Underground water-storage vault and method for installing same

A vault for storing water underground, the vault comprising:

• a sump formed of flexible, water impermeable membrane material that has been joined together at weld seams to form a box having an internal volume that defines a reservoir for water, said box including a base with a perimeter, a plurality of sidewalls extending upwardly from the perimeter of the base, said internal volume being sized to receive a predetermined amount of structural support matrices;
• and
• a lid configured to cover an opening at a top of the sump, said lid comprising a plurality of flaps that extend from a perimeter of a top portion of the lid, said plurality of flaps not being attached directly to each other;
• wherein fasteners are provided on the sump and corresponding fasteners are provided on the flaps of the lid for releasably securing the flaps to the sump,
• wherein at least one of the sump or the lid includes at least one aperture that provides fluid communication between the internal volume reservoir and an exterior of the sump, and
• wherein the sump and lid are in a folded or rolled condition and contained within a container for shipment.