. Manitoba, for example invested in the construction of the Red River Floodway, but without it, the 1997 flood would have left 80 percent of the city underwater, and forced the evacuation of more than 500,000 residents.
Drought
Drought is a condition of abnormal dry weather resulting in a serious water shortage, with consequences on crops, humans and livestock. They are caused by anomalous weather patterns when shifts in the jet stream block storm systems from reaching an area. As a result, large high-pressure cells may dominate a region for a prolonged period, thus reducing precipitation.
This natural hazard differs from others in several ways. First, there is no universally accepted definition of drought. Second, drought onset and recovery are usually slow, and in Canada primarily affects the prairies. Third, droughts also can cover a much larger area and last many times longer than most other natural hazards. Fourth, they are part of the natural variability. Due to these differences many communities have neglected to include this hazard in their disaster management plans.
Around the world droughts cause many deaths, but their impact on Canada is primarily economic by comparison. In Canada droughts are the most costly hazard, though they rank fourth in frequency. Droughts affect agriculture where losses to crops and livestock have reached the billions of dollars. They have also caused extensive environmental problems through increased degradation and erosion of soil, destruction of the ecological habitats and deterioration of lakes. It is important to note that with climate warming more frequent drought is a likely consequence.
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