Rational Method vs. SCS Method

How does the Rational method compare to the SCS method? If you’re a civil engineer involved in stormwater modeling, at some point you’ve asked yourself this age-old question. When one jurisdiction favors the Rational method, and another jurisdiction favors the SCS. Some allow both. Some prefer the Modified Rational method. It’s a valid question. So which is correct? Which method should I use?

After an internet search you’ll find many articles comparing these methodologies, from a theoretical viewpoint, that basically conclude one produces more volume than the other and more volume means a conservative analysis. Bigger is better, right?

Okay. So what. A better question focuses on the practical viewpoint, “How do these methods affect my detention pond designs?” Most assume their final designed detention pond will be much, much larger using the higher-volume method, the SCS, a.k.a. NRCS.

We set out to find an answer to this question by actually comparing the Rational against the SCS method in a hypothetical pre- and post-development scenario. The Basin Model schematic in Hydrology Studio is shown below.

We started out by setting some ground rules and data inputs with a goal to minimize the variables by holding as many features constant as we can.

A 10-Acre Site in Atlanta, Ga

Our scenario is based on modeling a pre- and post development project on a ten-acre site in Atlanta, GA that transforms a “Meadow in good condition” to an “Industrial site”. We modeled using NOAA intensity and precipitation rainfall for the 2, 10 and 100-year return periods. Pre- and post-developed runoff hydrographs were created using both methods on the same 10-acre site. The post-developed hydrographs were routed through newly-designed detention ponds using the pre-developed peak Qs as target release rates.

To give the Rational method a fighting chance, we threw the Modified Rational method into the ring. Below are the data inputs.

Rational vs NRCS Method Data Inputs

Scenario		Rational	Modified Rational	NRCS - Type II
	Area (ac)	10.00	10.00	10.00
Pre-dev	C / CN	0.30	0.30	71
	Tc (min)	30	30	30
Post-dev	C / CN	0.85	0.85	91
	Tc (min)	12	12	12

Below are the resulting Pre and Post Rational method hydrographs, 100-yr. The Post Modified Rational was constructed using the peak Qs from the Pre Rational hydrograph.

Below are the not-so-comparable 100-yr Pre and Post NRCS hydrographs.

Here are the graphs presented in a numerical table.

Rational vs NRCS Method Hydrographs

Scenario	100-year Event	Rational	Modified Rational	NRCS - Type II
Pre-dev	Q peak (cfs)	14.97	14.97	40.92
	T peak	30 min	30 min	12.18 hrs
	Volume (cuft)	26,946	26,946	163,304
Post-dev	Q peak	69.41	21.97	83.58
	T peak (min)	12 min	12 min	12.02 hrs
	Volume (cuft)	49,978	113,889	230,508

Wow! The NRCS has a huge volume of over 230,000 cubic feet. Even the Modified Rational can’t come close at less than half the volume. The NRCS is going to make for a very large detention pond compared to the Rationals.

Or is it?

Lets design detention ponds for each of these scenarios and find out. We will hold as many pond design features as we can constant in order to simplify the comparison. For example, the ground rules below will make the only differences in pond sizes to be the bottom area.

Pond Design Constraints

1. Ponds must be 8 feet deep.
2. Final routing must provide 1 foot of freeboard.
3. Ponds must be trapezoidal shape with 2:1 side slopes.
4. Outlet Culvert to be 50 feet in length @ 0.50% slope.
5. Must design to match pre-developed flows for the 2-, 10- and 100-year events.

We’ll use Hydrology Studio and its three-step detention pond design procedure:

1. Estimate the required pond storage by reducing post-developed flows to the pre.
2. Build the storage pond to satisfy the required storage from Step 1.
3. Add a multi-stage outlet structure to satisfy the three target release rates. Perform trial routings, adjusting outlet structures until the routed outflows match the pre-developed, target Qs.

Any combination of orifices, weirs, etc. Culvert must be 50′ @ 0.50% slope.

Final Detention Pond Designs

The following designs were produced: (drum roll please…)

Standard Rational Method Design

Modified Rational Method Design

NRCS Method Design

Conclusion

There you have it. Same site. Three identical pond shapes. Three different methods. Three basic outlet configurations.

The multi-stage outlets are nearly identical. Each use a standard 4′ x 4′ box riser and a couple of secondary orifices and/or weirs. The only significant difference is the SCS method needed a 30-inch culvert compared to 18’s in the Rational methods. Either way, you still have to dig a trench and lay some pipe. The cost differences are insignificant in the grand scheme.

The NRCS method produced a post-hydrograph volume of over 4.5 times that of the Rational method and more than double the Modified Rational. But the resulting differences in pond storage utilization was far less.

As shown in the table below, the NRCS method produced a post-hydrograph volume of over 4.5 times that of the Rational method and more than double the Modified Rational. But the resulting differences in pond storage utilization was far less. SCS’s pond took up only 5 additional feet in bottom length and width compared to the Modified Rational method’s footprint, requiring only 12 percent more volume.

Method	Pond Bottom Dimensions	Inflow Hydrograph Volume	Total Storage Used	Peak Outflow
Rational	60' x 60'	49,978 cuft	39,067 cuft	15 cfs
Mod Rational	80' x 80'	113,889 cuft	62,229 cuft	15 cfs
NRCS	85' x 85'	230,508 cuft	69,625 cuft	41 cfs

The Standard Rational method didn’t fare so well. Based on these results it would be a tough argument justifying using it with a conservative, SCS-type mindset. What we like about the Standard Rational is how closely the routed outflow hydrograph matches the existing, pre-developed. The Modified Rational routed outflow isn’t anywhere close to the original. NRCS gets the blue ribbon on this one. Its 100-year outflow is almost an exact match of the original.

In Summary:

1. The Standard Rational probably shouldn’t be used for detention pond designs.
2. Five feet and a couple of standard sizes of culvert pipe are all that separated the Modified Rational from SCS method in this particular study.
3. Thumbs-up to SCS because of how well it mimics the original pre-developed hydrograph… except for that pesky volume between 12.5 and 15 hours.

So the next time a client or colleague asks, “How will these methodology differences affect my land development project?” Your reply can be:

1. Not much in the way of outlet structures but SCS commands a larger Culvert.
2. SCS detention ponds will be about 75% larger than Standard Rational method ponds and about 10% larger than Modified Rational ponds.
3. If your goal is to minimize costs, and your local drainage authority allows it, use the Modified Rational method.

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