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A Moving Experience
Hydrodynamic forces are among the five main causes of flood damage. The other four primary causes of
flood damage are debris impact, hydrostatic forces, sediment and contaminants, and soaking.
Moving water creates a hydrodynamic force that can damage a building’s walls in three ways
(see below):
- Frontal impact, as water strikes the structure.
- Drag effect, as water runs along the sides of a structure.
- 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.
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