These are stormwater flows that escape from a pipe system or a detention storage for one of these reasons:

• flows reaching a pit cannot be accepted by the pit inlet, and flow away as bypass flow,

• flows are forced to upwell out of a pit because the downstream pipe has insufficient flow capacity,

•  flows cannot enter an open channel because it is completely filled,

• a detention storage becomes completely full.
  

Overflows may be due to a combination of effects. In DRAINS they are directed along overflow routes to downstream pits or out of the system altogether.