This window allows you to set up a data base of inlet capacity relationships for common pit types or families, with different sizes in each type. This format is required for the design method used in DRAINS.

The Add and Properties buttons in the top part open a dialog box in which the Pit Type Name is entered:

The corresponding buttons in the lower part of the dialog box open a dialog box for a particular size, which must be selected and highlighted before the box can be opened. The Pit Size dialog box is in three parts. The first has boxes for the Size Name and for comments:

The second provides space to enter a table of on-grade inlet capacities, related to the approach flow:

The relationship can be typed directly into this dialog box, or it can be developed using the wizard implemented by clicking the HEC22 Wizard button. They can also be read in from a spreadsheet via the Windows Clipboard. After preparing an inlet capacity relationship in two columns of a spreadsheet and copying these, the relationship can be  transferred to DRAINS using the Paste Data button.

A similar relationship applies for sag pits:

By entering the pit perimeter, clear opening area and a maximum depth of ponding, you can have a relationship defined and placed in the dialog box from which it is called.  This is based on relationships that allow for weir and orifice flows into sag pits, and the transition between these.  It is based on the relationships from the U.S. Federal Highway Administration Hydraulic Engineering Circular 22, Urban Drainage Design Manual, 3rd Edition, 2009 (available from  www.fhwa.dot.gov/bridge/hyd.htm).

Note that DRAINS applies a curve-fitting procedure to the points in these relationships. It only uses straight line interpolations when two points are given. If you have a relationship with a sharp change of curvature, for example, due to the change from weir to orifice control shown in the diagram above, you should locate points close together near the change of curvature.

Regional pit inlet capacity relationships for common pits used in New South Wales, Queensland and Victoria, the ACT and South Australia are supplied in the DRAINS examples and data base files.

(Note that there are restrictions on changes to data bases once pits and pipes are entered in the Main Window for a DRAINS model. It is then not possible to install a new default data base, or to remove pipe or pit types, though it is possible to add additional pipe and pit types into the data base, and to alter data in the pipe, pit and overflow route data bases.)  The data bases defined for a DRAINS model remain with that model, being stored in the model’s .drn file.)