Gutters for Storm Sewers

Among other data, each inlet has a gutter cross section that consists of a width, cross-slope, compound gutter cross slopes, Sw and Sx, and an optional local depression, a. HEC-22 -type inlets can be very confusing and hopefully this article will help to simplify.

Storm Sewer Gutters
Section at the upstream end of inlet. The dotted line is the gutter slope at the inlet face. Depression, a, plus Throat Height equals the total opening.


Sx is the cross-slope of the pavement section between the center of the pavement and the edge of the gutter. Common value is 2% or 0.02 feet per feet.


Sw refers to the cross-slope of the gutter section just upstream of the inlet opening. This is not necessarily the cross-slope of the gutter section at the face of the inlet. See Local Depression below. Common values of Sw hover around 5% or 0.05 feet per feet.

Local Depression, a

The local depression describes the gutter slope at the face of the inlet. But instead of describing it as a slope, it is described as the vertical distance between a projection of Sx and the invert of the inlet opening. Note the heavy dotted line in the sketch above. Typical values range from 1 to 3 inches.

If Sw > Sx, then HEC-22 considers it to be a depressed inlet. Depressed inlets will have smaller spreads at the face of the inlet but the depression has no affect on the computed spreads upstream in the gutter section.

Required Inputs

The following describes individual data items for inlets and gutters. Note that certain cells on the input grid will be marked out with “n/a“. This indicates that data is not required for that particular junction. Simply [Tab] through those cells.

Cross Slope, Sx
Enter the transverse slope of the pavement section only, Sx. See figure above. This item is not required for Manholes, Headwalls or None inlet types.

Cross Slope, Sw
Enter the transverse slope of the gutter section only, Sw. This item is not required for Manholes, Headwalls, None and Drop inlet types.

Gutter Width
Enter the width of the gutter section. This is the width as it corresponds to the Sw value, if specified, and should not be less than any grate widths specified for this Line.

If this is a Drop Grate inlet, you should select a width wide enough to contain the entire grate width. This item is not required on Manholes, Headwalls, None, and Drop Curb inlet types.

Local Depression
Enter any local depression amount. This value is measured vertically from the projection of Sx. See figure above. If this inlet has no local depression, enter zero. If Sw <> Sx then Stormwater Studio will automatically compute a local depression and insert it for you.

Longitudinal Slope (on grade only)
Required for all inlets on grade. Enter the gutter slope, or longitudinal slope of this inlet. Not required for inlets in a sag or Manholes, Headwalls, None, Drop Curb inlet types.

Manning’s n
Select an n value for the gutter section. Typically 0.013. This is not required on any inlet in a sag, Manholes, Headwalls, None and Drop Curb inlet types. See Useful Tables for suggestions.

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