This example shows how DRAINS might model an established stormwater drainage system where overland flows are blocked by fences and other barriers.  A detention basin is used to model an unintended stormwater storage area above a pit located behind a commercial building.

The DRAINS model is shown below. The location is occupied by small factory and warehouse units. The fall of land is from the north-west to the south-east. The site shown hatched represents a building re-development that has shut off an overland flow path, causing surface stormwater to pond beside the building and to escape by two possible routes. This situation is modelled as a detention basin with one low-level pit/sump outlet and two high-level weir outlets.

All pipe inverts in the system must be fully specified, and the run is made as an Analysis run with major storms. The last pipe in the system has a rectangular cross-section. Note how overflow routes are linked via a node.

The next figure shows the median storm results obtained from DRAINS. These show instabilities, which mainly occur at low water levels inside a pit.