Flood frequency analysis excel

In this assignment, you will be creating a flood frequency curve for a California river, the Merced River. A flood frequency curve is used in hydrological studies, to get an idea of the statistical likelihood of floods of a given size on a river. Obviously, there are a modest number of small floods, a moderate number of large floods, and a very small number of gigantic floods. But what streamflow would translate as “small”, large”, or “gigantic” on a particular river? And if you could attach streamflow measurements to various floods, how frequently do those streamflows occur? That is the purpose of the flood frequency curve. In this lab you will process raw Excel data from the USGS to construct a flood frequency curve.

You are expected to revisit your Picture Yourself Learning Microsoft Excel 2010 textbook chapters (as assigned in the syllabus) to find the techniques necessary to do the manipulations outlined in this assignment. I will guide you through the hydrological work, and tell you what to do accomplish in your Excel workbook, but not provide every step of how to accomplish the required spreadsheet tasks; those are for you to have learned from your readings, and apply here.

Hydrological background

A flood frequency curve is a graph which summarizes the statistical behavior of peak river runoff. A sample flood frequency curve is shown below.

The annual probability is the chance for a flood of that size (or smaller) occurring in any particular year. The formula for the annual probability p is given as:

1

The letter m is the rank of the annual peak discharge (or streamflow, in cubic feet per second), and n is data points represented in the analysis. In a flood frequency analysis, all years are treated independently, so there will only be one peak runoff per year. Thus, if there are 70 years of data, there should be n=70 data values being analyzed. The smaller the size of m, the greater the flood. The largest flood is ranked number 1.

The quantity represented by p-1 is the return period. This is the duration of time, on average, between flood events of a certain discharge. The return period is an alternate way to express the statistical likelihood, as a whole number. For example, if the annual probability p is 0.0625 (6¼ %), the return period would be p-1 , or 16 years. Thus the water quantity representing the 6.25% annual probability would alternatively be known as the 16-year flood. The smaller the annual probability—or longer the return period—the greater the flood discharge. But it is not a linear proportionality between probability and discharge, which is why the need for the flood frequency curve arises.

Necessary steps

1. Import the data file into Excel, by opening it from within Excel. Choose the tab-delimited option.

2. Copy the geographic and data information notes into a new worksheet. Rename this worksheet with the name metadata. Rename the original worksheet data with the station number given in the metadata, and color its tab light blue.

3. Insert the column headings of Date and Discharge (cfs). 4. Delete the entries from 10/1/1916 through 12/31/1916, and 1/1/2002 through 9/30/2002. 5. Insert a new column, Year, between Date and Discharge. 6. Use the year function to have Excel compute the year for each row. 7. Perform (auto)filtering to have Excel display only one year. Then click the discharge

column to display the individual choices available for that year. The data will be sorted in ascending order. Note the maximum discharge and type it into a new worksheet tab, named Annual Peaks.

8. Next to the discharge, type the associated year. Click in the cell entry, then use the Fill (Series) button to automatically fill in every year of record.

9. Repeat step 7 for every year. 10. In the Annual Peaks worksheet, add a new column for the annual probability. Program it

using an Excel formula, and compute the annual probability for every discharge. 11. In a separate worksheet, generate a hydrograph for the annual discharges. Remember the

important convention for independent and dependent variables. Make sure you label all axes, and title the figure.

12. On a separate worksheet, generate a flood frequency curve for the data. You should use markers for your data points. The flood frequency data should be expressed using return periods. Again, use the proper convention for independent and dependent variables, and label and title your work. One of the axes should be plotted on a semilogarithmic scale (traditional for this type of graph, for compact viewing).

13. Use your graph to estimate the return period for a discharge of 14,800 cfs. State this value, and label it on your flood frequency graph, using a textbox callout.

14. Save your Excel file with your name and FF (for flood frequency), and submit it to the instructor. For example, Ashley Smith would submit ASmithFF.xls.