Analysis Techniques: Flood Analysis Tutorial with Daily Data (LogPerason Type III Distribution)

Information to get started:
 The lesson below contains stepbystep instructions and "snapshots" of what each step
looks like when carried out in a Microsoft Excel workbook. Blue shading of information
in the Excel illustrations denotes changes made from the previous step. Dots placed in
three consecutive rows indicate that a portion of data is hidden from sight.
 You can download an Excel workbook containing the complete data set by clicking on the
"Download Data" link below. It contains
each calculation step on a separate worksheet. To move between steps, click on the
tabs at the bottom of the excel window.
 When you download the file, it may open in your browser window. You may wish to use the
"save as" function to save the file to a local drive and then reopen it in Excel. This
will make it easier to flip between the online lesson and the example workbook.
 Finally, we want to remind you that the techniques explained on this site are statistically
based; therefore results must be viewed as predictions and not as facts. Please use
the techniques and the information obtained from them responsibly!

Step 1: Obtain streamflow data
 Obtain daily streamflow data from the USGS web site.
 Go to http://oregon.usgs.gov
 Select Historical Water Data
 Select Surface Water
 Select Daily Data
 Check box under Site Identifier for Site Name and Submit
 Type in Alsea under Site Name and select match any part and Submit
 Select gage at TIDEWATER (140306500)
 Select Tabseparated data
 For the tutorial, copy the data for water years 1990 through 2000
into an Excel worksheet
 Paste special as text (this will separate the data into columns
 Calculate the maximum discharge for each water year in the period of record.
 The AVERAGE, MAX, and MIN functions in excel can be used to calculate
these values for each water year. It may be more efficient to calculate
the mean, maximum, and minimum flows for each water year in the period
of record at one time.
Step 2: Organize the information in a table.
Step 3: Rank the data from largest discharge
to smallest discharge. 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 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.
variance =
standard deviation =
skew coefficient =
Step 12: Calculate k values
 Use the frequency factor table
and the skew coefficient to find the k values
for the 2,5,10,25,50,100, and 200 recurrence intervals
 If the skew coefficient
is between two given skew coefficients in the table than you can linearly
extrapolate between the two numbers to get the appropriate k value. To
view the frequency factor table click on the button below.
Step 13: Using the general equation, list the discharges
associated with each recurrence interval
general equation =
_{}
Step 14: Create table of Discharge values found using
the log – Pearson analysis
Step 15: 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 instantaneous peak discharge data, the result would have
been a more conservative estimation of the discharges associated with each
return period.
