Using probability to understand 100-year storms

1 + Using probability to understand 100-year storms Grade Level: 5-7 Activity Duration: 30-45 minutes Overview: I. Storm stories II. 100-year Storm Probability III. Delineating Storms in NE Minnesota Summary: Students will remember the biggest storm they ve ever experienced and share their stories with each other. The teacher will introduce the term 100-year storm. To understand the probability of a 100-year storm happening, students will document the results of a storm probability spinner with a team and discuss the likelihood of a 100-year storm based on their results. Then, using a table of storm data, students will compare two large storms that have hit Duluth, one in 1972 and one in 2012, to determine if they qualify as 100- year storms. Topic: weather, storms, 100-year storm, probability Theme: 100-year storms have a 1 in 100 chance of happening on any given day. These large storms and rare storms have significant impact on the land and people. Goals: Students will understand the probability of a 100-year storm happening on any given day. Objectives: 1. Students will freewrite about their storm experiences. 2. Students will define 100-year storm. 3. Students will calculate percentages of how often a certain form of storm occurred. 4. Students will explain why it is possible for100-year storms to occur intervals shorter than 100 years. 5. Students will use a data table to determine what qualifies as a 100-year storm. Lesson Adapted From: Floods, Floodplains, and Flood Probabilities. Purdue University Extension. Retrieved from http://www.four h.purdue.edu/natural_resources/resources/wat 1/Floods.pdf A Chance of Rain? - Science Institute 2012-

2 Standards suggested: Minnesota Science Standards: 5.1.3.4.1 The Nature of Science and Engineering Interactions Among Science, Technology, Engineering, Mathematics, and Society Use appropriate tools and techniques in gathering, analyzing, and interpreting data. Minnesota Math Standards: 5.IV.A.2 Data Analysis, Statistics and Probability Use fractions and percentages to compare data sets. 6.IV.A.1 Data Analysis, Statistics and Probability Collect, organize, and represent categorical and numerical data with tables and bar graphs. 6.IV.B.1 Data Analysis, Statistics and Probability Generate and display data in graphs and table to estimate experimental probabilities. 7.IV.B.1 Data Analysis, Statistics and Probability Express probabilities as percentages, fractions, proportions, and decimals. Environmental Literacy Scope and Sequence Benchmarks: Social and natural systems are made of parts. (K-2) When the parts of social systems and natural systems are put together, they can do things they couldn t do by themselves. (K-2) In social systems that consist of many parts, the parts usually influence each one another. (3-5) Social and natural systems can include processes as well as things. (6-8) Social and natural systems are connected to each other and to other larger and smaller systems. (6-8) Concepts addressed in this lesson: abiotic factors, cause and effect, cycles, patterns, probability For the full Environmental Literacy Scope and Sequence, see: www.seek.state.mn.us/eemn_c.cfm Great Lakes L Literacy Principles The Great Lakes influence local and regional weather and climate. Much remains to be learned about the Great Lakes. For a complete list of and more information about the Great Lakes Literacy Principles, visit: http://greatlakesliteracy.net/ A Chance of Rain? - Science Institute 2012-

3 Materials: Storm Stories Paper for each student (not included in kit) Writing utensil for each student (not included in kit) 100-year Storm Probability Activity 6 Storm Spinners Storm Probabilities Worksheet for each student in class (Master copy included in kit) 6 calculators Delineating Storms in Minnesota 6 laminated Sectional Mean Frequency Distributions for Storm Periods cards 6 laminated Precipitation in Duluth Reports for 1972 and 2012 storms 6 Calculators Storm Probabilities Worksheet (used in previous activity) Pencil with eraser for each students (not included in kit) A Chance of Rain? - Science Institute 2012-

