Since statistics are on my mind, and every time there’s a major river flood, someone demands to know how a hundred-year-flood can be happening only thirty years after the last one…
What’s a hundred-year, or five-hundred year flood? And why don’t they live up to their names, at least in terms of frequency of occurrence?
A quick review of riverine anatomy to begin with might be good. The part of the river that is always under water (or is if you live east of the 100th Meridian) is the river, and the deepest part is the thalweg. The dry land that touches the river are the banks. Bankfull discharge, which is critical knowledge if you are measuring river flow, flood frequency, or are planning a river analysis and restoration, is the point where the river rises to every year just before it reaches the flood plain. The flood plain is the flat or flatter area that is covered in water only if the river reaches flood stage. We’ll be talking about flood plains in particular.
As they say in the business, “If you don’t know bankfull, you don’t know sh*t.”
Flood frequency is based on the likelihood of the water reaching that point in any give year. The flood plain has a one year flood frequency or more, depending on the river. You know, that bit of the riverbank park that always goes under water if you get more than a quarter-inch of rain? That’s a one-year (or six-months) flood, and there is a one hundred percent chance of a flood in any given year. The hundred-year flood plain is the area where, statistically, the water may reach once every hundred years, or where there is a one percent chance of a flood in any given year.
That’s it. A flood that, based on historical records, has a one percent likelihood of happening is a once-in-a-hundred-year flood, or “hundred year flood.” If the historic and sometimes archaeological record says “Once in 500 years, the water reached here” you have a 500 year flood, or one that has a 1-500 chance of happening this year. Of course, given that at best we have accurate records going back 300, saying one percent or point one percent is a touch optimistic. Especially on rivers that have been highly modified by man.
If you happen to be the person baling water out of the basement, or ripping out sheetrock that got soaked in muddy water for the second time in twenty years, the odds of “high water in any given year” don’t really matter at that moment. For urban planning, flood insurance, and plans for river remediation, the flood incidence is more important.
Some rivers flood more often than others. Some rivers have flash floods, where water rises on very short notice, surges down the channel and flood plains, then goes away for a relatively long period of time. The hydrographic curve for the event looks more like a saw-tooth than a nice, smooth bell-curve. Those are the streams that tend to eat unwary hikers and drivers.
Others, like the Red River of the North, start to rise in spring and sprawl so far that “flood plain” seems more like a synonym for “North Dakota and Manitoba” than a specific designation. The Red River of the North also has the problem (for flood prevention) that instead of snow melting from the mouth toward the headwaters, like the Missouri, it melts from upstream to downstream, adding more and more water just as the snow to the north starts thinning out, and also flooding frozen ground. Not a happy series of events.
Some rivers had two flood surges, one with spring melt at their headwaters and a second during the rainy season. Each river’s a little different, although there are ways to classify and categorize them when you need to.
Rivers flood, unless humans try to prevent them, and even then, rivers are prone to cocking-a-snock at engineers and real estate developers. The more you straighten a river, the faster the water moves, and the more prone to eating the banks it is. If you build levees, you put a venturi on the river, so that the moment it spreads out, you have fast water that gobbles sediment even faster. When you let water out of a dam, it is “hungry” and carries off sediment just downstream of the dam. Potentially, worst case, it could possible “eat” back to the base of the dam.
A river’s going to do what it wants to. If there was one thing I learned while I was working on my dissertation and subsequent research, it is that rivers and streams do not give a dam about people’s plans. The best thing to do is work with the stream, give it room to meander if it is that kind of stream, and only build things on the floodplain that you are willing to see wash away every so often.
So that once-in-five-hundred-years flood can happen again the very next year, if conditions are right (or wrong). And sometimes the water comes from the wrong direction, as hit Houston and southern Texas this year. Floods are supposed to come downstream, not all at once from the clouds.