A sandbag dike must be built properly to prevent or reduce flood damage.

Bags are made from various materials, including woven polypropylene and treated burlap. They usually measure about 14 inches wide and 24 inches long. Other sizes are available, but bags are easier to handle if weight is limited to 35-40 pounds.

Sand is the easiest material for filling and shaping sandbags. Silt and clay in bags will form a good dike but are more difficult to work with. Fill sandbags about one-half to two-thirds full.

Estimate Sandbags Needed

Build the dike at least one foot higher than the projected crest level to allow for fluctuations in the water level. Local experience will assist in determining the amount of free board to provide. The base of the dike should be about three times as wide as the dike is high to withstand the forces exerted by the flood water.

Each foot of finished dike length requires 1 bag; each foot of height requires 3 bags; and each 2.5 feet of width require 3 bags.

Use the following equation to estimate the number of 30-pound bags required per linear foot of dike.

N = (3 x H + 9 x H x H) / 2

N – Number of bags required per linear foot of dike

H – Dike height (ft.)

Example:

Estimate the number of bags required per linear foot for a dike three feet tall. N = (3 x 3 + 9 x 3 x 3) / 2 = 45 bags

The estimated number of bags needed for 100 linear feet of dike is shown in the following table:

Contact your county emergency management office for information on where to obtain sandbags.

Estimated cubic yards of sand needed per 100 feet of dike length for various dike heights and a dike width about three times dike height are shown in the following table.

A cubic yard will fill about one hundred 30-pound sandbags or about eighty 40-pound bags. Sand weighs 100-130 pounds per cubic foot, depending on moisture content and packing.

Site Selection

Select the location for the dike, taking 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, due to the forces the dike may place on the building. Leave at least 8 feet to maneuver between the dike and buildings for observation, pumping seepage water, and other activities.

Since friction holds a dike from sliding, create a good bond between the ground and the dike. Remove ice and snow or anything else that is “slippery.” If the dike is to be more than about 3 feet high, dig a bonding trench where the dike will be placed. The trench should be at least 4 to 6 inches deep and a couple feet wide.

Filling Sandbags

Fill the bags one-half to two-thirds full and do not tie them closed. Untied bags form into the dike better than tied bags. Filling sandbags is usually a two-person operation. One member of the team holds the bag on the ground slightly in front of his or her spread feet, and the second shovels the sand into the bag. Use gloves to protect the hands of the bag holder. The use of safety goggles is desirable, especially during dry and windy days. For large-scale operations, filling sandbags can be expedited by using bag-holding racks, funnels on the back of dump trucks used for sanding operations, and various power loading equipment. However, this special equipment is not always available during an emergency.

Stacking Sandbags

Lift bags using your legs, and limit twisting of your back.

A triangular or pyramid shape for the dike is not necessary if the height will be 1 foot or less. Support the wall of sandbags with “clusters” of bags every 5 feet to stabilize the sandbag wall so it does not tip over due to the force exerted by the floodwater.

Place the first layer of bags lengthwise (parallel to the flow), lapping the bags so the filled portion of one bag lies on the unfilled portion of the next, with the untied open ends of the bags facing downstream. Offset adjacent rows or layers by one-half bag lengths to eliminate continuous joints, similar to what is done laying bricks. Compact and shape each bag by walking on it. Continue to walk on the bags as succeeding layers are placed to eliminate voids and form a tight seal.

The base of the dike should be about three times as wide as the dike is high to provide adequate friction surface area. A tremendous force is exerted by the water against the dike. Estimate the force on the dike base by multiplying the water depth by 62 pounds. For example, the force exerted on the base of a dike by five feet of water is about 310 pounds per square foot. Flowing water striking the dike exerts even more force on the dike. A triangular shape is desirable because it permits the weight of the water to push down on the dike to help hold it in place. Place the same number of rows on the bottom layer as there will be vertical courses of bags, to obtain the triangular shape. The second layer of bags should be perpendicular to the first layer, then alternate the direction of the bags to the top of the dike, alternating header courses (bags placed crosswise) and stretcher courses (bags placed lengthwise).

Lapping Sandbags

Stacking Sandbags

Sealing the Dike

The finished dike should be sealed with a sheet of plastic to improve water tightness. Spread a layer of soil or sand 1 inch deep and about 1 foot wide along the bottom of the dike on the waterside. Lay polyethylene plastic sheeting so the bottom extends 1 foot beyond the bottom edge of the dike over the loose soil or sand. The upper edge should extend over the top of the dike. Poly sheeting at least 6 mils thick is preferred. It is generally available in 100-foot rolls from construction supply firms, lumberyards and farm stores. Do not put plastic sheeting under the bags, since that will increase the potential for the dike to slide.

Lay the plastic sheeting down very loosely. The pressure of the water will then make the plastic conform easily to the sandbag surface. If the plastic is stretched too tightly, the water force could puncture it. Place a row of sandbags on the bottom edge of the plastic to form a watertight seal along the water side. Place sandbags at about 6-foot intervals to hold down the top edge of the plastic. Place boards or dirt between the bags on the top to prevent winds from disturbing the plastic. Avoid puncturing the plastic with sharp objects or by walking on it.

Use a sump or skimmer pump to remove water that permeates through the dike. Use ground fault circuit interrupters on circuits or extension cords to reduce risks from using electricity

Sealing a Dike

Managing Volunteers

A dike will fail if not built correctly, so it is very important that people placing sandbags be trained on proper procedures. In the rush, volunteers will do something, but the result is frequently a dike that performs poorly or fails. Put a high priority on planning and organization. Identifying a supervisor for the project is recommended.