4 Background 1, 1, 2, 3, 4 : What is a 100-year storm? One of the more misleading phrases used in meteorology and hydrology is 100- year storm. The phrase implies that an intense rainstorm, dubbed as a 100-year event, dropped rainfall totals heretofore unseen for 100 years, and not to be experienced again for another century. This is a logical, but incorrect conclusion to draw from the phrase. More precisely worded, a 100-year storm drops rainfall totals that had a one percent probability of occurring at that location that year. Encountering a 100-year storm on one day does nothing to change the probability of receiving the same amount of precipitation the very next day. A better way to describe these unusual events is to refer to a one percent probability storm. Given this, what could you call a 50-year storm? How about a 500 year storm (Table 1)? Recurrence interval and probabilities of occurrences Recurrence intervals in years Probability of occurrence in any given year Percent chance of in any given year 500 1 in 500 0.2 100 1 in 100 1 50 1 in 50 2 25 1 in 25 4 10 1 in 10 10 5 1 in 5 20 Table 1. The relationship between recurrence intervals, probability and percent chance of a storm event occurring. How much rain needs to fall to be called a 100-year storm? Currently in Minnesota, a 24-hour duration 100-year storm for most communities is roughly six inches. In Duluth, 5.2 inches of rain in a 24-hour period is a 100-year storm. These criteria are set by looking at peak rain events, and measuring the length of time between events. This length of time, how often an event happens, is called a recurrence interval. As data is collected over a greater period of time, recurrence intervals can change, thereby redefining what makes a 100 year storm. For recurrence intervals and the storms they signify for regions of Minnesota, see pages 130-132 of the Rainfall Frequency Atlas of the Midwest: http://www.isws.illinois.edu/pubdoc/b/iswsb- 71.pdf A Chance of Rain? - Science Institute 2012-

5 100 year storms in the Duluth area Two one hundred year storms occurred in Duluth within 40 years. During the storm of 2012, 6.9 inches fell in a 24-hour period (Fig. 1). In August of 1972, the Central Hillside neighborhood of Duluth, MN received close to three inches of rain fall in just a two hour period between 3:00 am to 4:30 am on August 20 th (Fig. 2). A mass of debris including furniture, bricks, rocks, mud, and telephone poles were washed down the Duluth hill during the 1972 storm (Fig. 3). The severity of the damage was attributed to the steep terrain of the Duluth area; a rise of 800 feet within less than a mile. Figure 1. Flooding rains of June 19-20, 2012. Figure 2. Heavy rains in August of 1972. An additional factor in this flood event was the 6.04 inches of rain that fell during the previous two weeks of August; with 3.85 inches falling on the 15th and 16th. These rains saturated and weakened many streets and alleys and left many sewers clogged. A Chance of Rain? - Science Institute 2012-

6 Figure 3. Flood damage caused by the storms of 1972. Like the flood of 2012, previous rain events in 1972 set the stage for large scale flood damage. Photos courtesy of perfectduluthday.com Only a month later the initial heavy rainfall, another round of rain brought a swath of 4-5.5 inches of rain to the Duluth community between the hours of 4:00 am and 2:00 pm (Fig. 4). The Duluth airport recorded a rainfall amount of 3.42 inches during this eight hour period. There were two fatalities with this storm and nearly 100 graves were washed up. Below is a map of the rainfall over the Duluth area on September 20th, 1972. Values are in inches. Although data is widely scattered, it is believed that similar rainfall amounts extended along the North Shore. President Richard Nixon declared the area a Federal Disaster. Damages total $18 million. Figure 4. The 100-year rain event of 1972. Some areas received as much as 5.5 inches of rain in a ten hour period. A Chance of Rain? - Science Institute 2012-

7 Why does it seem like 100-year storms occur more frequently than every 100 years? 100-year storms appear to be occurring with greater frequency. Increased population density, improved precipitation monitoring networks, and radar-based precipitation estimation have increased the likelihood of capturing (measuring) heavy and often geographically isolated rain events. Additionally, improved communication allows for faster and more complete transfer of weather information. When the neighboring county is walloped by a 100-year storm, we hear about it quickly. Invariably we will vicariously "experience" the event and wonder why 100-year storms seem to be occurring every other week! Will a 100-year storm result in a 100-year flood? A 100-year storm does not necessarily create a 100-year flood. Several factors can independently influence the cause-and-effect relationship between rainfall and streamflow: 1. Extent of rainfall in the watershed: When rainfall data are collected at a point within a stream basin, it is highly unlikely that this same amount of rainfall occurred uniformly throughout the entire basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly from the rainfall amount measured at the location of the rain gage. Some parts of the basin may even remain dry, supplying no additional runoff to the streamflow and lessening the impact of the storm. 2. Soil saturation before the storm: Existing conditions prior to the storm can influence the amount of stormwater runoff into the stream system. Dry soil allows greater infiltration of rainfall and reduces the amount of runoff entering the stream. Conversely, soil that is already wet from previous rains has a lower capacity for infiltration, allowing more runoff to enter the stream. 3. Relation between the size of the watershed and duration of the storm: Another factor to consider is the relation between the duration of the storm and the size of the stream basin in which the storm occurs. For example, a 100-year storm of 30-minutes duration in a 1-square-mile (mi 2 ) basin will have a more significant effect on streamflow than the same storm in a 50-mi 2 basin. Generally, streams with larger drainage areas require storms of longer duration for a significant increase in streamflow to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood. A Chance of Rain? - Science Institute 2012-

