Flood Recurrence Intervals and the 100 Year Flood Concept Bruce F. Rueger, Department of Geology, Colby College, Waterville, ME width (m) depth (m) velocity = m s 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, 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) References Pipkin, Trent and Hazlett Geology and the Environment (4e), Chapter 9 Chernicoff and Whitney Geology (4e) Chapter Year Discharge in cubic feet per second (cfs) ,000* , , , ,000 *Estimated Peak Streamflow (cfs) Rank (M) Recurrence Interval (R) log R Peak Streamflow (cfs) Rank (M) Recurrence Interval (R) log R 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. 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: From there they select a site. This one illustrates the example 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 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.