Force of Floods

In the United States there is hardly a place where natural disasters do not occur. No matter where you live, there is always the threat of a natural disaster. Through recorded history we have accounts of disasters created by volcanoes such as Mt. Vesuvius and Krakatau. Storm chasers have brought us dramatic video footage of disastrous tornadoes and hurricanes. The nightly news seems to regularly feature disastrous floods from over flowing rivers that follow large accumulations of rain or the thawing of ice and snow.

Of all the natural disasters floods are the most costly in terms of human hardship and economic loss. Most communities in the United States do experience some kind of flooding. In the United States the floods of 1889 (Johnstown), 1927 (Mississippi River), 1976 (Big Thompson River near Denver), 1993 (Mississippi River), and 1997 (Western Tributary System of the Mississippi River) are well known and documented disasters. Globally the most significant flood in terms of loss of human life is the Huang He River in China where approximately 3.7 million people were killed in 1931. In 1939 the same river flooded again, killing 500,000 people. In 1970 a cyclone and a seismic wave in the Bay of Bengal killed 266,000 people. These serve as terrible reminders that floods may be devastating.

1. Coastal Flood – May be brought on by hurricanes and tropical storms, seismic waves or tsunamis, or astronomically driven high flood tides.
2. Flash Flood – This is the most dangerous of flooding situations because this type of flood comes as a surprise. It generally occurs after a heavy rain and the natural or municipal drainage system is overwhelmed. The failure of a dam is another type of flash flood.
3. Urban Flood – The concrete and asphalt sprawl of a city reduces drainage areas. Water has nowhere to go after a heavy rain.
4. River Flood – Flooding along rivers is a natural event. The encroachment of urban and suburban development in the floodplains of rivers turns a natural event into a natural disaster.

Floodwaters can be extremely dangerous. The force of six inches of swiftly moving water can knock people off their feet. Flash floods move at very fast speeds and can roll boulders, tear out trees, destroy buildings, and wipe out bridges. Cars can be swept away in just 2 feet of water.

It is difficult to predict a flood, especially a flash flood. Several factors must be considered, such as current river levels and the amount of recent of precipitation. The frequency of flooding is often stated as a statistical chance. This chance is stated as the number of floods per one hundred years. For example, a certain area floods about every thirty-three years. Then there is a one in thirty-three chance of it flooding there. This is stated as a 33-year flood or the 33 percent chance of flooding. It is based on historical records and surrounding topography. The highest or largest flood that may occur in an area is considered to have a 1% chance of occurring in 100 years.

This type of flood is called a 100-year flood. It doesn’t mean that there will only be an extreme flood every 100 years. It means there is a 1% chance of having such a flood anytime. One of the interesting questions about floods is why are we seeing a greater occurrence of 100-year floods over larger areas, such as the 1927, 1993, and 1997 floods of the Mississippi River system.

People once had the option of settling in the best protected areas, but are now forced into less than desirable areas. As people live and build in floodplains, coastal zones, and dry washes they must assess their risk of flooding. How much risk is acceptable? Should you consider living in a house that is subject to 10-year flooding or 50-year flooding? Likewise, when living along the coast do you live in an area that is subject to 10-year storms or 100-year storms? How do you weigh the risk to human life? It seems that any life-threatening event should be considered too much risk. However, people continue to live and build in areas susceptible to flooding. Another consideration is the economics of living and working in a flood plain. What level of loss can one tolerate before going broke? How much should the federal government pay in disaster relief to people and businesses located in flood zones? These are all very tough questions to try to answer. What do you think?

The great advances in engineering and science that came with the turn of 19th century made us believe we could control rivers and stop excessive flooding along with improving navigation. The Army Corps of Engineers built levees, diversion outlets, and dams to control rivers. For a period of time, that seemed to be the answer, but what happens when a levee or dam fails? Were the levees built to 100-year flood heights? Did human activities alter the hydrology of a local area so that 100-year floods are now coming every 10-years? The use of diversion outlets is controversial because of potential environmental problems such a diversion may create.

Since it would be impossible to move people and businesses out of all the floodplains, a better warning system needs to be developed. Several government agencies are working on plans to reduce the occurrence of national disasters. This planning is called the U.S. National Disaster Reduction Initiative. The major activities associated with this initiative are to: (1) provide the best possible warning and information to prevent damage and allow escape from harm, and (2) provide information and techniques to lower the vulnerability and increase the resiliency of people and property after floods. The United States Geological Survey is developing new GIS (Geographic Information System) materials that allow planners to model flood scenarios. One thing they can do with these models is identify which roads or bridges may be cut off by flood waters and suggest alternative escape routes. These agencies will also make recommendations as to the type of construction that should occur in certain areas and work with local governments to create zones for various business activities. For example, having a ballpark in a flood zone would be less risky than having a hospital in a flood zone.

It is clear that the government cannot continue to subsidize disaster. It is important for citizens to recognize risks associated with floods and plan and prepare wisely. Further research is necessary in order to gain a better understanding of climate and global change. Hopefully, new and better technologies will evolve to help us with our predictive abilities to recognize when the water is on the rise.