8 Vocabulary 5 : Storm any disturbed state of the atmosphere and strongly implying destructive and otherwise unpleasant weather. Storms often indicate precipitation. Flood any high flow, overflow, or inundation by water which causes or threatens damage 100-year storm a storm that has a 1 in 100 chance of occurring on a given day. 100-year flood a flood with a magnitude that can be expected to occur on average with a frequency of once every 100 years at a given point or reach on a river. The 100-year flood is usually developed from a statistical distribution that is based on historical floods. Procedure Storm Stories 1. Ask students to get out a piece of paper and a writing utensil. 2. Ask students to close their eyes and remember the biggest, craziest, most historical storm they ve ever experienced. 3. Tell students that you would like them to freewrite about everything they remember about that storm for 3 solid minutes. a. Explain that there is no wrong way to freewrite. The goal is simply to keep moving your pencil and write everything you can think of. If you are stuck, try describing a thing that you remember. i. You can write about the date, the clouds, the people you were with, really big lightning that flashed, what your pets did in response to the storm, what people said about the storm, how you felt during the storm (or after), what you saw on the news, etc. 4. Once students have had time to write, have them verbally share with a partner what they remember from the storm. They can choose to read their freewrite if they would like. 5. Explain to students that some of the storms people described were so big that they earned special names like 50-year storm or 100-year storm. a. Ask students if they have heard these terms before. b. Ask students if they can guess what makes a storm significant enough to earn a name like 50-year storm or 100-year storm. 6. Explain that these names are won based on how much water falls during a certain amount of time. a. 2 inches of rain in one day is different than 2 inches of rain in one hour. A Chance of Rain? - Science Institute 2012-

9 b. Share the Sectional Mean Frequency Distributions for Storm Periods cards with students. Explain that a 100-year storm means that a certain number of inches of rain fell in specified time period. i. Ask the students to look on together. 1. Ask students to follow the 100-year storm a storm that has a 1 in 100 chance of occurring on a given day. 5-minute row all the way to the 100-year column. This (0.62) is how much rain needs to fall in 5 minutes to be called a 100-year storm. a. If you have students move one row up in the 100- year column, you see how much rain needs to fall in a 10-minute period to be called a 100-year storm. And so on. ii. Tell students that we will use this table again at the end of the lesson, but for now, we will set it aside. 7. One challenge with names like 50-year storm or 100-year storm is that people think a 100-year storm can only happen once every hundred years. That s not how it works. A 100-year storm simply has a 1 in 100 chance of happening on any given day. a. This means that there is a 1 in 100 chance the storm will happen today. And, even if a 100-year storm occurs today, tomorrow will have a 1 in 100 chance of having a 100-year storm. 100-year Storm Probability Activity 1. To help us understand this idea of how a 100-year storm could occur, we are going to play with probability 2. Introduce students to the Storm Spinner. a. Students will use the Storm Spinner to study why events with higher probabilities occur more frequently. The areas on the spinner are roughly equivalent to the chances of having a 2, 5, 10 or 100-year storm. 3. Handout the Storm Probabilities Worksheet. 4. Explain to students that you are going to break them into 6 groups. Within their group, a. they will spin the spinner 100 times and tally the results on a worksheet b. calculate the percentage the group got for each storm, c. You may need to review how to calculate percentages. i. Formula for calculating percentages (example): total number of 2-year storms/total number of storms = percentage of 2-year storms A Chance of Rain? - Science Institute 2012-

