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.