How Sandbags Work

Before you start scooping sand from the local playground's sandbox into a plastic grocery bag, we should tell you a few things about how sandbags are put together. First, sandbags are changing with the times. Burlap once was a popular material to use, but today's bag of choice is polypropylene plastic. This kind of plastic is similar to the kind you might see in a fertilizer bag. Polypropylene bags win out over other material because they won't decompose as easily when subjected to abuse by the elements.

Second, the common sizes for today's sandbags measure around 14 inches wide by 26 inches long (36 centimeters by 66 centimeters), 13 inches by 30 inches (33 centimeters by 76 centimeters) or something close to these measurements. If you can't quite picture that, a USA Today newspaper, folded out, measures 12 inches by 22 inches (30 centimeters by 56 centimeters). Bags with other dimensions will work, but you don't want them to be too large. An optimally-filled sandbag should weigh around 40 pounds (18 kilograms). An overstuffed sandbag doesn't have enough give, and you want the filled bag to mold into the gaps in the sandbag wall.

Sometimes, you're not going to have access to sand -- especially if the rest of the people living in your town are scrambling for it, too. In an emergency situation, like when an overflowing river is about to convert your den into a swimming pool, you could turn to clay and gravel, but these materials are not preferred. Why? Remember that you're going to have volunteers filling these bags as quickly as they can. Clay and gravel are harder to manipulate and handle, so it's going to take the volunteers longer to do their job. When the river is knocking at your door, time is certainly of the essence.

As we discuss more about how sandbags do their job, you'll learn why sand works as well as it does.

People have used sandbags for several hundred years, going back as far as the Revolutionary War [source: Leibenluft]. With science advancing in so many areas, why do we still use this low-tech solution to control floodwaters? Here are three simple reasons:

First, sandbags are easy to use. To prepare sandbags, it only takes a few volunteers with shovels, gloves and some knowledge of safety precautions. When a flood is impending, everyone has to work fast, so you don't have the opportunity to bring in experts and engineers, let them scratch their chins and figure out the best thing to do. You need quick action; you want all hands on deck and supplies that are easy to get. It's hard work to make sandbags, but it's not rocket science.

Second, sandbags are inexpensive. The bags and the sand are cheap to buy, so most people can use these. You can buy the actual bags in quantities ranging from 10 to thousands per box. The average person who is about to be knee-deep in water in his or her living room may not have thought to order sandbags in advance, but this isn't a cause for concern. You'll find the bags at a Home Depot or any similar type of store. For example, you can get 500 sandbags at a Home Depot for $179, or 36 cents a bag [source: Home Depot]. When you go to buy your sand, ideally, you want to buy a coarse-grained type of sand. Think of sandbox or playground sand as high quality and scale down from there. If you need a better guideline, try this one: Unused sand from sandbags ends up in gardens as soil or fill. People also save it to sand icy roads in wintertime. That's the kind of sand you're looking for; however, in an emergency, sand is sand. Use what's available.

Third, sandbags work. They've been working for years, and if it's not broken, don't fix it. So now we know why we rely on sandbags, but how do they work?

If you've ever gone to the beach and let a handful of sand flow through your fingers, you've probably noticed that sand is made up of lots of tiny bits of rock that have been broken down over time, thanks to the elements. And if you've been to more than one beach, you may have observed that the sand you encountered at each different beach was different. It's no surprise that the composition of sand varies in different geographic areas, but there are certain common minerals found in most sand, such as quartz, a very hard and durable mineral.

Mineral hardness is measured by a standard called the Mohs scale. Quartz has a Mohs rating of 7, with 1 being the softest and 10 being a mineral of perfect hardness, like a diamond. Quartz is chemically inert, meaning it keeps its original form and doesn't react with most substances, so it changes little with time and weathering (including barrages of floodwater).

Porosity is another important attribute of sand. The trait refers to a substance's ability to let light, air or water flow through it. The more porous something is, the more water can pass through it. Sand is a coarse-textured material, and the coarser the texture of the material, the less porous it is. Not a bad quality if you're talking about a material that's used to help control flooding.

In a sandbag, the sand itself doesn't hold back the water; there are several different things going on within and around the bag. Floodwaters, filled with fine particles like silt and clay, are flowing into the sandbags. The sand basically "catches" these particles. The clay and silt fill the gaps in the sand, actually making the sand a better barrier. As more water flows through, the sand gets muddier and muddier, which is a good cycle, as it allows less and less water to seep through the bag. In addition, the sandbag is getting heavier and heavier, so if it's placed in the right area, the floodwaters won't wash the bag away.

