EENS 3050 | Natural Disasters |
Tulane University | Prof. Stephen A. Nelson |
Flooding Hazards, Prediction & Human Intervention |
Hazards Associated with Flooding Hazards associated with flooding can be divided into primary hazards that occur due to contact with water, secondary effects that occur because of the flooding, such as disruption of services, health impacts such as famine and disease, and tertiary effects such as changes in the position of river channels. Throughout the last century flooding has been one of the most costly disasters in terms of both property damage and human casualties. Major floods in China, for example, killed about 2 million people in 1887, nearly 4 million in 1931, and about 1 million in 1938 The 1993 flood on the upper Mississippi River and Midwest killed only 47 people, but the U.S. Army Corps of Engineers estimates the total economic loss at between 15 and 20 billion dollars. Primary Effects
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Secondary and Tertiary Effects
Predicting River Flooding Floods can be such devastating disasters that anyone can be affected at
almost anytime. As we have seen, when water falls on the surface of the Earth, it
has to go somewhere. In order to reduce the risk due to floods, three main
approaches are taken to flood prediction. Statistical studies can be undertaken to
attempt to determine the probability and frequency of high discharges of streams that
cause flooding. Floods can be modeled and maps can be made to determine the extent
of possible flooding when it occurs in the future. And, since the main causes
of flooding are abnormal amounts of rainfall and sudden thawing of snow or ice, storms and
snow levels can be monitored to provide short-term flood prediction. |
Frequency of Flooding
R = (n+1)/m |
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Pe = m/(n+1)
Two important points emerge:
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Flood Hazard Mapping Food hazard mapping is used to determine the areas susceptible to flooding when discharge of a stream exceeds the bank-full stage. Using historical data on river stages and discharge of previous floods, along with topographic data, maps can be constructed to show areas expected to be covered with floodwaters for various discharges or stages. |
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In constructing such maps aerial photographs and satellite images of prior
floods are studied to help to determine the areas that would be covered. The
illustration above shows a possible hazard map based on estimated discharges or river
stages for a hypothetical 10-year flood, 50-year flood, and 100-year flood.
In addition, scale models are often constructed of areas prone to flooding. Such models only work if they are kept up to date with current flood prevention structures and drainage modifications. Monitoring the Progress of Storms |
In conjunction with the National Weather Service, various agencies in the U.S. provide information on the internet that forecast potential floods. For example - http://water.weather.gov/ahps/ Human Intervention Humans can modify the landscape in many ways. Sometimes humans
attempt to modify drainage systems to prevent flooding, but sometimes these efforts have
adverse effects and actually help to cause flooding in other areas. Any modification
of the landscape has the potential to cause changes in the drainage system, and such
changes can have severe consequences. Channel Modifications
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Effects of Development on Flood Hazard |
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If the channelization results in decreasing the cross-sectional area of the stream, as in the example above, then the same discharge that may not have produced flooding prior to channelization, may overflow the banks and cause extensive flooding after channelization. |
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Mitigation of Flood Hazards
Engineering Approaches
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Regulatory Approaches to Reduce Vulnerability With a better understanding of the behavior of streams, the probability of flooding, and areas likely to be flooded during high discharge, humans can undertake measures to reduce vulnerability to flooding. Among these non-structural measures are:
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Examples of questions on this material that could be asked on an exam
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