At many places in drainage systems, including commonly occurring sag pits, surface water can form ponds due to obstructions or barriers.  Some of these may be intended, such as detention basins, while many are unintentional and may only operate in large storms. Barriers may be low, such as at road crowns, or high, such as walls, fences and embankments.  Allowable levels of ponding are defined in the Pit property sheet, as shown below for major and minor design storm events:

The safe depths for minor or major events can coincide with the depth at which the pond spills.  A system is considered to have failed if the nominated depths are exceeded, and the run reports will note this.

Appropriate depths can be set by considering the effects of excessive extents of ponding, which may impede traffic, prevent pedestrians from crossing a road, cover footpaths and enter adjoining properties.  In major storms, safe ponding depths greater than the pond spill depth may be allowed, permitting flows over road centrelines in major storm events.  The choice is up to the designer.

Usual pond depths are of the order of 0.1 to 0.3 m, and ponding volumes are 1 to 5 m³.  A typical safe depth for minor storms might be 0.12 or 0.15 m.  (If these values are set too low, the corresponding sag pit inlet capacities may be impracticably small, so some compromise may be necessary, accepting greater depths.)  A typical value for major storms might be 0.2 m, representing a 50 mm depth above a road crest with a 0.15 m spill depth.

If the safe depths set for a sag pit are exceeded, warning messages may appear in the run report.  This facility is also used in DRAINS as part of the automatic design procedure that determines the pit and pipe sizes needed to ensure that the safe depths will not be exceeded in manor and major flow conditions.

There may be clashes between the allowable minor and major flow depths set for overflow routes running into sag pits, and the corresponding depths set for these pits.  For example, if the overflow route with the property sheet shown below flows into a pit with the characteristic shown,

The following warning will appear:

And if Yes is selected, the minor safe level at the pit will be changed from 0.2 m to 0.15 m. Calculations can proceed with or without this adjustment.