Analysis Techniques: Flood Analysis Tutorial with Instaneous Peak Flow Data (LogPearson Type III Distribution)
Download DataView and print this webpage as a pdf file.Step 1: Obtain streamflow data
Step 2: Organize the information in a table.
Step 3: Rank the data from largest discharge to smallest discharge using the "sort" command. Add a column for Rank and number each streamflow value from 1 to n (the total number of values in your dataset).
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 the 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 probability of 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 skewnessShow 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

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