The special weir object in DRAINS can be used as an alternative to an overflow route at sag pits when the hydraulic control is a parabolic weir. For example, if a user has access to the Full unsteady hydraulic model option in DRAINS, the overflow from a sag pit shown below can be replaced by a weir.

Instead of overflow OF 1, a weir can be inserted.

The results will be the same for these two arrangements, but the weir has some advantages:

- It does not include any volumes of overflow routes, avoiding possible double-counting of storage volumes (the volumes specified for the sag pit or detention basin plus the volume inferred from the cross-section and depth in the overflow route).

- It is not necessary to include a cross-section for the overflow route, which can be confusing when overflows travel a short distance, such as situation where flows cross over the crown of a road.

Using the Full unsteady hydraulic model calculations, backwater effects due to high water levels downstream can influence the flows across the special weir, in a similar way that they influence water levels and flows in overflow routes.

Note that the other weir types (rectangular, v-notch, etc.) cannot be used for surface flows from a sag pit.  Overflow routes must be used with these.