It may have been coincidence and convenience that caused us to create sandbags in the beginning, but composition and a bit of geology have kept this tool around.

Ready to start fighting back floodwaters? First, you'll want to have an idea of how many sandbags you need. Using specifications set up by the U.S. Army Corps of Engineers, imagine this. You want to build a wall that's 4 feet (1.2 meters) high by 10 feet (3 meters) wide. The wall should be in a pyramid shape -- we'll discuss the reason you build a pyramid in the next section. What's important to know here is that you will need 78 sandbags for each foot of that wall [source: Leibenluft].

Now, let's get filling. Sandbags should only be filled one-half to two-thirds full so there will be room to tie the bag or fold over the top. Again, you want your bag to weigh around 40 pounds (18 kilograms) so that the sand has room to move around. This is key when the bags are laid down, as you want them to mold together into a seamless wall. Teams of two to three volunteers can easily fill sandbags, taking simple safety precautions, such as wearing gloves to handle the chemically treated bags. Goggles are helpful, too, especially if it's a windy day. After that, it's just common sense -- one person holds the bag open with the second person adding the sand to the bag. If there are three people on the team, the third person can pile the filled sandbags.

You may wonder how many grains of sand it takes to make a 40-pound sandbag. It depends on how fine or coarse the sand is. But to get an idea, one measuring cup holds about 12.5 ounces (0.35 kilograms) of sand, give or take. You'd need to scoop around 51 cups of sand to fill your bag. Thankfully, volunteers use shovels, not measuring cups!

Though manually filling sandbags is the easiest and least expensive method, it's also the most time-consuming. Automated options exist for times when you need to quickly produce a substantial number of sandbags. Bag-holding racks, funnels on the backs of dump trucks, and other commercial equipment can speed up the process. This equipment isn't always available -- especially in emergency situations or areas that are difficult to reach. Numerous companies sell prefilled sandbags, and an online search yields multiple opportunities to buy them, in case you live in a flood-prone area and want to stack some away for a rainy day.

When you think about preparing for a flood and battening down the hatches, you might envision piles and piles of the sandbags you just pulled together stacked in front of your home - the bigger the better. Wrong. You don't want your wall to be too high or it will lose effectiveness and not withstand the water. Here are the steps you may want to follow when placing sandbags:

[sources: State of California, The Resource Agency, Department of Water Resources; FEMA]

Keep in mind that you also want to use sandbags in doorways and other openings in a home.

Like most things, sandbags aren't perfect. They won't save you in all flood circumstances. You don't want to place them in standing water, so if an area is already flooded, it's too late for sandbags. Instead, focus on getting your family and your belongings out to a safer area. Sandbags are set up to help with what is called "low-flow protection," which protects against waters that are just a few feet high. Anything higher than that needs a larger, more sophisticated form of protection. If ongoing flooding is the issue, sandbags will struggle to hold up over time, because the bags themselves do decompose. Sandbags can be a great tool, but when dealing with Mother Nature, our solutions have their limits.

In a severe flood situation, thousands and thousands of sandbags may line a vulnerable area, sometimes upwards of millions. What do you do with them once the water's gone? Another shortcoming of sandbags is the disposal dilemma. Getting rid of sandbags poses a problem for several reasons. Wet or otherwise used, sandbags often contain contaminated sand, as floodwaters are filled with lots of unclean materials, so the sand in these bags can't be reused. Often, cities set up distribution sites to which residents can bring their sandbags. Disposal companies often don't want sandbags because they take up too much space in landfills [source: Santa Clara Valley Water District]. Unused sandbags can be stored, or the sand can be used for routine uses like filling sandboxes or play areas.

From mid-May until the end of November, names like Alex, Gustav and Katrina scatter across the weather ticker as people wait to hear which will be the Big One. The National Oceanic and Atmospheric Administration (NOAA) says U.S. floods are the largest severe weather killer. Flood control has become a concern for people all over, and while sandbags are an imperfect, labor-intensive, low-tech option, they've stood the test of time. Though people have developed different technologies to do the job of the sandbag, nothing has replaced it yet, and maybe nothing ever will.

Keep reading for more links on flood control and disaster management.