Rainfall/Runoff Determinations for outside Colorado

[David Carpenter]

Hello,

I am working on a project in another state for which I need to model a watershed. Due to the lack of studies and regulations in this state, we need to develop new IDF curves.

My question is whether or not the procedures outlined in Chapter 4 and UD-Raincurve will work for calculations in another state using the NOAA atlas, or are they Colorado-specific only?  I assume the CUHP is Colorado-specific as well for my SWMM model, so I'll have to develop my own hydrographs.

I would appreciate any assistance you can provide.  Thank you for your help.

Regards,

David Carpenter, PE
Accurate EngiSurv LLC

[Ben-UWLLC]

David,

 

The CUHP model is a unit hydrograph based model that has been calibrated for runoff in urban areas.  It also has been successfully applied outside of Colorado (e.g., Rapid City, SD has adopted its use and “blind” testing by an ASCE committee in the Seattle, WA area produced more accurate results than were obtained using HEC-HMS and EPA SWMM.  In addition, MS thesis by a student at university of Minnesota some 20 years ago showed a successful application of it in that state.  Also, an uncalibrated SWMM and other distributed kinematic and dynamic wave models can produce results dramatically different from reality. 

 

The key to successful application of CUHP is the use an appropriately designed “Design Storm”, which vary with the nature of local meteorology.  However, the Design Storms that are built into the UDFCD version of CUHP should work well for any semi-arid and arid climate zones as long as you can input the NOAA Atlas one-hour depth for the return period you are modeling.  However, at this time, CUHP is not designed for continuous simulation and is to be used primarily with individual Design Storms. 

 

Ben Urbonas, PE, D.WRE

Urban Watersheds, LLC

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Denver, CO 80209

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[Lori Moore]

Can you help me with this one or how you might go about figuring it out…

I need to know the odds of having two 100 year storms occur in the same location within 130 days of each other. 

It would also be nice to know what the odds of a 100 year storm and a 10 year or 5 year in the same location within 130 days of each other.

 

Lori Moore

[Craig]

The probability of having a 100-year storm in any given year is 1/100.

The probability of having two 100-year storms in any given year (at the same location) is (1/100)*(1/100) = 1/10,000

That's the probabilistic theory.  Is this a probabilistic question?

I would only apply this logic to measured streamflows, not to rainfall data.  Rainfall can vary so greatly in intensity and distribution, that comparisons are meaningless.  This is how news media can make statements like "This area has had three one-hundred year storms in the past year."

Hope this helps.

Craig Perl, PE, C.F.M.

Phelps Engineering

[Ken MacKenzie]

Thanks for fielding this, Craig. Lori, I think the group would be interested in knowing why you're asking the question. I would also add that, given the seasonal nature of flooding, at least in our region, the 130 day probability is close enough to the annual exceedance probability. This sounds like the type of question usually asked by an elected official!

- Ken MacKenzie (mobile)

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[Fred Ernst]

Hi Lori,

 

The probability of having two events that exceed the 100, 10 and 5 year events in 130 days is 0.0000128, 0.0013565, 0.0058369 respectively.

 

Fred

 

 

Frederick Charles Ernst, P.E.

Principal

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