Problem

What is the "Kr" coefficient in the context of a headwall or culvert?

When computing a model, the following user notification appears: "Kr (reverse flow entrance loss coefficient) was not specified. Kr was set to same value as Ke= #.##."

Solution

The Kr coefficient and the message above is usually in the context of culverts/headwalls. It is the same thing as the culvert entrance loss coefficient (Ke), except for reverse flow. Meaning, if flow reverses through the culvert, you can have a different value for Kr than Ke to have different entrance loss characteristics during reverse flow conditions. In other words, when the flow is in the forward direction, it uses the Ke and when the flow is reverse it uses the Kr. The entrance loss component is equal to the coefficient multiplied by the velocity head. So, for forward flow: Ke*(V^2/2g) and for reverse flow: Kr*(V^2/2g). This is mentioned on page 34 of the FHWA HDS-5 manual and table 12 on page 223 has a list of common coefficients.

The user notification message above is related to culvert or headwall settings in your model, either for a conduit that is being modeled as a culvert or for a culvert in a composite outlet structure. This simply means that no value for the reverse flow entrance loss coefficient, Kr, was included in the culvert properties. In such a case, the program will automatically assign the Kr coefficient the same value as the coefficient Ke. If you require a different value for Kr, browse to the culvert and enter the appropriate value. Otherwise if you are OK with the program using the same Ke during reverse flow conditions, no action is needed.