## Modified Rational vs. Rational Method

## Modified Rational vs. Rational Method

(OP)

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!!

## RE: Modified Rational vs. Rational Method

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 stage:outlet 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

## RE: Modified Rational vs. Rational Method

## RE: Modified Rational vs. Rational Method

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

## RE: Modified Rational vs. Rational Method

## RE: Modified Rational vs. Rational Method

## RE: Modified Rational vs. Rational Method

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

## RE: Modified Rational vs. Rational Method

## RE: Modified Rational vs. Rational Method

## RE: Modified Rational vs. Rational Method

## RE: Modified Rational vs. Rational Method