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Flood Recurrence Intervals and the 100 Year Flood ConceptBruce F. Rueger, Department of Geology, Colby College, Waterville, ME 04901 [email protected]
width (m)
depth (m)
velocity = m s-1
1. March 14, 1936, ice jams at Swan Island and Browns Island 2. April 2, 1987, no ice jams 3. March 2, 1896, ice jam at Swan Island and at other points above Swan Island 4. February 20, 1870 5. March 26, 1826, ice jams at Browns Island and 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)
ReferencesPipkin, Trent and Hazlett. 2005 Geology and the Environment (4e), Chapter 9Chernicoff and Whitney. 2007. Geology (4e) Chapter 15http://waterdata.usgs.gov/nwis/sw http://me.water.usga.gov/kenmon.html http://www.sciencecourseware.com/VirtualRiver/
YearDischarge in cubic feet
per second (cfs)
1987 194,000*
1902 157,000
1936 154,000
1923 135,000
1896 113,000
*Estimated
Peak Streamflow
(cfs)
Rank (M)
Recurrence Interval (R)
log R
15500 1 97.00 1.99
13800 2 48.50 1.69
12400 3 32.33 1.51
11800 4 24.25 1.38
10900 5 19.40 1.29
8870 6 16.17 1.21
8770 7 13.86 1.14
8580 8 12.13 1.08
7780 9 10.78 1.03
6940 10 9.70 0.99
6720 11 8.82 0.95
6700 12 8.08 0.91
6520 13 7.46 0.87
6410 14 6.93 0.84
6220 15 6.47 0.81
6170 16 6.06 0.78
6050 17 5.71 0.76
5650 18 5.39 0.73
5230 19 5.11 0.71
5200 20 4.85 0.69
5140 21 4.62 0.66
4750 22 4.41 0.64
4480 23 4.22 0.63
4340 24 4.04 0.61
4330 25 3.88 0.59
4330 26 3.73 0.57
4200 27 3.59 0.56
4200 28 3.46 0.54
4150 29 3.34 0.52
4150 30 3.23 0.51
4100 31 3.13 0.50
4060 32 3.03 0.48
3800 33 2.94 0.47
3650 34 2.85 0.46
3450 35 2.77 0.44
3450 36 2.69 0.43
3450 37 2.62 0.42
3450 38 2.55 0.41
3380 39 2.49 0.40
3370 40 2.43 0.38
3340 41 2.37 0.37
3300 42 2.31 0.36
3270 43 2.26 0.35
3250 44 2.20 0.34
3250 45 2.16 0.33
3120 46 2.11 0.32
3100 47 2.06 0.31
3000 48 2.02 0.31
Peak Streamflow
(cfs)
Rank (M)
Recurrence Interval (R)
log R
2940 49 1.98 0.30
2810 50 1.94 0.29
2810 51 1.90 0.28
2740 52 1.87 0.27
2740 53 1.83 0.26
2630 54 1.80 0.25
2580 55 1.76 0.25
2570 56 1.73 0.24
2560 57 1.70 0.23
2560 58 1.67 0.22
2520 59 1.64 0.22
2520 60 1.62 0.21
2480 61 1.59 0.20
2410 62 1.56 0.19
2360 63 1.54 0.19
2300 64 1.52 0.18
2220 65 1.49 0.17
2090 66 1.47 0.17
2050 67 1.45 0.16
2030 68 1.43 0.15
1980 69 1.41 0.15
1950 70 1.39 0.14
1940 71 1.37 0.14
1930 72 1.35 0.13
1920 73 1.33 0.12
1880 74 1.31 0.12
1860 75 1.29 0.11
1850 76 1.28 0.11
1830 77 1.26 0.10
1810 78 1.24 0.09
1790 79 1.23 0.09
1620 80 1.21 0.08
1590 81 1.20 0.08
1560 82 1.18 0.07
1560 83 1.17 0.07
1520 84 1.15 0.06
1490 85 1.14 0.06
1420 86 1.13 0.05
1400 87 1.11 0.05
1390 88 1.10 0.04
1380 89 1.09 0.04
1370 90 1.08 0.03
1360 91 1.07 0.03
1290 92 1.05 0.02
1260 93 1.04 0.02
1180 94 1.03 0.01
1100 95 1.02 0.01
1040 96 1.01 0.00
Flood Recurrence Interval Ramapo River, NJ
y = 6980.9x + 853.14
R2 = 0.9814
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
0.00 0.50 1.00 1.50 2.00 2.50
Log R
Pea
k Str
eam
flow
(cf
s)
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 theybegin to get an understanding of flood recurrence intervals, hydrographs and the magnitudeof 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 thestream. Velocity is measured by use of a velocity meter. The resultant values are theinserted 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 anotherlevel by introducing the flood recurrence interval. Here they get data from the internet and manipulate it to create something that can be interpreted.
From there we introduce the concept of the “100-year Flood and how it relates to real-life situations, such as in insurance premiums.
To determine flood recurrence interval, students are directed to the USGS website:http://waterdata.usgs.gov/nwis/sw
From there they select a site. This one illustrates theexample in their lab exercise.
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.
A Local Example, the Flood of 1987 (a 500 year flood)
100-Year Flood - The Concept
• A flood that occurs, on average, one time in a 100-year period
• Has one chance in 100 (1%) of occurring in any given year
• Used as the basis for most flood protection plans
Flood Recurrence Interval Kennebec River
y = 28607x + 15582
R2 = 0.9425
0
10000
20000
30000
40000
50000
60000
70000
80000
0.00 0.50 1.00 1.50 2.00
Log R
Pea
k St
ream
flow
(cf
s)
Using the equation created by the graphingexercise, the discharge for any flood of anyrecurrence 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 yearfloods.
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 forthese events would be in the neighborhoodof 72,796, 92,821 and 101,403 cfs, respectively.
