Floodwaters on the Werribee River at Cottrell St Ford. Werribee, Melbourne, Victoria, Australia. November 28, 2010.
Floods showcase nature’s power in an immensely destructive way. Although modern engineering and planning has allowed us to shape, change, and attempt to control nature, we still remain powerless when nature’s laws dictate that an extreme event, like a flood, must occur to restore the environment to equilibrium.
Floods are powerful in a way that most of us cannot fully appreciate. For example, if water is flowing fast, it only takes 6 inches of water to knock a person over, while maybe even more impressively, 2 feet of water is enough to lift a large vehicle, like a bus, off the ground and make it float.
Great Johnstown Flood aftermath. Pennsylvania, USA 1889.
Flooding in modern times highlights the tension between human engineering ability and nature’s raw power. One example, the Johnstown Flood, took place in western Pennsylvania in 1889 when an artificial dam failed after a period of extreme rainfall. The dam had been used to create the man-made Lake Conemaugh, part of a private recreational club used by the industrial age’s elite - people who had made their mark using technology to impose human will over nature.
The dam proved too weak to hold back the rapidly rising lake-water, and it broke, releasing a pent-up 20 million tons of water with a current so strong that it temporarily equaled the flow rate of the Mississippi River. This water swept downstream, overwhelming the industrial workers’ town of Johnstown and killing 2,209 people in the deadliest man-made disaster in U.S. history until the September 11, 2001 terrorist attacks.
Since 1900, over 10,000 people have died in the U.S. due to floods. Unfortunately, we often lack the knowledge that would keep us safe when flooding occurs, as is illustrated by the sad statistic that in recent years, approximately two-thirds of people who die in floods are in vehicles – likely attempting to drive through flood water.
What causes a flood?
Floods result from an imbalance in nature’s hydrological system. The way that the water cycle should work is: rain falls from clouds, lands in the soil, and flows through streams and rivers to the oceans. From there, water returns to form new clouds, and the cycle begins again. However, when that system gets out of balance, a flood occurs, since more water enters and flows through an area’s hydrological system than that system is able to distribute through its network of waterways.
Many causes of floods are actually quite intuitive, once you think about them. Heavy rainstorms and large volumes of snowmelt saturate an area’s soil and waterways with more water than they are able to handle. When soil gets saturated, it stops soaking up water and, instead, acts as a pathway for the water. Frozen ground has a similar effect – acting as a conduit for water rather than a drainage system. As this excess water reaches streams and rivers, they overflow, causing a flood.
Flooding in Brisbane, Queensland, Australia. January 10, 2011.
Blockages, such as ice, in streams and rivers can also create floods. Large pieces of ice get caught and stack up, near bridges or bends in the waterway.
Schuylkill River Flood. Mohrsville, Pennsylvania, USA. June 28, 2006.
Urbanization also plays a role. More streets, buildings, and parking lots mean less natural soil area for drainage. Instead of grass, dirt, and sand to soak into, water finds itself on impermeable concrete and pavement, which then provide conduits for the floodwater. Of course, municipalities plan out ways to direct rainwater using drainage ditches and storm sewers. However, when precipitation or snowmelt exceeds city planners’ expectations, these systems overload. Without nature’s soil drainage system, cities can get flooded by an amount of rain that would have been easily absorbed and recycled in a non-urban area.
As wetlands along rivers get destroyed or drained, the river actually becomes more likely to flood. The mud and soil of this swampland, which often sits at the edge of a river, act as a storage vehicle for excess water. When these areas have been eliminated or altered, possibly for farming or housing developments, water from a river overflow is no longer soaked up by the riverside swamp.
Normal floods from rivers occur gradually, as the water rises over time, eventually exceeding the banks of the waterway. Compared to flash floods (discussed below), river floods are much easier to predict, although they do tend to cause more property damage when they take place. The area around a river, called a watershed, act as a funnel, collecting water and directing it down into the river. When there has been a large amount of rain or snowmelt runoff in the watershed, the river rises to unusually high levels. This visible sign serves as an early warning, so that by the time the river overflows its banks, hopefully local officials and residents have anticipated the flood and are prepared. Flood predictions can happen days, weeks, or sometimes even months in advance.
A flash flood, by contrast, happens instantly - a “wall of water” rushing over an area. Flash floods are much more dangerous to people than normal floods, because there is significantly less warning, and people might not be able to evacuate quickly enough. Nearly three quarters of flooding deaths in the US are due to flash floods. Although modern Doppler radar technology has improved our ability to estimate the possibility of flash floods, they often still come as a surprise because of the speed with which they start, when unusually heavy rain hits terrain conducive to a flash flood.
"Hundred Year" Floods
Flash Flood. Ridgecrest, California, USA. July 20, 2008.
Flood experts use data on an area’s topography, precipitation, and flood history to estimate the likelihood of flooding in that area. They classify floods by the likelihood that they would occur in a particular period of time. For example a significant event like a ‘twenty year flood’ is theoretically expected to occur once every 20 years, while an immensely destructive “100 year flood” is expected to occur once every 100 years.
This terminology can seem misleading to people, however. Some might think that if a severe hundred-year flood occurred recently, it might not be due again for another 100 years. That’s not what the term means, though. The term hundred-year flood indicates that this type of flood has a 1% chance of happening in any given year, which gives it the statistical expectancy of occurring once every hundred years. One way to visualize this concept is to imagine rolling a 100-sided die each year, with 1 of the sides representing the occurrence of a flood. So, regardless of whether a large flood occurred recently, there is still a 1% chance that a hundred-year flood would take place in a particular year.
Although nature’s power will no doubt continue to amaze and frighten us in the futures, as it has in the past, scientists have gleaned valuable information on flooding over the decades. As long as we continue to learn about the causes of flooding and put this information to good use through smart environmental policy, while improving our flood forecasting techniques, we should be better suited to handle floods as the future unfolds.
Sources: History.com, Weather.com, and National Geographic
Image sources: Rexness, Johnstown Area Heritage Association, Martin, David~O, and Jack
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This article is intended for informational purposes only. Before beginning any construction project at your home or taking steps to prepare for an emergency, please ensure that you take necessary safety precautions; consult construction professionals, your local authorities, and disaster safety experts whenever necessary. Water Damage Defense accepts no responsibility for the actions you take during an emergency or as you prepare for one.