Rainfall Intensity Curves 1

[fuz]

Is there a specific or general equation for determining the intensity vs. Time of Concentration graphs local to Southern New Jersey? If so, where could they be found?

 

[BillHolt]

You can construct them yourself for any location from the TP-40 (or whatever the later version is named) precip/freq/duration maps. For each frequency, note the series of precip amounts for each duration. Divide precip by duration to obtain the series of maximum average intensities for each frequency. Plot and connect the dots for each freq curve.

Yes, there are generalized equations which will closely approximate the same result, but they require that you solve for a series of coefficients by a iterative routine that's probably not worth your time to set up for a single case. Due to some work I've done in the past, I'm set up for it and will gladly kick the equations back to you if you'll provide an exact location.

Bill Holt

 

The area is Southern New Jersey. We have the intensities from the graphs, we are just trying to set up a storm sewer program in house that calculates the intensity after you plug in a Tc. We have it working right now but it is based on all data points not an equation of a line. It was very time consuming but it works. The equation of the line would be quicker to work with and probably more exact. But whatever you can give us is great. Thanks

 

[BillHolt]

Good news, bad news. I found my original work on this stuff (done a number of years ago for the same reason you're doing it) but don't seem to be able to locate my TP-40!

Here's the general equation. Other forms are possible, but I've found this to generate good result with an average error accross the table that is usually less than 1%.

Intensity = (LOG10(freq*mr+br)/(duration^B+C)

where:
intensity is in inches per hour
frequency is in years
duration is in minutes

For Nashville, TN, the coefficients are:
mr = 29.72093365
br = 45.42764707
B = 0.810701783
C = 5.414353427

Of course, you can multiply the intensities by the durations to return the rainfall amounts.

Give me the rainfall amounts (in inches) for the 6 hour storm at your location (for f = 2, 5, 10, 25, 50, and 100 year) and I'll run your coefficients.

Bill Holt

 

Here are the rainfall amounts in inches for the 24 hr storm event:
2 = 3.5
5 = 4.5
10 = 5.5
25 = 6.2
50 = 6.8
100 = 7.6
I couldn't find the 6 hour storm and we rarely use that, we use Type III, 24 hour storm events. Hopefully this can work.
Thanks
Bob Watkins

 

[BillHolt]

Note: Working out the coefficients from the 100 year data is probably the weakest approach, but my hunch is that it will probably still be ok. Your precision in reading from the map is a factor, as is the accuracy of the map.

mr = 36.31055551
br = 44.4089066
B = 0.81070128
C = 5.414311558

Be sure to check the results across a wide range of the mapped values. I'm accustomed to working in areas where the type 2 distribution is preferable, but my limited experience is that the results are also good for type 3 areas. (In fact, I've only had real problems coming up with a good set of coefficients for areas of the West Coast.)

Bill Holt

 

[cheb]

I have a FORTRAN program that fits IDF curves to several common forms, you choose the best fit from the results. You have to supply (duration, intesity) pairs for a range of duration values (say, 5 min - 240 min, as from IDF curves). Works very nicely. I wrote the program so I could automate these calcs in spreadsheets. I'll e-mail it to you if you wish.

 

[juany]

I'm trying to find the unit cumulative increments for the 24-hour 15-min type III storm.. can anybody suggest where I can find this?