Flood Risks to Life, Home and Property

Risks to Life, Home and Property

At the earliest possible moment when you know or suspect that flood water is coming your way, move to higher ground. Once the flood is in view, you will almost certainly not be able to outrun the moving water which usually flows at 6 to 12 miles per hour. For comparison, when the U.S. Geological survey measured the speed of water in the Colorado River using dye-movement, it was found to move at from about one mile per hour up to a little over four miles per hour.


Above: The Kaua‘i Marriott flooded on March 12, 2006. (Photo: Kaua‘i Civil Defense)
Do not enter or swim in flood water. The speed of the moving flood water is not the only danger. It is impossible to know with certainty what is in or under the surface of flood water. Any number of hidden risks from debris to chemical or sewage pollution may be present.

If anything porous enough to absorb flood water becomes wet, you will probably want to discard the object rather than risk contaminating your home, family or new/dry items that may come in contact with it. Any food that is at all affected by contact with flood water should be discarded. Even flood water that seems clean is almost sure to be carrying some contaminants. Hawaii's rivers, streams and waterfall pools are often dangerously contaminated with leptospirosis due to the presence of wild pigs at higher elevations, and flood waters are often washing down the same slopes.

Top

Risks to Drivers and Vehicles

Any discussion of flood risk involving automobiles is very much a discussion of risk to life. Many web based resources, including numerous pages of the National Weather Service's websites, cover the dangers of driving during flooding. Here is an excerpt from an essay by a Kentucky Army National Guard officer that summarizes the dangers very clearly:

"City streets can become rivers in seconds.

"In looking at the statistics, another fact becomes apparent. Most flood-related deaths occur in automobiles. Sadly, many of these fatalities were avoidable. In many cases, if the driver had turned around instead of driving onto a flooded roadway, one or more lives would have been spared.

"The reason for the high number of automobile-related deaths is that moving flood waters contain an incredible amount of force. For each foot of flood water, 1500 pounds of an automobile's weight is displaced. This means that two feet of water has more than enough energy to send most automobiles floating helplessly downstream.

"Escaping from a vehicle once flood waters have carried it away is very difficult, and in some cases nearly impossible. Among the problems: water pressure on the outside of the vehicle prevents occupants from opening doors; the vehicle could overturn into a ditch or ravine and become inundated; and even if a person were able to get out of the vehicle, the strong current and undertow of the flood waters would likely be too much to overcome in attempting to swim to safety.

"The safest practice during a flood or flash flood is to avoid driving onto water-covered roadways, even if the water depth appears low. Water depth is very difficult to estimate on roads, especially at night, when many flood deaths occur. Also, in the case of a flash flood, waters rise very quickly. Water that covered a road by only 6 inches at one moment could easily be 2 to 3 feet deep just seconds later!"

Even after a flood has subsided, the danger to drivers and people in automobiles is not gone. Wet roads and roads covered with silt or strewn with debris can be very dangerous.

While the water is present, the National Weather Service urges, "Turn Around, Don't Drown." After the water has drained away, extreme caution should still be exercised and driving should be avoided until local officials announce that free movement in vehicles is again safe.

Top

Risks in Coastal-Outlet Areas

People on relatively broad, flat coastal land might think they have little or no reason to be concerned about flooding. Runoff from higher places, whether it comes to the shore in gulches or as a sheet of water spread over a wide area, is unlikely to rise at the coast unless there is an obvious barrier to prevent it draining into the sea. Nonetheless, there are risks specific to coastal-outlet areas.

Wherever the ground is out of level enough to allow water to flow, it tends to move at 6 to 12 miles an hour, spreading as widely as the terrain permits. At the coast, the spread may be substantial, but prolonged movement of water across the land, even if it is spread till it is no more than an inch deep, can cause erosion. Eroded foundations or tree roots, ground under surface-anchored sheds or smaller paved areas can create dangerous situations. Be watchful.

Water rushing over open coastal ground can also carry away light items that are not anchored or placed in higher areas.

Risks Associated with Debris in Water

Debris in flood water can accumulate in tight passages creating temporary dams that cause flooding above the dam and then flash flooding below when the jam breaks. One of the effects of this kind of forceful and sudden movement of water, or of very prolonged flooding or sheets flooding is that boulders from mauka (higher ground) can be transported makai (to lower ground). Not only do the moving boulders and debris constitute a danger while in motion, but they can come to rest under water. When this happens, a road, path or open area that you may know to be flat or unimpeded, is actually turned into an obstacle course, although it is possible that none of the dangerous obstacles will be visible through the murky flood waters.

Moving debris carried by even very shallow water can also be dangerous. A large tree limb or whole tree, for example, may break windows, dislodge plantings or otherwise create risky or directly dangerous situations.

The rule for safety is simply this: Do not walk, swim or drive into flood waters. Even boating in flood waters should be avoided if at all possible.

Risks Associated with Trees and Falling Objects

Even if objects such as trees, sheds, barbecues and carport roofs are not carried away by flood waters, they can be destabilized and fall. Be watchful. When conditions allow, check the stability of anything on your property that is tall enough or high enough to be a danger if it falls. Trees or structures supported by poles in the ground might fall because of flood damage hours, days or even weeks after the waters have receded.

Top

Risks Associated with Utility Services

Downed power lines are a very significant danger during and after flooding. It is important to understand that various power lines carry higher or lower levels of power and require more of less protective equipment when they are handled. Leave the handling of power lines to professionals. If a power line is going to be down for a while, set up a "do not cross" perimeter around the downed line and the wet zone surrounding it. Even touching a tree limb when another part of the tree is in contact with a downed line or the saturated ground around the line could result in shock, injury or electrocution.

Leaking natural gas is not a hazard in most areas of Hawaii simply because there are no gas lines underground. However, an increasing number of subdivisions are providing piped in natural gas. If you have this utility serving your home, the smartest thing to do is to turn off the gas when a flood warning is issued for your area. If you do not do that, there is a risk of a broken gas fitting or pipe. Natural gas ignites easily from any spark, flame or intense heat source if it becomes traps and concentrated. Be aware of the danger and alert to the scent of gas in the air. If you detect or suspect natural gas is building up in a building, evacuate and stay out until a professional or qualified responder says it is safe to return.

A broken water line may add little more than additional water to a flood, but it can also cause sink holes in open ground or under paved areas and, less often, under structures such as your home. If you see a stream of clearer water flowing in flood water, suspect a broken water line and act accordingly. If the origin on the stream seems, for example, to be in an area you know to be a submerged roadway, expect a sinkhole or extreme instability in that roadway.

A sewer line break or sewer/manhole backup is not usually obvious right away if the effluent is escaping into flood water. This is one reason that you must not walk, swim or drive into flood water. Like a water line break, a sewer break can destabilize the surrounding ground and/or create a sinkhole, and it always pollutes the already dangerous flood waters with bacteria and other potentially harmful organisms and chemicals.

Top

Special Web Resource

Although the site seems to be addressed primarily to utility company employees and emergency responders, you will find more detailed information about the risks created by damaged utility services at the U.S. Department of Labor's Occupational Safety & Health Administration (OSHA) site.

Flood