NDSU Extension Service agricultural engineer Ken Hellevang has advice for building dikes, if river levels rise as predicted this spring. "Knowing the proper procedures for placing sandbags is very important," he said. "Put a high priority on planning and identify a supervisor for the project."
Woven polypropylene bags are the most commonly used type in this area. The bags usually are about 14 inches wide and 24 to 26 inches long. Other sizes also are available, but the bag is easier to handle if limited to 35 or 40 pounds with filling material in it.
Sand is the easiest material for filling and shaping sandbags. Silt and clay in bags will form a good dike, but working with those materials is more difficult.
Hellevang has this advice for building a sandbag dike:
• Fill sandbags about one-half full and do not tie bags closed. Untied bags form into the dike better. Lift the bags using your legs to limit the strain on your back.
• Build the dike at least one foot higher than the projected crest level and take advantage of natural land features that keep the dike as short and low as possible. Avoid obstructions that would weaken the dike.
• Do not build the dike against a building wall because of the forces the dike may place on the building. Leave at least eight feet to maneuver between the dike and buildings.
• Remove ice and snow from the ground before starting to build a dike or the dike will leak when the snow and ice melt. Do not put plastic sheeting under the bags since it will increase the potential for the dike to slide. If the dike is to be more than about three feet high, dig a bonding trench where the dike will be placed. The trench should be at least four to six inches deep and 18 to 24 inches wide.
• Use a sump or skimmer pump to remove water that permeates through the dike.
The U.S. Army Corps of Engineers recommends building a dike three times as wide at the base as it will be tall. For example, a four-foot-high dike would have a base width of 12 feet.
Here's how to build a dike, according to Hellevang:
• Place the first layer of bags lengthwise (parallel to the water flow), overlapping the bags so the filled portion of one bag lies on the unfilled portion of the next, with the untied open end facing downstream.
• Offset adjacent rows or layers by one-half bag length to eliminate continuous joints – similar to laying bricks.
• Compact and shape each bag by walking on it. Continue to walk on the bags as you place succeeding layers to eliminate voids and form a tight seal.
• Seal the completed dike with a sheet of plastic to improve water tightness. Spread a layer of soil or sand one inch deep and about one foot wide along the bottom of the dike on the water side. Lay polyethylene plastic sheeting over the loose soil or sand with the bottom extending one foot beyond the bottom edge of the dike. Place a row of sandbags on the bottom edge of the plastic to form a watertight seal along the water side. The upper edge should extend over the top of the dike. Place sandbags to hold down the top edge of the plastic.
For more information, visit www.ag.ndsu.edu/disaster/flood.html.
After the flood, do not use sand from bags exposed to floodwater for children's sand boxes because floodwater generally is considered polluted, Hellevang said.