Flow velocity is the speed at which floodwaters move. It is usually measured in feet per second (fps). Flow velocities during riverine floods can easily reach 5 to 10 fps and, in some situations, may be even greater. Expressing velocities in fps is common in floodplain studies and engineering analyses. It may be helpful to relate fps to a more familiar unit of measure. For example, 10 fps is roughly equal to 7 miles per hour (mph).
The velocity of riverine floodwaters depends on a number of factors; one of the most important is the slope of the stream channel and floodplain. As you might expect, floodwaters will generally move much faster along streams in steep mountainous areas than streams in flatter areas. Even within the same floodplain, however, flow velocity can still vary. As water flows over the ground, its velocity depends largely on the roughness of the ground surface. For example, water will flow more swiftly over parking lots, roads, and other paved surfaces, and will flow more slowly over ground covered with large rocks, trees, dense vegetation, or other obstacles. Also, flow velocities in the floodplain will usually be higher nearer the stream channel than at the outermost fringes of the floodplain, where water may flow very slowly or not at all. In areas subject to coastal flooding, velocities depend largely on the speed of the wind and, like riverine flow velocities, on the slope and roughness of the ground surface.
If your home is in an area where floodwaters are flowing, especially if they are moving more than about 5 fps, the flow velocity is important for several reasons. Flowing water pushes harder on the walls of a building than still water. So instead of just the hydrostatic pressure caused by the weight of the floodwater resting against the walls of your home, you have the additional pressure of moving water, referred to as “hydrodynamic pressure” (Figure 2-10). As water flows around your home, it pushes against the side of the home that faces the flow (the upstream side). As it flows past the sides of the home, it creates friction that can tear at wall coverings, such as siding. On the side of the home that faces away from the flow (the downstream side), the water creates a suction that pulls on walls.
In some situations, the combination of these forces can destroy one or more walls, cause the home to shift on its foundation, or even sweep the home away.
Flowing water can also cause erosion and scour. As previously discussed, erosion refers to a general lowering of the ground surface over a wide area. Scour refers to a localized loss of soil, often around a foundation element. Both erosion and scour can weaken the structure of a home by removing supporting soil and undermining the foundation. In general, the extent and depth of erosion and scour increase as the flow velocity and size of the home increase. Also, keep in mind that any objects being carried by floodwaters will be moving at roughly the same speed as the water.