What do the different "width" and "length" fields represent, for a catalog inlet?

  Product(s): StormCAD, CivilStorm, SewerGEMS
  Version(s): 08.11.XX.XX, 10.00.00.XX
  Area: Calculations


What do the different "width" and "length" fields represent, for a catalog inlet?

How are inlet structure width and length used in the computations with the StormCAD (GVF-Rational solver)? What hydraulic results are affected by increasing/decreasing length and width?

Problem ID#: 87805


Width and Length in the inlet catalog

For a catchbasin, the Width and Length fields inside the inlet catalog affect the structure losses hydraulically, when using the HEC-22 2nd edition headloss method. They are the inside dimensions of the subsurface vault and are taken into account in the Ko factor, which is defined as the initial headloss coefficient based on the relative junction size.

The initial headloss coefficient, which is based on relative junction size, is calculated as:

θ = Deflection angle between inflow and outflow pipes
• b = Equivalent diameter of the structure (m, ft)
• De = Equivalent diameter of the outlet pipe (m, ft)

The HEC-22 2nd edition method computes a Ko term with the structure equivalent circular diameter, which means there is is a prerequisite step to convert a box vault structure into the equivalent diameter length.

Please not that the new HEC-22 3rd edition junction loss method does not compute a Ko factor.

The "Grate width" in the catalog inlet (for a combination or grate inlet) represents how far the grate extends into the road, and the "Grate Length" in the catalog inlet represents how long it is along the road. See diagram below (click to enlarge):

Width and Length in catchbasin properties

If the "Structure Type" is set to "Box Structure", then you will see additional Width and Length fields in the catchbasin properties. These represent the dimensions of the vault for reference and visual purposes - the Length field determines the size of the structure in profile view.

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