NOAA Atlas 14 - precipitation intensity 2

[WaterMagna]

I normally work in really large drainage areas, so I may be overthinking this...

I have a small site, less than 5 acres. I need to calculate pre-post project stormwater calcs.

My plan is to use rational equation and use NOAA Atlas 14 for precipitation intensity (I) in the equation. Looking at the in/hr values in the table, it seems like the discharges are really small. Am I missing something?

 

[cvg]

assume you are using PFDS?

https://hdsc.nws.noaa.gov/hdsc/pfds/

your Tc is probably 10 minutes, so you should be seeing some fairly high intensities
make sure you are looking at intensity and not depth ("data type" at the top of the screen)

 

[WaterMagna]

The subdivision regulations state calculations should be completed for 25-year 24-hour event, which is like 0.2 in/hr. Is that really my intensity for design?

Yes, I am using the precipitation estimates.

 

[cvg]

ok
say you are in Sedgewick Kansas
time of concentration is 10 minutes for your site
25-year, 10-minute rainfall intensity is 6.89 inch/hour

Q=CIA will give the peak discharge

Rational method doesn't use the total storm duration to determine peak discharge

 

[psmart]

Check the regs carefully. Using a 24-hour duration with the Rational method would not be common practice. A reference to a 24-hour storm generally implies the SCS method, which is most often used with a 24-hour synthetic rainfall distribution.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

Agree with psmart. Storm duration usually kicks out rational and puts you in a hydrograph method.

 

[WaterMagna]

The reference to the 24-hour event is in regards to the storm water detention, the NOAA Atlas 14 "I" value is regarding culverts/ditches. If the contributing basin from top to outfall (about 2.5 acres) is all sheet flow with no defined channel, around 550 feet long, is it still applicable to use kinematic wave to determine Tc?