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Floods

 

Overview

It's hard to believe that a small, slow-running stream could cause harm. During a flood, however, even the calmest brook can become a torrent and cause unforeseen misery and damage. Many people like to live near a body of running fresh water. Rivers and streams provide recreation, food, and, of course, drinking water. But when rain or melting snow produces large amounts of water, rivers and streams may flood, causing problems for those who live nearby. Although we describe floods as disasters, they are, in fact, natural phenomena. Floods occur when a watershed receives so much water that its waterways cannot drain it off properly. A watershed is an area of land (usually quite large) that drains into a river or stream. A small river will drain several thousand or hundreds of thousands of acres of land. Within any one watershed, excess rain will cause increased water levels downstream. What occurs at any point along a river can affect not only that point but also the entire watershed. To minimize the effects of a flood, engineers build levees to constrict the overflow of rivers. As more communities build levees, the water in a river is forced to run at a higher level because it cannot spread out. As the water runs at a higher level, it deposits sediment and raises the riverbed. The situation worsens as the water rushes downstream. The water level can only continue to rise, eventually spilling over the levees. During prolonged periods of flooding, many levees give way because they are under pressure from the swollen river and are being undercut by water seepage. Floods in undeveloped areas are not as damaging as the floods in developed areas. First of all, many natural areas have thousands of acres of wetlands which act as giant sponges to soak up excess water. Second, many rivers overflow into the floodplain--a low, flat area on either side of the river. If a river is allowed to spread out onto its floodplain, the flow downstream is slowed. A river's floodplain can accommodate huge amounts of water which are diverted from the main channel and held back. Allowed to flood in this way, the river creates less damage downstream. If humans do not interfere with it, a stream or river produces its own flood control system.

Activity

The presence of a natural floodplain is crucial to the management of floods. When water is allowed to spread out away from the main channel of a river, the river can accommodate more water. If, on the other hand, a river's flow is prevented from spreading, the only place for the water to go is up. An increase in water height means higher water pressure and an increase in damage. How does the spreading out of river water make a difference? How does it decrease water pressure? Try the following activity to get the "feel" of water at different depths.

Materials

  • roll of clear plastic (poly), 6 mil or thicker, wide enough to produce at least two 3.7 meter x 3.7 meter (12'x 12') sheets
  • garden hose
  • supply of water
Number of people: Eight or more. If you have fewer, cut down on the size of the plastic sheet.
  1. Find a level, soft surface outdoors that is close to an outdoor water spigot. Lay the plastic sheet flat on the ground. Depending on the thickness of your plastic, you may want to use two sheets to increase strength.
  2. Organize participants around the perimeter of the sheet so that everybody is at equal distances from each other.
  3. Each participant should firmly grab the plastic sheet with both hands.
  4. All participants should come together towards the center to form a large "bowl" out of the plastic. Come together as close as possible. You can bunch up the plastic a little on your legs. There should be about 1.2 meters (4') between you and the person directly across from you on the plastic sheet.
  5. Begin filling the plastic with water from the hose.
  6. From here on, the goal is to put as much water as possible into the plastic without letting any escape. You can move about in any way you want to increase the capacity for water. See what you learn about the force of water. How much water can you hold?

    Questions

    1. How does this activity compare to actual watershed problems? What are the similarities and differences?
    2. What is the best arrangement to hold the greatest amount of water? What arrangement requires the least effort? How deep a pool of water can you create? Can you devise a way to estimate the total number of gallons you can hold?
    3. What would it take to create a better structure for holding more water?

Resources

  • Snyder, G. (1992, May/June) The watershed. Sierra, pp. 24-26.

Additional sources of information


American Water Resources Assn.
5410 Grosvenor Lane, Suite 220
Bethesda, MD 20814-2192

America's Clean Water Foundation
750 First St. NE, Suite 911
Washington, DC 20002-4241

Freshwater Foundation
725 Country Road 6
Wayzata, MN 55391-9611
National Water InformationClearinghouse
U.S. Geological Survey
423 National Center
Reston, VA 22092-0001
Water Education Foundation
717 K St., Suite 517
Sacramento, CA 95814-3408

Community resources


Local Army Corps of Engineers
Soil Conservation Service field office for your area