Flood Recurrence Intervals and the 100 Year Flood

Flood Recurrence Intervals and the 100 Year Flood Concept Bruce F. Rueger, Department of Geology, Colby College, Waterville, ME 04901 A Local Example, the Flood of 1987 (a 500 year flood) The purpose of this exercise is to expose students to using data obtained from the internet, manipulating it into an useful form and interpreting it. At the same time we hope that they begin to get an understanding of flood recurrence intervals, hydrographs and the magnitude of floods and how they are categorized. We begin by taking the students into the field and by use of some very simple tools, i. e. , a tape measure, a concrete weight on a rope and a stream velocity meter, we allow them to determine stream discharge. This is accomplished by calculating the cross-sectional area of the stream by measuring the width of the stream and determining the average depth of the stream. Velocity is measured by use of a velocity meter. The resultant values are the inserted into the equation Q= W x D x V, where Q is the discharge, W is the average width, D is the average depth and V is the average velocity. We do this at two very different locations on the stream to demonstrate the variability and the problems associated with the “appearance” of the stream. In the following week’s lab we take the concept of discharge to another level by introducing the flood recurrence interval. Here they get data from the internet and manipulate it to create something that can be interpreted. width (m) depth (m) From there we introduce the concept of the “ 100 -year Flood and how it relates to real-life situations, such as in insurance premiums. velocity = m s-1 To determine flood recurrence interval, students are directed to the USGS website: http: //waterdata. usgs. gov/nwis/sw bfrueger@colby. edu From there they select a site. This one illustrates the example in their lab exercise. Peak Streamflow (cfs) Rank (M) Recurrence Interval (R) log R 15500 1 97. 00 1. 99 2940 49 1. 98 0. 30 13800 2 48. 50 1. 69 2810 50 1. 94 0. 29 12400 3 32. 33 1. 51 2810 51 1. 90 0. 28 11800 4 24. 25 1. 38 2740 52 1. 87 0. 27 10900 5 19. 40 1. 29 2740 53 1. 83 0. 26 8870 6 16. 17 1. 21 2630 54 1. 80 0. 25 8770 7 13. 86 1. 14 2580 55 1. 76 0. 25 8580 8 12. 13 1. 08 2570 56 1. 73 0. 24 7780 9 10. 78 1. 03 2560 57 1. 70 0. 23 6940 10 9. 70 0. 99 2560 58 1. 67 0. 22 6720 11 8. 82 0. 95 2520 59 1. 64 0. 22 6700 12 8. 08 0. 91 2520 60 1. 62 0. 21 6520 13 7. 46 0. 87 2480 61 1. 59 0. 20 6410 14 6. 93 0. 84 2410 62 1. 56 0. 19 6220 15 6. 47 0. 81 2360 63 1. 54 0. 19 6170 16 6. 06 0. 78 2300 64 1. 52 0. 18 6050 17 5. 71 0. 76 2220 65 1. 49 0. 17 5650 18 5. 39 0. 73 2090 66 1. 47 0. 17 5230 19 5. 11 0. 71 2050 67 1. 45 0. 16 5200 20 4. 85 0. 69 2030 68 1. 43 0. 15 5140 21 4. 62 0. 66 1980 69 1. 41 0. 15 4750 22 4. 41 0. 64 1950 70 1. 39 0. 14 4480 23 4. 22 0. 63 1940 71 1. 37 0. 14 4340 24 4. 04 0. 61 1930 72 1. 35 0. 13 4330 25 3. 88 0. 59 1920 73 1. 33 0. 12 4330 26 3. 73 0. 57 1880 74 1. 31 0. 12 4200 27 3. 59 0. 56 1860 75 1. 29 0. 11 4200 28 3. 46 0. 54 1850 76 1. 28 0. 11 4150 29 3. 34 0. 52 1830 77 1. 26 0. 10 4150 30 3. 23 0. 51 1810 78 1. 24 0. 09 4100 31 3. 13 0. 50 1790 79 1. 23 0. 09 4060 32 3. 03 0. 48 1620 80 1. 21 0. 08 3800 33 2. 94 0. 47 1590 81 1. 20 0. 08 3650 34 2. 85 0. 46 1560 82 1. 18 0. 07 3450 35 2. 77 0. 44 1560 83 1. 17 0. 07 3450 36 2. 69 0. 43 1520 84 1. 15 0. 06 3450 37 2. 62 0. 42 1490 85 1. 14 0. 06 3450 38 2. 55 0. 41 1420 86 1. 13 0. 05 3380 39 2. 49 0. 40 1400 87 1. 11 0. 05 3370 40 2. 43 0. 38 1390 88 1. 10 0. 04 3340 41 2. 37 0. 37 1380 89 1. 09 0. 04 3300 42 2. 31 0. 36 1370 90 1. 08 0. 03 3270 43 2. 26 0. 35 1360 91 1. 07 0. 03 3250 44 2. 20 0. 34 1290 92 1. 05 0. 02 3250 45 2. 16 0. 33 1260 93 1. 04 0. 02 3120 46 2. 11 0. 32 1180 94 1. 03 0. 01 3100 47 2. 06 0. 31 1100 95 1. 02 0. 01 3000 48 2. 02 0. 31 1040 96 1. 01 0. 00 1. 2. 3. 4. 5. March 14, 1936, ice jams at Swan Island Browns Island April 2, 1987, no ice jams March 2, 1896, ice jam at Swan Island at other points above Swan Island February 20, 1870 March 26, 1826, ice jams at Browns Island Swan Island Kennebec River at Hallowell, ME (heights marked at 136 Water St. ) Five largest historic peak flows measured at the Waterville, ME gage on the lower Kennebec River (table below right) Discharge in cubic feet Year per second (cfs) 1987 1902 1936 1923 1896 194, 000* 157, 000 154, 000 135, 000 113, 000 *Estimated • A flood that occurs, on average, one time in a 100 -year period 100 -Year Flood - The Concept • Has one chance in 100 (1%) of occurring in any given year • Used as the basis for most flood protection plans Using the equation created by the graphing exercise, the discharge for any flood of any recurrence interval can be determined. Since the historical record of the gauging station on the Kennebec at Bingham only goes back ~75 years, we ask students to determine The discharge of the 100, 500 and 1000 year floods. Knowing that the log of 100, 500 and 1000 are 2, 2. 7 and 3 respectively, by inserting them in the equation of the line on the graph, they can determine that the discharge for these events would be in the neighborhood of 72, 796, 92, 821 and 101, 403 cfs, respectively. Here is site specific data from which a table can be downloaded. This figures illustrates the table that can then be saved as a text document and opened with Excel. References Pipkin, Trent and Hazlett. 2005 Geology and the Environment (4 e), Chapter 9 Chernicoff and Whitney. 2007. Geology (4 e) Chapter 15 http: //waterdata. usgs. gov/nwis/sw http: //me. water. usga. gov/kenmon. html http: //www. sciencecourseware. com/Virtual. River/