Rational and SCS comparsion 1

[cip]

I am new to stormwater and been told that peak flow rates for 24 hour storms are always higher wtih the SCS method. I am reviewing a submission which uses both methods for predevelopment. The SCS method shows lower peak flows for the 2, 5 and 10 year storms than the rational method. However, the SCS method show higher peak flows for the 25, 50 and 100 year storms than the rational method.

Is this possible and how? I checked all the obvious things the C # the CN # and TC and do not see anything wrong.

thanks

 

[psmart]

Are your calculations just for runoff, or is there also pond routing involved? The former will compare only the peak flows, while the latter will depend on the total runoff VOLUME.

In any case, its dangerous to generalize about SCS vs Rational runoff values - Either one can be higher. That said, make sure that your rational intensity really corresponds to the SCS storm distribution and depth you are using. (This is not an easy comparison.)

If there is storage-routing involved, you need to be sure that the rainfall volumes are comparable. The problem here is that the SCS method considers an entire rainfall event (such as a synthetic storm) while the Rational method usually considers only a (shorter) critical duration. For this reason, the rational method is generally not recommended for storage calculations.

 

[BillHolt]

The biggest factor is in the storm distributions. The rational method uses a constant intensity storm for its model. The SCS method uses a storm that is initially gentle but that has a period of much greater intensity at some time during the event.

The second largest factor is the difference in the runoff factors. In the rational method, constant runoff ratio is assumed while in the SCS method the ratio of runoff to precip is a variable that increases as the storm goes on, kind of like real life. The coefficients for the rational method, developed mostly for the design of culverts and ditches, are generally conservative and, when projected to larger areas (where longer storms are appropriate) tend to be excessive.

Bottom line, while they both may be fruit, one is apple and the other is orange. You can only equate them in the sense that you can grow to understand more fully what each does and its limitations.

 

[sta7240]

You will always have a great debate about the Rational Method verses TR55. Personally, I would limit the use of the Rational method to drainage areas up to say 20 acres. Beyond that, I would use TR55 because I believe it provides better detail in describing the physical characteristics of the watershed. Peak discharge is very sensitive to time of concentration. Make sure you have Tc nailed and have not made a mistake.

Regarding your question, if your drainage area is larger than 20 acres, or you are routing through ponds or culverts, you may want to re-evaluate your calculations to make sure you have fully accounted for travel times and Tc. As you move into the 10, 25, 50, and 100 year storms, the Tc will change because there is generally more flow out of bank, in the floodplain, than there is the channel. Therefore your velocities will be different than an in-bank flow situation. Also, water will pond behind culverts, ponds, etc, and be detained, whereas a 2-5 storm will generally pass through. Sometimes for large watersheds, the big storms need separate calculations from the small storms.