Street inlets and catch basins

Street inlets

Street inlets are openings in the road kerb or gutter that let storm water from the road surface enter the underground storm drain or combined sewer. Types: curb inlets (vertical opening in the kerb), gutter inlets (horizontal grating in the gutter) and combined inlets (both together).

 Curb inlet          Gutter inlet
   |                 ________
   |____ opening     |=grating=|
   |   |             |         |
   |   v chamber     |  chamber|

Inlets are placed at 30-60 m spacing, at corners and at low points of the road, and they connect to the sewer by a 150-300 mm pipe.

Catch basin

A catch basin is a small masonry or RCC chamber, usually 0.6-0.9 m in diameter and 0.9-1.5 m deep below the outlet, built in the line from a street inlet to the sewer to collect silt, sand, grit and floating rubbish which would otherwise settle in the sewer.

      Street inlet grating
          |
     _____v_____
    |           |  ground level
    |           |
    |      ___  |-- hooded outlet (dips in water)
    |     |   | |
    |_____|silt|_|

Construction: brick or concrete walls plastered inside, a concrete floor, a cast iron cover, and an outlet pipe placed 0.3 m or more above the floor so that the heavy matter settles below it. A hood (bend) on the outlet traps floating matter and gives a water seal against foul gas. Working: the velocity falls in the basin, the silt settles, and the clear water goes into the sewer. The basin is cleaned regularly (every few weeks, and more often in the rainy season). It is not provided in modern sewers where velocity can carry the silt and where cleaning is difficult; instead gully traps are used.