Flood Water Hazards - Threat to Life
Generally people should avoid walking or driving though water or getting too close to watercourses surging with floodwaters.
Assume walking in moving water deeper than 0.15 metres (6 inches) is potentially dangerous.
Vehicles including SUV’s are commonly carried off roadways in as little as 0.6 metres (2 feet) of moving water.
Flood waters pose a far greater risk to life and injury than most people realize. In this respect the following discussion is an attempt to educate people about the hazards of flood waters to save lives and prevent injuries and personal trauma.
According to Natural Resources Canada, there have been at least 168 flooding disasters in Canada during the 20 th Century resulting in the deaths of at least 195 people (up to 1997, but not including storm surges).
Clearly the threat to human life from flooding is significant even today despite our North American standards in housing, flood plain development planning, flood warning and emergency response. A significant factor in these deaths is the continuing lack of awareness or appreciation of the hazards of flood waters by the public.
Loss of Stability (losing your footing and being swept away)
Most people recognize the risk of drowning or injury from moving water. The faster the water is flowing, the shallower the depth required to sweep people off their feet. However most people fail to recognize just how little current or depth is required to create a hazard.
It is not uncommon for people to be swept off their feet by moving flood waters in water depths below their knees, even ankle deep in some cases. In deeper water, less current is required to make one unstable due to increased buoyancy.
The recommended approach is to assume walking in moving water deeper than 0.15 metres (6 inches) is potentially dangerous. In addition the depth of water is hard to determine because of the high turbidity (muddiness) of flood waters. Vehicles including SUV’s are commonly carried off roadways in as little as 0.6 metres (2 feet) of moving water.
Strainers and Entrapments
The biggest hazard from moving water is what is referred to as strainers. These are natural obstructions like log jams or sweepers (trees fallen into the stream along the banks of a watercourse) or obstructions like fences and grates. Strainers allow water to pass through, but not solid objects, including people.
The force of moving water is not fully appreciated by most people. Once a person becomes tangled in a strainer the force of moving water will in most cases make self-rescue nearly impossible and assisted rescue very difficult and dangerous. Most natural strainers force people under water and drowning will occur nearly immediately.
In natural streams, particularly streams with rocky or boulder bottoms, foot entrapments are a danger when trying to wade across or attempting to stand up after falling into the water. In an urban environment other obstructions such as fences may entrap a person’s foot when attempting to wade or to stand up after falling.
In natural or urban environments, wading across moving water is not recommended unless you can see what you are stepping into and it is absolutely necessary. It is unlikely during a flood event that you will be able to see where you are wading as flood waters are normally very muddy.
Stationary Objects in Moving Water
When people are swept into moving water they may strike stationary objects like boulders, bridge piers or telephone poles with enough force to injure them or render them unconscious. A boat or canoe striking a stationary object may be pinned causing the boat to collapse, sink or be crushed. People in the boat could become trapped in the boat or thrown into the water.
If swept into moving flood waters there are two recommended methods of avoiding hazards downstream. One is to float on your back with your feet up facing downstream and using your feet and legs to push away from stationary objects like rocks, boulders, cars or telephone poles. The other technique is to swim hard on your stomach angling upstream and away from the hazard. Only experience or quick judgment will determine the appropriate action.
In addition, flood waters in British Columbia normally carry debris like branches, trees, lumber or even houses. Debris can either injure an individual just from being struck or can crush or pin a person against another stationary object. People standing on the banks of flood swollen rivers have been struck and knocked into the water by large floating debris like trees.
Flood waters are often polluted with sewage, carcasses, fuel and other contaminants. A mouthful of this water may cause illness from some form of water borne disease. Even immersion in polluted flood waters may be hazardous to your health. Food that has been contaminated by flood waters should be discarded.
Electrocution and Gas Hazards
Downed power lines and other electrical sources are an obvious threat. Leaking gas lines are another threat. Good reasons not to re-enter one’s home until given the OK by the authorities.
Cold Water Hazards
(Most of the following information has been condensed from Transport Canada, Survival in Cold Waters)
Flood waters are hazardous because of the normally cold temperatures associated with them. This is especially true in British Columbia where the majority of floods are from winter rains or snow melt. Temperatures associated with ice jam flooding are even more frigid with water temperatures being extremely cold (close to or at 0 oC).
Cold Shock (from immersion)
There is a shock reaction that incapacitates many people as soon as they are immersed in cold water. Contrary to intuition, this shock reaction is at a maximum at water temperatures between 10 - 15 C. There is a gasp reflex followed by hyperventilation, muscle spasm and drowning. Basically this is an inability to control breathing and breath hold. In addition to these effects there are also massive increases in heart rate and blood pressure that can cause death as well. These effects last for the first two to three minutes.
Between three to thirty minutes after immersion in cold water death often occurs from an inability to swim even in people who are fit and are good swimmers. Although not well understood, it is attributed to the effects of cold water on one’s respiratory (breathing) and cardiovascular (heart rate and pressure) responses. There are numerous recorded cases of fit, young and very capable swimmers failing to swim even relatively short distances (50 metres or less) to safety in cold water.
Muscular Failure Including Dexterity
Before hypothermia takes effect, cold water will quickly reduce the ability of muscles to perform work. This is why so many people are unable to rescue themselves. The ability to climb on to life rafts, hang on to safety lines, inflate life jackets, activate flares or a host of other life saving tasks just becomes impossible.
If the first thirty minutes doesn’t prove fatal prolonged immersion in cold water will eventually result in hypothermia. Death results from drowning through incapacitation or cardiac arrest. Survival times for hypothermia is primarily a function of water temperature, but other factors including clothing (insulation), agitation of the water, body heat production through exercise or shivering, ratio of body mass to surface area, subcutaneous fat thickness, physical fitness, diet prior to immersion and physical behaviour and body posture in the water. The table below from “Cold Water Survival”, Canadian Red Cross, shows survival time versus water temperature. Unfortunately many people will die from hypothermia even after rescue if not promptly and correctly treated.
Flooding as a Threat to Life , in Appendix 6, Technical Guide River & Stream Systems: Flooding Hazard Limit, Ontario Ministry of Natural Resources, Water Resources Section, 300 Water Street, 5th Floor, South Tower, P.O. Box 7000, Peterborough, Ontario K9J 8M5
Hydrodynamic Models of Human Stability in a Flood, Niels Lind, Desmond Hartford, and Hamed Assaf, in Journal of the American Water Resources Association, February 2004.