How to create customized/personal defined rainfall event with 11 hours' precipitation

[galaxy2014]

Hi guys,

I have precipitation depth for 11 hours and I want to use this storm data to create a rainfall event to do the analysis, does anyone know how to do that?

I attached my HydroCAD model and the last thing to do is to create the rainfall event.

Besides, what is the P2(2yr-24hr Rain) I should type in for my sheetflow if I create my personal defined rainfall event.

Thank you.

 

[zelgar]

I don't think you have enough data to calculate the 2yr-24 hour storm. You have only 11 hours worth of data and you're trying to predict the amount of rain for a 24 hour period that will occur 50% during any storm event. Your data indicated 2" of rain in 11 hours, but who knows how that relates to an 11 hour storm. To get the 24 hour storm could you calculate it by 2"/11 hr x 24 hours = 4.36"? Actual precipitation data can be obtained at the following website:

https://hdsc.nws.noaa.gov/pfds/pfds_map_cont.html?bkmrk=ga

 

[galaxy2014]

Hi Zelgar,

Thank you for your reply.

We have 11 hours of precipitation data from 12:54 AM to 11:54 PM on 9/23/2023, obtained from the National Weather Service. Our team also measured the flow at one culvert outfall onsite on the same day.

We aim to use this 11-hour precipitation data to define a rainfall event in HydroCAD and simulate the flow at the culvert for comparison with our field measurements. Could you guide us on how to create this event in HydroCAD?

Specifically, is it possible to input the rainfall depth directly for the 11-hour period and then simulate the flows in HydroCAD?

What what we can do is to adjust the rainfall depth as shown in the image (2"/11 hr x 24 hours = 4.36 inches)?

Your advice on setting this up would be greatly appreciated.

 

[psmart]

You can use the "Sample F" rainfall file to define a rainfall distribution with uneven time steps as discussed at

https://www.hydrocad.net/rainfall/samples.htm

You will need to convert the time-of-day values to elapsed time, starting at zero. You can enter decimal hours -or- convert to minutes and use "timeunits=minutes"

When you select the new file HydroCAD will set the overall duration and depth automatically. Do not override these values!

I would use your normal 2yr-24hr depth, unless you can justify otherwise.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[galaxy2014]

Thank you, Peter.
Sample F is a 24-hour rainfall, but I want to simulate 11-hour rainfall, may I ask how does it work?
Can I just change 24 to 11, like follows?


name=Sample F 11-hr
comment=Incremental rainfall w/arbitrary time step
smoothing=false
timeunits=hours
depthunits=inches

// The following lines are required only to override
// the inherent duration and depth of the supplied data
// duration=11
// stormdepth=1.29

// first time value sets origin, depth must be zero
timeincr= 0 .00
timeincr= 1 .00
timeincr= 2 .00
timeincr= 3 .00
timeincr= 4 .00
timeincr= 5 .10
timeincr= 6 .10
timeincr= 7 .20
timeincr= 8 .30
timeincr= 9 .35
timeincr= 10 .45
timeincr= 11 .52

 

[psmart]

The duration will be set automatically based on your data. Remember to add the incremental depth on each line, like the sample.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[galaxy2014]

Hi Peter,

Thank you for your reply and guidance.

I just want to clarify the following sentence: "The duration will be set automatically based on your data."

Does this mean I should leave the duration field blank, and HydroCAD will automatically set it to 11 hours based on the 11 "timeincr" entries below it?

Or should I input "duration = 11" and HydroCAD will automatically update the duration based on the number of "timeincr" entries I have below it? For example, if I have 24 "timeincr" entries below the "duration = 11", will HydroCAD update the duration to 24 hours even if I initially set it to 11?

Your clarification on this would be greatly appreciated.

Best,
Shuo

 

[psmart]

The duration parameter is used only if you want to rescale the data you provide. Otherwise do not set the duration parameter, and your last time value of 11.52 will set the duration.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[galaxy2014]

Thank you, Peter.

Now if I know the storm duration is 11 hours, and the total depth for these 11 hours is 1.29 inches. Do you think it is accurate to do it in the following way (as shown in the snapshot)?
Or I have to use the "Sample F" rainfall file to define a rainfall distribution with uneven time steps.

 

[psmart]

No! Scaling a Type II 24-hour storm will NOT give you the intended rainfall distribution. You need to use the custom rainfall distribution you've been working on. Just add your rainfall depth on each timeincr line and you're all set. Very easy. Note that your overall duration will be 11.52 hours, since that is your last time value.

Peter Smart
HydroCAD Software

http://www.hydrocad.net