10 d. and answer the following questions on the worksheet. 5. Break students up into 6 groups. a. Give each group a spinner. 6. Allow time for groups to complete the task. Walk around and ask questions throughout. a. Have you gotten a 100-year storm yet? b. Why do you think the 2-year storm has so much space on the spinner? c. If you spun a 100-year storm on one spin, could you spin it again on the next spin? 7. Once all students have collected their data, calculated percentages, and answered the questions, gather the attention of the students in a large group. 8. Draw the percentages of each storm type box on the board from the student worksheet. 9. Have one student from each group write the percentage of each storm type on the board. 10. Discuss the percentages and answers to the questions on the worksheet. a. Ask students if the breakdown was what they expected. Delineating Storms in Minnesota (Data from 6 and 7 on the reference list.) 1. Explain that figuring out how often these large storms occur is more difficult that it seems. Explain that students will use real data sets to determine what to call powerful storms that have occurred in Duluth, MN. 2. Break students into 6 groups. 3. Hand out one Sectional Mean Frequency Distributions for Storm Periods card to each group. 4. Hand out precipitation reports for 1972 and 2012 storms in Duluth. 5. Look at the daily precipitation report for May 23, 2012. (1.79 inches) Find where this storm falls on the Sectional Mean Frequency Distributions for Storm Periods, North East Minnesota chart 24 hour duration. (9-month) 6. Continue to look at the precipitation data for 2012. Are there any other precipitation events that can be designated as a storm event (2-month, 1-year, 100-year etc)? (Yes, in May there was a 4-month storm, in June there was a 25-year and a 5/10-year storm, July had one 5-month storm) 7. Add the precipitation for May 23 & 24, 2012 together (2.93 inches) and compare the 48-hour rainfall record to the Sectional Mean Frequency Distributions for Storm Periods, North East Minnesota chart 48 hour duration. Does this change the classification of the storm? (Yes, it becomes a 2/5-year storm) 8. Do the same for June 19 and 20, 2012. (7.25 inches, over a 100-year storm) 9. In 1972 there were two significant storm events in Duluth. A Chance of Rain? - Science Institute 2012-

11 10. Look at the daily rainfall for August 15-21. Compare the daily 24-hour precipitation to the Sectional Mean Frequency Distributions for Storm Periods, North East Minnesota chart. a. How does the classification change when you add the daily precipitation records for August 15-21 and look at the 5-day duration? (Aug 15-3-month storm, Aug 16 2-5 year storm, Aug 20 5-10 year storm, Aug 21 1-month storm; Aug 15-21, 7.91 inches well over a 100-year storm) 11. The second rainfall event happened in September. Which day was the second big rainfall event? (9-20-1972) What does this event classify as for a 24-hour duration? (10/25-year storm) 12. Both of these storms created catastrophic flooding, even though the second storm wasn t as big as the first one. What factors may have played into the significance of the damage caused by the second storm? Saturated soils, previous destruction, flooded streams, etc. 13. What is different between the 1972 and 2012 storms? 14. The historical reports of the 1972 storm and flood did not indicate any flooding on August 15-16, the flooding happened after August 20. What factors may have influenced the flooding? 15. Why do multiple 5-year events happen in the same year? Same week? More than one day in a row? Extensions: Delineation of storm categories is based on location. Compare the rainfall needed to qualify as a 50 or 100-year storm in Northeastern Minnesota to East Southeastern Minnesota (included in the kit). Have students brainstorm and research reasons why there are differences in the amount of rainfall to earn these big names. A Chance of Rain? - Science Institute 2012-

12 References: 1. Minnesota Climatology Working Group. (2012). 100-Year Storms. http://climate.umn.edu/doc/misc/100year.htm. Accessed March 5th,. 2. United States Geological Survey.(). Floods: Recurrence intervals and 100-year floods. http://ga.water.usgs.gov/edu/100yearflood.html. Accessed March 5th,. 3. National Oceanic and Atmospheric Administration, National Weather Service. (). A look back at Historic Floods of 1972. http://www.crh.noaa.gov/dlh/?n=1972_floods. Accessed March 5th,. 4. National Oceanic and Atmospheric Administration, National Weather Service. (). Historic Duluth and Northland Flooding 19-20 June 2012. http://www.crh.noaa.gov/dlh/?n=june2012_duluth_flood. Accessed March 5th,. 5. National Oceanic and Atmospheric Administration, National Weather Service. (). Glossary. Retrieved from http://w1.weather.gov/glossary/index.php?letter=number Accessed March 6,. 6. Huff, F.A. & Angel, J.R. (1992). Rainfall Frequency Atlas of the Midwest. Midwest Climate Center, Climate Analysis Center, National Weather Service, National Oceanic and Atmospheric Administration, and Illinois State Water Survey. Pages 130-132. Retrieved from http://www.isws.illinois.edu/pubdoc/b/iswsb-71.pdf Accessed February 28,. 7. Minnesota Climatology Working Group. (January 2, ). Preliminary Local Climatological Data Duluth, MN. Retrieved from http://climate.umn.edu/doc/prelim_lcd_dlh.htm Accessed February 28,. Brought to you by Great Lakes Aquarium, Minnesota DNR MinnAqua Program, Minnesota Sea Grant, and Wolf Ridge ELC. This project is funded in part by the Coastal Zone Management Act, by NOAA s Office of Ocean and Coastal Resource Management, in conjunction with Minnesota s Lake Superior Coastal Program. A Chance of Rain? - Science Institute 2012-