Depth and spread calculations for grate inlet in sag

  Applies To 
  Product(s): StormCAD, FlowMaster
  Area:  Calculations
  Original Author: Jesse Dringoli, Bentley Technical Support Group


How does StormCAD and Flowmaster compute the depth and spread for a grate inlet in sag? How does the clogging factor come into play?

Problem Number: 33582


A sag inlet always captures 100% of the approach flow. What the program first does is figures out the head (depth) necesary to push all of this flow through the inlet, using standard HEC-22 methodology. Then, based on this depth and the road cross slope, it finds the spread.  The equation used to figure out the depth is a rearranged form of
HEC-22's 4-26.   This equation is found in the StormCAD 5.6 help under Contents > Appendix B > Inlet Hydraulics > Inlet Analysis > Inlets in Sag > "Grate inlet in sag" or in a similar location in the V8 help. Note that in some conditions (higher depths), orifice flow can occur, in which case a different equation is used (see the help.) These equations from the help are shown in the below image:

The important note when using this equation is that the value for depth, d, is at the middle of the grate. StormCAD and Flowmaster report the depth at the curb. (where the maximum is.) So basically StormCAD uses this equation to find the depth at the middle of the grate, based on the flow, then uses this along with the geometry based on the road cross slope to figure out the depth at the curb, and reports that.

When using a clogging factor, the Width (W) part of the equation is multiplied.

It then uses spread = depth/slope to find the spread. (the slope value being in units of ft/ft such as 0.05ft/ft)

See Also

Click here to see "How does the clogging factor work in StormCAD?"