A Barrier Is a Pump Specification: Hydrostatic Load, Sandbag Count, and Leakage at a Household Flood Door

Household flood protection is usually sold and discussed as a barrier: a board, a gate, or a wall of sandbags fitted across a doorway. This paper computes the three quantities that decide whether such a barrier succeeds — the hydrostatic load it must resist, the material needed to build it in sandbags, and the water that passes it anyway. Across a 0.90 m doorway, floodwater 0.6 m deep exerts a resultant force of 1,589 N, equivalent to the weight of 162 kg, four times the 397 N exerted at 0.3 m depth, because load grows with the square of depth. A free-standing sandbag wall built to resist 0.6 m of water plus freeboard requires about 93 bags per metre of frontage, or roughly 139 bags for a doorway with returns. Most importantly, leakage is not a defect but a design quantity: even a well-compressed seal, represented by an equivalent uniform gap of 0.1 mm, passes 11.1 L/min across the doorway at 0.6 m depth, and an aged seal at 0.5 mm equivalent gap passes about 56 L/min across the door — rates that no amount of barrier strength addresses. A flood door specification is therefore incomplete unless it states a leakage class and pairs the barrier with a pump or bailing capacity that exceeds it. All results are simulated, not measured.