Product(s):	OpenRoads Designer
Version(s):	2024+
Area:	Drainage & Utilities
Original Author:	Holly Herring

Question: 

Why do hand calculations for the 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?

Answer: 

Most engineers working with the Rational Method in US customary units use the equation:

Q = CIA

where I in in/hr, and A in acres. They are expecting Q to be in cfs

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 ac-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. Check your hand calculations to make sure you incorporated this conversion factor.

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. On a Catchment Flex Table change the units as shown below it you do not want to see the 1.008 conversion value, but the value in in-ac/hr:

Display Precision

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.