Now let's look at what happens when the city gets too much water than the stormwater system can handle.  Once in a while it will rain too hard or the snow will melt too fast for the drains, pipes, streams, detention ponds, and canals of the stormwater system to handle.  When there is too much water for the system to carry away, it builds up in unwanted areas.  The result is a flood.

STEP 1:  You may remember a large flood that hit Fort Collins a few years ago. In the summer of 1997 Fort Collins experienced a very large flood.  It was the result of a rare thunderstorm that created approximately 10 inches of rain in some areas around the city within only a few hours. The 1997 flood resulted in extreme damage throughout many parts of Fort Collins.

• Open ArcView if it is not already open.

• Open "lesson3.apr."  The following themes should be active: "JES and WJHS", "Rivers and Streams", "Canals", and "Stormwater Basins."

Floods occur along areas where water collects during drainage.  In other words, flooding usually occurs along rivers, streams, lakes, detention ponds, and other low spots.  During a flood these areas get more water than they can handle.  The water no longer fits within the banks of the stream and begins to overflow into the surrounding area.  This surrounding area is called the floodplain.

• Turn on "Floodplains."

Flood plains are flat areas on either side of a river or stream.  During normal times these areas are dry, but during a particularly big storm or season these areas become flooded.
Sometimes a floodplain will be flooded about once a year.  Sometimes a floodplain will only flood once in a long time, like 100 years.  Floodplains will always get flooded... it is just a matter of when.

The picture below shows a drawing of a floodplain.

When an area gets more water then even its floodplains can hold the result is a horrible flood. When this happens all the extra water can't fit in the main floodplains of the local streams.  This is what happened in Fort Collins in 1997.

STEP 2: Now let's look at the location of the damage caused by the 1997 flood.

• Turn off "Stormwater Basins", so we can clearly see the flood of 1997.
• Turn the theme on "Flood Damage."  This theme consists of many dots representing locations of major damage during the 1997 Flood.

Think about the following questions.

• Was the damage within the floodplains?
• What kinds of things were close to these areas?
• Do you notice that most of the damage is close to streams and canals?
• Is there anything that could have been done to avoid the flood?

• Fort Collins Floods

In the following section we'll look at how wetlands can help control flooding and pollution.

• Close ArcView by clicking "exit" from the File menu.
• Click "No" when asked to save changes.

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Lesson 3 Worksheet

Lesson 4