Unreinforced masonry (URM) structures in less developed areas are at great risk from the rapid flow of water resulting from flash floods. This study investigates how susceptible unreinforced masonry walls are to the loads produced by flash floods via testing, simulating, and gathering field measurements of existing URM structures. Rather than simulate everything on the computer, the study compiled and reviewed published documents that reported results pertaining to performance trends of different wall configurations, as well as slenderness ratio, strength of mortar, and other boundary conditions, and developed an understanding of performance trends among URM walls. The earliest damage begins with the combined action of hydrostatic and hydrodynamic pressure, which has the greatest effect on slender walls with weak mortar. Walls that contained more reinforced concrete or brick demonstrated better performance, with longer crack propagation before catastrophic failure and higher critical flow rates before cracking. The assessment of fragility determined that the likelihood of a catastrophic failure increases greatly when the water flow velocity exceeds a certain threshold, and that how the structure is designed (geometry) and what it is made of (material quality) are two of the key factors in determining the likelihood of failure during a flash flood event. The results of this study indicate the need to develop new methods for designing and selecting retrofitting systems that will improve the level of flood resistance for masonry structures in areas susceptible to flooding.