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Size and Occurrence of Floods

Topics: Floods, watersheds

Length: One class period

Type of Resource Being Described: Hands on activity


The size or magnitude of a flood is described by the term "recurrence interval", which is determined by studying a long period of flow records for a stream. This activity demonstrates the occurrence of a particular flood event and shows that floods do not necessarily occur every 10, 50, or 100 years, as is commonly thought. In this activity, students use macaroni or beans to calculate the statistics of flood recurrence (see back side of poster).


Students should understand:
• How many times a 100-year, 50-year, 10-year, or 5-year flood is likely to occur
• Discuss ways in which to protect individuals and property from floods


Each group will need:

  • One hundred items the same size and shape, but five different colors. Suggestions for items include marbles, dried beans, or multicolored macaroni. The items need to be organized according to color and number as follows:
    • One item its own color. This item represents a 100-year flood event, 100-year recurrence interval.
    • Two items their own color. These items represent a 50-year flood event, 50-year recurrence interval.
    • Ten items their own color. These items represent a 10-year flood event, 10-year recurrence interval.
    • Twenty items their own color. These items represent a 5-year flood event, 5-year recurrence interval.
    • Sixty-seven items their own color. These items represent no flooding.
  • A small paper bag or large coffee an to contain the items. The opening at the top of he container must be large enough for the students to place their hand in for selection of an item
  • Paper and pencil. 

If sufficient material are available, divide the class into small groups. Provide each group with a container of 100 items. Explain to the students that each item in the container represents a flood event. The single items represents the 100-year flood event, the two items of the same color represent the 50-year flood event, the 10 items of the same color represent the 10-year flood even, the 20 items of the same color represent he 5-year flood event, and the 67 items of the same color represent no flooding. Student will be selecting 100 different items from the container, simulating 100 years of stream flow.

Identify one student in the group as the  note keeper. The note keeper is to make a table with column headings of 100, 50, 10, 5, and 1. This individual needs to keep track of each item that is selected from the container. There will be a total of 100 items selected from the container. 

Designate one student to hold the container above the heads of the other students in the group while they take turns selecting one item at a time from the container. After the note keeper has tallied the selection, the student should return the item back to the container prior to selection nof the next item. Students will make a total of 100 selections from the container, after the 100th selection, the note keeper is to total all the selections by individual flood events and make a grand total of all selections. 

When all the groups have finished, make a master list of each groups' results on the chalkboard so all the students can share the results.

Interpretive Questions

  1. How many times did the 100-year flood occur during the 100 selections? How many times did the 50-year, 10-year, or 5-year floods occur?
  2. If the 100-year flood happened last year, what are the chances of it happening again this year? Answer: The same chance each year, one in 100.
  3. What are several ways to protect individuals and property from floods? Possible Answers: Develop flood-warning systems to alert citizens when water in stream and rivers reaches a certain height. Limit development and land use in flood plains. Construction of flood control structures such as dams and levees.