Unexpected spread and depth results for a curb inlet in sag with local depression

Product(s): StormCAD, SewerGEMS, CivilStorm
Version(s): 10.XX.XX.XX
Area: Calculations


Unexpected spread and depth results for a curb inlet in sag with local depression. Or, the results do not match GEOPAK.


In general, inlet capture, depth and spread calculations build upon the HEC-22 standards. (see HEC-22 manual) GEOPAK (or other applications) may be making more simplified (or different) assumptions or interpretations of HEC-22 for certain situations.

The first thing to note is that the gutter depth reported at the catchbasin may be different depending on the version you're using. Starting with the CONNECT Edition ( and greater), the gutter depth is always measured from the bottom of the local depression. This can differ from other applications if they do not account for the local depression in the gutter depth figure. If you are using V8i or earlier, upgrade to the CONNECT Edition as this change should result in more logical results.

The second point to consider is that the dataum point for weir flow calculations is based on the bottom of the gutter depression, as

the bottom (datum point) of the weir is essentially the top of the local depression. Meaning, the beginning of the local depression slope:

Gutter flow "spills" into this weir and the head in the weir equation is measured from that point.

As the local depression is increased, the bottom/low point is lowered, but the datum point above which the weir head is measured is still the top of the local depression (see red line above) So, the spread would stay the same (water surface elevation stays the same) but the depth would increase (bottom point is lower).

Here is the general flow of how the program (as of CONNECT Edition) is computing depth and spread for this type of situation, using a 4" depression as an example:

1) Two versions of the weir equation are compared and the one with the largest depth is selected

  • Checks HEC-22 equation 4-28
  • Checks HEC-22 equation 4-30 (applicable to curb inlets without depression) and subtracts the local depression

2) Gutter depression height is added. The weir equations 4-28 and 4-30 solve for a HEC-22 "d", which does yet include gutter depression.

3) Orifice flow is considered, with local depression subtracted

4) The larger of the weir (step 2) and orifice (step 3) depths is selected

5) Local depression height is added

6) Compute spread based on cross slopes.

  • See help topic "Composite Gutter Section"
  • With a composite gutter section with multiple different slopes, the spread is computed in steps, and depending on whether it extends past the local and/or gutter depression, it determines which slope(s) need to be considered to compute the final spread.

See Also

Abnormal large spread on curb inlet in sag

Where is the Gutter Depth measured from?

Unexpected results with long curb opening length