If you do not enter a user-defined length in the attributes for conduits, channels, and gutters, the length used in Bentley StormCAD is the plan view distance between the coordinates at each end of the link element. This length is used as the actual length in hydraulic calculations. However, as the slope increases, the difference between the plan length and the actual length also increases as shown below.

The table below shows the difference between the actual and plan length as a function of slope. Note that for most reasonable slopes, the difference between the actual and plan view length is less than one percent. (100% slope is 1:1 slope.) As the slope approaches vertical, you must enter the actual length.

Actual and Plan Length as a Function of Slope

* The model’s generalized friction formulation is only valid for slopes less than 10%.

If you are not satisfied with the plan view length, you can enter a user-defined length, which you can determine using the following equation: