**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.

**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.

1. Draw a straight line (best-fit,

2. What is the discharge for a 100 year flood? A 20-year flood?

3. What is the recurrence interval for a flood with

4. What is the average annual flood (i.e., the arithmetic mean of maximum annual discharge for length of record?

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.

B. Saini-Eidukat

NDSU

Environmental Geology

Geology 300