Considering Flood Risk

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Students analyze and interpret data on a map of floodplains to assess risk of flooding and inform decision-making that will mitigate the effects of flooding.

Learning Objectives

  • Students learn that the risk of flooding varies by location and activity.
  • Students learn to assess and communicate risk of flooding.
  • Students analyze and interpret data on a map.




  • Show the map to all students and discuss the key. Define what is meant by the 100-year and 500-year floodplain. Consider doing the Flood Chances activity as part of the introduction to help students develop a more in-depth understanding of these terms.
  • Explain that this area was flooded during an unusually heavy rainstorm in September 2013. Now, people who live in the area have questions about their flood risk. Tell students that in this activity, they will help answer hypothetical residents' questions about flood risk by interpreting the map, which contains information about flood probability and scenarios that contain information about potential consequences for people should a flood occur.


  • Have students use the map to help them answer the questions on Considering Flood Risk student page. The locations given for each scenario are identifiable on the map even though every street is not labeled.
  • Provide guidelines about how much students should write for each answer. You may wish to have younger students answer each scenario with one complete sentence. High school students may write a paragraph, email, or letter to the person or people in each scenario to address both their interpretation of the flood map and the specific decisions that should be made. See the answer key for points that students should mention in their answers.


  • Ask students to share their answers with each other, then discuss that the factors that inform decision-making are partly related to the floodplain map and partly related to the situation. For example, the protected land for a bird sanctuary would not be vulnerable to a flood while a furnished basement would be vulnerable.



Risk is defined as a combination of the probability of an event occurring and the consequences if it does occur. For example, the probability of a flood occurring in a low-lying area next to a river might be quite high, but if it is a natural setting with no negative consequence from a flood aside from the deposition of some sediment, then the risk is low. If, on the other hand, a town is next to the river, then there would be negative consequences for it and its residents should a flood occur. Risk would then be high.

The probability of a flood occurring in the 100-year floodplain is one in 100 or 1 percent each year. It does not mean that a flood happens every 100 years. With a 1-percent probability, floods could happen two years in a row. Likewise, the 500-year floodplain is the area where the probability of flooding is one in 500 or 0.2 percent each year.

Boulder, Colorado is at the foot of the Rocky Mountains. Several narrow valleys lie to the west of the city. When rainstorms drop large amounts of water in one of these valleys, it flows downhill into the city. Often thunderstorms in the area are isolated to a particular region, meaning that flooding could happen in a creek that comes from one valley where the rainstorm has stalled and not another. However, in September 2013, a very large rainstorm parked itself over the entire region for several days causing widespread flooding of all the waterways that pass through the city.

The probability of flooding can also be affected In various areas of the worldby differences in topography, rapid snowmelt, ice jams in rivers, the permeability of the land, and proximity to coasts that are vulnerable to flooding during storms like hurricanes.


Have students investigate where the floodplain is in their local area on the Floodsmart website or by acquiring floodplain maps from local government offices.