Geology 127 (Hydrogeology)

California State University, Sacramento

Week 3: Flood frequency estimates, flooding in Sacramento



Upon completion of this unit students should be able to:

  • Describe how hydrologists measure stream velocity and discharge

  • Use the Manning equation as an indirect method of estimating stream discharge

  • Describe the tools and methods that hydrogeologists use to estimate flood probability.

  • Discuss assumptions and potential errors with flood frequency analysis.

  • Describe the levee and reservoir systems that control the American (and Sacramento) rivers.

  • Evaluate the probability of a major flood in the American River basin.

  • Read and understand a 100 year flood map.

  • Describe the specific inflow and outflow volumes that indicate flood hazards in the American river system, including information from inundation maps and the City of Sacramento's Comprehensive Flood Management Plan.


Reading Assignments:

Fetter,  pp. 55-60 and class handouts (diagrams)


Class notes:

Lecture 4: Stream gauging, hydrographs

Lecture 5: flooding in northern California

Additional resources:

  • City of Sacramento Comprehensive Flood Management Plan, Sacramento Dept. of Utilities, Sept. 1995. Available through Gary Reents, Engineering Services Manager, Dept. of Utilities, tel # 433-6633.

Note: on-line versions of flood maps are available at:

  • USGS Bulletin #17B of the Hydrology subcommittee, 1982, Guidelines for determining flood flow frequency, Interagency advisory committee on water data, US Dept. of the Interior, Reston VA.
  • Sauer, V. B., and Meyer, R. W., 1992, Determination of error in individual discharge measurements, USGS Open File Report #92-144, Norcross, GA, 21 p.
  • Waanenen, A. O., and Crippen, J. R., 1977, Magnitude and frequency of floods in California, USGS Water-Resources Investigations 77-21, 96 p.
  • Cudworth, A. G., 1992, Flood Hydrology Manual, United States Department of the Interior, Denver, Water Resources Technical Publication, 243 p.
  • CDEC Web site (home page): has links to river flow and reservoir storage data for major waterways in California. Also includes weather forecasts, storm warnings, temperature and precipitation data, and snow pack information.
  • CDEC river stage definitions: a good, non-technical reminder about the differences between flood monitoring, project flood stage (design capacity) and flood danger stage.
  • CDEC list of monitoring and flood stage elevations for rivers of California. This helps interpret news media reports, which often give river stage information without additional data about levee or river capacity.
  • CDEC direct link to American River data: shows inflow, storage and outflow in the American River system. Higher levels of the Web site have information for other California rivers. Updated several times per day, usually very current and accurate.
  • Sacramento Bee, section B, back page: Contains daily storage and flow information for the American River system. Usually current to within 24 hours of present.

Homework Assignment:

Homework Assignment:

Note:  This assignment counts as 200 points, or double the normal homework value.


Flood magnitude/frequency curves, flood probability.


Access a web site, and download annual peak discharge for a stream gauge that has at least 60 years of record. Plot the data, and discuss your results.  See below for details.  You can choose any stream or data set, but your stream gauge or data set must be different than the data sets used by your classmates.

CDEC Web site:

The address for the CDEC (California data exchange center) is given below. This Web site is useful for obtaining stream flow data in California.

USGS Web site:

The USGS maintains a Web site with stream flow and water quality information.  This can be accessed through the main USGS page (

At this site you will need to choose a Geographic area (California?), then narrow your request until you find a river for you homework assignment.  One option is to select the State, then a County.  You can also pull up a list of site I.D.'s, and type in the site I.D.  Make sure you choose "Tab-separated data" when you download your data.  This format will be easier to load into Excel.

Guidelines and point value for the homework assignment:

Your report should be structured like a formal lab write-up or technical report.  Text should be word-processed and spell-checked.  Use a technical writing style- simple, direct presentation of the data and calculations, then a detailed discussion of results.  Each section should include an introductory sentence or paragraph before you present data.  Each figure and table must have a number, and each figure or table must be referred to in the text. 

Include the following sections when you hand in your assignment:

1) Title or cover page

2) Introduction and purpose  5 points

3) Methods  5 points

4) Sample calculations-  Show sample calculations for placement (probability) and recurrence interval.  Label and discuss this sample calculation so that I know what year/value you are using for your example.    20 points

This equation will help you calculate probability:

% probability = (1/period of recurrence) x 100

Example: Calculate the probability of a flood with 2 year period of recurrence:

% probability = (1/2) x 100

% probability = 50%

extra probability paper

5) Plot a graph of peak flow vs. magnitude/ probability.  This graph should have probability on the x-axis, and discharge on the y-axis.  You will need to plot this graph by hand, using the special probability paper that I hand out in class.  Label the following events on your graph:

2 year flood, 50 year flood, 100 year flood, 200 year flood

Use dashed or colored lines to label your graph.  Show how each period of recurrence (x axis) is represented by a flow (y axis).  25 points

6) Results:  What flows would be associated with each of these events:  a 2 yr flood, a 50 yr flood, a 100 yr flood and a 200 yr flood?   Include the results in a table in your written report.  Also include the sorted data set with annual peak flow and probability in your report.  Include year, rank, peak flow and placement values that you used to plot your graph. 25 points

7) Discussion of results.  As part of your discussion, answer this question:  What uncertainties are involved in estimating the magnitude of a 100 or a 200 year flood?   Use these questions to guide your answer:

  • Discuss potential biases or problems with the mathematical method that we used.  What other methods could we have used to analyze or plot the data?
  • What effect would urbanization have on flood frequency? 
  • What effects could a dam have on the hydrograph? 
  • How confident are you that this method provides an accurate estimate of low probability events (i.e. a 50 yr, 100 yr or 200 yr event)? 
  • What would happen if your short record (usually about 100 years) contained an event that was less common, i.e. a 200 year flood or a 500 year flood?  What would this do to the curve and your estimates??
  • How certain is a flood map drawn on a 100 year flood estimate? 

Put your answer in the form of an essay or paragraph.  I want to see insight into the method that you have just used.  20 points

Due date:  Wednesday, Sept. 19