Hurricane Sandy and its Sandbags

News photos show a great range of sandbagging work on the Mid-Atlantic seacoast in the last few days -- this montage from Google images:

New York Magazine named Goldman Sachs as Top Sandbagger for this fortification (photo, Greg Roumeliotis)

I've heaved my share of sandbags, most recently at a flood zone along the Mississippi, but it's been a few years, so I needed to refresh my recollection of sandbagging dos and don'ts. Here's a website with very good info.

The following is gleaned from North Dakota State U at Fargo, an area that sees almost annual sandbagging due to spring flooding along the Red River of the North. 

Because most news photos of Sandy-sandbagging don't indicate whether a particular job was underway or thought to be complete and ready for the storm surge, I won't pass judgment on the attempts, but here are a few sandbag pointers.

Good sandbags are polypropylene weave and filled only a little more than half full of sand. And they don't need to be tied shut. Why? When half full and left untied, or half-full and tied at the top with lots of room left inside, the sand can shift to close up voids between the bags, which prevents leaks through the wall. The sand in a full bag, like a plastic bag of sand bought from the home-improvement store, can't do that. 

They're laid down parallel with the water flow, the open ends pointing downstream:

If a wall is to stand less than a foot high, construction can be as simple as a long row of bags, stacked atop each other, braced intermittently with columns of bags stacked on the dry side. 

But ... the labor requirement goes up exponentially if the bag wall is to stand over a foot high. That's because the sandbag wall has to take on a pyramidical cross section -- otherwise it will topple from the water pressure. And if it's to stand more than three or four feet high, it probably needs a "key" trench to keep the whole mass from sliding due to the water pressure:

The number of sandbags required for a neck-high wall is sobering. A hundred-foot stretch of wall that is five feet high, with a base ten feet wide, will need at least 9,000 bags weighing 35 to 40 pounds. And that's the bare minimum: the US Army Corps of Engineers recommends that the base be closer to fifteen feet wide.

Here's a diagram on making a sandbag wall more water-resistant, with a length of anchored plastic sheeting on the wet side: