For sag pits, in the Pit property sheet, DRAINS offers two choices regarding storage volumes: an inverted pyramid assumption and an elevation - surface area table:

With the first option, in the Lite hydraulic model, DRAINS considers ponding at sag pits up to the specified ponding depth and the associated volume supplied.  It assumes that the storage over sag pits is the shape of half an inverted pyramid, and does not allow for volumes greater than the specified value.

In the Full unsteady hydraulic model , the pyramid shape is extrapolated upwards and allowance is made for the additional storage when the defined storage depth is exceeded and overflows occur.  A maximum depth of ponding is recorded.  In flat terrain, this depth may also be affected by backwater from the downstream flow path. If downstream levels are high, it will increase the ponding depth and the available storage.

If more precise modelling of the storage is required, the second, tabular option for pond geometry can be chosen. The table can be set up in a spreadsheet as two columns that can be copied and brought into DRAINS using the Paste Table Data key.  (Note that this table cannot be used in design calculations, but it can be used in subsequent analyses of a designed system.)

More general types of storages can be modelled as detention basins, with multiple outlets if required.