Why do hand calculations for Rational Method flow in U.S. Customary units differ from the values presented in the software?
Why does the Q=CIA calculation appear to be slightly off from what is expected?
There are two possible reasons why you may see different numbers when comparing Rational Method flows to hand calculations: Unit conversion and Display precision
Most engineers working with the Rational Method in US customary units use the equation:
Q = CIA
With Q in cfs, I in in/hr, and A in acres.
Looking at the units however, you will notice that the multiplication yields a unit of acre-in/hr, not cfs, as most people assume.
The factor that converts acre'in/hr to cfs is 1.008. Since this conversion factor is very close to 1.0, most engineers ignore the unit difference when doing calculations by hand - but the Bentley software for the true units. Meaning, the Bentley product is not applying a special conversion factor but rather it is displaying the true value in the units specified.
If you do not wish to do this and want to use the 1.0 factor, then you will need to change to the correct flow units of acre-in/hr. See: Changing units in Properties, Flextables, graphs or profiles
When comparing results to hand calculations, also be aware that the software displays values that are rounded based on display precision (the number of significant digits after the decimal).
For example, a value of 1.23456789 would display as 1.23 at a precision of 2, or as 1.234568 at a precision of 6. Either way, the underlying value is still stored at the highest possible precision.
To change the display precision, just right-click on the field with the value of interest and select Units and Formatting from the context menu.
When using the Modified Rational Method, the Q=CIA calculation is based on a critical storm duration. See related article: Understanding the Modified Rational Method
StormCAD TechNotes and FAQs