What happens to Tookany Creek when it rains?

Tookany Creek, Tacony Creek in Philadelphia, has both  flooding and creek trash problems that affect Cheltenham and Philadelphia residents.  I have written about the Tookany -Tacony Creek trash problems here, herehere and here.

Today, I am starting a series on the Tookany Creek flooding situation to address 2 keys questions:

  1. Is global warming changing precipitation patterns in the Philadelphia area
  2. What are Tookany Creek flood flow trends

This post discusses Philadelphia area  precipitation trends to address the question on how perceived increases in rainfall intensity may be affecting flooding in Cheltenham Township. A later post will review the Creek flow data.

Rainfall Data at Philadelphia International Airport (KPHL)

Daily rainfall data dating back to 1874 is available at the Phillyweather.net site where users can download an Excel workbook with daily data (link). I used the Phillyweather.net data to evaluate changes in precipitation patterns. Here is what I found:

  • Frequency of high intensity rain storms has increased since 1960. There were only two 4-inch or more rain storm events in the 86 year period 1874-1959,  compared to 12 storm events in the 52 year period 1960-2012.
  • The calculated 100 year return period daily precipitation in the 1874-1959 period was 5.11 inches, that value increased to 6.86 inches in the 1960-2012 period.
  • 9 of top 10 rain storms in the 1874-2012 period occurred since 1960

Details

Most people have heard about the “100 year storm“.  It is actually the storm that has a 1% chance of occurring in a given year.  The figure bellow shows how we can use precipitation frequency analysis to estimate the the 100 year storm for Philadelphia International Airport. Tthe plot shows the peak rain storm in inches per day for each year from 187 4 to  2012 plotted on a Gumbel probability grid (link). This allows us to identify the rainfall for any return period, say 10 years or 100 years.

KPHL_gumbel_1974_2012

The estimated storm 100-year return period, isng the entire 1874-2012 data period is 5.83 inches. Since we   know that  rain intensity has been changing, I’ve  also calculated the 100 year storm estimates for the pre-1960 and post 1960 periods, as shown in this 2nd chart.

KPHL_gumber_comparison

The calculated 100 year storm is affected by the period included in the precipitation frequency analysis.  If we just consider the 1874 to 1959 period, we get a 100 year return period storm of 5.11 inches. If we include just the 1960 to 2021, we get 6.86 inches. Clearly we are seeing higher intensity storms since the 1960s.

NOAA,  using more sophisticated techniques, estimates the Philadelphia Airport 100 year – 24 hour storm to be  7.72 inches (link). Since the maximum 1874-2012 24 hour rain was 6.83 inches, Philadelphia Airport has not seen the NOAA 100 year storm yet.

Perspective on Variation in Rainfall Across our Region

This NOAA map shows the 100 year return period rain for the mid-Atlantic region, including Pennsylvania, Delaware and New jersey. Notice the green – yellow areas in western Pa and the purple areas in Delaware and New jersey.  The green – yellow ares have estimated 100 year storms of 5 – 6 inches while the purple areas have storms of 10 – 11 inches. The Philadelphia area is right in the middle at 7.01 – 8 inches.

(Click image to enlarge)noaa_atlas_14

You can access this NOAA map here.

The next post will discuss Creek flow data from the USGS  gaging station at Adams Avenue.

Do you have data related questions about rainfall and creek flow? If you do, let me know and I’ll see what I can do. Just click in the the Reply box and submit your question/ comment.

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