Pits (manholes, gullies, inlets, catchbasins)

In piped stormwater drainage systems pits serve several purposes. They act as inlets for stormwater, points where pipes can conveniently change their size, slope or direction, and inspection and maintenance openings.

An important distinction is between on-grade pits and sag pits.  Pits of the former type are located on a slope, so that any flows that bypass the pit move away and travel on to the next pit. Sag pits are located in a depression or sag, so that water cannot readily escape.  If the capacity of the pit is not sufficient to accept all flows arriving at the pit, stormwater ponds over the pit until it becomes high enough to cross some barrier such as the crown of a road.

Pits can have various shapes and types of inlets; in Australian separate stormwater systems they are usually rectangular boxes constructed of in-situ or precast concrete. The main inlets used are kerb inlets, slots located in the kerb face, or grates. A sump (a bottom of a pit lower than the invert of the outlet pipe) may be provided to catch sediments, but most Australian pits do not provide this.

Sealed pits can allow the hydraulic grade line to rise above the ground surface.  They prevent water from entering and from leaving the pit. A large set of information on pits used in various parts of Australia is supplied with DRAINS. It is the responsibility of users to assess the suitability of this information for their needs.

In DRAINS, pits can have two outlet pipes in cases where there is a branched outlet, or where the network is looped. Normally, there will only be one outlet from a pit. After a run, peak water levels for pits and nodes are presented in the Main Window. If multiple storms are used, the results for a particular storm can be selected using the pull-down menu that appears in the top-left corner of the Window. The Customise Text option in the View menu allows you to vary the on-screen display.

The main point of interest at pits is likely to be the level of the hydraulic grade line. The HGLs at a pit can be examined using the Pop-Up (right mouse button) menu that provides options for plotting HGLs calculated during a run, or for displaying the numerical values.

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