Question

Lab Assignment

Part B - Recurrence Intervals

Data from the chart below was collected at the USGS site and includes the 20 largest discharge events for Sweetwater Creek at station 02337000 from January 1, 2008 through May 1, 2015, excluding the dramatic 2009 flood (we will learn more about that later). In order to create a flood-frequency graph, you must first calculate the recurrence interval (one is calculated below as an example). A recurrence interval refers to the average time period within which a given flood event will be equaled or exceeded once. To calculate it, first evaluate the rank of the flood, with a “1” going to the highest discharge event and a “20” going to the lowest discharge event. Calculate the recurrence interval using the following equation:

RI = (n+1) ÷ m

where, RI = Recurrence Interval (years)

n = number of years of records (in this case, 8)

m = rank of flood. Peak Discharge Date	Discharge (cubic ft/sec = cfs)	Rank	Recurrence Interval (years)
08/27/2008	5,140
03/02/2009	2,360
10/13/2009	3,290
11/12/2009	6,120	1	9
12/03/2009	2,860
12/10/2009	2,170
12/19/2009	3,830
12/26/2009	2,650
01/25/2010	2,500
02/06/2010	3,680
03/12/2010	3,600
03/10/2011	2,350
04/17/2011	3,100
02/24/2013	2,060
02/27/2013	2,190
05/06/2013	3,610
12/23/2013	3,790
04/08/2014	4,170
01/05/2015	3,970
04/20/2015	2,940

Answer 1

At first I have arranged the Discharge amounts in descending order and got the ranks. The maximum discharge amount i.e., 6120 holds the first rank. The second largest amount, i.e., 5140 is at the second position and so on.Then I have calculated the RI (Recurrence Interval) using the formula RI = (number of years of records + 1) ÷ rank.

In this case numbers of years of records is 8, since the data is from 2008 to 2015.

For the first row, RI = (8 + 1) ÷ 2 = 4.50

For the second row, RI = (8 + 1) ÷ 16 = 0.56

Similarly I have calculated for all the 20 rows as follows

For the 3rd row, RI = (8 + 1) ÷ 10 = 0.90

For the 4th row, RI = (8 + 1) ÷ 1 = 9.00

For the 5th row, RI = (8 + 1) ÷ 13 = 0.69

For the 6th row, RI = (8 + 1) ÷ 19 = 0.67

For the 7th row, RI = (8 + 1) ÷ 5 = 1.80

For the 8th row, RI = (8 + 1) ÷ 14 = 0.64

For the 9th row, RI = (8 + 1) ÷ 15 = 0.60

For the 10th row, RI = (8 + 1) ÷ 7 = 1.29

For the 11th row, RI = (8 + 1) ÷ 9 = 1.00

For the 12th row, RI = (8 + 1) ÷ 17 = 0.53

For the 13th row, RI = (8 + 1) ÷ 11 = 0.82

For the 14th row, RI = (8 + 1) ÷ 20 = 0.45

For the 15th row, RI = (8 + 1) ÷ 18 = 0.50

For the 16th row, RI = (8 + 1) ÷ 8 = 1.13

For the 17th row, RI = (8 + 1) ÷ 6 = 1.50

For the 18th row, RI = (8 + 1) ÷ 3 = 3.00

For the 19th row, RI = (8 + 1) ÷ 4 = 2.25

For the 20th row, RI = (8 + 1) ÷ 12 = 0.75

**If it clears your concept, please let me know.