×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Rainfall Intensity Curves

## Rainfall Intensity Curves

(OP)
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?

### RE: Rainfall Intensity Curves

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

### RE: Rainfall Intensity Curves

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

### RE: Rainfall Intensity Curves

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

### RE: Rainfall Intensity Curves

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

### RE: Rainfall Intensity Curves

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

### RE: Rainfall Intensity Curves

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.

### RE: Rainfall Intensity Curves

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?

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!