ROCK DOC - Floodplains brook disaster for unwary

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You have certainly done business on them, and you may well have lived within their boundaries. Whether you are reading this in the desert West or the soggier regions of the country, floodplains are a part of the landscape around you - and they can be highly dangerous places to be.

A floodplain is the flat part of the Earth beside and around a river. It's also the place we like to build houses and schools and stores because it's easier to build on flat ground rather than a steep hillside. Especially in the old days, when materials came by boat to a town or perhaps to rail yards built on level floodplains, entire towns were built on lowlands with hills around them sometimes left less populated.

It's a recipe that works well enough, most days. But when conditions veer from normal, of course, it's a disaster for whole towns and their inhabitants.

A river can only move so much water downstream. If it's full to the brim and moving as fast as it can, it doesn't have the capacity for more. The technical term is "bank-full" and the stream cannot accommodate more water without flooding.

A flood is a natural event in which the stream simply occupies the floodplain as well as the stream channel. To put it another way, the floodplain is a natural part of the stream system that a stream or river occupies only occasionally, rather than every day.

But that's the part it's tough for us humans to graciously accept. We see flat land and we tend to start building: hotels, stores, hospitals, libraries and more.

Most of the time, we get by reasonably well with our decisions of where to locate our installations, ranging from concrete monstrosities to basic campgrounds.

In a normal flood, water overtops its banks in a fairly predictable way, one in which the authorities can warn people about. But flash floods are very different because they hit so quickly. They often affect smaller streams. If a deluge occurs above part of such a stream, the water level will rise rapidly.

In the case of a deadly flash flood in Arkansas, water levels rose up to 8 feet an hour. Because the rainstorm - a true Noah-like deluge - occurred in the night, campers never saw it coming. Even if they had been awakened with warning bells of some sort, many of them still would have lost their lives because the slopes of the hills they were in were so steep and some roads out of the area were blocked by floodwaters. The violence of the torrent increased as the night went along - with asphalt being torn off roads by the currents at one point.

Flash floods occur in the driest deserts, too. That's because of two factors. One is that rainfall in deserts tends to be either non-existent or extreme. In other words, there aren't many gentle rains in the desert. And the ground in the desert is often covered with a crust-like material that slows or even prevents water from percolating into the ground - natural pavement, if you will.

So if you camp in the desert this spring or summer be sure - doubly sure - not to pitch your tent in an arroyo or any other type of "dry stream." That's been a fatal mistake made by too many people not thinking ahead about what heavy rainstorms upstream can mean for local conditions.

Rural Northwest native E. Kirsten Peters, Ph.D., trained as a geologist. Her column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University. Follow her at rockdoc.wsu.edu. "Planet Rock Doc," a collection of her columns, is available via wsupress.wsu.edu or 1-800-354-7360.

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