This is a pit with a bolted-down lid, which can pressurise so that the hydraulic grade line can rise above the ground surface.

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This can be quite complicated when there are branches and flows must be distributed among the pipes leading into a pit so that pipe flows and overflows can be determined. DRAINS can deal with these situations. To specify a sealed pit, it is necessary to tick the Pit has bolt down lid box in the Pit property sheet.