Inlets

In a stormwater system, an inlet is a device (a structure) that intercepts the surface runoff, and directing it into the underground sewer system. Apart from their appropriate location and spacing, inlets should be provided with sufficient capacity to entrap and direct surface water as fast as possible with a view to avoid backups on the ground surface after taking into account factors like clogging, nuisance to traffic, hydraulic capacity, and desired safety. It is within this context that flooding across street intersections is avoided.

There are three basic types of inlets that are in use, such as, kerb inlets, gutter (or grate) inlets and combined inlets. A kerb inlet has a vertical opening along the kerb itself. It is through this opening that the runoff flow finds entry into the sewer system. With reference to child safety the kerb opening should be less than 150 mm high, which, however, limits the intake capacity of the inlet. A gutter inlet is a horizontal opening provided directly in the pavement itself; and, it is covered with cast iron grating to obstruct the entry of large-size debris. The gutter type inlet, however, obstructs the smooth flow of traffic (particularly bicycles, etc.), and the grating is subject to being plugged with debris.

A combination inlet incorporates both a kerb and a gutter opening in its design. The gutter grating can be depressed to gain additional hydraulic capacity. The combination inlet is, obviously, the least subject to the menace of clogging.

If the drain invert happens to be above the bottom of the inlet basin, then this inlet basin is termed catch basin. In fact, the catch basin is meant to trap the grit, sand and leaves etc. that may be washed into it. Sometimes catch basins are discouraged to be incorporated as a feature of stormwater collection design due to requirement for periodic cleaning, odour menace and mosquito problem, and, therefore, only inlet basins are adopted as the design feature of the collection system at the inlet point. Appropriate design and adopting self-cleansing drain slopes would in that case maintain the collection system free from blockage problems.