Floodplains and Floods (Manitoba Flow 1997)

A flood occurs when a river overflows and due to environmental and/or human features and water flows onto the land surrounding it; also known as the floodplain.

A floodplain is the flat piece of land found beside and surrounding a stream or river that will be periodically flooded. Its features a floodway, which consists of a stream channel, flood fringe, which are covered by the flood. The curves found as the river moves through the floodplain are referred to as meanders: the board curves in the river as it winds back and forth.

These will form as a result of erosion occurring the most rapidly outside of each river bend; due to this factor, the meandering rivers are always changing. Oxbow lakes are curved bodies of water located next to the river but separated by sediment. An oxbow lake is formed when a river creates a meander due to the river eroding the banks through hydraulic action and abrasion/corrosion.

After a long period of time, this meander becomes very curved, and eventually, the neck of the meander will touch the opposite side and the river will cut through the neck, cutting off the meander to form the oxbow lake.  A Yazoo stream is a term given to a tributary stream that runs parallel to a larger river which it eventually joins within the floodplain. Thick buildups along the sides of the river are referred to as natural levees; these occur during periods of flood in which large quantities of sediment are cared onto the floodplain, once the river slows the suspended sediments are deposited.

In 1997 the Red River Flood occurred in between North Dakota, Minnesota, and Southern Manitoba. It reached metropolitan areas such as Winnipeg/ Fargo and was the most severe flood to occur in the region since 1826. The areas of Grand Forks and East Grand Forks were the most affected and were completely submerged in by the flooding; the total damage of the flood was over 3.5 billion dollars. Environment factors including a large amount of snowfall and drastic temperature fluctuations caused the snowfall to melt rapidly and caused flooding.

Having experienced a flood prior to this, Manitoba was able to react to this flood with several natural and artificial methods. Winnipeg and several towns south of it had permanent clay dikes built around them. Other major flood control structures that also existed were the Portage Diversion/ Shellmouth Dam found on the Assiniboine River.

However, even with all these preexisting flood prevention, the flood caused nearly half a billion dollars in damage to property and infrastructure. Even though they had experienced flooding of this magnitude before, Manitoba had very poorly devised emergency flood plans which were later improved after the flooding. Also, the protective levee system located around Winnipeg and several other towns were remade with better materials and support structures. If Manitoba had implemented these protocols and structures previously then damage and life loss could have been drastically reduced.

Several towns located upriver in the province of Manitoba were warned after seeing the devastating damage occurring in Grand Forks Towns and built river dikes to protect their homes and properties. These were constructed with a wide range of materials such as sandbags and other improvised material that the citizens were able to find. Almost all of the ring dikes around the towns held, except for Ste. Agathe. The town’s dike system was prepared for the river approaching from the south, but the river had spread and swamped the town from the west. Thousands of people were evacuated to regions above the river’s floodplain to camps organized and controlled by the Canadian military.

What Causes floods?

Most river floods are caused by the rapid melting of winter snow and/or long-lasting rainfall. Flash floods, single cloudburst, occur over a narrow valley of a young mountain stream.

Consequently, towns located within the vicinity of the base of such a valley will suffer the most severely from flash flooding. Larger rivers will be less prone to flashing flooding but may receive flooding after thaws in the winter and early spring. The formation of dams will flood the valley and therefore the river up to the height of the dam.

Naturally occurring dams such as ice dams or dams caused by ash, cinders, or lava can form across streams. It is understandable that the failure of these reservoir dams can lead to flooding and often large-scale damage.

Human activity such as covering the land with buildings/ pavement does not allow it to absorb water, increasing the runoff. Also, the removal of vegetation along steep slopes will increase runoff because the vegetation plays a crucial role in the absorption of water. The continuing spread of urban sprawl and agricultural development also displaces many wetlands which act as “natural sponges” and help contain runoff.

Flood Prevention

Numerous forms of human activity are employed by humans in order to prevent and control flooding; each with its own advantages and disadvantages. First, restoring natural vegetation or wetlands that human activity has destroyed can decrease the amount of runoff but doesn’t entirely prevent flooding. Manmade dams are often utilized to control and prevent flooding; built across large rivers, they store all the excess runoff.

However dams do possess a threat if they break to cause even greater flooding and when they trap too much sediment, their ability to control flooding is greatly reduced. In addition, artificial levees (ie. Sandbags) can be placed at the tops of natural levees to control flooding; rising the banks around the river and stopping runoff but can be destroyed by increased current caused by a deeper river. Finally, spillways can be used, which run parallel to the main river can catch excess runoff and guide it toward swamps and seas.

Compare and Contrast

Bibliography

“A History of Flooding in the Red River Basin” (PDF). USGS. <http://pubs.usgs.gov/gip/2007/55/pdf/finalWebGIP55.pdf>. Retrieved October 3, 2007.

Amanda Ripley (2005-10-23). “Hurricane Katrina: How the Coast Guard Got it Right”. Time Magazine. <http://www.time.com/time/magazine/article/0,9171,1122007-2,00.html>.

“CBC Archives”. A city submerged: Winnipeg and the flood of 1950. <http://archives.radio-canada.ca/IDC-1-70-670-3783/disasters_tragedies/manitoba_floods/clip1>. Retrieved September 17, 2007.

“CBC News”. 1997’s “Z-dike” showcased province’s flood-fighting machine. April 25, 2007.

Spaulding, Nancy E. Earth Science. Boston: McDougal Littell, 2003. 290-92. Print.

“USAToday.com”. Flood of criticism from 1997 floods: Did faulty forecasts add to disaster?. November 10, 1999. <http://www.usatoday.com/weather/wrednws.htm> Retrieved 2007-11-03.