Pit property sheet

This sheet appears as:

(Layouts that apply in earlier versions of DRAINS are described later in this topic.)

(a) Inputs

With the latest sheet, it is necessary to enter the following information in the main Pit Properties page:

• a pit name of up to 10 characters,

• the pit surface elevation (m), to the datum being used,

• a pit type and size, selected from two drop-down menus containing choices set up in the Pit Data Base property sheet,

• a pit pressure change factor for full pipe flows.

If  a pit size is not specified, DRAINS will accept this and place '??' before the pit name in the main window,  It will be possible to perform a Design run with this, but not an Analysis run.

Pit inlet capacity relationships for common pits used in New South Wales, Queensland, Victoria, South Australia, Western Australia and the ACT are supplied in the DRAINS examples and as data base files in the DRAINS Project folder (C:\Program Files\Drains\Program or C:\Program Files (x86)\Drains\Program).

The pressure change coefficient varies with the geometry of the pit and other circumstances and is usually determined from design charts such as those in the Queensland Urban Drainage Manual (QUDM).  Most of these charts require flowrates through the pit to be specified.  There are two options in the Run menu for DRAINS to define full-pipe pit pressure coefficients after an initial run that estimates flowrates through a pipe system.

You also have the options to specify:

• whether the pit is sealed with a bolt down lid,

• whether the pit is new or existing (a new pit can be changed to a smaller or larger size in a DRAINS design run),

• a fall across the pit that is different to the default value set in the Project → Options property sheet,  and
  

• a blocking factor between 0 (clear or unblocked) and 1 (fully blocked), different from he default values set in the Project → Options property sheet.

This is all the information required if the pit is an on-grade pit, but if it is a sag pit, the following information must be entered on the Pond Properties page.

Ponding depths are described in more detail here, and the specification of volumes is described here.

The third tag opens the QUDM page:

This is used when determining pipe pressure changes from charts in the Queensland Urban Drainage Manual, using the procedure in the Run menu.  The alignment of pits and pipes is used to select between various design charts.  The pit width is that of the face in which the outgoing pipe is located.  If the QUDM method of determining pit pressure changes is not being used, no entries are required in the QUDM page.

In the main Pit Properties page you can also enter a baseflow or user-provided hydrograph. If you click the Baseflow button, an additional Baseflow property sheet will appear. If you click the Inflow Hydrograph button, the User-Provided Hydrograph property sheet appears and the ordinates can be entered.

(b) Earlier Versions

In very old DRAINS files you may come across the ILLUDAS type pit, which is not available in current versions of DRAINS because it can cause errors in some circumstances, and its effects can be modelled in other ways.

Formerly, the pit property sheet included three choices of how to handle part-full pit pressure changes. Only the default model that applied then is now available.

Handling pressure changes at pits when flows are part-full is difficult because there is little literature or guidance on this subject.  In the Lite and Full Unsteady hydraulic models in DRAINS, the part-full flow ku factor is assumed to be the same as the specified full-pipe flow ku factor.  This is probably conservative, but helps avoid unstable changes of HGL as pipe systems change from part-full to full-pipe flow and other oscillations occur.

The safe depth requirement was introduced in 2017. Prior to this, design change for pit sizes or a warning in Analysis runs only occurred when a depth greater than zero occurred in the downstream flow path.  Effectively, the safe depth was taken as the crest of the weir at the sag pit, the maximum pond depth. Users wishing to meet lower depth requirements had to work by trial and error. Now, zero levels should not be specified for flow paths.

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