Building a sandbag barrier
When a flood threatens, a wall of sandbags can hold back shallow water and redirect flow away from doors, garages and low walls. Sandbags are cheap and easy to handle, but a barrier takes far more of them than most people expect, and filling them takes time. The calculator estimates how many bags and how much sand you need for a barrier of a given length and height.
How the estimate works
A sandbag levee is built as a pyramid: its base should be about three times as wide as its height, so it resists the push of the water and does not topple. The calculator works out the cross-section of that pyramid, multiplies it by the length of the barrier and divides by the volume of one filled bag. A standard empty bag measures about 14 × 26 inches; filled one-half to two-thirds full, it takes up about a third of a cubic foot once flattened in place and weighs 30 to 40 pounds.
Example
A barrier 100 feet long and 1 foot high, with a base 3 feet wide:
- About 600 bags, or six bags per foot of barrier
- About 20,000 pounds of sand, roughly 10 tons or 7.4 cubic yards
Add 10 to 20% for gaps, repairs and settling. A wall 2 feet high takes three to four times as many bags as one 1 foot high, because both the height and the base width double.
Laying the bags
- Clear the ground of debris and dig a shallow trench, one bag deep and two bags wide, to key the barrier into the soil.
- Lay the bags lengthwise along the direction of flow, with the folded tops tucked under and facing upstream.
- Stagger the joints like brickwork and tamp each bag into place.
- Lay a plastic sheet on the water side for barriers over three courses high.
Limits
Sandbags slow water but rarely make a complete seal, and they are best for heights up to about 3 feet. For anything higher, levee work needs engineering advice. Contaminated floodwater makes used bags hazardous waste in many places, so ask your local emergency management office how to dispose of them. Many counties hand out free bags and sand when a flood watch is issued.