Modified Rational vs. Rational Method

[rventura23]

I computed a stotage volume for a site using the rational method(Q-ciA). The site is only 2 acres. It was brought to my attention that I MUST use toe Modified Rational Method, because it will give a larger volume to be stored. What is the difference between the two? Why will it yield a much large volume? Thanks in advance!!

 

[LHA]

Rational Method uses intensity to get a peak flow, only. Rational doesn't give a volume at all, only a flow.

Modified Rational gives a volume, sort of. Modified rational method uses the Rational peak, modified rational does not yield a peak.

You can't use peak flow by itself to analyze volume. You can't detain a peak flow, you can only detain a volume, and the delta volume is determined by the dynamic Qin:Qout, not by Qp alone.

Detention/Retention volumes must be sized using an inflow rate and routed (again, at a rate, not a volume) through outlet devices, given a storage volume. It is a dynamic system; as such, you can't size a storage volume based on a static peak volume in - volume out equation. You need the above hydrograph in, the stageutlet ratios (governed by the outlet structure) and the volume's stage:storage values.

Using the Modified Rational approach, the Rational peak is ascended to constantly over a multiple of Tc. It lasts at that peak for a duration, then descends constantly to zero, again, over a multiple of Tc. There are many approaches; some hold the peak for the duration of the rainfall, others hold it for a critical intensity duration, others triangulate at the peak. None are incorrect, but any given one must be substantiated.


Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[rventura23]

Steve, thank you for the timely response. With the site in question, I am using seepage pits to store the runoff. So I only have and outflow(overflow) for the 100 year storm. Even with the overflow, the postdeveloment runoff is less then the predevelopment. To get my volume, I used the peak flow calculated, and figured the volume by using just the time of concentration. Should I use a larger duration of time to increase this volume? Maybe twice the Tc?

 

[LHA]

You didn't get a realistic volume that way. Your assumption says the storm starts at tiem=0 at full intensity (which may or may not be valid), and lasts only until Tc, then instantly stops. You got a rectangular hydrograph, again, maybe OK, but you have to justify, explain and stand by that.

Your intensity must ascend to Qp, last a duration, then decend. That is where Modified allows a hydrograph of a triangular or (better yet) trapezoidal shape.

This "looks more like a rain event, at least in my region of the world. Your storm characteristics may be different, so make sure your hydrograph reflects that.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[cvg]

If you are being required by others to use a particular method, I would venture a guess there are also required methods for calculation of the volume. As stated previously, there are many ways to approach it and many give useful approximations of the volume, but your reviewer may only accept a particular one. I would ask the reviewing engineer for the approved method.

 

[rventura23]

He is requiring the Modified Rational Method. I think doing the triangular hydrograph will probably work. Would that be using Tc to the peak, and then Tc back to zero? That would in essence be 2 x Tc. Thanks for all the responses. Steve, what part of the country are you in?

 

[LHA]

Southcentral PA, USA

A commonly accepted triangular approach here uses:
1. Ascending leg from 0,0 to 3Tc,Qp
2. Descending leg from 3Tc,Qp to 9Tc,0.05Qp

But, as cvg said, that means nothing if it is not an acceptable approximation to the reviewer. Run this by him or her first.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[rventura23]

Thanks for all your help. The engineer seems to want a triangle with a 2TC leg up and a 3TC leg down. When I do this it just seems like way to much volume, especially since I am just storing the impervious areas in seepage pits....

 

[bltseattle]

If you set up a time step model, for example with a spreadsheet, you can account for the ongoing infiltration that occurs as the hydrograph flow develops.

 

[rventura23]

Thanks again. I decided to use the 1 x TC to the peak and then 2 x TC back to zero, so I have a triangular hydrograph. The reviewing engineer allowed my initial assumption of just using the TC multipled by the peak flow, but an engineer was brought in as opposition. I will just have to stand by my assumption, why I used this Triangular Hydrograh.

 

[acschlosser]

As IHA (Steve) indicated, the rational method gives you the peak flow rate based on A, c and I (which is based on tc). The modified rational method attempts to create an accurate hydrograph from this method because the rational method is so much more accurate than other methods for small watersheds. In the original modified rational method, the tc was considered a variable and was to be increased until the maximum volume was achieved. The shape of the hydrograph was an (equallateral?) triangle. Many programs forgo that because it is too hard to program in. Some programers fixed the total length of the hydrograph to be either 1*tc or 3*tc. Neither is technically correct. I believe the 1*tc up and 2*tc down was made popular by terramodle but I don't know where it came from. The larger question is whether or not infiltration is a viable method for stormwater management. Here in S.E. PA, many areas have a high water table and as such the storage devise fills with ground water before it even starts raining. As a result, sites with underground systems with ground water shed 100% of the rainfall into the facility with no detention at all. Make a mental note the next time you hear complaints from increased flooding downstream from a site that "conforms" to the new DEP requirements. I'm already seeing flooding problems from a site designed under the new requirements. If you want background on methodologies, try getting your hands on "Recommended Hydrologic Procedures for Computing Urvan Runoff from Small Watersheds in Pennsylvania" by Penn State in 1982.