Analysis Techniques: Flood Analysis Example with Instaneous Peak Flow Data (LogPearson Type III Distribution)
Download DataView and print this webpage as a pdf file.Step 1: Obtain streamflow dataObtain instantaneous peak streamflow data from the USGS web site.
Step 2: Organize the information in a table.
Step 3: Rank the data from largest discharge to smallest discharge.
Step 4: Create a column with the log of each max or peak streamflow using the Excel formula {log (Q)} and copy command.
Step 5: Calculate the Average Max Q or Peak Q and the Average of the log (Q)
Step 6: Create a column with the excel formula {(log Q – avg(logQ))^2}
Step 7: Create a column with the excel formula {(log Q – avg(logQ))^3
Step 8: Create a column with the return period (Tr) for each discharge using Excel formula {(n+1)/m}. Where n = the number of values in the dataset and m = the rank.
Step 9: Complete the table with a final column showing the exceedence probabilityof each discharge using the excel formula {=1/Return Period or 1/Tr} and the copy command.Step 10: Calculate the Sum for the {(logQ – avg(logQ))^2} and the {(logQ – avg(logQ))^3} columns.Step 11: Calculate the variance, standard deviation, and skew coefficient as follows: variance =
standard deviation =
skew coefficient =
Excel functions can also be used to calculate the variance (=VAR( ) ), standard deviation (=STDEV( ) ), and skewness coefficient (=SKEW( ) ). Note that you use these formulas with the data in the log(Q) column.
Step 12: Calculate weighted skewness
Show MeStep 13: Calculate k values
Show Me
Step 14: Using the general equation, list the discharges associated with each recurrence intervalgeneral equation = _{}
Step 15: Create table of Discharge values found using the log – Pearson analysis
Step 16: Create Plot
Below is a comparison of flood frequency analysis completed using mean daily data versus instantaneous discharge data. As can be seen, had you completed this analysis using mean daily data, the result would have been an underestimation of the discharges associated with each return period.

This website was developed by Oregon State University's Civil, Construction, and Environmental Engineering Department with support from the state water institutes program of the U.S. Geological Survey.  
Copyright © 20022005 Oregon State University 
Web Disclaimer Web Address: http://water.oregonstate.edu/streamflow/ Send Comments to: Peter Klingeman 