Stormwater Drainage design for Cloud Bursts 1

[zahid106]

I need to evaluate a drainage system for the capacity to capture and convey stormwater runoff from cloud bursts.
Is there any guidance available on what storm frequency and duration we should use to capture intensity and volume from a typical cloud burst.

Zahid

 

[psmart]

By "cloud burst", I assume you're referring to the rainfall from a short duration event. The shortest duration that is commonly tabulated is 5 minutes. You can get this information from a local IDF curve, or you can use an online source such as the NOAA Precipitation Frequency Data Server:

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

5 minute storm seems a bit crazy and for what recurrence?

suggest that it is a relatively common event such as a 2-, 5- or 10-year, 1-hour storm
a 1-hour storm will easily contain the "5-minute cloudburst".

 

[dik]

I suspect a lot of the climate data may change in the not too distant future to reflect climate change. We'll have to wait and find out.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[psmart]

I was assuming a very simple Rational method calculation. I suggested 5 minutes since it will produce the highest peak runoff from the roof. Unless the Tc for the roof is more than 5 minutes, you would commonly use a 5 minute duration.

For a more rigorous analysis you could generate a full runoff hydrograph using a complete rainfall distribution, but I'm not sure what the OP is looking for.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[zahid106]

Thank you Peter and Cvg for you input
I need to develop a rainfall distribution for this evaluation.
Here is some information on the problem
This is landfill site with stormwater swales along the slopes that collect water along slopes and carry to a downchute.
The swales were designed for 25 year storm event using rational method, many years ago.
The swales have some settlements and erosion from the time they were built.
A recent storm of 2.31" in three hours has overtopped the swale bank on top of landfill, and resulted in a cascading failure of swales downslope and some damage to landfill slopes.
We are trying to evaluate how far we should go with repair and/or rehab of these channels.
We have done new topographic survey. The swales in their present state do not have clean geometrical cross sections or constants slopes.
We are thinking about building a 2d model with either HECRAS or PCSWMM and one of the scenarios we want to test is a short duration intense storm like a cloud burst.

 

[cvg]

pay particular attention to low spots in the swales. thats where you will lose capacity and overtop. assume you will have a lot more settlement in the future. assume the cover is nearly impervious. 2D model is a good idea.

a 1 hour storm is probably adequate. the rainfall distribution is up to you.