The Mississippi River breached more than a dozen levees in the St. Louis, Mo., area Thursday as flooding continued to spread across the Midwest. To mitigate the damage, the U.S. Army Corps of Engineers alone has distributed nearly 13 million sandbags, most of which have been filled and laid down by local residents. Why do we still use sandbags?
Because they're cheap, easy to use, and usually effective. The familiar image of the burlap sack stuffed with sand goes back at least as far as the Revolutionary War—when they were used to build makeshift forts—and they have long been deployed as a defense against deluges like the Great Mississippi Flood of 1927. These days, the bags used to hold back rising floodwaters are more likely to be made of polypropylene plastic, often taken from the scraps of textile manufacturers. They cost about a quarter apiece, and they are packed for delivery by the thousands to flood-stricken areas.
Then locals have to find sand to put in the bags. In Iowa, it has come from local quarries that normally serve as suppliers for construction. Sand has the benefits of being inexpensive, plentiful, and easy for untrained volunteers to handle and clean up. (Clay might be more effective at holding back a flood, but it's more difficult to bag and stack quickly—and to remove when the danger is past.) If for some reason sand weren't available, the Army Corps of Engineers says you could use silt or gravel in an absolute emergency (PDF).
Sandbags remain so popular because they are low-tech—all you need are some bags and shovels, manpower, and a whole lot of sand. They're also effective in most cases, having proven reliable for dikes from 4 to 6 feet tall. The best sandbag protection usually comes from packing them about halfway full (which keeps the bags from getting too heavy) and leaving them untied with the top folded over (which makes them easier to stack). The most stable arrangement is to stack them in a pyramid. (Using Army Corps of Engineers specifications, a 4-foot-high pyramid would be about 10 feet wide; for every foot along the river, the pyramid would require about 78 bags.) But experts say that proper construction makes a big difference: Researchers at the University of Manitoba conducted an experiment (PDF) in which they asked two groups—one made up of professional engineers, the other of volunteers given standard instructions—to construct a dike using standard sandbags. The professionals were able to create a sandbag dike 12 feet tall that proved quite effective. But the 6-foot-tall dike prepared by the unsupervised volunteers failed when the water reached its peak level.
Sandbags pose another problem when it's time to get rid of them. If the plastic bags have been out in the sun for a few weeks, they may start to fall apart from exposure to ultraviolet radiation. As a result, the bags themselves can't be easily reused. If the sand is wet, there's the added risk that it has become contaminated by unsafe materials in the floodwater. In Johnson County, Iowa—the site of some of the state's worst flooding—local agencies are trying to figure out what to do with as many as 6 million leftover sandbags. State authorities do not recommend (PDF) dumping the bags into your kid's sandbox or on beaches, but they do say the material can be stored to sand streets in the winter or used as fill material under roads or buildings.
There are some viable alternatives. One example is the HESCO Bastion barrier, which has supplemented sandbags in Iowa City. Consisting of a mesh wire frame wrapped in polypropylene, the barrier—a version of which is used to fortify troops abroad—can be filled with gravel or other material using a front-loader. As long as heavy machinery and trained installers can get to the site of the flooding, the barriers can be set up in far less time and with far less labor than sandbags—and then later reused. Along with the HESCO barriers, the Army Corps of Engineers has also tested barrier devices like the Portadam and the Rapid Deployment Flood Wall; when properly constructed, all three showed lower seepage rates than sandbags under lab conditions.
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Explainer thanks James Blatz of the University of Manitoba, Ron Fournier and Fred Pinkard of the U.S. Army Corps of Engineers, Jennifer Jordan of the Iowa City Landfill and Recycling Center, Mike Sullivan of the Johnson County Emergency Management Agency, and Stephanie Victory of HESCO Bastion USA.