Ten-year floods have a 10% probability of happening in any given year (Pe = 0.10); 500-year floods have a 0.20% chance of occurring in any given year (Pe = 0.002); and so on. The probability of experiencing an X-year flood in a single year is 100/X. A similar methodology is routinely used with data on coastal flooding or rainfall. The term "once in 100 years" has no statistical meaning and should not be quoted as the frequency of occurrence of some event.
A ten-year flood is one that has a 10% chance of being experienced in any given decade. Thus, it's possible to experience two, three, or even four ten-year floods in a row. In fact, since a ten-year flood will happen once every ten years on average, it's very likely that at least one ten-year flood will occur during any given decade. A hundred-year flood is expected to occur once in 100 years. A thousand-year flood is said to have a 1 in 1000 year chance of occurring. Note that this definition of a thousand-year flood does not include the likelihood of experiencing multiple such floods in a row.
The chances of experiencing various levels of flooding within any given year or over time vary depending on where you live. Ten-year floods are most common in large cities like New York, Chicago, Houston, and Philadelphia. They're less common but still likely in smaller towns and rural areas. A hundred-year flood is rare everywhere except near coasts or other large bodies of water.
A 10-year flood, for example, has a 1/10 = 0.1 or 10% probability of being surpassed in any given year, but a 50-year flood has a 0.02 or 2% chance of being exceeded in any given year. A 30-year flood has a 0.1% chance of being exceeded.
Floods are the number one cause of death from natural disasters, accounting for approximately 19,000 deaths per year. Flooding can have many different causes; however, all floods share some common characteristics: they occur when it rains enough to raise water levels behind a dam or over another obstruction, and those rising waters reach locations where people live or work. Flood damage ranges from small local flooding to massive national catastrophes. In fact, eight of the top ten deadliest floods in history have been caused by rain rather than snow or ice.
The most deadly floods in history have killed hundreds of people per day for several days straight. For example, the 1994 Chicago flood was the worst single-day flood disaster in United States history until it was surpassed by the 2008 Illinois River flood. The flood killed 251 people and injured more than 500 others.
The deadliest flood in Canadian history occurred in 1837. The Great Montreal Fire destroyed 4 million acres (16,777 km2) of land, including much of downtown Montreal, and killed an estimated 13,000 people.
The term "1,000-year flood" refers to the statistical likelihood of a flood of that magnitude (or higher) occurring in any given year. The 1,000-year flood has a 0.1 percent chance of occurring in any given year, according to statistics. These statistical numbers are based on data that has been collected. They do not take into account other factors that may increase or decrease the chances of a flood, such as local geography and climate.
The human mind is prone to panic at the thought of disaster, so the phrase "1,000-year flood" has become associated with floods that cause immense damage and loss of life. However, a much larger number of people would have to experience simultaneous floods than what would be required to produce a "1000-year" level of damage. A single flood of less than $100 million dollars would not even register on the scale of magnitude used to describe a "1000-year" flood!
There have been many large floods throughout history. Some regions are generally more susceptible to flooding than others. For example, areas near oceans tend to experience more frequent severe storms and hurricanes, which can result in increased levels of precipitation that lead to floods. Climate change may also play a role in increasing the frequency and severity of floods. For example, studies have shown that sea level rise has led to an increase in the frequency of large hurricanes like Sandy.
What exactly do we mean when we say a ten-year deluge has occurred? It indicates that the flood with the largest discharge in the past ten years has happened. The likelihood of this occuring each year is one-tenth of one percent. Thus, the term "ten-year flood" is somewhat of a misnomer, because it implies that such events are likely to happen more than once every ten years.
The correct terminology would be "one-in-ten-years flood" or "one-in-100-years flood". A flood of this magnitude happens about every 200 years!
The frequency with which these floods occur depends on how often large rivers change course or overflow their banks. If they were to do so every ten years, then there would be no danger from river flooding. But since this does not happen very often, scientists use other factors to predict what might happen during a given period of time. They note that large storms at least occasionally hit the same region of the country, and when they do so water flows in the same direction, so they can estimate how much rain fell where by measuring the levels of rivers or lakes. This is how they know that such floods are not likely to happen more than once every ten years.
A "500-year flood" has an AEP of 0.2 percent, which indicates that a flood of that size or bigger has a 0.2 percent probability (or 1 in 500 chance) of occuring in any given year. Floods are measured in feet, not inches.
The figure comes from using data on the historical distribution of floods across the United States to estimate what might be expected at Lake Mead. The lake provides more than 90 million gallons of water per day for use by people in Las Vegas and other areas nearby. It also serves as one of the main sources of water for Southern Nevada's growing population.
In general, the likelihood of a given event happening at least once in 500 years is called its "frequency". So a "500-year flood" means that such a flood has occurred somewhere in the United States every 500 years on average. There are no official guidelines for what would make a river "at risk of flooding" from mining activity, but government agencies generally consider activities that raise the level of water in a river by 10 feet or more within 50 miles of its source to be significant threats to public safety. The federal government's Bureau of Reclamation estimates that about 35 percent of the land in the Western United States is currently subject to such risks.
A "1 in 200 year flood" has a one in 200, or 0.5 percent, probability of being equaled or exceeded in any given year. Such floods are so rare that they occur approximately every 20 years. A flood of this magnitude would cause $20 million in damages.
A single flood is not likely to produce such severe damage. The likelihood increases when there are several floods during a season or series of seasons. For example, if 10 inches of rain falls in one month and another 10 inches in another month, there is a 1 in 2 chance that the total for the year will be 20 inches. If a major river keeps flooding after several months of dry weather, you can assume that there's a good chance that the total for the year will be significant.
Total rainfall of 20 inches within a month is very unusual. However, if this happens twice in a season, then you're looking at a 1 in 200 year event. Three such events in a season makes it a 1 in 2000 year event. And so on.
The precise number depends on the specific river, but generally, a 1 in 200 year event is considered very large.