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
there are multiple kinds of sand; some sand works better than others; especially when wet.
sand bags are very malliable which makes them great for stacking around corners and bumps and curved areas
sandbags are NOT water tight and arent designed to be; since water will take the path of least resistance the sandbags help “guide” the water away from where we dont want it to go
Sandbags are really only used to divert floodwater into a specific direction, typically away from a house or a building. They don’t require any real knowledge to use them, they’re cheap, and they’re malleable. They are not water proof, and there is definitely bound to be leaking. But if you have a situation develop quickly and need to divert water, sandbags are very useful.
One sandbag doesn’t do much, because as you said, sand is porous. However, put a few 50 pound bags of sand on top of each other and they are much less porous, as they are compacted.
Water will follow the path of least resistance
If you put a pile of sandbags down, it’s much harder for the water for get through than just going around it, so the water will do that and go around it
It’s not 100% perfect but it will stop most of the water
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.
Sand bags work by creating a physical barrier that redirects the flow of water, whereas sand used for drainage allows water to filter through and be absorbed by the soil underneath. It’s all about the placement and purpose of the sand!
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