A Moving Experience

1. Frontal impact, as water strikes the structure.
2. Drag effect, as water runs along the sides of a structure.
3. Eddies or negative pressures, created as water passes the downstream side.

The speed of moving water is called velocity, a force that is measured in feet per second. The faster water moves, the more pressure it puts on a structure and the more it will erode stream banks and scour the earth around a building’s foundation.

Floodwaters moving faster than 5 feet per second comprise a high-velocity flood, requiring special design considerations for buildings, roads, bridges and other manmade structures in its path.

While velocity is one factor in determining the potential harm of a flood, the total impact of moving water is also related to the depth of the flooding. Research demonstrates that deep water and low velocities can cause as much damage as shallow water and high velocities.

People are more susceptible to damage than buildings: Studies have shown that it doesn’t take much depth or velocity to knock a person over. Thus, no areas with moving floodwater can be considered safe for walking.

A car will float in only 2 feet of moving water, which is one reason floods kill more people trapped in vehicles than anywhere else. Often victims put themselves in perilous situations by ignoring warnings about travel or mistakenly thinking that a washed-out bridge is still open.

This article is based on information from " Floodplain Management Requirements: A Study Guide and Desk Reference for Local Officials," a FEMA publication that is accessible online.