Modified Rational vs NRCS?

[geosavvy]

I work for a municipality and currentlly our storm water manual does not list the Modified Rational as an option for using during pond design.

I am currently reveiwing a set of calculations where the civil engineer used the Rational Method (within pond pack) for sizing the detention vault.

I want to use this as an opportunity to evaluate the Modified Rational Method (since I am new to stormwater and am relatively unfamiliar with the method).

Typically the engineers will use Rational Method to calculate peak flows for sites under 10 acres, then use NRCS unit hydrograph methods to create the inflow hydrograph.

My main concern is that the Modified Rational Method does not utilize the Type II rainfall distribution that we require to be used. Instead is uses storm events based on intensity.

On a small site (under 10 acres), how does the Modified Rational Method compare to the NRCS unit hydrograph method with respect to storage volumes?

Basically, I want to compare:

Modified Rational (peak flow + hydrograph)

vs.

Rational (peak flow) + NRCS (hydrograph)

I have been attempting to run through the NRCS calcs by hand, but since I don't do this regularly its taking more time than I currently have to work the learning curve.

Thanks for the help.

 

[RWF7437]

Suggest you forget the "modified Rational method' It is probably bogus and can't be "proven" ( i.e. calibrated ) anyway.

With good rainfall data, the NRCS method should yield reasonable results for all but the largest basins.

Rather than waste time on small, "local" detention basins, spend some time and money on regional basins and on stream and rain gages so you can calibrate whatever model you use

 

[geosavvy]

The Modified Rational Method has been around awhile. That doesnt mean it's used very often; and I assuming its not. Nonetheless, its used enough for me to take an interest in it. Despite your recommendation to 'forget about the modified rational method', I would still like some feedback from people who are relatively familiar with it.

As for acquiring easements at close to $1 mil/acre to build regional detention ponds... I'm not sure that's in the foreseeable future. I can't even imagine what it would take to get the council to approve of that in the budget. I'm not sure I understand whats so bad about having developers detain their own runoff. Even in my short time working in this area ive seen bad flooding problems get fixed by having the developer go back and install his small, local detention pond upstream.

Im sure regional ponds have their merit. In fact we are working on a couple right now that we happened to have worked out land swap deals on; however, they only seem practical in specific situations.

Just my thoughts; albeit, relatively unfounded.

 

[psmart]

In fact, the trend is towards smaller, local runoff management. It is often more effective and less expensive to manage runoff at the source. This also encourages low-impact development techniques, rather than just pushing the problem downstream.

As to the rational method (modified or otherwise), it just doesn't provide a reliable prediction of runoff VOLUME, and therefore should not be used for detention pond design. (You will find many earlier threads in this forum to that effect.) Stick with the NRCS method, with is well suited to detention pond design.

As to comparing the methods, you would need to do this on a case-by-case basis. Generalities about one method being higher or lower than the other are usually wrong. It depends on many factors. You will need to compare the actual results for your specific situation. However, you will still have the problematic issue of determining the correct rainfall duration (and hence the intensity) to use with the Rational method. The NRCS procedure has the advantage of analyzing all durations in a single run, while the rational method requires the selection of a specific critical duration. For a single subcat this is just the time-of-concentration, but for multiple subcatchments things get much more complex. For details please see

http://www.hydrocad.net/rational.htm

If you really want to compare the results, use a program like HydroCAD that provides both runoff methods, making it easier to compare. You can get a free Sampler at

http://www.hydrocad.net

 

[geosavvy]

Ok. thanks for your response.

I guess that leads me to another questions. When people say the Rational Method is only good for small basins, do they mean to say that its better than TR-55 for small Basins? Or is TR-55 always going ot give you a decent answer, small or large?

Our manual basically has guidelines that dictates using Rational Method for sites less than 10 acres and Tc less than 15 min, otherwise use TR-55.

I'm wondering now if that was written simply because it was assumed a designer would want to use Rational due to the ease of hand-calculating. With computers, all methods seem to be equally as easy to input. Thus, if TR-55 is accurate on a 2 acre site, then why even bother using Rational at all (other than maybe sizing pipes, etc).

------------------------------------

I guess im still unclear on this:

How does installing many local detention ponds "push the problem downstream"? Intuitively thinking, if the entire city gets 4 inches of rain dumped on it in 1 hour, why is it bad to throttle that water back so that its released over a longer duration at smaller flowrates? It just seems like handling it in regional ponds would require that very large amounts of water get piped to these ponds, requiring substantially large pipes and other infrastructure?

Once again, I'm new at this, so i'm just asking questions. I know there are factors im overlooking.

Thanks again.

 

[psmart]

geosavvy,

I agree, local stormwater management (close to the source) is often more effective and less expensive. If you don't manage at the source, you push the problem downstream where more drastic and less effective measures are often required.

As you suggest, the prevailing approach to stormwater management is to build detention ponds in order to release the (increased) runoff over a longer time. The problem is that the overall volume will still increase as the land is developed, so your site will take longer to discharge, and will cause increased flooding at some point downstream when combined with other sites. To resolve this, some authorities are now requiring that the discharge peak AND volume be held at pre-development levels. This generally requires on-site infiltration to offset the effects of the increased curve number.

Rational method is often recommended for small sites for the sake of simplicity, but with good software providing the NRCS method this argument is hard to justify. The original TR-55 software also had a lower limit on the time-of-concentration, which precluded it's use on very small sites. But the current NRCS Unit Hydrograph procedure (as used in TR-20, Win-TR-55, and others) can be used with very small Tc values, so this method is commonly used for small sites as well. The lower Tc limit is actually dependent on the quality of the rainfall distribution, rather than on the runoff method itself.

 

[geosavvy]

So would Rational method potentially be more accurate when Tc is much lower than 0.1 hr (which I think is the limit in the current version of Win TR-55).

 

[psmart]

It depends on the exact program - and the quality of the rainfall distribution it uses. Traditional tabular rainfall tables don't have enough resolution to give meaningful intensity information for very short Tc values. This is part of the reason by TR-55 has a lower Tc limit. But other programs (such as HydroCAD) use high quality polynomial-based rainfall tables that can be used at small durations. In fact, you can even use a Tc of zero for instantaneous runoff.

However, this gets into another issue of how you calculate the Tc, and the fact that many localities specify a minimum Tc value, partly to avoid the complexities and inconsistencies of how these short values are handled.

 

[env21tech]

There is no need to use Tc execept to adjust the runoff coefficient used in the Rational Method.
Rainfall intensity will be determined by atmospheric conditions. These conditions are available from rain gage data for 5,15,and 60 minute durations.
This data is used to generate IDF Curves. A raincloud 500-1000 acres in size is needed to create the minimum duration of five minutes.