Entering Roughness Coefficients


The roughness of a culvert will have a significant effect on the flow through the culvert. The energy loss due to the hydraulic resistance or roughness of a culvert is part of the "hydraulic price" of flow through a pipe. Two commonly used equations to describe the flow as a function roughness in a culvert are the Darcy equation and the Manning equation. In these equations the roughness is represented with a resistance or  roughness coefficient, either Manning's n or the Darcy Friction Factor.

Manning's n

FishXing uses Manning's n as a default resistance coefficient for both culvert material and bottom material, when different. FishXing will require you to enter a Bottom Roughness if the culvert installation is Embedded or if the selected culvert shape is an open bottom structure (High /Low Profile Arch, Single Radius Arch or Metal Box). FishXing then computes a Composite Roughness for determining flow resistance in the hydraulic calculations.

To select a roughness coefficient for the Culvert and for the Bottom Material use the drop down menu or by entering your own estimate. FishXing provides default values of roughness only when using Manning's n.

An expanded table of Manning's n values is available by pressing the Ellipsis Button to the right of the field. These are provided as a reference and are not directly selectable. You must type in the desired value or copy and paste from the table using the Ctrl+C and Ctrl+V commands respectively.

Darcy and Chezy Friction Factor

You can change the resistance method used by for hydraulic calculations by selecting Roughness Calculations in the Options menu on the Navigation Bar. If the Darcy-Weisbach Friction Factor or the Chezy Coefficient is selected you may only enter a single roughness coefficient for the culvert, even if it is embedded. When values are available the Darcy or Chezy methods can be used to model Baffle Hydraulics in the culvert.