Creating a HydroCAD Rainfall file 1

[H2onymous]

I ran into a problem when trying to create a rainfall file for in HydroCAD v9.10. I want to create a 24 hour Chicago storm using a,b,c coefficients. The formula HydroCAD uses calculates depth = t*a/(b+t)^c. This is depth = time*intensity. The problem is that the a, b,c coeffs I have require t, to be in minutes, but generate an intensity in mm/hr. Therefore the t, on the top needs to be in hours, while the t on the bottom in minutes. Is it correct to adjust the a and b coeffs so that they use a t in hours? Or will this alter my Chicago storm. Or should I leave the coeffs as they are and manually input a depth when executing the rainfall file in HydroCAD. (The default depth is grossly incorrect).
Note, adjusting the coeffs allows HydroCAD to accurately provide a default rain depth.

Thanks for the help!

 

[psmart]

The IDF file can be configured to accommodate your coefficients so you don't have to make manual adjustment.

These are the default IDF units, as shown in the sample IDF file:

depthunits=inches
timeunits=hours
duratunits=minutes

In your case you can use:

depthunits=mm
timeunits=hours
duratunits=minutes

In brief, just add "depthunits=mm" to your file.

Please note that the resulting IDF file can be used with a HydroCAD project that is configured for Metric or English - HydroCAD will make the units conversions automatically.

For further details please see

http://www.hydrocad.net/rainfall/idf.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[H2onymous]

Thanks Peter,

Let me clarify by reiterating my question. I'm trying to create a rainfall file, I have a working IDF file. I need to create a 24 hr Chicago storm distribution. I was using the file: Sample D 24-hr to create a 24hr Chicago storm based on IDF coefficients.

Can I add duratunits to this file? I haven't been successful so far.

Thanks

 

[psmart]

Sorry, my comments pertained to an IDF (HCI) file.

For a rainfall distribution (HCR file) the answer to your questions is contained in the "Sample D" file you referenced. Specifically, you will need to include:

timeunits=minutes
depthunits=mm

duration=24*60
peaktime=12*60

The *60 factor converts the duration and peaktime values from hours to minutes.

This will create a dimmensionless rainfall distribution. If you want to automatically preset the total rainfall depth you can also include:

stormdepth=xx.xx

If you have further questions please post the file or email it to HydroCAD support.



Peter Smart
HydroCAD Software

http://www.hydrocad.net