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Middle Atlantic River Forecast Center
 
 
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Calculation Information

Several statistics have been computed from MARFC's flood climatology data. We hope this page will help you to understand the method behind each calculation. The calculations explained on this page are:

1. Frequency of flood exceedance in each month during the period of record.

2. Percent of total occurrences of flood stage exceedance in each month during the period of record.

3. Return period between flood stage exceedance and floods per year.

4. Flood probability of flood stage exceedance for 1, 5, 10, 15 and 20 year periods.

5. Height above flood stage.

The results of this analysis is available for each of MARFC's forecast points by using the menu above. The method for each calculation is described below.

1. Frequency of flood exceedance in each month during the period of record

This chart shows how many times each site has exceeded flood stage during the period of record.

Monthly Flood Exceedence
Site FS Years J F M A M J J A S O N D Total
Farmville 16 1928-1998 10 8 12 8 5 2 2 4 9 6 4 9 79
Matoaca 10 1969-1998 0 2 6 1 1 1 0 0 1 2 1 0 15
Mattoax 19 1936-1998 16 12 16 12 7 5 5 3 8 6 5 11 106

In this example, you see that Matoaca has flooded 6 times during the month of March.

2. Percent of total occurrences of flood stage exceedance in each month during the period of record

This table shows how many floods have occurred each month as a percentage of the total number of flood events at a particular location.

Percentage for Flood Exceedence
Site FS J F M A M J J A S O N D
Color Code 0-.9 1-19.9 20-39.9 40-59.9 60-79.9 80-99.9 100
Farmville 16 12.7 10.1 15.2 10.1 6.3 2.5 2.5 5.1 11.4 7.6 5.1 11.4
Matoaca 10 0.0 13.3 40.0 6.7 6.7 6.7 0.0 0.0 6.7 13.3 6.7 0.0
Mattoax 19 15.1 11.3 15.1 11.3 6.6 4.7 4.7 2.8 7.6 5.7 4.7 10.4

Percent of Exceedence = [(number of floods per month)/(total number of floods)]*100

In this example, the percentage of flood exceedence in March for Matoaca, VA is [(15 floods in March)/(40 total floods)*100] = 40%.

3. Return period between flood stage exceedance

This chart shows how many floods occur per year and return periods of floods. This data is not exactly like a return period based on by the USGS since the data may not span 100 years. However, it still gives the user an idea of many years for the next flood on average.

Return Period Between Floods and Floods per Year
Site FS Period of Record Years Floods Return Period Floods per Year
Farmville 16 11/29/1928 - 09/30/1998 69.84 79 0.88 Years 1.13
Matoaca 10 10/30/1969 - 09/30/1998 28.92 15 1.93 Years 0.52
Mattoax 19 10/01/1936 - 09/30/1998 62.00 106 0.58 Years 1.71

return period = (period of record in years)/(number of floods)

floods per year = (number of floods)/(period of record in years)

In this example for Matoaca, there is an average of 1 flood every 1.93 years, or 0.52 floods per year. In Farmville, there is an average of 1 flood every 0.88 years, or 1.13 floods per year.

4. Average flood stage exceedance per year.

This chart shows the probability of exceeding flood stage for 1, 5 and 10 year periods.

Probability of Exceeding Flood Stage
Site FS Period of Record Years Floods 1 Year 5 Years 10 Years
Farmville, VA 16 11/29/1928- 09/30/1998 69.84 79 100% 100% 100%
Matoaca, VA 10 10/30/1969 - 09/30/1998 28.92 15 52% 97% 100%
Mattoax, VA 19 10/01/1936 - 09/30/1998 62.00 106 100% 100% 100%

Probability of Exceedence = [(Number of Floods)/((Period of Record in Years/(Year Range))]*100

For example, the table shows Matoaca had 15 floods in 28.92 years. In 1 year, the probability of exceedence is (15 floods)/[(28.92 years)/(1 year)]* 100 = 52%. In 5 years, the probability of exceedence is (15 floods)/[(28.92 years)/(5 years)]* 100 = 97%.

5. Height Above Flood Stage

For this calculation, the data was broken into one foot intervals of how high a flood has exceeded flood stage. The table shows the number of occurrences, percent of occurrence and cumulative percent of occurrence.

Height Above Flood Stage Number of Occurrences Percent of Occurrence Cumulative Percent of Occurrence
Total 15    
0 to 1 ft 2 13.3 100.0
1 to 2ft 5 33.3 86.7
2 to 3 ft 4 26.7 53.3
3 to 4 ft 1 6.7 26.7
4 to 5 ft 2 13.3 20.0
5 to 6 ft 0 0.0 6.7
6 to 7 ft 0 0.0 6.7
7 to 8 ft 0 0.0 6.7
8 to 9 ft 1 6.7 6.7
9 to 10 ft 0 0.0 0.0
10 to 11 ft 0 0.0 0.0
11 to 12 ft 0 0.0 0.0
12 to 13 ft 0 0.0 0.0
13 to 14 ft 0 0.0 0.0
14 to 15 ft 0 0.0 0.0
15 to 16 ft 0 0.0 0.0
16 to 17 ft 0 0.0 0.0

Number of Occurrences - The number of times a flood crest occurred. In this example, 4 flood crests have occurred 2-3 ft above flood stage.

Percent of Occurrence - The number of times a flood crest has occurred divided by the total number of floods turned into a percentage. In this example, (4 crests/15 total floods) = 26.7%.

Cumulative Percent of Occurrence - Cumulative percentage of the chance that height above flood stage has been reached. It begins at the highest mark above flood stage and works it way up to the top of chart accumulating floods and percentages as it goes up. In this example, (6.7% + 13.3% + 6.7 % + 26.7 %) = 53.3% of the floods have crested at least 2-3 feet above flood stage.


Data provided by the U.S. Geological Survey

Additional support for this project was provided by the Susquehanna River Basin Commission, National Weather Service and Penn State University (PSU) Department of Meteorology. Special thanks to Michael Kozar (PSU student) for writing numerous weather summaries in the Floods by Events section.

Questions should be directed to Charles Chillag or Alaina MacFarlane.


NOAA's National Weather Service
Middle Atlantic River Forecast Center
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State College, PA 16803
Page Authors: Charles Chillag & Alaina MacFarlane
Webmaster's E-mail: marfc.webmaster@noaa.gov
Page last modified: December 31, 2012
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