Example 1. Systematic Record – Moose River at Victory, VT

Example 1 illustrates the computation of a peak flow frequency curve using EMA and Bulletin 17C procedures with an annual maximum series comprised of systematic flood peaks. For this example, USGS gage 01134500 Moose River at Victory, Vermont is used. The Moose River is located in the northeastern part of the state and flows mostly from north to south through very hilly terrain. The Moose River basin is approximately 75 square miles of nearly all forest (England, et al., 2018). Historically, it was an important logging area and some logging still continues today. Attempts at farming in the basin have generally failed due to the presence of shallow rocky soil. There are a small number of villages in the basin, but overall it is sparsely populated with only a few miles of paved roadway. There is also a large bog approximately a third of a mile upstream from the gage. The bog is part of the 5,000 acre Victory Basin Wildlife management area. While there is no streamflow regulation in the basin, the bog attenuates peaks in the basin. The Moose River at Victory, VT stream gage has an annual peak record consisting of 68 peaks beginning in 1947 and ending in 2014, as shown in the Figure below. The annual peak flow record is tabulated within the Table below the Figure.

Table. Moose River at Victory, VT Annual Peak Flow Record.

A Bulletin 17 Analysis using EMA and Bulletin 17C procedures has been developed for this example. To open the analysis, click on the analysis labeled B17C Example 1 from the Study Explorer panel on the left side of the project window. When B17C Example 1 is selected, the Bulletin 17 analysis editor will appear as shown in Figure below. Click on the Guided Setup button to step through the analysis settings.

Click on Steps 1-6 of the Analysis, as shown in the Figure below. Step 1, General Information, shows the dataset used in the study.

The analysis spans 68 water years (WY 1947 - WY 2014) as shown in the Time Window (Step 2) in Figure below.

As shown below, no low outliers have been calculated with Multiple Grubbs-Beck test in Step 3. Low Outlier Detection.

No changes to the EMA Data (Step 5) are necessary (see Figure below). Since this example uses an annual maximum series consisting entirely of systematic data with a complete record, a single zero – inf perception threshold is adequate. No modifications to the default flow ranges and data types are necessary.

As shown in Figure below, the Skew option (Step 5) was set to use the Weighted Skew. To use the weighted skew option, the user must enter a value for the Regional Skew and the Regional Skew MSE.

In this example, a regional skew coefficient of 0.44 was used along with a Regional Skew MSE of 0.078.

The Output Frequency Ordinates have been entered in Step 6, as shown below.

The user can now press the Compute button to perform the analysis. Once the computations have been completed, the Results window will open, containing, from top to bottom, Inputs, Result Tables, and Frequency Curve Plot.

The Inputs, shown below, contain the summary of inputs for the analysis.

The Results Tables should resemble the one in Figure below. It contains Fitted Distribution Parameters and Years of Record on the left, and Frequency Curve Ordinates on the right. 

<<<<Update figure when the correct Equivalent Record Length is calculated.>>>>

The Fitted Distribution Parameters table for this example is shown in the Figure below. It contains:

• Statistical metrics for the observed station data:
  • mean,
  • standard deviation, and
  • station skew
• Regional adjustment:
  • regional skew and weighted skew.
• <<<Additionally, the computed EMA Estimate of MSE (G at-site) and Grubbs-Beck Critical Value are shown within this table.>>>>

The Years of Records table below shows:

• Number of systematic, historical and censored events used in the analysis
• Number of high outliers, low outliers and zero flows found
• Number of missing flows, and
• The Equivalent Record Length (ERL) in years

<<<TODO: update when ERL an the field above it are fixed >>>>.

The Frequency Curve Ordinates table, shown in the Figure below, contains the following results:

• Annual Exceedance Probability
• Computed Flow (Log Pearson Type III results)
• Confidence Limits (5% and 95% chance exceedance)

In addition to the tabular results, a Frequency Curve Plot, a graphical plot of the computed frequency curves, is added on bottom of the results window (see Figure below). The plot contains Log Pearson Type III distribution fit using EMA and Bulletin 17C procedures to the input annual maximum flow data set, the 5% and 95% confidence limits, and the computed Hirsch/Stedinger plotting positions.

Because the annual peak flow record contains only systematic peaks with no historic information, no censored peaks, and no low outliers identified using the Multiple Grubbs-Beck Test, the fitted peak flow frequency curve using EMA and Bulletin 17C procedures is the same as that obtained using Bulletin 17B procedures. However, due to the appropriate consideration of the uncertainty in the skew coefficient within the Bulletin 17C procedures, the 5% and 95% confidence limits are different when compared against results obtained using Bulletin 17B procedures.

<<In addition to the tabular and graphical results, there is a report file that shows the order in which the calculations were performed. To review the report file, press the View Report button at the bottom of the analysis window. When this button is selected, a text viewer will open the file and display it on the screen. Shown in Figure 6 is the report file. The report file contains a listing of the input data, preliminary results, outlier and historical data tests, additional calculations needed, and the final frequency curve results. Different types and amounts of information will show up in the report file depending on the data and the options that have been selected for the analysis. The user should review the report file to understand how the Bulletin 17C procedures were applied.>>>

Figure 6. Report File for B17C Example 1.

Continue to:  Example 2.

Return to: Bulletin 17C Examples.