how sand bags help prevent flooding, yet sand is also used for drainage since it’s porous and water easily flows through it?
In: Engineering
You can use them in two ways:
1. Sandbags only. The bags aren’t completely full and so can be formed to fit together pretty snugly. They’re not totally waterproof, but they can direct the vast majority of water in the desired direction. Some water leaks through, but dealing with a much-reduced amount of seeping water is much easier than dealing with a rushing torrent and debris.
2. Sandbags + a layer of plastic sheeting. The sheeting is impermeable to water. The sandbags provide a dike-like structure to resist the force of the water (which keeps the sheet from being washed away) and protect the sheet, and the sheet provides a waterproof barrier. See [this](https://www.ndsu.edu/agriculture/extension/publications/sandbagging-flood-protection) for an example.
In my local flood training course they had us build the dike, add the sheeting, and then cover the sheeting with a layer or two of sandbags to protect it from damage from debris flowing by. In the example above the sheeting is exposed. Your mileage may vary.
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