How high’s the water, mama?
Three foot high and rising.
I have no idea of the origin of a song I remember from my childhood, but flooding is by no means a new problem. Furthermore, due to the very nature of the event, it is even more of a wildcard than tornadoes.
I would endeavor to propose that there is no area of land on the planet, save in permafrost regions, where flooding does not pose a threat. This is because flooding has so many potential causes. Abnormally heavy snowfall, followed by a sudden bump in average temperatures can cause six or seven feet of snow to melt at a rate which overwhelms a drainage basin. A tropical storm can push water ahead of it up river channels in a surge like those that inundated New Orleans during Katrina or left the subterranean commuter routes between the New York City burroughs filled to the brim. Sometimes, its simply an unusually wet season in a region unaccustomed to it, like occassionally occurs in the Mojave Desert, refilling the faint dotted blue lines that mark dry lake beds on the maps. Or a flash flood can concentrate all of the water that falls in a large area into a single drainage channel, throwing trees and boulders into anything in its path.
Storm surges are a unique type of flooding, and the mechanics that cause them are completely different from “freshwater” flooding. For information on the origins and dangers, please refer to my post New Normal: Hurricanes.
All other floods work in the same basic way. They are driven by a localized precipitation, soil conditions, and terrain. Let’s look at the basic math. There are 128 ounces in a gallon. Since ounces are equivalent to 30 mL or cubic centimeters, and one cubic inch is equivalent to 16.387 mL, we know that one gallon of water contains 2097.5 cubic inches of water. Since we measure rainfall in the US in tenths of an inch, and not feet, we can divide 2097.5 cubic inches by 12 inches in the vertical foot to surmise that one inch of rain falling over 174.79 square feet leaves us with one gallon of rain water. To simplify slightly, we’ll round this last figure to 175 square feet per gallon. One acre is 43560 square feet, which means that if a single acre receives an inch of rain, nearly 250 gallons of rain have fallen.
Rain storms rarely limit themselves to single acres, so lets look at the next aspect, soil conditions. The type of soil on which your home is built, and on which your community, and region rest, influences flooding. This is because the more dense the soil, either because of the size of the particles or the amount of compacting, the less water it can hold. The property I grew up on had been scraped of its topsoil to use as fill dirt at some point, leaving us with little more than yellow clay, which absorbs little water. When I was in grade school, then, anything more than a couple of inches of rain would leave us with enormous puddles to play in. In the mid 1980s, my dad had a few trucks of fill dirt put in to give us topsoil, and the flooding problem ceased because the loam and organic material could absorb more water.
Volcanic ash is very light, and unable to absorb water, making areas where ash is the primary component very prone to either flooding on the plains, or closer to the mountains, to mudslides. Concrete, of course, can absorb essentially zero water because of its dense nature, which is why street flooding is more common in urban areas than in the suburbs. You can gauge the absorbancy of your local soil every time you overwater your lawn. The less water it takes to form puddles (taking into account recent rainfall) the denser the soil.
The final consideration in flooding chances has to do with terrain and topography. The Rocky Mountains are famous for their narrow, fast moving streams, while slow moving rivers wind through the heartland. At the risk of oversimplfying things, the farther down the food chain of your particular drainage you live, the more brittle your flooding chances are. A small creek passing through farmland can break out of its banks with the addition of only a few inches of rain. If a storm drops an inch of water over a square mile, that’s 250 gallons * 640 acres, or 160,000 gallons. Again, the speed is important. The Mississippi River flows past New Orleans at a rate of approximately 450,000 gallons per second. That’s 162 million gallons per hour. A rainstorm that drops an inch of rain on a square mile area of the river outside NOLA in an hour would only increase the amount of water by about 1/10%. At Lake Itasca, where the river originates, the rate is only about 16,200 gallons per hour.
Thus, an inch of runoff falling just below Lake Itasca would cause some flooding, as the water in the river is increased by 1000%, while an inch of rain near its mouth would be more than just figuratively a drop in the bucket.
Flash floods are similarly problematic, often caused by rainstorms that occur miles away, over terrain where there is only a limited amount of drainage. An inch of rain falling over 100 acres in the mountains may quite easily force all 1.6 million gallons of water down a canyon only a dozen yards wide, and the dry river bed can quite rapidly be several feet deep, and extremely forceful because of the same concept that causes a gentle rain to force water out of your rain gutters hard enough to cause the soil around the drain spout to erode, only on a much more epic scale.
In truth, precipitaiton is quite variable, although they seem to run in cycles. St. Louis, MO, for example, has been monitoring precipitation amounts since 1837, yet six of their top seven wettest years have occurred since 1980, including the #1 and #5 in 2008 and 2009 respectively, both with over 50″ of moisture. 1980 ranks as the #9 driest year on record, with only 27.5″ falling, and along with 1976, at #2, are the only in the top ten dry years to occur during my lifetime.
The problem we’re facing, as with many of the other types of disasters, is due to development. Not only do nearly all of our major cities sit on flood plains, many on the very banks of a river, but our efforts to protect these population centers may well be compounding the problem.
To prevent the Mississippi from flooding the corn fields of Iowa, Illinois, and Missouri, for example, we’ve installed wing dams, levees, and other structures to keep the water within its banks. 150 years ago, major snowfall in the Dakotas or Minnesota would flood locally, and then systematically push water a mile or two into the croplands of the surrounding states. Now, instead, we hold all of that water within the banks of the river until, fed by the Ohio and Missouri systems, it breeches into Memphis, Baton Rouge, or New Orleans.
The worst flood in recorded history along this river was in 1927, when rainfall was so extreme across the Midwest that flooding was not contained to the Mississippi, but rather pushed up the Missouri, Arkansas, Ohio, and Tennesee Rivers, flooding 7 states, and displacing millions. A single breech in the levee system, near Greenville, MS, was 100 feet deep and half a mile wide, and in the course of 10 days put a million or more acres under ten feet of water. For those doing the math, that’s an area of approximately 1562 square miles. By comparison, Long Island, New York, covers approximately 1400 square miles.
To sum up this suddenly very long post, flooding can occur anywhere that temperatures rise above freezing. Your chances of being flooded hinge on the amount of moisture you get, how permeable the ground is, and the type of terrain you live in. Whether you live on the vast plains of the Midwest, the swamps of the Deep South, or the concrete jungle, flooding is a real possibility, and one that’s difficult to predict until after the rain starts to fall. So be prepared. Get a Plan, pack a kit, stockpile some food.
Because this is the new normal.