The hydraulic head, Dh, is equal to zero when:

The hydraulic head, Dh, represents the height of water above a reference point and is an important parameter in assessing the potential energy of water in a system. It is calculated by considering factors like elevation head, pressure head, and velocity head. For Dh to be equal to zero, it indicates that there is no pressure or elevation head contributing to any water potential.

When overflow along the roof edge is present, it suggests that there is excess water that is unable to drain properly, often leading to a situation where water accumulates at the roof edge. In this case, the hydraulic head at that point is effectively zero because the flow is not being contained or supported by a functional system – it indicates an overflow condition rather than a functional drainage situation.

Other factors mentioned in the choices wouldn't directly drive the hydraulic head to zero. The absence of snow load or the presence of a fully functional drainage system would not necessarily result in a hydraulic head of zero, as these conditions might still allow water to exist or flow through a system. Likewise, if water is completely absent, while it might suggest zero hydraulic pressure, it doesn’t specifically reflect the conditions relating to the functioning of the drainage system or the overflowing scenario which defines operational hydraulic head.

Thus, the presence