Whether the flooding at a building results from storm surge, riverine flooding, or urban flooding, the physical forces of the floodwaters which act on the structure are generally divided into three load cases. These load cases are hydrostatic loads, hydrodynamic loads, and impact loads. These load cases can often be exacerbated by the effects of water scouring soil from around and below the foundation.
The hydrostatic loads are both lateral (pressures) and vertical (buoyant) in nature. The lateral forces result from differences in interior and exterior water surface elevations. As the floodwaters rise, the higher water on the exterior of the building acts inward against the walls of the building. Similarly though less common, a rapid drawdown of exterior floodwaters may result in outward pressures on the walls of a building as the retained indoor floodwater tried to escape.