Department of Geosciences
North Dakota State University
Geology 300:
ENVIRONMENTAL GEOLOGY
Flood Frequency Exercise
(Portion of a Landsat Thematic Mapper (TM) image of part of the Missouri River Floodplain near Glasgow, Missouri. For the original and other images, see http://edcwww2.cr.usgs.gov/moberly.html.
According to Dr. Laurence C. Smith, UCLA Department of Geography, "a volcanically-triggered glacial outburst flood or jökulhlaup occurred on the Skeidararsandur, Iceland November 5, 1996, the largest ever recorded in Iceland (at peak flow the discharge rivaled that of the Congo, the worlds second largest river, and was comparable with that estimated for the late Quaternary outburst flood from Glacial Lake Missoula). The flood eroded and deposited up to 9.1 m of sediment, removed major stretches of the most important highway in Iceland, destroyed two bridges, and caused damage estimated at U.S. $15,000,000. The total volume of water released from the glacier is estimated at 3.5 km³, and was the most rapid jškulhlaup recorded for the area. This site is still rapidly adjusting to the extreme erosional and depositional changes caused by this extreme event. For general information and great photographs of this event, see the May 1997 issue of National Geographic (vol. 191, no. 5)". (info from http://lena.sscnet.ucla.edu/iceland.html).

Exercise on River Flood Frequency

[See the publication: Saini-Eidukat, B. (1998) A WWW and spreadsheet based exercise in river flood frequency analysis. Journal of Geoscience Education, v. 46, pp. 154-156.]

Find historical Peak Flow Data on a river of your choice (Excluding the Red River at Fargo), using the U.S.G.S. National Water Information System (NWIS).

Be sure that the river you choose has Peak Flow Data available for a time period of over 40 years.

Here are the steps to take:

• In a web browser, go to the U.S.G.S. National Water Information System (NWIS).
• Choose a river. For example, you may wish to search for records on a river near your home town. You may search the list of gaging stations using station information such as a partial station name or the county where it is located.
• Choose a gaging station on your river.
• Retrieve peak flow values for your station. Enter the maximum range of years to retrieve data. Use calendar years, choose "Annual peaks", and for "Output Format", choose "Tab-delimited text data file" - Date format: MM/DD/YYYY. Click on "Retrieve Data" to receive the data.
• Save the data to file by using the menu command "Save As..." and give the file a relevant name.
• Now, start up a Spreadsheet program such as Excel, and import the data file you have just saved. Do any clean-up or arrangement that is necessary.
  
• In your spreadsheet, use a menu command to sort the data in descending order based on discharge.
• In a new column, create a Data Series from 1 to the number of data points you have, n. These values are referred to as m for "magnitudes."
• In a new column, calculate Recurrence Interval for all data points (each n), using the following formula. Ludgren, 1986, p. 177-180 describes the process of using the Weibull Equation:

  RI = (n + 1)/m,

  which is the average time interval between the occurrence of two discharge events of a given or greater size.

• Using the charting functions of your spreadsheet, or those of a separate plotting program, create a Discharge Frequency Curve. Plot Discharge on the y-axis vs. RI on the x-axis. Although in practice, this is done using special graph paper, for this exercise plotting the X-axis of your curve using a logarithmic scale is acceptable.
• Use your spreadsheet to calculate the average and the median of the recurrence interval. The median is the data point in the middle of a histogram of the data - there are as many measurements above the median as below it.

You may work together with a partner, but each partner should hand in an analysis of a different river. Print out your raw data file, and include the river name and gaging station number.