Hourglassing

Hourglassing is a term used to describe a phenomenon where a structure, particularly a sand dune or a sandbagged barrier, begins to lose its integrity and collapse in a shape resembling an hourglass. This typically occurs when the material supporting the structure from below is eroded or removed, causing the upper portion to slump downwards.

In the context of sand dunes, hourglassing can happen when a significant portion of the base of the dune is undercut by wind or water. As the supporting material is lost, the dune's shape changes, with the upper part of the dune appearing to sag inwards, creating a narrow waist before widening out again at the base. This can lead to a visible break in the dune's structure.

Similarly, when sandbags are used to create temporary flood defenses, hourglassing can occur if the water pressure or flow erodes the ground behind or beneath the sandbag wall. The bags at the base may shift or sink, causing the entire structure to bend and potentially fail in an hourglass formation. This signifies a critical point of weakness in the defense. Understanding hourglassing is important for predicting and preventing structural failures in environments where granular materials are used for support or construction.