Flood Debris Impact and Other Hazards

Floodwaters can pick up and carry objects of all types – from small to large, from light to heavy – including trees, portions of flood-damaged buildings, automobiles, boats, storage tanks, mobile homes, and even entire homes. In cold climates, wintertime floods can also carry large pieces of ice. Dirt and other substances such as oil, gasoline, sewage, and various chemicals can also be carried by floodwaters. All of these types of debris add to the dangers of flooding. Even when flow velocity is relatively low, large objects carried by floodwaters can easily damage , doors, walls, and, more importantly, critical structural components of your home. As velocity increases, so does the danger of greater damage from debris. If floodwaters carrying large amounts of dirt or hazardous substances enter your home, damages may be greater. In addition, your cleanup costs are likely to be higher and your cleanup time greater.

Other Hazards

Two more hazards you should be aware of are high winds (including hurricanes) and earthquakes. For homes in areas subject to these hazards, some retrofitting methods are more appropriate than others. But, regardless of the method you choose, if your home is in a high-wind or hazard area, your professional or contractor must ensure that all structural changes made can withstand not only the expected flood forces, but the expected forces of winds or earthquakes as well.

Wind is similar to flowing water in that it pushes against the side of the home that faces the wind and pulls on the side that faces away (Figure 2-12). Wind passing over a home can exert a lifting force on the home. The combination of push, pull, and lift acts on the home, including the foundation, and can result in extensive damage if the structural system and building envelope are not adequately designed and constructed.

The building envelope is the entire exterior of a building (including walls, doors, and windows) that encloses or envelopes the space within.

The ability of the wind to damage a building is increased if the wind or windborne debris breaches the building envelope by breaking windows, collapsing doors, or puncturing walls. Once the envelope is breached, wind will enter the building and the pressure on the walls and roof will increase, as shown in Figure 2-12. Wind and flood forces can combine in different ways, depending on the directions of the wind and flood flow. When the wind and flood flow direction are the same, the load on the home is greater than the load from either wind or flood alone.


Figure 2-12. Wind forces on a sealed building and on a building with an opening.

The movement of the ground during an earthquake can place large horizontal and vertical loads on a home (Figure 2-13). Like the loads that result from flood flow and wind, earthquake loads can cause extensive damage to a home if they have not been accounted for in the structural design.

High-wind and earthquake hazards vary throughout the .


Figure 2-13. Earthquake forces act in both horizontal and vertical directions.