One factor to keep in mind when protecting your home and property in the event of a flood is that sandbags are simply meant to divert water and debris away from the home, but do not create a water-tight seal. Proper placement will, however, significantly reduce the amount of water that seeps in and there are ways to keep as much water out as possible.
First, you will need to prepare the site where the bags will be placed. Whether you will be placing them against the building or creating a barrier or a wall, you should clear the area where they’ll be placed by removing debris such as large sticks, etc. These may cause gaps in your structure. Also clear the site of any ice or snow. Your barrier won’t serve you any good if the ground is too slippery and your sandbags all slide away when the water flows so you need to be sure there is enough friction to keep them in place.
Some bags will come with ties to close them once they have been filled. If your bags need to be tied for transportation tie them towards the top end of the bag and flare out or flatten the tied end. This will give the sand or the soil flexibility to move within the bag which will create a better barrier when laying them down atop of one another. For un-tied sacks, fold over the area of the sack that is not filled and tuck it underneath the sack so that the weight of the bag keeps it closed off. The bags should be placed so that the opened end will be facing against the expected water flow. Likewise, they should be stacked parallel to the direction of the water flow.
When you are creating a sandbag barrier, it’s best to keep it to just two layers. The exception to this would be if you were planning to create a pyramid barrier. The barrier should be slightly wider at the base than height to give it more stability. Complete one layer at a time and tamp down each bag into place You can do this by stepping on top of each bag to compact it. The tighter the bags are packed together, the less chance of water seeping through. As you place the bags for the next layer, stagger the seams as you would if you were laying down bricks.
To protect buildings from water getting in where the water levels are expected to rise you would stack the sandbags against the building. For sliding glass doors, windows, vents or other entries you can create a seal to prevent water intrusion by placing a plastic sheet (such as visqueen) on the ground and up against the wall then stack the sandbags on top. If you will be creating a barrier around buildings be sure to allow a path for the water and debris to flow between the buildings.