NRCS applicability outside US 1

[waseem19]

Hi,
I've never worked in hydrology before and just got an opportunity to work on storm water management system project which will include lots of modelling of routing structures, pipes, channel and possibly reservoirs.
I’m doing text books readings and have the following questions for a start

1- I work outside the US so does this mean that the NRCS four curves (type 1 to 4) are not applicable?

2-the curve number method looks like something that could be applied outside the US (assuming similar soil conditions) correct?

3-I'm assuming that the SCS synthetic unit hydrograph is also limited to the US, correct?

4- If my assumptions are correct and the NRCS method is not usable outside US then what should I use (knowing that rational method is also not possible due to catchment area and need to design storage systems) ? I only have IDF curves to start with.

5- I did some readings on the available free software that I can use and looks like SWMM is my best start but again I’m worried that it might have other built in features custom made to the US. Please tell me your opinion.

Any help is greatly appreciated, i would be more than happy if you answer me in the same format of my questions.
Waseem

 

[psmart]

The SCS/NRCS method is used in many countries outside the United States. In most cases, a local rainfall distribution is used in place of the standard SCS storms. A custom Unit Hydrograph may also be employed, but this is less common.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[waseem19]

psmart,

thanks for your feedback but if a local rainfall distribution and custom unit hydrograph are used then what's left of the NRCS method ?

I have come across a method called alternating block model which might be useful cause i only have IDF curves, can you point me to text books or web site that gives more details about this and other methods? I'm also looking for steps of using the IDF curves in SWMM model.

thanks.

 

[psmart]

The NRCS method is not just a set of rainfall data, it's the entire procedure for generating a runoff hydrograph, including curve number classification, the SCS runoff equation, Tc calculation, and runoff generation based on a unit hydrograph.

Each of these elements can be modified to suit a specific need, but the overall framework remains the same.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Ryb01]

Not sure about other areas but have used the NRSCS methods in Canada and Mexico.

Ryb01

 

[sujanshakya]

The regression equation method is statistically based and have a large margin of error-up to 57% for 100-yr discharge compared to NRCS unit hydrograph method which has more input parameters unique to watershed. I still used the regression method for my hec-ras analysis.Had I used the NRCS values, the proposed bridge would have flooded. Is there any other reason for not using NRCS method?

 

[cvg]

USGS regression equations are regional equations prepared for the US and do not apply in other countries. Regression equations based on local data would need to be generated for your project.

As stated before, the NRCS methods would have to be used with local precipitation data and possibly with a locally approved unit hydrograph. The proposed unit hydrograph and precip data would need to be approved by the local agency reviewing the hydrology study. In addition, AMC conditions, recommended curve numbers, Tc calculation or the runoff calculation methods used in the NRCS method may or may not be approved by the reviewer. I'm not saying it is a bad method, just that it is not a universally accepted method.

Given that you do not know what is acceptable, I would first check meet with the reviewing agency to determine what methods would be acceptable and go from there. Since you have not given the name of the country where your project is located, we all would be guessing what might be acceptable.