Street inlets and their types

Street inlets (storm water inlets) are openings, with a grating or a slot, built at the edge of a road or in the gutter to admit surface storm water into the storm drain or combined sewer.

Types of street inlets

Street inlets are of three main types: curb inlet, gutter inlet and combined inlet.

1. Curb inlet

A vertical opening in the curb (kerb) face through which water enters the chamber. It is most effective on flat roads, at sag (low) points and where leaves and debris are heavy, because the vertical opening is not blocked easily (but silt can still collect). On steep roads much of the fast gutter flow runs past it.

  Road   Gutter  Curb
  ______         |
        \_______ |  <- opening (10-15 cm high)
                 |_____ Inlet chamber
         ---------> to sewer

2. Gutter inlet

A horizontal opening in the gutter covered by a cast iron grating (cast iron bars parallel to the flow). Water falls through the grating into the chamber. It is more efficient than a curb inlet on continuous (moderate to steep) grades and can handle a large flow, although some fast flow may skip over the grating. It may be blocked by leaves, paper and debris, and it is a risk to cyclists.

   Curb |  Gutter
        |__[#######]__    <- grating
        |  |       |
        |  | chamber|----> to sewer
        |  |________|

3. Combined inlet

A curb inlet and a gutter inlet are provided together, so that water enters through both. If the grating is blocked, the curb opening continues to work. It has high capacity and is widely used on busy city roads.

Other points

• The inlet is connected to the sewer by a 150-300 mm pipe, and a catch basin (silt pit with 0.6-0.9 m deep sump) or a trapped inlet is often provided to stop grit and smell.
• Inlets are spaced at 30-60 m on a road, depending on road slope, width and rainfall, and they are placed at street corners, low points and before pedestrian crossings.